Satellite laser ranging (SLR) stations in the International Laser Ranging Service (ILRS) global tracking network come in different shapes and sizes and were built by different institutions at different times using different technologies. In addition, those stations that have upgraded their systems and equipment are often operating a complementary mix of old and new. Such variety reduces the risk of systematic errors across all ILRS stations, and an operational advantage at one station can inform the direction and choices at another station. This paper describes the evolution of the ILRS network and the emergence of a new generation of SLR station, operating at kHz repetition rates, firing ultra-short laser pulses that are timestamped by epoch timers accurate to a few picoseconds. It discusses current trends, such as increased automation, higher repetition rate SLR and the challenges of eliminating systematic biases, and highlights possibilities in new technology. In addition to meeting the growing demand for laser tracking support from an increasing number of SLR targets, including a variety of Global Navigation Satellite Systems satellites, ILRS stations are striving to: meet the millimetre range accuracy science goals of the Global Geodetic Observing System; make laser range measurements to space debris objects in the absence of high optical cross-sectional retro-reflectors; further advances in deep space laser ranging and laser communications; and demonstrate accurate laser time transfer between continents.
Technical advances in satellite laser ranging (SLR) towards achieving higher measurement precision, eliminating systematic biases and meeting increasing demand have brought about a new generation of SLR station. A technique that was first performed over 50 years ago is responding to the expanding needs of Earth science and finding new science and engineering applications.
Reflected laser pulses, fired from a ground station to a satellite orbiting the Earth, were first detected in 1964 at NASA Goddard Space Flight Center and at Organ Pass, New Mexico (Plotkin et al. 1965; Snyder et al. 1965). Using a pulsed ruby laser, observing the photomultiplier return signal on an oscilloscope and using a range gate generator and a time interval unit, the teams were able to make time-of-flight measurements to the Beacon Explorer 22 (BE-B) satellite that were accurate to a few metres. In the years that followed, many satellites were launched carrying retro-reflector targets, a global network of SLR stations was established and improved technologies brought greater accuracy, precision and capability (Degnan 1994a). Satellite laser ranging contributed to studies in tectonic plate motion, crustal deformation, Earth orientation, the Earth’s gravity field and the relativistic Lense-Thirring frame dragging effect. Today, it continues to maintain the International Terrestrial Reference Frame (ITRF) and to support Earth observation satellites, including altimetry missions, through precise orbit determination and validation.
The current International Laser Ranging Service (ILRS, Pearlman et al. 2002) network, as shown in Fig. 1, is made up of 15 European stations (plus the German, BKG AGGO system relocated to La Plata, Argentina), 11 Russian stations (8 on Russian territory plus a station in Brazil, one in Kazakhstan and a newly installed station in Hartebeesthoek, South Africa), 8 American NASA stations (4 on US territory plus stations in Hartebeesthoek in South Africa, Yarragadee in Australia, Arequipa in Peru and Tahiti in French Polynesia), 6 Chinese stations (5 on Chinese territory plus a station in San Juan, Argentina), 3 Japanese stations, 2 stations in South Korea and a second station in Australia at Mt Stromlo. Stations currently performing routine lunar laser ranging (LLR) to retro-reflector arrays on the surface of the Moon are the Apache Point Observatory, New Mexico, USA, and the Observatoire de la Côte d’Azur, France (Murphy 2013). The McDonald Observatory in Texas, USA, last performed LLR in March 2015.
The network exists to make accurate and precise laser range measurements to satellites on the ILRS target list, currently over 80 objects, including geodetic spheres, Earth observation satellites and Global Navigational Satellite System (GNSS) satellites. Range measurements to the passive, spherical LAGEOS and Etalon geodetic satellites are used in global solutions to define the ITRF centre-of-mass origin and scale (GM), determine precise orbits and station coordinates, monitor tectonic motion and deformation and calculate the Earth gravity field and Earth Orientation Parameters (EOP). The solutions from ILRS Analysis Centres are produced and combined under the guidance of the ILRS Analysis Standing Committee (ASC), which submits the laser technique contribution to the formation of the ITRF (Altamimi et al. 2016) along with submissions from the GNSS, Very Long Baseline Interferometry (VLBI) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) techniques.
The Earth observation satellites supported by the ILRS require precise orbits and a well defined and stable ITRF (Couhert et al. 2015). These include: the Jason oceanography missions that monitor global ocean circulation, sea level rise and climate change (Zelensky et al. 2010); Cryosat-2 that measures ice thickness in the polar regions (Schrama 2018); Sentinel-3a that conducts radar altimetry to study sea surface topography; the three Swarm satellites that monitor the geomagnetic field (van den Ijssel et al. 2015) and SARAL that performs altimetry for oceanography (Zelensky et al. 2015). Laser ranges are used for direct independent validation of the radial component of precise orbits that were derived using onboard GNSS or DORIS systems. These missions put the greatest demand on the ILRS network for accuracy and the Global Geodetic Observing System (GGOS, Gross et al. 2009) has set a goal for geodetic techniques to support an ITRF that is defined with accuracy of 1 mm and stability of 0.1 mm/year.
The GNSS satellites tracked by the ILRS network include the Russian GLONASS, the Chinese Beidou and the European Galileo constellations, the Japanese Quasi-Zenith Satellite System (QZSS) satellites and the Indian Regional Navigation Satellite System (IRNSS) satellites. In addition, the GPS-III generation of Global Positioning System (GPS) satellites will once again carry retro-reflector arrays. Laser ranges to GNSS satellites are used for independent validation of precise orbits, to improve the modelling of radiation pressure forces (Arnold et al. 2015; Sósnica et al. 2015) and are important in the separation of clock offsets from radial orbit errors. As new satellite missions are launched and older spacecraft retire, the number of satellites supported by the ILRS network varies. However, Fig. 2 shows a steady increase over time, and this will increase further if all future GNSS satellites carry retro-reflector targets.
As is clear from Fig. 1, the ILRS network is unevenly distributed globally with most stations in the northern hemisphere and a large number located in Europe. This situation is exacerbated by the relative productivity and stability of stations, resulting in a core subset making the bulk contribution to the ITRF (Pavlis et al. 2017). A SLR station must track 3500 satellite passes, including 600 LAGEOS passes, per year to be considered a well performing ILRS station. Across the network, the variability in technological make-up (including laser functionality, telescope design, timing method and detector type) results in differences in station capability and long-term stability. Therefore, the existing ILRS network could be strengthened through hardware upgrades and the wider adoption of best practices.
The ideal SLR station would be secure on a stable monument mounted on bedrock and its measurements would be of the highest accuracy and precision, limited only by the available technology. It would perform regular system calibrations to a stable, precisely engineered and accurately surveyed terrestrial target to keep track of any changes in system delay. It would keep good records of all system hardware or software changes, archived in the ILRS data centres. It should be capable of tracking all ILRS targets, including the fast moving targets at low altitude, such as the GRACE gravity field missions at about 500 km, and the distant GNSS targets at heights around 20,000 km. The ILRS now supports missions at heights as far as around 36,000 km, such as the Japanese QZSS, the Indian IRNSS and the Chinese Beidou geosynchronous and geostationary GNSS satellites. The station should be able to quickly acquire laser returns and collect sufficient data to form 1 mm precision normal points so that it is free to move to the next target. The station would operate continuously at a site not overly restricted by cloud cover. It would be able to operate during daylight hours using temporal, spatial and spectral noise filtering and overcome any pointing issues by adjusting the angular divergence of the transmitted laser beam and/or using search routines. Ideally, the ranges would be free from systematic biases and any occurring biases would be detected early and eliminated or measured and a correction applied in software. The epoch timestamps would be held close to UTC using GNSS timing receivers steered by a stable frequency source such as an oven-controlled quartz, rubidium or caesium standard or a hydrogen maser.
In the sections that follow, we describe the points of progress at stations that led to changes to well-established systems across the network and the advantages of the new generation of SLR stations. We highlight other areas of research activity in the network before looking at the status of sub-networks and then look at the remaining challenges for the future.
