ACS Architecture
ACS includes three separate spectrometers. On the TGO spacecraft the instrument occupies the slot at the upper deck (Fig. 2). ACS has several optical openings allowing observations in nadir (\(\overrightarrow{-Y},\ \overrightarrow{-X}\) spacecraft coordinates, Fig. 2). The accuracy of spacecraft attitude control is ±1 mrad. The main opening of NIR is pointed to the nadir direction. The scanner mirror rotates within full \(360^{\circ}\) with minimal step \(0.1^{\circ}\), allowing to point in nadir, to the internal blackbody, and to the open space, which is necessary for absolute calibration of TIRVIM. TIRVIM is also equipped with a separate solar port.
The ACS block diagram is shown in Fig. 4, and its concept design—in Fig. 5. The instrument consists of four blocks bolted together and sharing the mechanical and thermal interfaces to the spacecraft. Roughly two thirds of its mass allocation of 33.5 kg is dedicated to larger channels, MIR and TIRVIM. The remaining mass is shared between the smaller NIR channel, the main electronics (Block of Electronics, BE), and the mechanical structure, thermal insulation, fixation brackets, etc. The BE is the only electrical interface of the ACS with TGO in terms of power, commands and data. The science channels can be operated together or standalone, or in any combination. The BE, MIR and NIR channels have two fully redundant sets of electronics. Either set of electronics in a science channel can be used with either set in the BE. The redundant set of the TIRVIM electronics could not be accommodated because of limited mass allocation. The ACS-BE turns on the scientific channels powering redundant 24-V lines. These internal power lines are conditioned and isolated from the spacecraft 28-V power line.
General measurement and interface parameters of ACS are summarized in Table 2.
Table 2 Measurement and interface parameters of the three scientific channels of ACS and the overall values for the whole instrument
Following the TGO requirements, the ACS suite regulates its thermal characteristics, minimizing the thermal flux to the spacecraft. Low-conductive titanium brackets are used for the mechanical fixation at the TGO honeycomb deck. The whole instrument is covered by MLI thermal blanket manufactured and tailored by NIIKAM (Russia). The MLI has a good conductivity, and can resist temperature loads during TGO aerobreaking. The thermal control is provided by several radiators placed at the upper plane of TIRVIM and MIR channels (+X axis of the spacecraft, see Fig. 1), and by means of several independent operational and survival thermal control systems. The radiators are covered by EKOM-1 enamel by Kompozit (Russia), and are solar-blind.
During the cruise and aerobraking phases the BE is OFF, and the temperature in the scientific channels is maintained at acceptable level by dedicated survival heaters subsystems (Fig. 4). These systems include thermosensors and heaters and are powered via dedicated power line (20–40 V) directly from the spacecraft, bypassing the BE.
TIRVIM is thermally insulated from other ACS constructions by means of plastic brackets and MLI patches. Some parts of TIRVIM could not be covered by MLI, and in order to diminish the thermal emission they were gold-coated by EKOMET (Russia). One of the two TIRVIM radiators drains the heat from the Stirling cooler, and the other regulates the overall instrument’s temperature. For maintaining a stable thermal environment it is planned that TIRVIM will be permanently ON when in TGO science orbit around Mars. It will predominantly perform nadir measurements, and will occasionally measure while the spacecraft is in solar occultation attitude.
NIR Channel
Concept
The ACS NIR channel employs the same optical layout as the SPICAV/SOIR (Solar Occultation Infrared) instrument on the ESA Venus Express mission, that is a combination of an echelle spectrometer and an acousto-optic tunable filter (AOTF) for the selection of diffraction orders. In space research this scheme was proposed and prototyped for the measurements of the terrestrial atmosphere (Korablev et al. 2002) and first implemented in space for Venus Express (Nevejans et al. 2006). NOMAD on TGO (Vandaele et al., this issue) employs the same principle and includes two similar SOIR-type spectrometers. The echelle-AOTF combination allows one to reach high spectral resolution in a compact and robust design. Several diffraction orders can be sequentially acquired during one measurement and they can be located anywhere within the entire spectral range thanks to the AOTF tuning versatility. Within the NIR range, atmospheric spectra are generally featureless, dominated by dust particle continuum properties, yet a number of spectral intervals can be used to cover interesting species. The main disadvantage of this design is the narrowness of the selected spectral intervals. In addition, sidelobes of the sinc function representative of the AOTF spectral bandpass imply that adjacent diffraction orders superimpose on top of each other. Also, the echelle grating efficiency quickly decreases away from the blaze angle (center of the detector) and, combined with a similar effect of the AOTF bandpass curve, induces a significant reduction of signal near the edges of the detector.