Evolution of the ILRS network
Prior to 2004, ILRS stations had largely settled on a set of tried and tested instruments. Frequency-doubled mode-locked Nd:YAG lasers produced moderate energy (20–100 mJ), short duration (35–200 ps) pulses at a wavelength of 532 nm at rates of about 10 Hz. Time interval units measured the two-way time-of-flight (Degnan 1985), which meant that the laser firing rate had to drop 4–5 Hz for the more distant GNSS targets with values over 0.1 s. There were some early adopters of event timers, specifically those with lunar laser ranging capability such as in Grasse in France, Matera in Italy and McDonald in the USA due to the considerably longer time-of-flight intervals of around 2.5 s. The frequency sources supplied to these timers were mostly from crystal oscillator sources, synchronised by GPS. The compensated single-photon avalanche diode (C-SPAD, Kirchner and Koidl 1999) and micro-channel plate photomultiplier (MCP-PMT, Degnan 1985) detectors were already widely in use. The compensation circuit on the C-SPAD was introduced to significantly reduce the range error ‘time-walk’ in the diodes caused by greater signal strengths (Gibbs and Wood 2000). Some legacy stations, such as the NASA MOBLAS systems and WLRS in Wettzell, Germany, operate higher laser energies and identify potential returns using a detection threshold on the MCP-PMT detector voltage signal. Constant fraction discriminators (CFDs) are used to minimise range errors due to changing pulse amplitudes. Others, such as Herstmonceux, UK, adopted the ‘single-photon’ mode of operation whereby return energy levels were kept at low levels at all times so to avoid SPAD detector time-walk and to make a more consistent distribution of measurements across the retro-reflector array (Otsubo et al. 2015). The C-SPAD and MCP-PMT detectors each had jitters of approximately 30 ps. The most accurate time interval units were the Stanford SR620 and the Hewlett-Packard 5370B, which had reported precisions of 25 ps and 35 ps, respectively. These timers operate by starting and stopping a precise counter, driven by an external frequency source. The linearity of the timer is critical, and the SR620 was known to contain some range dependent bias. This variability would average out over the rapidly changing ranges measured over a satellite pass, but this was not the case for terrestrial laser ranging to fixed calibration targets (Gibbs et al. 2006).
Estimates of one-way system precision were made by combining the laser pulse lengths, the precisions of the time-of-flight recorders and the detector jitter values taken from ILRS site log entries. Figure 3 shows these precisions for previous and new configurations of the Graz, Potsdam, Beijing, Changchun, Shanghai, Herstmonceux and Zimmerwald stations. The original systems had calibration RMS values of a few centimetres, while the upgraded systems have values of a centimetre or less. Even greater precision is achievable in the formation of normal points from such kHz stations, at the level of 1 mm, by averaging many observation residuals at a mean epoch.
The event timers built from Thales/Dassault modules and the EventTech A033-ET, built in Riga, Latvia have precision specifications of less than 5 ps and no nonlinear range dependency. A further benefit of using an event timer is that when recording an epoch on a laser fire it is not caught waiting for the returning signal to stop the counter, thereby enabling multiple shots to be ‘in flight’ at a time, making high repetition rate SLR possible. Additionally, many stations have introduced higher standard frequency sources, such as hydrogen masers, to record more stable epochs.
In 1994, a largely autonomous, lower cost, eye-safe, low pulse energy, high repetition rate, event timing SLR system called SLR2000 was proposed to NASA to track LAGEOS and lower altitude satellites (Degnan 1994b). This was a radical departure from the legacy high energy, low repetition rate NASA MOBLAS systems and all other stations in routine operation in the ILRS network. To help meet the challenging eye-safe requirement for aircraft and personnel, the energy of the 100 ps pulse laser was reduced by over two orders of magnitude to 0.25 mJ and the pulse rate was increased to 2 kHz, resulting in approximately the same average power output as the MOBLAS systems. In order to comply with a NASA requirement that the system be eye-safe at the output of the telescope, SLR2000 was designed so that the transmitter and receiver shared the full 40-cm telescope aperture. Prototype design and construction began in 1997. Unfortunately, an unforeseen change in ANSI laser safety guidelines in 2000 resulted in a factor of 13 lower acceptable peak laser power at the emitter than was originally anticipated. This required a reduction in the laser pulse energy to 60 μJ and an increase in pulse width to 300 ps. In spite of these setbacks, the first successful kHz SLR returns were detected at NASA Goddard Space Flight Center, USA, in March 2004 (McGarry et al. 2004). SLR2000 was a precursor to later NASA kHz station designs.
The Graz SLR station, operated by the Austrian Academy of Sciences, upgraded its timing system from several interval timers operating in parallel, 3 Stanford SR620 plus a HP5370, to an event timer, constructed from Thales/Dassault modules, in August 2000. A High-Q solid-state, diode-pumped laser was installed in 2004 that fires 10 ps, 0.4 mJ pulses at a repetition rate of 2 kHz. Hardware control of the C-SPAD detector gating was necessary to provide exactly timed signals to coincide with the arrival of each returning laser pulse. This range gate generator was designed and built by the Graz team using a FPGA board. An issue of laser backscatter in the atmosphere coinciding with returning signal times was anticipated and overcome using the range gate generator to adjust the laser firing frequency to avoid this overlap. The system had to cope with greater data volumes from kHz SLR, and changes were made to data transfer, storage and the observer display (Kirchner and Koidl 2004).
In 2008, the SLR station in Herstmonceux, UK, followed the Graz example and successfully upgraded to 2 kHz SLR using the epoch timer and laser from the same manufacturers and taking advantage of the range gate generator, developed in Graz. The Chinese stations at Changchun, Shanghai and Kunming upgraded to kHz operations in 2009 and Beijing in 2010. The SLR station in Potsdam, Germany, began 2 kHz SLR in 2011 using an in-house range gate generator design and soon after upgraded their software control to a modern real-time Linux system (Kloth et al. 2014). The station in Zimmerwald, Switzerland, upgraded to 110 Hz in 2008. Unlike these other stations, it has a common transmit and receive coudé path and so to achieve high repetition rate SLR a perforated mirror was installed to separate the two light paths, in place of a mechanically rotating mirror, along with a rotating shutter to screen the detector from backscatter. The Mt Stromlo SLR station in Australia went through a similar upgrade to 60 Hz in 2007 but retained use of a rotating mirror. The Russian station in Arkhyz began SLR in 2006 at 300 Hz, and the station in Altay was upgraded to 300 Hz in 2008. The stations at Badary, Baikonur, Irkutsk, Komsomolsk, Mendeleevo and Zelenchukskaya have since been similarly upgraded.
An advantage of high repetition rate SLR is greater agility. Acquiring a new target can be done more quickly as the high data rates result in more responsive search routines. This leads to a further advantage once a target is acquired in that the upper requirement for a normal point of 1000 laser range measurements or a precision of 1 mm can be obtained quickly and this frees up the SLR telescope to interleave more rapidly between targets. This is particularly true for GNSS targets at high altitudes, which are allocated normal point intervals of 300 s. It is also important that these more difficult targets carry sufficient numbers of retro-reflector cubes to meet the ILRS cross-sectional guideline of 100 million square metres (Wilkinson and Appleby 2011).
With ultra-short laser pulse lengths and the high density of data points common with these kHz systems, it is possible to distinguish individual retro-reflector cubes in range residuals. Figure 4 shows flattened range residuals from a Stella pass observed at the Herstmonceux SLR station. During a pass, individual cubes become visible and disappear as the relative position and orientation of the satellite and laser station changes. Studies using high repetition rate data have been able to measure changing spin rates and spin axis orientation of the LAGEOS (Kucharski et al. 2007), Ajisai (Kucharski et al. 2010), Etalon (Kucharski et al. 2014) and the millimetre level signature BLITS (Kucharski et al. 2011) satellites.
SLR stations must actively prevent the laser firing in the direction of an aircraft overhead to avoid startling the pilot or potentially damaging a human eye. Active radar systems are in operation at many stations to automatically block or inhibit the laser upon detection of an aircraft approaching the laser beam. Since microwave signals emitted by the radar transceivers can potentially interfere with the new, more sensitive, co-located VLBI Global Observing System (VGOS) antennas, alternative safety systems based on multispectral cameras and pattern recognition are being developed. Many stations have introduced Automatic Dependent Surveillance-Broadcast (ADS-B) receivers to use the real-time position and velocity information broadcast from nearby aircraft to monitor the skies and inhibit laser firing.