The closest analogue of NIR is the RUSALKA instrument (Russian acronym for Manual Spectral Analyzer of the Atmosphere Constituents), flown on the International Space Station (ISS) in 2009–2012 (Korablev et al. 2011). RUSALKA covered the spectral range of 0.7–1.65 μm with a resolving power of \(\lambda/\Delta \lambda \approx 20{,}000\), and demonstrated its capability to measure terrestrial CO2 and CH4 lines in a nadir viewing. Still, cosmonauts operated RUSALKA in the habitable compartment of the ISS, and the instrument was built according to requirements far less stringent than those imposed for a deep space mission such as TGO. ACS NIR is an adaptation of the RUSALKA design with several modifications aimed to improve the sensitivity and the optical quality of the instrument.
ACS NIR can perform nadir and occultation observations using two optical ports. A part of the nadir aperture of the instrument (Ø20 mm) is blocked by a periscope mirror, forming the solar port.
Optical Scheme
The optical scheme of the instrument consists of two main parts (see Fig. 6). The entry part, illuminating the slit of the echelle spectrometer includes the foreoptics, the telescope, and the AOTF with a number of auxiliary elements such as polarizers, diaphragms, etc. The entrance lens doublet has a focal length of 120 mm. A red filter cancels out all wavelengths shorter than 0.7 μm.
The AOTF is an optical filter based on the principle of acousto-optic diffraction in an anisotropic medium. The central wavelength of the filter spectral response function (SRF) is determined by the frequency of the ultrasonic wave in a birefringent crystal excited by an acoustic transducer. A non-collinear TeO2 AOTF is employed (Chang 1974). The acoustic wave modifies the refractive index, and diffraction at the resulting distributed structure separates the incident ray into two diffracted beams and a zero-order beam. The acousto-optic interaction length is 27 mm. The diffracted and non-diffracted rays are polarized athwart, permitting the use of polarizers to suppress the unwanted zero order. Besides, the non-diffracted light is separated angularly from the main throughput.
In contrast to previous AOTF-echelle based designs, this AOTF is placed in a converging beam. Therefore, the AOTF unit is a part of the foreoptics: it includes the polarizers and some folding mirrors (Figs. 6a, 7a). The FOV of the instrument is determined by the slit dimensions. The entrance optics, including the AOTF crystal preserves the image of the observed scene with a good quality over the full spectral range. This allows the use of NIR as a hyperspectral imager both with good spectral and spatial resolution. Undesired stray light in the entry part of the optical scheme is suppressed inside a long baffle that also holds the foreoptics elements.
The beam filtered in by the AOTF passes through the slit into the echelle-spectrometer. The slit has a size of \(4~\mbox{mm}\times 40~\upmu \mbox{m}\), forming a FOV of approximately \(2^{\circ}\times 0.02^{\circ}\) (\(35\times0.35\) mrad). The slit is inclined at \(5.5^{\circ}\) around the optical axis to compensate for the tilt of the slit image on the detector. The echelle-spectrometer employs a Littrow auto-collimation scheme, with a single off-axis parabolic mirror. The separation of the incident and the dispersed light is made in the plane perpendicular to the dispersion (along the slit height). A custom-size echelle grating from Spectra Physics Corp., Richardson Grating Laboratories (USA) on a Zerodur® substrate has a blaze angle of \(70^{\circ}\) (R3), 24.35 grooves/mm, and a useful area of \(46\times 102~\mbox{mm}^{2}\). The parabolic collimator mirror is fabricated by diamond turning from aluminum alloy (it has a focal length of 200 mm, \(10^{\circ}\) off-axis, size \(58\times53~\mbox{mm}^{2}\)). Its surface is additionally polished and coated with copper (Fig. 7b).
Compared with the heritage design of RUSALKA, NIR features a higher slit in combination with a 2D array detector (instead of a linear detector on RUSALKA), which captures the dispersed light along the full dimension of the slit. We adapted for space the infrared camera module XSW-640 from Xenics (Belgium) based on a thermo-electrically cooled (Peltier element) InGaAs array of \(640\times512\) pixels (Fig. 7c). This detector features a pixel size of \(20\times 20~\upmu \mbox{m}^{2}\), and a spectral range defined by the InGaAs sensitivity domain between 0.4 and 1.7 μm. The slit is imaged onto the detector with a ratio of 1:1, so only a part of the detector’s height (4 mm or ∼200 out of 512 pixel lines) is illuminated.
The main specification of the RUSALKA-SOIR design is that the AOTF spectral response function (SRF) width must be aligned with the free spectral range of the echelle spectrometer. Expressed in wavenumbers, the free spectral range is independent to wavelength:
$$ \Delta\nu= \frac{1}{2a\boldsymbol{\cdot} \sin \theta}, $$
(1)
where \(1/a\) is the number of grooves per cm, and \(\theta \) is the blaze angle (in autocollimation). If one ignores the dispersion of the TeO2, the width of the SRF is approximately constant in wavenumbers as well. By design, the half-width of the AOTF SRF is 70 cm−1.