The introduction of new technology and ongoing innovation and development at stations has paid off with greater accuracy, precision and capability. An advancement demonstrated by one station can influence the development of the network, as is encouraged by the ILRS Networks and Engineering Standing Committee (NESC) and the new NESC online forum (http://sgf.rgo.ac.uk/forumNESC). This can also lead to new opportunities and applications in other scientific or engineering activities.
Emerging applications and diversifying SLR station operations
The methods in operation at SLR stations, for example: high precision timing; fast and accurate telescope control; laser operation and high productivity scheduling, can be used in activities beyond traditional satellite laser ranging. Having additional tasks, alongside the ILRS tracking schedule, can be useful for individual SLR stations for maintenance, development and future funding. Some adaptation, upgrade or system calibration may well be required before beginning the new work. Here we highlight a few areas of activity.
Orbiting space debris laser ranging
The ILRS target list is limited to those satellites carrying a sufficient number of retro-reflecting corner cubes to give a detectable return signal. In 2011, using a more energetic (1 kHz, 10 ns, 25 mJ) laser, the Space Research Institute of the Austrian Academy of Sciences in Graz detected returns from diffuse scatter from a number of ‘uncooperative’ debris targets, i.e. without retro-reflectors, (Kirchner et al. 2013b). Today, a number of SLR stations, mainly in Europe, Asia, and Australia, can track such targets by switching to more powerful lasers and single-photon detection with high quantum efficiency (Lejba et al. 2018). The main goal of these measurements is to improve the orbital prediction accuracy of selected objects (Sang et al. 2014), for example, to refine orbital predictions of a debris object that could potentially collide with an active satellite.
Standard SLR stations use laser powers of around 1 W for high precision laser ranging to ‘cooperative targets’ (i.e. with retro-reflectors): e.g. 0.4 mJ per shot at 2 kHz or 100 mJ at 10 Hz. This is insufficient power for debris tracking. Successful debris tracking has been demonstrated with laser powers of at least 16 W, up to 60 W, at slant ranges of about 3000 km for targets radar cross sections (RCS) larger than about 0.5 m2 with laser wavelengths of 532 and 1064 nm. Figure 5 shows, for example, debris object tracking carried out at the Shanghai station using a 200 Hz, 8 ns, 60 W laser. The Graz SLR station now has a dedicated space debris laser (16 W; 200 Hz; ≤ 10 ns pulses; 532 and 1064 nm) mounted on the main SLR telescope. This approach avoids the problems and limitations of a conventional coudé path, such as transmit/receive path separation for high repetition rates, mirror coatings for high power and multiple wavelengths and pointing stability. It can quickly switch between the lasers and detectors used and can simultaneously record light curves with a single-photon counting module to give spin parameters of debris targets.
The achievable range accuracy is in the order of 0.5 m RMS and is limited by the longer laser pulses and large target sizes (e.g. 10 × 3 m cylinders of rocket upper stages). Another limitation is the inaccurate orbit predictions, where the along-track biases can be a few kilometres and it is therefore necessary to visualise the target. This restricts observation time slots to the terminator periods, where there is darkness at the SLR station, but the target is still illuminated by the Sun. However, work is in progress to extend debris laser ranging to full day and night capability by improving predictions and target visibility.
An extension of single-station debris ranging is the concept of multi-static debris laser ranging, where one active SLR station fires laser pulses at the target and the diffusely reflected photons are detected by other passive receive-only stations, allowing more than one measurement from different locations with one single laser shot. The active station fires the laser at predefined epochs so that the receiving stations can predict the arrival of the laser signal. This technique was demonstrated with Graz acting as the active station and Wettzell, Zimmerwald and Herstmonceux as passive stations (Kirchner et al. 2013a). Such multi-static measurements allow a significant improvement in precise orbit determination from only a small number of passes. A new concept of ‘Stare and Chase’ was successfully tested at Graz (Steindorfer et al. 2015) in which the telescopes ‘stares’ at a fixed position of the sky and waits until a completely unknown, but sunlit, object passes through the field of view. The telescope begins tracking this object and pointing vectors are acquired, which are used to calculate an orbit. This is then used to acquire the target with the space debris laser (‘chase’) without any a priori knowledge of the target’s orbit. Space debris laser tracking activities within the ILRS are coordinated by the Space Debris Study Group (SDSG).
Independently of this group, over many years EOS Space Systems has been developing high energy space debris tracking facilities, co-located with the Mt Stromlo SLR station. These activities led to the establishment, in 2014, of the SERC, an Australian research centre for space environment management (www.serc.org.au). SERC is supported by an international consortium of aerospace industries and universities and has several research programmes, including improving tracking sensors and technologies, space object characterisation and identification, orbit analysis and modelling, and catalogue development. These programmes have led to improved orbit predictions, improved passive and active tracking of space objects, better conjunction assessments and the development of an optimised space asset management service. A programme to investigate the potential use of laser radiation to modify debris orbits has also been a key objective of the SERC. These programmes commenced using the resources of the research and development facility at Mt Stromlo, which included a 1.8 m telescope and high power laser (Ritchie 2013). Other operational debris tracking sites followed, starting in 2018 with a debris tracking facility near Exmouth in Western Australia.
Laser time transfer
If, during SLR, laser pulses are detected at the satellite and the epoch of arrival is accurately recorded, time can be transferred by comparing the onboard clock to the ground clock of the laser station. This unique property is known in the literature as the Einstein synchronisation (Einstein 1905). It is based on the assumption that the velocity of light is isotropic (µ = 1/2) and requires the reflection of light at the satellite target to not add a time delay to the measurement. The ground station emits a laser pulse at t1 in the local timescale. It reflects and is simultaneously detected at time t2 at the satellite. The returning laser pulse is timed again on the ground station at t3. The time tagged epoch t2′ in the timescale of the satellite then compares to the instance of reflection t2 in the timescale of the ground station as t2 = t1+ µ (t3 − t1). Figure 6 illustrates this relationship in a time-range diagram. Due to the motion of the satellite the uplink time may be different from the downlink time. However, for a near Earth orbit, this does not matter at the level of ps-timing resolution and can be ignored.
The first demonstration of picosecond resolution laser time transfer was carried out in 1975 by Professor Carroll Alley and his graduate students at the University of Maryland in the USA (Degnan 2007). In this landmark experiment, two matching ensembles of atomic clocks were synchronised over several days before being placed in both a ground laser station and a P3 aircraft. Laser pulses from a mode-locked, 100 ps, 30 Hz laser were directed at the aircraft, which were detected and the times of arrival were measured by the airborne ensemble of clocks. The aircraft also carried a single retro-reflector which returned the pulses to the ground station detector, and the times of arrival were measured by the ground-based clock ensemble. Over the approximate 15 h duration of the experiment, the total measured time shift between the two ensembles was 47.1 ns, which included a gravitational redshift of 52.8 ns and a time dilation correction of—5.7 ns, in excellent agreement with Einstein’s relativistic predictions of 47.1 ± 0.25 ns. Furthermore, as the aircraft used up fuel, it was able to fly at higher altitudes where the lower gravity field resulted in a larger redshift that could be easily observed in the relative time data.
The LAser Synchronisation from Stationary Orbit (LASSO) experiment onboard the geostationary MeteoSat-P2 satellite achieved intercontinental optical time transfer with high precision (Fridelance and Veillet 1995). The SLR stations in Grasse, France and Graz, Austria conducted the first experiments soon after its launch in 1988. Then in 1992, the Grasse and the McDonald, USA, laser stations demonstrated trans-Atlantic time transfer with a precision better than 100 ps.
The T2L2 time transfer experiment launched on Jason-2 in 2008 has impressively demonstrated the applicability of this concept (Samain et al. 2014). The mission is tracked routinely by the whole ILRS network, enabling clock comparisons between SLR stations, including those with ultra-stable frequency sources such as active hydrogen masers. Ground-to-ground synchronisation of less than 1 ns uncertainty was achieved between stations in common view (Exertier et al. 2014) and in non-common view (Samain et al. 2018). Non-common view comparisons required the modelling of the satellite clock oscillator to produce time series of the local reference times at SLR stations (Exertier et al. 2017).