The spectral range of the instrument (0.73–1.6 μm) corresponds to the echelle diffraction orders from 48 through 105. The spectral resolution is limited by the slit width and aberrations of the off-axis parabolic collimator. Following the sampling theorem (2 pixels per resolution element) the resolving power of an aberration-free spectrometer reaches \(R \approx 27{,}500\). The resolving power is variable over the detector and also depends on the diffraction order, but remains always ≥20,000 (see Sect. 4.1 below).
Electronics and Software
The NIR electronics consists of a power board with DC/DC converters and power switch, a processor module board, the XSW-640 detector module and the AOTF control electronics. These three modules of ACS NIR instrument are functionally independent devices and combined only with power lines and digital communication interfaces. Power board and processor module board are fully redundant. Thermal regulation systems are physically and logically separated from the electronics itself. Survival heaters are powered via fully independent line and they are always ON when the rest of the electronics is OFF.
The NIR control electronics is built around the TCM-BF537 processor module. Parallel Port Interface (PPI) of the processor module is used to input video data from the detector. Switchable line serial ports UART0 and UART1 are used to control the detector and AOM, as well as for communication with ACS BE. Serial Clock frequency is 129.17 MHz, supporting 8 Mb/s interface speed with BE and 115.2 kb/s with AOTF and the detector.
An AOTF module includes a microprocessor control system, a digitally controlled frequency synthesizer and a RF power amplifier. It is powered throughout the session, but the RF power to the crystal is sent only by the external command from the NIR control electronics. The same command also sets the frequency and the power of the RF signal. The frequency is set within a range of 64–156 MHz with a 50 kHz step. A limit of 100 ms for the duration of a single turn ON is used to protect crystal from overheating.
The detector module is working in the Integrate Then Read (ITR) mode, exposure starts with an external signal. A built-in Peltier cooling system of the detector can operate in the temperature stabilization mode and the temperature can be set with an accuracy of 3 K.
Operations
Each NIR observation session starts with a dedicated telecommand. A 64-byte command contains all the necessary parameters, including timing (overall time of observation, pauses, exposition time etc.), AOTF and detector settings, and also frame processing customization.
The instrument can be programmed to register sequentially up to ten diffraction orders (10 different AOTF tunings, i.e. acoustic frequencies and therefore 10 relatively small portions of the whole 0.73–1.6 μm range).
Depending on the telecommand, the NIR electronics commands the AOTF module to set the required frequency and triggers a detector exposure. After the integration time a single frame is read out within 9 ms (the ITR mode). At each frequency the instrument always performs two integrations, one with the AOTF ON and another with the AOTF OFF to allow dark subtraction subsequently. Either both frames are transmitted or only their difference.
The exposure time can be tuned from 0.45 ms to 100 ms depending on the observation regime. Furthermore, an on-board image averaging up to 96 single frames can be programmed in flight. The full frames are accumulated and averaged onboard by the NIR electronics for each frequency. In this case the control electronics continues to measure pairs of frames, stacking them in the internal memory. The resulting frame is scaled to fit each pixel value into 16 bits and then can be cut into up to five bands of programmable position and height (5 different areas on the detector). Each band contains a 640-pixel spectrum, corresponding to a certain portion of the slit height. Further co-adding along the height of each band into a line is also possible to reduce the data downlink volume.
Measurements for all given frequencies continue in a loop with a programmed duty factor, which will be used to regulate the data rate during nadir measurements.
MIR Channel
Concept
The MIR channel is a cross-dispersion spectrometer working in the 2.3–4.2 μm spectral range and acquiring simultaneously up to 0.3 μm per measurement. The echelle orders are dispersed along the \(x\)-axis and separated along the \(y\)-axis of the focal plane by a secondary dispersion element, making full use of the 2-D detector array. This concept is used in astronomy and has been employed in planetary missions with VIRTIS-H (Visible and InfraRed Thermal Imaging Spectrometer with High resolution) instrument flown on Rosetta and Venus Express (Coradini et al. 2007). VIRTIS-H acquires the spectral range of 2–5 μm instantaneously using an echelle grating and a prism. With a number of diffraction orders of ∼10, the resolving power of VIRTIS-H is \(\lambda / \Delta \lambda \approx 2000\). Targeting much higher resolving power, ACS-MIR features an echelle-spectrometer with a higher dispersion, and a steerable secondary dispersion element (reflecting grating). Working at high diffraction orders the instrument captures from 10 to 30 adjacent diffraction orders at once, covering approximately 15% of the spectral range in a single integration. Steering the secondary disperser allows one to switch from one group of diffraction orders to another, alternating between desired positions during one measurement sequence.
By analogy with MATMOS, several photometric channels for the wavelengths from 200 to 900 nm were studied for inclusion in MIR (Korablev et al. 2014). Their implementation would have supported aerosols characterization, and allow for measurements of UV-absorbing species. However, this unit did not fit into the initial allocated mass budget of ACS. Instead, aerosol studies will be carried out by combining the measurements made by all ACS channels (and potentially with NOMAD UV-visible spectra).
The Optical Scheme
The optical scheme of MIR may be separated into three main parts, the entry optics, the echelle spectrometer, and the secondary disperser (see Fig. 8). All the refractive elements are made in ZnSe and CaF2, transparent in the visible. Reflective elements are fabricated of aluminum alloy by diamond turning.
Radiation from the sun, passing through the Martian atmosphere, is directed inside the instrument by a periscope, which is imposed by mechanical constraints. A longpass entry filter is made of AR-coated Si 5-mm slab. The telescope forms the image of the solar disk on the slit (\(400~\upmu \mbox{m} \times 30~\upmu \mbox{m}\)). The FOV of \(0.3 \times 4.1~\mbox{mrad}\) is determined by the slit dimensions.
Similar to the ACS-NIR the echelle-spectrometer of MIR employs the autocollimation Littrow scheme. Two conditions are needed to achieve a high spectral resolution: firstly, the echelle grating must be large enough and have a high linear dispersion. The diffraction-limited resolving power is given by
$$ R_{\mathit{DL}} = \frac{\lambda}{\Delta \lambda} \approx \frac{2 D}{\lambda} \tan \theta, $$
(2)
where \(D\) is the collimator diameter and \(\theta \) is the blaze angle. For the worst case of 4.3 μm and \(\tan \theta = 2\) the ACS requirement of \(R \geq 50{,}000\) implies \(D \geq 54~\mbox{mm}\) that stays within allocations. The linear dispersion determines a slit-limited resolving power
$$ R_{\mathit{Sl}} =2 \tan \theta \frac{f_{\mathit{col}}}{\delta_{\mathit{Sl}}}, $$
(3)
where \(f_{ \mathit{col} }\) is the collimator focal length, and \(\delta _{\mathit{sl}}\) is the slit width. IR arrays with a pitch of less than 15 μm are not available, and to satisfy minimal sampling the slit should be \(2\times 15~\upmu \mbox{m}\) wide. Then the requirement of \(R \geq 50{,}000\) results in \(f_{ \mathit{col} } \geq 500~\mbox{mm}\). This value is the principal design driver: it determines the size of the grating and the dimensions of the MIR channel in general. The large focal length in a reasonable volume is achieved using a two-mirror collimator designed and fabricated by NIIKI (Sosnovy Bor, Russia; see Fig. 9a). Both mirrors are aspherical. The aperture of the collimator is chosen as a compromise between the desired etendue of the instrument and the suitable size of the grating given \(f_{ \mathit{col} } \approx 550~\mbox{mm}\).
We use a large custom-made echelle grating from AMOS (Liège, Belgium) with a ruled area of \(107 \times 240~\mbox{mm}\). The grating is blazed at \(63.43^{\circ}\) (R2), and has 3.03 grooves per mm (untypically low number for most grating providers). The grating is ruled on an aluminum substrate using diamond turning. Following Eq. (1) the free spectral range for one diffraction order is \(\mbox{FSR} \approx 16~\mbox{cm}^{-1}\) (or 17.7 nm at 3.3 μm). The full spectral range is covered with 107 diffraction orders, from 142 to 258.
The quality of the imaging system is close to the diffraction limit: the spot diagram yields a root mean square (RMS) of 6.5 μm in the center of the diffraction orders with a typical degradation toward the edges. Therefore, the resolving power of the spectrometer is aberration and slit-limited. With a 30-μm slit a resolving power of \(\lambda/\Delta \lambda \geq 50{,}000\) at 3.3 μm is achieved. The optical design of the MIR channel, folded in 3-D, is sensitive to the thermal regime and the mechanical strains. Although verified during the integration and the alignment procedures, the resolving power will have to be confirmed during the science phase of the mission after the thermal and mechanical state of the instrument will be fully settled. Despite a number of Sun observations completed in flight (see Sect. 4), it is not yet possible to characterize completely the spectral resolution. This characterization is planned after completion of the first solar occultation sequence at Mars. In Fig. 9b we present the LSF for the MIR echelle-spectrometer simulated by ray tracing.
Two secondary cross-dispersion diffraction gratings from Richardson Grating Laboratories, USA (plain, custom size, 361 and 180 grooves per mm) are mounted back-to-back on a stepper motor axis. The design of the grating positioner designed and fabricated by Astron Electronics (Orel, Russia) capitalizes on MSASI/Bepi Colombo scanner development (Yoshikawa et al. 2010). Though the two-face grating assembly is much heavier than the MSASI scanning mirror, the mechanism moves with a characteristic time of ∼0.1 s. Changing the position of the secondary grating in calibrated angular steps of \({\sim}1.8^{\circ}\), from 7 to 17 echelle orders can be made during the same acquisition, the final number being wavelength dependent (see Table 3). As a result, for each position of the secondary grating (see Fig. 10) the detector area is covered with a number of stripes, each corresponding to a single echelle diffraction order (see below Sect. 4.2). To switch between the two gratings, the positioner supports fast \(180^{\circ}\) overturn.