Repeating these clock comparisons over a longer period of time allows a challenging test of the gravitational redshift, a consequence of the theory of general relativity. This test to a level of 2 ppm is one of the objectives of the Atomic Clock Ensemble in Space (ACES) (Heß et al. 2011) mission, currently under development and prepared for launch to the International Space Station in early 2020. Optical time transfer is an ideal technology for this purpose because it combines the estimation of the exact geometrical distance between the clocks under test at the time of the measurement and the high bandwidth interrogation of the respective event timers at both ends of the line of sight. Another advantage is the fact that, unlike microwave signals the propagation delay in the troposphere and ionosphere is much smaller, better established and subject to less variability. In order to achieve the best possible performance a few important requirements have to be satisfied. Both the ground clock and the space clock have to be stable at the level of ∆f/f ≈ 10−16 and the ground clocks should be traceable to UTC. All local system delays for the link between the station clock and the timer have to be both stable and known (Schreiber and Kodet 2017; Prochazka et al. 2017). Since optical time transfer starts at the instant when the laser pulse passes the reference point of the SLR system, the path delay of the laser pulse between the start diode and this reference point must be established with 1 ps accuracy. The epochs of each laser fire event must be taken with similar resolution. These requirements are different from regular SLR, where common mode biases of the clock cancel out and system delays are removed by calibration.
Time transfer to more distant satellite targets has the advantage of offering lines of sight to a larger segment of the global ILRS network for longer periods of time as has been demonstrated on Chinese Beidou (Wendong et al. 2013) and Russian GLONASS (Baryshnikov et al. 2015) satellites.
Deep space laser ranging
Roundtrip SLR return signal strength is reduced by a factor of range to the fourth power. As a result, metre-class telescopes and high laser energies (100 s of mJ) are typically required, for example, to obtain adequate single-photon returns from passive retro-reflector arrays on the lunar surface at an average distance of 385,000 km. However, if both ends of the link employed a laser transmitter and receiver, the signal strength in a one-way link would only be reduced by the range squared and therefore precise ranging and time transfer can be extended to much greater distances. These “laser transponders” can be fired asynchronously with incident shots detected at both terminals to obtain accurate ranges and clock offsets. Furthermore, the large telescope and high pulse energy can be confined to the Earth ground station, thereby minimising the size, weight, and power of the spaceborne terminal (Degnan 2002). This deep space capability was first demonstrated in May 2005 between the 1.2 m telescope at the NASA Goddard Geophysical and Astronomical Observatory (GGAO) and the Messenger Laser Altimeter (MLA) on a spacecraft en route to Mercury, over a 24 million km link with 20 cm precision (Smith et al. 2006). Three months later, a successful one-way 80 million km laser link from GGAO to the Mars Orbiter Laser Altimeter (MOLA) was established (Degnan 2007). In 2015, the Mt Stromlo debris tracking laser station and the Koganei SLR station supported the Japanese explorer mission, Hayabusa2. Uplink laser pulses from the Mt Stromlo station were successfully detected at the spacecraft at a distance of 6.6 million km, and this enabled the field-of-view direction of the laser altimeter to be determined, the link equation calculation to be confirmed and the calibration of the clock frequency offset (Noda et al. 2017).
The Lunar Reconnaissance Orbiter (LRO, Mao et al. 2017) reached the Moon in June 2009 and was placed in a near circular, mapping orbit with an average altitude of 50 km. Included onboard was the Lunar Observer Laser Altimeter (LOLA) for measuring the topography of the lunar surface with 10 cm resolution, which required precisely determined orbits. Over the lifetime of this experiment, 10 SLR stations in the ILRS network performed one-way laser range measurements to LRO as part of their observing schedule. The epochs of the laser fires were recorded at the station, and detections at the LRO spacecraft were viewed through a web-based display. Some SLR stations were able to synchronise the laser firing so that the arrival of the laser pulse coincided with the gating of the LOLA detector. Laser ranging data were used in combination with the S-band tracking data to improve the orbital accuracy in the radial direction to approximately 15 cm.
Science applications of interplanetary transponders include:
Solar System Science: Spacecraft missions equipped with transponders could better study the gravity field, internal mass distribution and rotation of the Sun and planets and other space objects. They would provide decimetre or better accuracy in spacecraft ranging to validate deep space microwave tracking. Onboard clock calibration could be achieved to better than 1 ns for improved synchronisation of Earth/spacecraft operations. Transponders on the moon, a planet or an asteroid would provide accurate ephemerides. A lunar transponder would enable LLR by small, low energy, ground stations. Transponders can also serve as independent self-locking beacons for collocated laser communications systems.
General Relativity: Transponder ranging to the planets would allow more accurate tests of relativity than LLR or microwave radar ranging including the precession of Mercury’s perihelion, constraints on the magnitude of G-dot, gravitational and velocity effects on spacecraft clocks and the Shapiro time delay.
Since the signal dependence on range is the same for both interplanetary transponders and laser communications, future prototype instruments can be simulated and tested by utilising existing SLR stations and current SLR satellites (Degnan 2006). In addition, the effect of the Earth’s atmospheric transmission and turbulence on signal strength (Dirkx et al. 2014) and communication Bit Error Rates (BERs) as a function of zenith angle can be assessed inexpensively prior to an actual flight mission. Table 1 indicates the interplanetary links that can be simulated with selected members of the current SLR constellation of satellites. Preliminary two-way transponder simulations were carried out at the Wettzell station in Germany using a lower energy 35 mJ laser and smaller receive telescope with an aperture of 10 cm, mounted on the top of the 0.75 m WLRS telescope (Schreiber et al. 2009).
Lasers are in wide use in communications, sending signals through optical cables and providing high-speed internet connection. As the amount of data collected by Earth observation satellites and deep space missions increases, free-air laser communications to and from satellites/spacecraft and between satellites has the potential to securely transfer greater data quantities at higher bandwidth, compared to microwave frequencies. The space segment can be small in terms of size, weight and power consumption and the narrow beams do not experience interference. Signals must, however, overcome atmospheric turbulence and cloud cover is a limiting factor.
Laser communications is a synergy application for SLR. Similar infrastructure is required since pointing, acquisition and tracking (PAT) functionalities are equally essential at the ground station. This was demonstrated during the ILRS support of the Japanese Optical Inter-orbit Communications Engineering Test Satellite (OICETS) satellite in 2006, when the NICT SLR station in Koganei, Japan, successfully conducted the first bidirectional optical communication demonstration to a low-Earth orbiting satellite (Toyoshima et al. 2007; Kunimori et al. 2012). The satellite was visible at night and was centred in the SLR telescope field of view using optical cameras. The optical antenna on the satellite was pointed at the ground station and the downlink laser beam with a wavelength of 847 nm, a transmit power of 53 mW and 5.5 µrad beam divergence was then positioned onto the station receiver. The uplink beacon was a continuous laser at 808 nm, 30 W (max) and 9 mrad divergence that enabled the satellite to locate its ground target. A second beam at 815 nm, 10 mW and 204 µrad was the communications uplink, which was split into four beams to reduce the impact of fluctuations from atmospheric turbulence. A mirror controlled by piezoelectric actuators was developed to further compensate for atmospheric turbulence.
The French LLR station has also participated in free space optical communication experiments with the NICT SOTA (Small Optical TrAnsponder) instrument flying on the low-Earth orbit satellite SOCRATES. Several links at 10 Mbps were achieved at 1549 and 976 nm, with the objective to study the laser beam propagation through the atmosphere and the design of all the sub-systems constituting an optical ground station (Samain et al. 2015).
More distant laser communication was achieved with the Lunar Reconnaissance Orbiter, which in 2012 received an image of the Mona Lisa from the NGSLR station at Goddard, USA, to the Moon at a rate of 300 bits/s (Sun et al. 2013), and in 2013 by the NASA Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, which received data at 622 Mbits/s (Boroson and Robinson 2015).
Transferring quantum information, or quantum keys, by laser link is being explored at the Matera SLR station (Vallone et al. 2015). Low energy, high repetition rate qubit laser pulses are synchronised with SLR laser pulses during tracking of low-Earth orbiting satellites. The targets carrying coated retro-reflector cubes preserve the transmitted polarizations on detection. NICT also confirmed quantum-limited communication characteristics using SOTA at the 1 m telescope in Koganei (Takenaka et al. 2017). In 2017, quantum keys were created and transferred by the Chinese Micius satellite to stations in Xinglon and Nanshan in China and the Graz SLR station in Austria (Liao et al. 2018). These entangled infrared photons were separated by polarisation at 0°, 45°, − 45° and 90° and detected using four SPADs. The key was then used in the transfer of secure data over optical fibres.