Table 3 Design wavelength coverage in function of angle of the secondary grating
The detector is a space-grade version of the standard Scorpio MW K508 Sofradir (France) product, with optimized spectral range. Adaptations with respect to this standard detector are: (i) the replacement of the cold filter (bandpass 3.7–4.8 μm) by a custom filter; (ii) customized AR coating of the entry window; and (iii) adaptation of the cryocooler K508 RICOR (Israel) for operation in space. This detector includes a \(640 \times 512\) MWIR retina made of MCT (Mercury Cadmium Telluride, HgCdTe) hybridized onto a silicon ROIC (read-out integrated circuit) by indium bumps. The pixel area is \(15\times 15~\upmu \mbox{m}^{2}\).
In order to decrease the thermal background, a number of reflecting diaphragms are located in critical areas near the detector, around the slit, and throughout the instrument, so as beyond its useful area the detector images mostly the reflection from the internal cold baffle.
Electronics
The MIR electronics can be separated into five functional modules: power board, control electronics, rotational unit, Scorpio detector, Stirling cryocooler and its own driving system. As stated previously, survival heaters are powered independently.
Similarly with NIR, the MIR control electronics is built around the TCM-BF537 processor module. For fast handling of the Scorpio detector frames it includes a Complex Programmable Logic Device (CPLD) XC2C256. The main function of the CPLD is to synchronize the Scorpio detector, the ADC video processor interface and the PPI. In addition, the CPLD provides a UART interface line switch. CPLD is controlled by register from processor module via the serial interface, running at a speed of 10 Mbit/s. As in NIR, the UART ports of the processor are used to control communication with ACS BE (8 Mb/s interface speed) and with the rotational unit (129.17 MHz interface speed).
Stirling cryocooler motor is controlled by the processor via a dedicated driver. Cooling power is set by a Pulse Width Modulation (PWM) signal (40 kHz). The maximum value of the speed control—about 190 pulses/s, the minimum acceptable value—50 pulses/s. The detector has two in-built temperature sensors. Its temperature is PID-stabilized by the processor software with an accuracy of ≤0.1 K.
The Sofradir detector has four video outputs, fed to four parallel ADCs. Data is transmitted to the PPI interface with an overall speed of \(4\times 8~\mbox{MHz} = 32~\mbox{MHz}\). The dynamic range of the detector is 13 bits. The ADC has a 16 bits resolution, but only 13 most significant are used. After stacking of frames each pixel is described by a 16-bits word, and they are transmitted to the ground.
The Rotational unit is commanded by its own FPGA-based electronics, which enables closed-loop PID regulator using the signal from Hall’s sensors and drives the VSS 33.200.0,6-HV vacuum-grade Phytron (USA) motor.
MIR’s power board, control electronics and the rotational unit electronics are fully redundant (including the main ADCs). Cross-redundancy is possible for the Stirling cryocooler driver, and with the rotational unit electronics; either set can be used with either set of the control electronics.
Measurement Sequence
Each MIR observation session starts with a dedicated telecommand from the TGO spacecraft. A 64-byte command contains all necessary timing parameters (overall time of observation, pauses, exposure time, etc.), the grating positioner commands (defining the spectral range), detector settings, and frame processing customization. After the detector precooling time of ∼8 min, and rotational unit self-calibration procedure of ∼40 s, the MIR channel is ready to record spectra.
Rotational unit receives a command to turn one of the two secondary gratings into the required position, and returns confirmation upon readiness. This triggers the measurement series. In order to increase the effective measurement time, thus improving the signal to noise ratio, the instrument is designed to minimize gaps between frames measurements. The detector is run in an Integrate While Read regime (IWR). While measuring a new frame, the previous one is being simultaneously transmitted to the MIR control electronics, which stacks all frames measured for the current secondary grating position. The readout of the full frame (\(640 \times 512\) pixels) using four detector’s outputs requires 10.25 ms. The exposure time can be set in the range 0.125–32 ms with a 0.125 ms accuracy. The real exposures when observing the Sun are within 3–6 ms, depending on the spectral range. The video signal of each pixel is digitized with resolution of 13 bits. The MIR processor can measure and sum up to 200 consecutive single frames. The result is accumulated in 32-bit words.
If an occultation session includes measurements with several secondary grating positions, the Rotational unit mechanism allows the spectral range to be shifted with a characteristic time of ∼0.1 s for adjacent positions, ∼0.12 s for positions separated by a step, etc. Up to five positions can be commanded and be measured during an acquisition, though we plan to use only one or two positions during a real occultation. The switch between the two gratings is only possible before a series of measurements, so during a single occultation only one grating can be used.
A single data frame will be accumulated as a result of stacking a number of shorter exposures. Accumulated frames are transmitted to ACS BE. The full frame requires ∼0.9 s to transfer. Each transmitted frame is accompanied by two housekeeping frames with board time, temperatures and setup parameters. If more than one grating position is measured the MIR control electronics transmits data from the previous position to BE while measuring the new position. In order to reduce the downlink volume and/or to increase the number of measured frames per second, lossless compression, and windowing of the frames can be used. Up to 5 bands of programmable position, height and length can be selected to isolate the most interesting spectra. Given the complexity of the diffraction pattern, no line co-adding is foreseen on-board, although the embedded software provides such possibility. Normally full or partial detector frames will be transmitted to the ground with lossless compression.
TIRVIM Channel
Concept
TIRVIM is a 2-inch double pendulum Fourier-transform spectrometer (FTS) operating in the 1.7–17 μm spectral range. The primary scientific goal of TIRVIM relates to the Climatological goal of ACS through the monitoring of key climatic parameters, like atmospheric temperature and aerosol abundance from nadir viewing. The instrument leverages from previous developments of IKI in Fourier-spectrometers (Moroz et al. 1990; Formisano et al. 1997; Korablev et al. 2009). One may note its resemblance to 2.5-inch PFS/Mars Express (Formisano et al. 2005), however the mass allocation for TIRVIM on TGO was 12 kg against 31.4 kg of PFS, and so simple copy of the dual channel PFS design was excluded.
By concept a FTS measures an interferogram that is then converted into a spectrum, which covers the spectral range limited by the detector and the transparency of the optics. In TIRVIM the whole spectral range is covered by one double-pendulum interferometer with KBr beamsplitter (KBr is transparent from 0.25 to 25 μm). The principal improvements in comparison to PFS are: (i) the possibility to observe in solar occultations, (ii) one-side maximal optical path difference (MOPD) increased to as much as 5 cm, allowing to reach apodized spectral resolution of 0.13 cm−1 (compared to \({\approx}1.3~\mbox{cm}^{-1}\) for PFS), and (iii) a PV-MCT detector cooled by a Stirling machine, thus increasing the sensitivity of the instrument by a factor of 10–20. The detector is sensitive from 1.7 to 17 μm and is employed both for nadir and during solar occultation observations.
While observing Mars in nadir TIRVIM operates as a spectro-radiometer, providing absolute radiometric calibration by periodically measuring the temperature of the internal Black-Body and of the space. While observing during a solar occultation TIRVIM operates as a spectrometer, self-calibrating the relative transmission of the atmosphere by observing the Sun outside the atmosphere.
Initially the instrument design included a short-wave optimization in the 3-μm range for the mapping of putative hydrocarbons in nadir (Korablev et al. 2014). This feature had to be de-scoped because of lack of mass (its implementation involved an additional mechanism, and significant electronics). For the same reason the redundant pyroelectric detector for solar occultation had to be cancelled as well. Eventually, main science requirement was put on nadir observations.
Optical Scheme
A simplified optical scheme of TIRVIM is presented in Fig. 11. It consists of the following main parts: a sun periscope, a scanner, a blackbody simulator, the interferometer with a reference channel, and the detector with its proximity optics.
The scanning device of TIRVIM (see Fig. 12) allows it to point its optical axis to nadir, to the limb, and also to the internal blackbody and to the open space to obtain absolute radiometric calibration. It is rigorously not a scanner but a one-dimensional positioner. The main element is the flat mirror (\(85\times 60~\mbox{mm}^{2}\)), which can be rotated within \(360^{\circ}\) with a step of \(0.1^{\circ}\). It can be pointed to three principal directions: nadir, open space, and the black body, but also to any intermediate position, including the direction to the Sun.
The second optical port of the instrument is the solar periscope. Its direction is fixed and co-aligned with solar ports of NIR and MIR channels, i.e. pointed to the direction of \(67^{\circ}\) with \(\overrightarrow{-Y}\) spacecraft axis (see Fig. 2). The solar periscope includes a blocking filter and two elements folding the solar radiation into the interferometer. The aperture of the solar port is ∼10 mm wide which constitutes ∼4% of the useful interferometer aperture.
The Black Body was fabricated in IKI, its emitting surface is painted with space-qualified EKOM-2 enamel by Kompozit (Russia). Its temperature is regulated by either the survival heaters, or the operational thermal control systems of TIRVIM.
The interferometer is the core unit of TIRVIM (see Fig. 13). Most of principal design solutions are inherited from AOST/Phobos-Grunt, but the TIRVIM interferometer is larger (2″ against 1″). The length of the double-pendulum arm (from the rotation center to the center of the corner-cubes) is 130 mm. The interferometer pendulum full swing results in MOPD of 5 cm, so the spectral resolution, if defined as 1/MOPD is 0.2 cm−1. The spectral resolution defined at FWHM of the instrument function is better (0.603/MOPD), \({\approx}0.13~\mbox{cm}^{-1}\) (see Table 2). For nadir observations the OPD is reduced to 0.8–1 cm (the spectral resolution is of 1.3–1 cm−1 or ≈0.8 at FWHM). The double pendulum is mounted pending on a Flexural Pivot by Riverhawk Company (USA) and propelled by a momentum motor built in IKI. The hollow retro-reflectors are from PLX Inc. (USA), modified in IKI so they can withstand mechanical loads during the launch. Their useful diameter is Ø50.8 mm. The beamsplitter and the compensator are made of KBr, Ø88 mm and are 12 mm thick.