Laser communications development and technology standards can be explored in the IEEE ICSOS proceedings (http://icsos2017.ieee-icsos.org) and through the Consultative Committee for Space Data Systems (CCSDS) Optical Communications working group (https://cwe.ccsds.org/sls/default.aspx).
Advancing the ILRS network
Existing SLR stations are planning upgrades, old stations are returning to SLR operations, new stations are under development and existing networks are expanding their activities.
The Russian SLR stations are divided into three groups depending on strategy and tracking priorities. The primary focus for the stations in Komsomolsk, Altay, Arhyz, Baikonur, Brasília in Brazil and Hartebeesthoek in South Africa is laser ranging to the Russian GLONASS navigation satellites. The priority for the Svetloe, Zelenchukskaya and Badary stations is the observation of geodetic satellites and contribution to the ITRF, as these sites are co-located with GNSS, VLBI sites and, in the case of Badary, DORIS. The main task for the SLR stations in Mendeleevo-2 and Irkutsk is time transfer. In 2018, work will begin on building a lunar laser ranging station at the Altay Optic-Laser Center (AOLC), where the Altay SLR station is located.
The SLR systems of all eleven stations are developed and manufactured by Precision Systems and Instruments (PSI). PSI provides equipment and maintenance, implements all improvements to increase accuracy, reliability and functionality and trains operators. Stations send raw measurements to the data-processing centre IAC PNT FSUE TSNIIMASH where normal points are calculated, CRD format files are generated and the files are sent to the European data-processing centre, EDC. The existing SLR stations will be upgraded to improve precision by replacing the current 300–400 ps lasers with lasers of 50 ps pulse duration. Also, the software designed for daytime laser ranging that is currently in operation at seven SLR stations will be implemented for the other sites in the network.
A new generation SLR station designed for millimetre precision, called ‘Tochka’, has been developed and is in the process of testing (Fig. 7). In addition to SLR, it will conduct one-way laser ranging to GLONASS navigation satellites equipped with laser pulse reception modules, which will link the laser pulse arrival time to the onboard clock time scale at a sub-nanosecond level of accuracy. These laser ranges will be taken alongside the GNSS pseudo-range navigation signals, allowing for the calibration of satellite antenna and ground receiver hardware delays (Sadovnikov and Shargorodskiy 2015a).
The Tochka design can run in a fully automated mode capable of performing SLR, correcting the laser beam pointing, calibration, control of the dome, making diagnostics checks and conducting search routines to acquire and optimise returns from the target (Sadovnikov and Shargorodskiy 2015b). A station will be installed in Mendeleevo at the State Service of Time and Frequency (SSTF), and a second will be installed at its branch in Irkutsk. Four more sites on international soil will be established, chosen from five sites currently under consideration: California in Mexico, Falda del Carmen in Argentina, Tahiti in French Polynesia, Java in Indonesia and the Spanish Gran Canaria island. The chosen sites will be able to see parts of satellite orbits that are inaccessible from the territory of the Russian Federation (the western and southern hemispheres of the Earth). They will have favourable weather conditions, local operator availability and opportunities for co-location with the other geodetic techniques.
The first laser ranging returns were detected at NASA Goddard Space Flight Center (GSFC) from the Beacon Explorer 22 satellite on 31 October 1964 (Plotkin et al. 1965). In the decades that followed, this new accurate ranging technique (Dunn et al. 1979) was exploited at more sites as more satellites were launched, including the geodetic spheres Starlette in 1975 and LAGEOS in 1976. The NASA designed Mobile Laser (MOBLAS) and Transportable Laser Ranging System (TLRS) stations occupied numerous sites of interest globally, such as near fault zones in Southern Europe and the Western US (Smith et al. 1979, 1994; Degnan 1994a). As the less expensive GPS networks took over regional measurements, these stations were assigned to fixed sites. The present NASA network consists of eight systems: Monument Peak, California (MOBLAS-4); Yarragadee, Western Australia (MOBLAS-5); Hartebeesthoek, South Africa (MOBLAS-6); Greenbelt, Maryland (MOBLAS-7); Tahiti, French Polynesia (MOBLAS-8); Arequipa, Peru (TLRS-3) and Haleakala, Hawaii (TLRS-4). The McDonald Observatory, Texas, (MLRS) operates a 75-cm telescope on Mt. Fowlkes to conduct both satellite and lunar laser ranging, but it has not conducted LLR operations since 2015.
Figure 8 shows the LAGEOS single shot precision improvement achieved by different component substitutions over time. The first three NASA Mobile Laser (MOBLAS 1–3) systems used Q-switched Nd:YAG lasers with pulse lengths of a few nanoseconds and achieved a precision of around 20 cm. Over time, the introduction of a 100 ps, mode-locked Nd:YAG lasers, newly developed MCP-PMT detectors and more precise HP5370 timer units (Degnan 1985) improved the single shot LAGEOS precision to 1 cm. Recently, the legacy HP5370 interval timers have been replaced by event timers with about 3 ps RMS precision (Varghese 2017).
Prior to international deployment, each new NASA system (and a few non-American systems) underwent collocation testing against MOBLAS-7. This is useful because the ranges from both SLR systems include the same atmospheric delay, and the vector between their respective telescope origins (“invariant points”) can be measured to within a few mm.
SLR2000 was a completely new station design that included single-photon detection, a high repetition rate laser and autonomous operation. It was required to be eye-safe not only for aircraft but, because of the automation goals, even to personnel looking into the transmit telescope window. The lower pulse energy and longer pulse length reduced the power density, but it was also necessary to use the full 40 cm telescope aperture to emit a large beam width (see Fig. 9). This was cleverly achieved in the design of the optical bench using polarising optics to isolate the transmitter from two oppositely polarised receiver channels, which were recombined prior to detection.
To increase satellite return rates, SLR2000 utilised a dual Risley prism transmitter point-ahead device and a single-photon-sensitive quadrant MCP-PMT. The transmitter point-ahead was computed from orbital characteristics so that a low-divergence Gaussian beam could be centred on the satellite position at its time of arrival. The telescope was pointed at where the satellite had been at the times of the previous pulses in order to detect the returning signal, which was centred by equalising the counts on each of the quadrants. Ultimately, due to the desire to track the growing constellation of high altitude GNSS satellites, the highly restrictive eye-safe requirement at the telescope output was rescinded by NASA. The 60 µJ, 300 ps laser was upgraded to 50 ps, 2.8 mJ pulses at 2 kHz, an active radar was installed and SLR2000 was renamed Next Generation Satellite Laser Ranging System (NGSLR). In 2013, NGSLR met all tracking and range accuracy requirements (including GNSS) in collocation experiments with MOBLAS-7 (McGarry et al. 2013) and currently serves as the prototype for the new Space Geodesy Satellite Laser Ranging (SGSLR) system destined to replace the current network of NASA stations and for installation in new locations globally. A detailed description of SGSLR can be found in a companion article in this journal (McGarry et al. 2018).
There are six SLR stations in China (Fumin 2001), including one mobile 1 m system and five in fixed locations: Shanghai, Changchun, Beijing and Wuhan with 0.6 m aperture receive telescopes and Kunming with a 0.5 m telescope, replacing the original 1.2 m telescope. In 2005, China also installed a 0.6 m SLR station in San Juan, Argentina as part of a cooperative effort between the two countries. The Chinese network began kHz SLR in 2009 and currently all stations now operate as kHz SLR stations, equipped with 1 kHz, 1 mJ pulsed lasers, C-SPAD detectors and EventTech event timers, except for the San Juan station, which is in the process of being upgraded with the support of the Changchun station.
To further the technical development of high repetition rate SLR, the Shanghai station successfully realised 10 kHz operation in 2014. A domestic 10 kHz laser system with an output power of 4–5 W was installed and the Shanghai team designed and developed a range gate generator and a controlling system (Zhongping et al. 2011). The SLR data yield within one normal point is increased by several times compared to that of 1 kHz SLR, which helps to better meet the requirement of 1 mm precision. Due to high data density, the 10 kHz data are also used to analyse the rotation rate of spherical satellites. The high dark noise generated by gating at 10 kHz should be addressed by developing a low dark current detector. Automation of the Shanghai kHz SLR station is also under development and will include processing predictions, return signal optimisation and post-processing (Haifeng et al. 2016). Additionally, aspects of the system can be remotely monitored and controlled. Recent work at the Shanghai and Changchun SLR stations improved short-term and long-term stability and included, for example, relocating the timer to a clean room, shielding the detector to prevent temperature changes and reducing signal intensity variation (Xue et al. 2016). Daylight SLR is more difficult for HEO satellites because of the weak echo signal, greater background noise and pointing errors. Up to now only the Shanghai and Changchun stations can perform SLR to geostationary orbit (36,000 km) and medium orbit GNSS (20,000 km) satellites in daylight hours by using high pulse energy lasers, ~ 20 arcsecond field of view and determining pointing offsets by monitoring star images in daylight. The other stations will be upgraded to daylight HEO tracking capability. To achieve greater detection performance, the Shanghai station was the first SLR station in the ILRS network to use Superconducting Nanowire Single-photon Detector (SNSPD) technology to successfully record SLR returns (Hao et al. 2016). This was achieved with a 532 nm laser. The Kunming station is also carrying out experiments with a SNSPD, focussing on 1064 nm ranging.
Due to the dated telescope and other equipment, SLR was last performed at the Wuhan station in 2006. A new 1 m aperture SLR telescope and 1 kHz laser system is under development with the help of the Shanghai station, see Fig. 10. On completion, hopefully in the summer of 2018, the capability will be dramatically improved and this site will play an important role in the Chinese network. The mobile SLR system, TROS1000, pictured in Fig. 11, has performed kHz SLR measurements at several sites (Tangyong et al. 2015). A SLR station is needed in the northwest of China and so it is planned to move this mobile SLR system to Urumqi Observatory for co-location with a nearby VLBI site. It will be then considered for applying to join the ILRS to track the priority list of satellites.
Space debris laser ranging is successfully performed at the Shanghai, Changchun and Kunming stations (Hao et al. 2015). The development of laser time transfer payloads for the Chinese Space Station (to be installed in 2022) is under way at the Shanghai SLR station and other stations will also participate in this activity (Wendong et al. 2016).
New and planned SLR stations
A SLR station is limited by its horizon, cloud cover and only being able to track one satellite at a time. The benefits of an additional accurate and productive SLR station located away from any other station, under a different sky and supporting a different part of the satellite orbits are clear. Any productive station would be able to significantly contribute to the products of the ILRS, but sites that improve SLR coverage by filling gaps in the network would be ideal and those that have the greatest impact, such as in Antarctica (Otsubo et al. 2016), would be desirable. To have a greater impact on the formation of the ITRF, SLR stations should be co-located with the other geodetic techniques: GNSS, VLBI, and DORIS. Sites that operate all of these techniques are likely to be GGOS Fundamental Stations.
The Geodetic Research Station in Metsähovi, Finland, operates these three other geodetic techniques along with superconducting and absolute gravimetry. SLR operations began in 1978 and ended in 2005. In 2012, work began to re-occupy the site with a kHz laser, event timer, bistatic 50 cm telescope, high-speed dome and new building. Likewise, to help achieve GGOS Fundamental Station status for the Yebes Observatory in Spain, a SLR station is planned as a high repetition rate bistatic system with a C-SPAD, event timer and automation. The South Korean SLR system ARGO-M began operations in Daedeok in 2012 and was then relocated to Sejong to be part of a GGOS Fundamental Station in 2015 (Choi et al. 2015). This station was upgraded to 5 kHz in 2016 using a range gate generator developed by Korea Astronomy and Space Science Institute (KASI) which is capable of operating at 10 kHz. A second South Korean station in Geochang joined the ILRS in 2017. It is an ARGO-F design with a 1 m aperture telescope with a common coudé firing 15 mJ pulses at 60 Hz. The Geodetic Earth Observatory in Ny-Ålesund, Norway will operate the highest latitude SLR station at 78º 55′ North, installing a NASA designed SGSLR station, and will also be a GGOS Fundamental Station. The Argentine-German Geodetic Observatory (AGGO) in La Plata Argentina will be a Fundamental Station once SLR operations begin using an upgraded telescope system. It operates a Ti:Sa, 100 Hz, 2 colour laser system that was formally named TIGO and previously operated in Conception, Chile. The GGOS Fundamental Stations in Wettzell, Germany, and Hartebeesthoek in South Africa are now operating second SLR systems. The high repetition rate SOS-W station (Riepl 2018) operates alongside the long-running WLRS station in Wettzell and is capable of generating laser light at the two unique Ti:SAP wavelengths of 850 nm and 425 nm. A Russian system (HRTL) began operations in 2017 next to the legacy NASA Moblas-6 station (HARL) in Hartebeesthoek. Additionally, two new stations are under construction in India, in Mount Abu and Ponmundi, operated by the Indian Space Research Organisation (ISRO) in support of the Indian Regional Navigation Satellite System (IRNSS).
A second laser telescope was installed alongside the Graz SLR station in Austria with a diameter of 80 cm (Kirchner et al. 2016). This telescope can potentially be equipped with multiple lasers, varying from the tiny SP-DART system (Kirchner et al. 2015), to ps lasers for millimetre measurements to space debris lasers. A new SLR station is under development alongside the SLR station in Matera (MLRO), Italy. It will be primarily for geodesy with a 0.6–0.8 m telescope, a > 1 kHz laser, automated tracking and an ultra-stable reference frequency. Having this station operational will leave MLRO to focus on other activities such as quantum communication, lunar laser ranging and space debris tracking.
Some early SLR stations were designed to be mobile so that they could support multiple locations and take part in regional campaigns. However, the accuracy eventually achieved by the more affordable, continuous GNSS sites, forming part of dense networks, resulted in SLR stations opting to occupy permanent sites. The ultra-portable French Transportable Laser Ranging Station (FTLRS, Nicolas et al. 2001) visited sites including Paris, Corsica, French Polynesia and Tasmania to support the oceanographic Jason-2 mission and the time transfer payload T2L2. The new Korean kHz SLR station design ARGO-M is transportable (Choi et al. 2015) and was moved from Daedeok to the fundamental station in Sejong City in 2015. The Chinese station TROS1000 (Tangyong et al. 2015), see Fig. 11, is built into a 15 × 2.5 m trailer and travelled over 1500 km from Hubei province to Shandong province to be installed in the Rongcheng seismographic station. A transportable system for space debris laser ranging called STAR-C has been designed at the German Aerospace Center in Stuttgart (Humbert et al. 2017).
The remaining scientific needs that might require transportable SLR are: co-location SLR; calibration of spaceborne microwave (such as the Topex/Poseidon and Jason satellites) and laser (ICESat-2) altimeters and laser time transfer. The costs associated with transportable SLR could be overcome by remotely controlled, automated systems using low-cost, commercially available laser, telescope and detector components.
Challenges for the next generation of SLR stations
Current limits to making further progress in laser ranging are challenging, but there would potentially be significant improvement to global SLR accuracy and productivity if they were overcome.
Systematic errors are likely to be present to varying degrees in the laser range measurements from every SLR station in the ILRS network. The GGOS goal for a reference frame of 1 mm accuracy and 0.1 mm/year stability is demanding and so stations must continue to seek out and eliminate all potential systematic errors (Otsubo et al. 2018) and particularly those indicated in analysis activities (Appleby et al. 2016).
A general method for identifying systematic bias is to make a comparison measurement with an alternative, independent and more accurate, or bias-free, technique. However, no such technique exists for SLR and so it is necessary to characterise all the individual error budget contributors, their precision and biases (Pearlman 1984). This list must be complete, taking into account and calibrating all hardware and environmental contributors. In general, greater SLR stability and precision is a prerequisite to reveal smaller biases, leading to higher accuracy.