The reference channel allows for a precise synchronization of the interferogram sampling, and the wavelength link of the retrieved spectrum. The reference signal is generated from the interference of radiation of a monomode laser diode within the main interferometer of TIRVIM. The fraction of the aperture used for the reference channel is small. A distributed-feedback laser diode for the wavelength of 760 nm was supplied by Eagleyard Photonics (Germany). The channel includes also a Si photodiode detector and a number of optical elements necessary to introduce the laser beam into the interferometer. In all the operation modes the digitalization of the two-side interferogram is performed at each one out of two zero crossings of the reference signal. Therefore the sampling theorem is satisfied for all wavelengths longer than 1.52 μm. The germanium inlet window blocks all the radiation below 1.7 μm, removing aliasing.
Radiation is focused onto the detector sensitive element with an aspheric plano-convex ZnSe lens. The photovoltaic MCT detector unit is custom-built in Russia. It is sensitive in the range of 1.7–17 μm; a germanium inlet window determines its short-wavelength cut-off. The diameter of the sensitive element is 1.5 mm, and the specific detectivity peaking at 14.5 μm is D∗ of \(1\times 10^{1 1}~\mbox{cm}\,\mbox{Hz}^{1/2}\mbox{W}^{-1}\). The sensitive element is mounted in a Dewar and cooled down to 65 K with a rotary Stirling cooler from RICOR (Israel), similar to that of MIR channel. The beamsplitter, AR-coated Ge windows, objective aspheric ZnSe lens and aluminum mirrors with gold coating are supplied by Spectral Systems LLC (USA).
The FOV is determined by the diameter of the sensitive element of the detector size, and the focal length of the detector’s proximity ZnSe lens. When observing in nadir the full FOV diameter of \(2^{\circ}\) is used. From the TGO orbit (∼400 km) the track width on the surface of Mars is 14 km. When pointed to the Sun the instrument’s FOV is limited by the diameter of the solar disk diameter (\({\sim}0.35^{\circ}\)).
Electronics
The electronics of the TIRVIM channel is organized around the Central processor. It interacts with BE through the LVDS interface, interprets its telecommands, acquires interferograms and housekeeping data, processes them, packs and sends them back to BE. It controls, directly or via two auxiliary processors, all TIRVIM subsystems, the thermal regime, the scanner, and other mechanisms.
The scanner operation is controlled using its encoder or backup Hall sensors. There are two multi-usable arresters; one is immobilizing the double pendulum, and the other the entire interferometer unit (Fig. 13) mounted on vibration dumpers. The operation thermal stabilization system controls, by means of the central processor, the thermal regime of all critical optical parts. It includes six collocated heater-sensor pairs placed at the blackbody, at the fixed mirror, at the “front wall”, in two locations within the interferometer unit, and near the body-fixed optical elements. The maximum dissipated power is 16 W.
The Interferometer processor provides closed-loop PID stabilization of the interferometer OPD alternation speed, controls temperature and current of the diode laser, and other interferometer functions. The temperature and the current of the diode laser are set by telecommand in the ranges of \(14^{\circ}\)–\(35\ {}^{\circ}\mbox{C}\) and 43–75 mA respectively. The embedded software allows for searching of optimal PID coefficients values in flight. The position of the interferometer pendulum arms is registered by a Hall sensor. It triggers at zero OPD, and the interferometer processor moves the pendulum with respect to this point. In fact, the Hall triggers not exactly at \(\mbox{OPD}=0\) but the shift, in units of the reference channel period, is known.
The Stirling processor provides PID-stabilization of the cryogenic detector temperature. It gets information about temperature in the cryogenic zone from a dedicated thermo-sensor. The nominal temperature in the cold zone is 68 K.
The reference channel detector (Si photodiode) registers the interference of the monochromatic laser light. If the OPD alternates evenly, the signal is a sine function, allowing one to close the loop of interferometer mirrors motion. The reference signal is transferred to the square wave, which serves to govern the pendulum motor and triggers the conversion of the main ADC.
The signal from the cooled MCT detector after a preamplifier goes to variable-gain amplifier (eight gain levels \(1, 2, 4, \ldots, 128\) set by telecommand) and bandpass shaper, which gets the signal ready for the AD-conversion. The main ADC provides the interferogram sampling in 18 bits, 16 of them being effectively used. The central processor governs the amplification coefficient and the frequency bandpass, gets data from the ADC and processes them.
The Stirling is powered directly from 24-V regulated voltage provided by the ACS-BE. A dedicated power PCB creates all other needed voltages (−9, \(+3.3\), \(+5\), \(+6.5\), \(+12\) V).