For a system calibration to a terrestrial target at a known distance, there are a number of key error budget contributors. A single retro-reflector calibration target with a delta function impulse should be precisely engineered so that a reference point is well defined. The survey to determine the distance from the target reference point to the telescope invariant reference point must be accurate. The system optics must be well aligned so that the calibration signal photons hit the detector active area. Although this seems to be an obvious condition, it is not easily fulfilled for near target laser ranging. SPAD detectors must be gated in advance to avoid any range error caused by close gating (Kodet et al. 2009). Timer nonlinearity can introduce a calibration range bias that is not present for SLR, which can be identified by calibrating to a set of multiple ground targets at different distances.
The hardware to produce and distribute the time scale at a SLR station must be characterised and calibrated. This includes the clock frequency and stability, phase purity and offset and the 1 pulse per second reference, its signal shape and the trigger levels used. Meteorological sensors at SLR stations record the local temperature, pressure and humidity to be used to calculate the atmospheric range delay correction (Degnan 1985). These instruments are critically important and must be calibrated periodically. The local site must also be stable so that the vectors between the SLR reference point, the calibration targets and the site ties to instrumentation for other geodetic techniques remain close to the values surveyed (Wilkinson et al. 2013).
Single-photon SLR is achieved by responsively attenuating the return signal at the detector to keep a consistent low level and by identifying and rejecting multi-photon returns. It eliminates detector time-walk effects and most of the problems of laser wavefront induced biases, where the ground target calibration is taken in the near-field diffraction pattern and the SLR target in the far-field. In addition, the single-photon approach can reduce significantly systematic errors related to the satellite retro-reflector array geometry (Otsubo and Appleby 2003). Threshold detection at multi-photon stations for the start or stop laser pulses can result in bias due to varying pulse amplitudes (especially in the stop channel). Many systems use constant fraction discriminators to drastically reduce range jitter. While this improves range precision, it does not eliminate range bias. It was recently suggested that centroid detection would allow kHz systems to operate at high return rates to yield both high precision and bias-free normal points (Degnan 2017).
New applications, such as one-way laser ranging over long distances, laser time transfer and bi- and multi-static laser ranging of orbiting space debris, require a one-way system calibration (Prochazka et al. 2017). The biases are strictly separated into transmitting and receiving channels. The epochs of signal transmission and reception must be established to a few ps and referred to a coordinated time scale, e.g. UTC, using a stable source with an accuracy better than 1 ns. This is about one hundred times more accurate than in standard SLR applications. Feedback to stations of clock comparisons from time transfer experiments are informative (Exertier et al. 2017) and can reveal discontinuities in the station time scale and offsets from UTC.
Routine aspects of SLR operations such as scheduling, orbit prediction, target acquisition, signal optimisation, data processing and safety and security monitoring are open to some degree of automation. The desired outcome is improved productivity, efficiency and reliability, reduced manpower, and high standards of security and safety. Most SLR stations incorporate some level of automation, for example, by the 1990s, the legacy NASA MOBLAS stations reduced their manpower from 4 person to 1 person shifts. High level, automated SLR has been performed successfully at the Zimmerwald station in Switzerland and the Mt Stromlo station in Australia for a long time, and more recent systems, such as the German SOS-W in Wettzell, the NASA SGSLR (McGarry et al. 2018) and the Russian Tochka are designed for full automation.
Following the installation of a new 1 m telescope at Zimmerwald in 1995, an independent, automated SLR system was developed using TCP/IP client/server architecture to send and receive command controls. Predictions received by email were used to create real-time optimised tracking schedules based on satellite priority and sky and weather conditions were monitored. Aircraft safety was ensured by an active tracking radar and a near-real-time feed from Swiss air traffic control. Work continues to maintain automation of this now legacy system and to include new activities and replace outdated hardware (Lauber et al. 2015).
The Mt Stromlo SLR station (near Canberra, Australia) was designed, from when it was rebuilt in 2003, to become a fully automated SLR station. It operates in a monostatic mode, firing laser pulses at approximately 60 Hz via a rotating disc. This provides multiplexing of the transmit and receive beams and directs the received photons to a C-SPAD detector. The Mt Stromlo laser, detector and timing sub-systems are all housed in a temperature controlled, clean environment. The 1 m Mersenne telescope is in an EOS Typhoon sealed telescope enclosure that is located within the envelope of a secure building, as seen in Fig. 12. This enclosure allows SLR operations to continue regardless of weather conditions and provides a clean, Sun shielded, isothermal environment. The enclosed dome and controlled environment at Mt Stromlo have been essential for its successful, reliable, continuous unmanned operations.
A sophisticated control system is used to manage all of the hardware sub-systems, allowing manual control but also providing autonomous management of higher level operational functions, including safety and security monitoring, target scheduling, target acquisition and tracking, signal optimisation, signal extraction and data processing, report generation and publication. Again, a TCP/IP client/server architecture was used to build up independent software components and allow an evolution from manual operations to the more complex automated and unmanned operations (Pearson 2006). This evolutionary approach was evident in the operational changes at the Mt Stromlo station, which in 2004 required 4 or 5 operators. Following the introduction of automated target scheduling and tracking in late 2006, the number of operators was reduced to 2. Subsequent introduction of automated file management, report generation and more capable post-processing facilities allowed the station to continuously operate unmanned for over many days and to require only part-time operator support. Fully automated post-processing and dynamic target scheduling further reduced operator requirements, essentially to a maintenance role. SLR at Mt Stromlo has shown that a station can operate around the clock and obtain useful data whenever there are breaks in sky cover, even in conditions that human operators would reject as being a waste of time (Moore 2006). The persistence of automation pays dividends.
An unmanned, automated SLR station must be designed to meet on-sky laser safety requirements at all times, incorporating multiple robust safety systems. Such technologies as radar, lidar, ADS-B receivers, passive transponder detection and multiple wavelength camera systems are used successfully at various SLR stations, making automated stations not only very productive but also very safe.
One potential source of error relates to the path of the laser pulses through a highly variable atmosphere. Atmospheric delay is modelled using local temperature, pressure and humidity readings (Marini and Murray 1973). Alternatively, by measuring the distance to the satellite target on two different optical frequencies simultaneously, one can obtain the atmospheric correction experimentally (Abshire and Gardner 1985; Degnan 1985) as:
where L1 and L2 are the obtained ranges at the respective wavelengths and γ accounts for the different refractive group delays, ng1 and ng2.
For the fundamental wavelength and the second harmonic of a Nd:YAG laser, γ assumes a value of about 21. Other pairs of wavelengths may have smaller factors, but in any case it turns out that the range values of L1 and L2 have to be measured at least one order of magnitude more accurately than the theoretically modelled refraction correction from local meteorological sensors. Assuming an error of the refraction correction of say 8 mm, this pushes the requirement for the simultaneous accuracy of L1 and L2 to values of about 1 ps. Averaging over many shots improves the resolution, but one has to cope with a changing correction as the laser follows the satellite along the sky. The atmospheric correction has a value of about 10 ns for the telescope pointing at zenith and this value increases to about 30 ns for elevations of 20°.
Dual colour ranging can be achieved in two different ways. The SLR system either has two at least partly independent observation channels and performs simultaneous tracking with the laser operated on the fundamental and the second or third harmonic frequency. Alternatively, the system could be set to record the different frequency pulses on a common sweep of a picosecond resolution streak camera. Both concepts have fundamental problems to deal with. The two-detector approach requires an exact and stable calibration, which among other problems also accounts for a different response time of the detector because of the difference in wavelength. The streak camera approach has the difficulty that a large receive telescope aperture is imaged on a very small sensitive area, so that angular turbulence induced dither of the detected echo maps straight into time, blurring the echo spots on the streak, which reduces the timing accuracy such that until now it has been impossible to achieve the required resolution for the differential time delay. Dual colour ranging still presents a significant challenge to SLR and awaits a practical solution.
An upgrade can advance SLR performance if it improves one or more of the following: accuracy or precision, acquisition time, reliability, cost and safety. Here we highlight some recent developments.
Event timers have improved the accuracy and precision of SLR measurements at numerous ILRS stations. The New Picosecond Event Timer (NPET) achieves epoch recording with a precision of less than 700 fs using a transversal surface acoustic wave filter as a time interpolator (Panek et al. 2010). It has a temperature dependence of less than 0.5 ps/K and stability expressed by time deviation better than 30 fs over measurement times of minutes and up to hours.
The motivation to increase laser firing rate depends on cost and availability; a favourable trade-off between laser repetition rate and pulse energy, so that output power remains high; high detector quantum efficiency and low dark current noise; and that the overlap between laser fire and return detection is avoidable. The immediate advantages would be greater normal point precision, faster target acquisition and faster normal point completion. Systems currently operating at 10 kHz include Sejong, Korea (Choi et al. 2015), Shanghai in China (Zhibo et al. 2014) and Stuttgart in Germany, which is investigating firing rates as high as 100 kHz. As well as performing infrared, 10 kHz SLR, the Stuttgart station, which was given ILRS ‘Engineering’ status in 2017, is unique in other ways: it uses only commercial, off-the-shelf components; in place of the coudé path it uses an optical fibre to direct the laser light to the transmitting telescope and it has near eye-safe operation (Hampf et al. 2016).
Higher repetition rate SLR could also have advantages with respect to the atmosphere. Optical turbulence causes signal path delay fluctuations up to frequencies of a few hundred Hertz and with amplitudes of a few hundred microns (Kral et al. 2005). Single-photon kHz SLR gives a measurement rate approximately equal to this frequency. Sampling at even higher rates would permit averaging over short time periods (< 0.01 s) to reach precisions that are better than the fluctuation induced by the atmosphere. This could inform atmospheric studies and have benefits for laser time transfer and two-colour SLR. Atmospheric turbulence distorts and spreads laser beams. For example, the laser beam diameter from the MeO LLR station in Grasse, France can be between 2 and 10 km on the lunar surface. If there was no atmosphere, or its effects were compensated for by an adaptive optics system, this spot would be diffraction limited at approximately 200 m. Adaptive optics systems sample light sources through the atmosphere to drive deformable mirrors at high speeds in order to correct for atmospheric wavefront distortions. For SLR there are 2 ways to use adaptive optics systems, by correcting the downlink or the uplink leading to an approximately 10 or 100 factor increase in intensity, respectively. Correcting both links would require different turbulence samples as satellite velocity aberration causes an angular difference. A light source, such as a laser guide star, is required and this restricts adaptive optics operations to night or good signal to noise conditions. Alternative light sources are possible, such as the sunlit satellite body or the lunar surface or the optical signal from a laser communications satellite.
Spectral filtering is necessary in daylight hours and greater transmission at the laser wavelength is possible with the latest improvements in optical coating designs. For example, Alluxa offers narrowband spectral filters with 0.5 nm FWHM, > 75% transmission at the selected wavelength and 6–7 OD blocking of other wavelengths. The majority of SLR stations operate a 532 nm green laser and a fast detector with high quantum efficiency. The benefits of operating at other wavelengths are restricted by the available detector technology. The Stuttgart station, operated by the German Aerospace Center (DLR), performs SLR in the near-infrared at 1064 nm using an InGaAs single-photon detector. These detectors have higher levels of dark noise, and so are kept at reduced diameters up to 80 µm, and have quantum efficiencies of around 30% and about 300 ps jitter. The lunar laser ranging station MeO in Grasse, France switched from 532 nm green to 1064 nm infrared laser ranging in 2015 (Courde et al. 2017). The IR LLR increased the station efficiency on all five lunar retro-reflector arrays and enabled detections on the three APOLLO targets during all lunar phases. Laser returns were successfully detected at the Shanghai SLR station using a superconducting nanowire (Hao et al. 2016), which offers an alternative single-photon detection technology. A wire of Niobium nitride was fabricated to track back and forth on a substrate of dielectric films with a sensitive diameter of 42 µm, a maximum detection efficiency of 75% and dark noise < 0.1 Hz. However, coupling the SLR telescope to this detector with an optical fibre was done with difficulty and considerable signal loss. It is anticipated that similar properties could be achieved for a superconducting nanowire detector sensitive in the infrared.
More stations will be able to begin SLR operations if the technology required is affordable, robust and easily available. Furthermore, existing telescope sites equipped with appropriate tracking mounts can be reliably modified to achieve accurate SLR. The need for a coudé path can be avoided either by using a fibre laser or by mounting a laser on to the telescope itself, as demonstrated by the Graz Single-Photon Detection, Alignment and Reference Tool (SP-DART) laser (Kirchner et al. 2015). The SP-DART consists of a 1 ns, 15 µJ, 2 kHz, tilt-resistant laser, a control unit, an event timer, a GNSS time and frequency receiver and meteorological devices. It is very compact and lightweight so that the laser transmit component can be mounted directly on to a receive telescope. It was mounted on the WLRS monostatic SLR telescope in Wettzell and successfully tracked targets, including LAGEOS 2 and GNSS satellites. It was also used to transform an astronomical telescope in Sandl, Austria (Fig. 13), to an operational SLR system within 6 h to successfully track targets at altitudes as high as a geostationary satellite (Wang et al. 2016).
Looking to the future, SLR stations, and the ILRS network as a whole, will continue to be judged in terms of productivity, accuracy and precision. The time it takes for a SLR station to acquire satellite returns reduces with greater laser pulse energy, repetition rate, telescope aperture, telescope pointing accuracy, satellite target response, detector sensitivity and local atmospheric transparency (Degnan 1993). Similarly, the time spent tracking a satellite so that 1 mm precision is achieved for a normal point depends again on the return signal strength and crucially the system precision and laser repetition rate. In the coming years, as the number of SLR targets continues to increase, reducing these two time intervals is the key to maximising a station’s capacity for SLR.
Ideally, a station that is secure and has robust aircraft safety systems can operate continuously according to satellite availability and local weather conditions. This might be best achieved through automation and remote monitoring with an optimised tracking schedule. Additional capacity could be found in the ILRS network through some central co-ordination and by dynamically setting station schedules. This approach might be more suited for the NASA and Russian sub-networks as they install and develop the SGSLR and Tochka designs that are capable of automation and remote operation. Further capacity is achievable through additional stations, including mounts alongside existing SLR stations, such as is in operation in Wettzell, Germany. The lower cost and greater commercial availability of components for SLR opens up the opportunity for many other observatories to convert suitable, existing telescope sites to active SLR stations. However, the requirements for fast and accurate pointing to low altitude targets and accurate calibration of a well-defined, invariant reference point may be challenging. However, the most productive station cannot impact ILRS products, such as the contribution to the ITRF, if it is not accurate and stable. The best performing and most stable stations are well known from ILRS reports, where possible, less achieving stations would ideally follow these examples.
SLR stations currently set their schedules according to the ILRS geodetic, Earth observation and GNSS satellite targets passing overhead. Included in these schedules could be targets that also simultaneously perform time transfer or telecommunications. SLR stations could also routinely make one-way laser ranges to distant targets in the solar system carrying detector or transponder technology. Based on the recent success in tracking space debris, SLR is no longer limited to targets carrying retro-reflectors, although these are required for more precise measurements. Therefore, the number of targets available to SLR is greatly increased, particularly in the hours before sunrise and after sunset, when lower altitude debris objects are illuminated by the Sun. Individual stations will continue to set their schedules to their own priorities and abilities.
There is an enduring need for the accurate optical measurements obtained by laser ranging. The contribution to the ITRF from laser ranging is unlikely to be replaced by other techniques. The independent validation of microwave derived orbits for GNSS operations and modelling and for oceanographic missions continue to be valuable. The challenge set by GGOS of a 1 mm accurate and 0.1 mm/year stable reference frame applies to SLR as it does to the other geodetic techniques. The new generation of SLR station was required to respond to this challenge for greater accuracy and also to support the increasing number of ILRS targets. To be successful in meeting these challenges, the next generation will have to take this further at the individual station level, but realistically they will be met by the global ILRS network as a whole.
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The authors would like to acknowledge all the design, analysis, maintenance, development and operational work undertaken by the great number individuals in the global ILRS community now and over the history of laser ranging. This work ensures the highest quality data and products and the greatest impact from SLR.
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Wilkinson, M., Schreiber, U., Procházka, I. et al. The next generation of satellite laser ranging systems. J Geod 93, 2227–2247 (2019). https://doi.org/10.1007/s00190-018-1196-1
- Satellite laser ranging
- Space geodesy
- ILRS network
- Next generation
- Laser transponders