The survival heaters are powered when the main TIRVIM electronics is OFF. This system includes three pairs of heaters-sensors (optical plate, interferometer and Black Body) and is powered directly from the spacecraft. The threshold values are set at \(+30\ {}^{\circ}\mbox{C}\), except for the Black Body (\(+60\ {}^{\circ}\mbox{C}\).) Higher temperature of the Black Body allows warming the rear side of the scanner mirror to avoid its contamination during spacecraft outgassing. The consumption is 8 W regardless of the voltage value (20–40 V).
Measurement Modes
The Fourier spectrometer subsystems allow for numerous settings and parameters, however few optimal combinations were selected and verified during calibrations. This parameter set is implemented in the embedded software of TIRVIM.
The interferometer pendulum can swing within its full range, resulting in a \(\mbox{MOPD}=5~\mbox{cm}\). Alternatively, a reduced swing with \(\mbox{MOPD}=0.8\mbox{--}1~\mbox{cm}\) can be employed for measuring weaker signals. The full swing is devoted to Sun observations, and the reduced one is devoted to nadir observations. Also, the interferometer mirrors can move with a slower or faster motion. The slow mode results in a reference channel frequency of ∼10 kHz, and the fast one in ∼80 kHz. With the change of the interferometer speed, the bandpass of the main tract is modified accordingly. The full swing is always implemented with the fast mode, and the reduced swing can use both speeds, resulting in three main interferometer modes.
To remove possible parasite signals (due to a variety of vibrations) the speed setting can be slightly modified: the slow mode may become 10% quicker, and the fast mode may become 10% slower. The 10% variation is enough to move the parasite signals along the wavenumber axis so they don’t distort the critical portions of the spectrum. So, in all, six interferometer modes (aliased as #0…#5) are possible.
TIRVIM interferograms are always two-sided. The interferograms recorded in the small-swing mode are much shorter, and the central processor can perform their on-board centering and stacking, increasing the SNR and decreasing the data downlink. The interferograms are averaged separately for forward and retrograde motion of the interferometer mirrors. It is possible to average \(2+2\) interferograms (2 “forward” and 2 “retrograde”), \(4+4\) or \(8+8\).
The central processor adds to every interferogram a header containing its parameters, onboard time, housekeeping and other data.
The TIRVIM parameters and observing modes are summarized in Table 4.
Table 4 Main parameters of TIRVIM in the most used modes of observation
Main Electronics
The main electronics (BE) serves as a single electrical interface of the ACS to the spacecraft. The spacecraft harness including power, commands, data lines and auxiliary interfaces is wired to the BE. The main electronics is fully redundant (see Fig. 14).
The BE consists of two PCBs, located in a separate unit. The Processor PCB (visible in Fig. 14b) includes two redundant sets of FPGA, 32 GB of flash-memory, auxiliary controller, interface controller, etc. The Power PCB carries two redundant sets of filters, DC/DC converters, and power switches. As one may note in Fig. 14b, the electronics occupies a smaller volume than the one available in the BE box. In fact, the volume allocation had to be decided before the final definition of the electronics, and some margins had to be added.
The power interface includes the main power switch, power conditioning, and specific switches for each scientific channel. All power switches and relays are controlled by High Voltage High Power Command (HV-HPC) spacecraft lines. A Bi-level Switch Monitor (BSM) interface is used for the TGO system to acquire a signal from the instrument switch loads. The final power distribution to the channels is done using regulated voltage lines from BE. Redundant sets of MIR and NIR internal electronics can be used with either set of the BE electronics. The choice of nominal or redundant set of MIR or NIR electronics to be turned ON is defined by a telecommand. Upon request, BE is generating HV-HPC for the dedicated channel. BE is able control any science channel in any combination; in particular both MIR and NIR can be operated at the same time, regardless of which set of electronics controls them.
The survival heaters in the scientific channels are powered by a dedicated TGO power line, bypassing the ACS BE. There is no survival heater in the BE; its thermal regime in cruise is maintained by thermal conduction from surrounding MIR and NIR channels (see Fig. 5).
The command and data interfaces to TGO are MIL1553 and SpaceWire, respectively.
After powered on, the BE receives 64-byte telecommands (TC) from TGO. MIR and NIR TCs include a small header interpreted by the BE to generate appropriate HV-HPC for the science channel and overall time of observation. TIRVIM will be permanently ON during the nominal science operations, and it periodically receives TC blocks, programming a mid-term operation sequence. The BE itself nominally doesn’t require any TC during routine science operation.
The data/command interfaces between the BE and the scientific channels are LVDS with a maximum speed of 8 Mbit/s. The data from the science channels can be acquired simultaneously. They are stored in blocks with certain system headers, specific for each channel, and are transferred via SpaceWire to the TGO (21 Mbit/s). The BE internal memory will be used only in case of the temporary unavailability of the TGO memory. The mass memory was also included to validate the design solution for the ExoMars 2020 Surface platform where a similar main electronics will be serving several instruments on the landing platform (Rodionov et al. 2017).