In the present section, we briefly review the background and previous work about SNR-based GNSS-R for sea-level monitoring. For ground-based installations, GNSS-R coverage can be adjusted by changing the antenna height \(H\), as the horizontal distance to reflection points is given by \(H/\mathrm{tan}e\), in terms of the satellite elevation angle \(e\). The detection range and location can be planned in terms of specular points and first Fresnel zones, the approximate sensing points and areas (Geremia-Nievinski et al. 2016).
Previous studies include Nievinski and Larson (2014b) on the theoretical side, who described a physically based model for GNSS multipath observables. A critical aspect is a dependence on the phase of reflections, making inapplicable much of GNSS-R studies developed for incoherent scattering. On the experimental side, about forty existing GNSS stations have been demonstrated for sea-level altimetry (Geremia-Nievinski et al. 2020), from which we highlight the first ones and those with the longest duration, as follows. The pioneer was Anderson (2000), after which there was a decade long hiatus, followed by two other demonstrations (Rodriguez-Alvarez et al. 2011a; Hongguang et al. 2012). These initial proofs-of-concept had all a duration of less than a week. Longer time series were presented first by Larson et al. (2013a), with 3-4 months at Onsala (Sweden) and Friday Harbor (Washington state), followed by Larson et al. (2013b), with 1-year observations obtained at Peterson Bay (Alaska). Their results showed that it is possible to measure sea level using geodetic receivers with a precision of 5–10 cm RMSE for raw retrievals (depending on the tidal range), with improved statistics (~ 2 cm) when forming daily means and neglecting longer-term variations. Finally, Larson et al. (2017) revisited Friday Harbor station for a much longer, 10-year period. Compared to a co-located tide gauge (300 m distance), it confirmed 12 cm and 2 cm RMS for raw retrievals and daily means, respectively. Modernized GPS signals, such L2C and L5, have superior performance in producing cleaner multipath signatures (Tabibi et al. 2015), although the legacy L1 C/A signal is also feasible for GNSS-R altimetry (Larson and Small 2016).
Previous hardware and software developments
Most studies on GNSS-R for coastal sea-level sensing used geodetic-quality commercial off-the-shelf (COTS) receivers and antennas, as reviewed by Geremia-Nievinski et al. (2020). Geodetic GNSS antennas are nearly hemispherical, thus less sensitive to reception from negative elevation angles. Despite their design, such antennas are still usable for GNSS-R, as they are unable to reject multipath at grazing incidence. This is because, away from zenith and nadir, there is little separation in direction of arrival and in polarization state between reflections and line-of-sight radio propagation (Nievinski and Larson 2014b). Unfortunately, these devices are relatively expensive, which limits the wider use of GNSS-R for environmental monitoring as the primary application, not just as a secondary role for GNSS CORS networks. Here we review hardware and software developed for SNR-based GNSS-R, with a focus on open-source and low-cost alternatives. A similar demonstration has been published recently (Williams et al. 2020), although the developed hardware and software system is not available as open source.
One of the pioneers in hardware alternatives for SNR-based GNSS-R was Rodriguez-Alvarez et al. (2011b), who named their device “Soil Moisture IPT Observations at L-band” (SMIGOL). Although developed originally for monitoring soil moisture and other terrestrial variables such as vegetation, SMIGOL was also demonstrated for water-level sensing in a reservoir (Rodriguez-Alvarez et al. 2011b) and later for sea level (Alonso-Arroyo et al. 2015). It employs a commercial receiver module (Trimble Lassen iQ) that offers 12 simultaneous tracking channels at L1 frequency and C/A code modulations in a 26 mm x 26 mm form factor (Rodríguez Álvarez 2011). The module is connected to an SD-card data logger (Sparkfun Logomatic and, later, Sparkfun Openlog) and a PIC microcontroller via a custom board. The antenna was most notably designed to be linearly (vertically) polarized for improved soil moisture sensing and manufactured from scratch as a microstrip patch. Although SMIGOL was highly successful for the intended use and its components had low cost, its custom manufacture seems labour intensive and the embedded code is not publicly available, which unfortunately hinders its replication by other researchers.
Another alternative of low-cost hardware for SNR-based GNSS-R was called “Free-Standing Receiver of Snow Depth” (FROS-D), developed by Adams et al. (2013) based on prototypes of, and for use by, Chen et al. (2017). The intended application was snow depth measurement, which is a type of altimetry, so the device would presumably be equally applicable for sea-level measurement. The GPS receiver used was a commercial module (GlobalTop Gmm-u2P) based on a chipset (MediaTek MT3339) that offers 22 tracking channels at L1 frequency and C/A modulation in a small form factor (9 mm x 13 mm module and 4.3 cm x 4.3 mm chip). A Raspberry Pi microprocessor provided control and storage. Finally, the antenna was custom designed (Chen 2016), based on a half-wavelength dipole; it is linearly polarized, normally vertical in upright installations, which authors have experimented changing to horizontal polarization by tipping the antenna sideways. The cost of the system, including autonomous power supply and supporting structure, was US$1200, which is lower than comparable COTS devices. Unfortunately, FROS-D had the same fate as SMIGOL: successful use by the developers involved but little replication by external researchers. Finally, we should mention that the same GPS chip (MediaTek MT3339) has found other environmental applications beyond GNSS-R. Rodrigues and Moraes (2019) employed it for ionospheric scintillation monitoring, and Rainville et al. (2019) used it for volcanic ash detection.
We should note that the antenna designed and manufactured by Chen (2016) has also been used with a custom software-defined receiver (SDR) for GNSS-R applications (Chen et al. 2014). Other open-source SDR was used for GNSS-R by Lestarquit et al. (2016) and Hobiger et al. (2016). Such GNSS-R applications are not instances of GNSS-IR/IPT, let alone of the more general GNSS-MR, because of the way that reflections are tracked; there is not a single replica for the composite, i.e. direct plus reflected signals. SDR is highly versatile, especially useful for prototyping, but it is still not practical for field applications because it requires storing extremely large datasets for post-processing or demanding higher power consumption in real-time processing.
Other related efforts are that of Rodrigues and Kasser (2014), who used low-cost equipment (LEA-6T u-blox receiver and Tallysman TW3430 antenna) for tracking the GPS L1 C/A signal. Although it is an instance of GNSS-R for water-level altimetry, it is not an instance of GNSS-IR/IPT, as they have relied on carrier phase observables instead of SNR. The main drawbacks are the need for a second receiver serving as a base station and also the vulnerability to incoherence of reflections due to surface roughness. Similar difficulties with wind-driven water waves in carrier-phase GNSS-R (despite using geodetic-quality equipment) were experienced by Löfgren et al. (2011), although in that case, the base station antenna was co-located on top of the GNSS-R antenna.
Low-cost single-frequency COTS GPS/GNSS antennas have the potential for improved performance in GNSS-MR in general and GNSS-IR/IPT in particular, because of their high susceptibility to multipath reception, even at high elevation angles. For example, Rover and Vitti (2019) evaluated two types of low-cost antennas (u-blox ANN-MS and Tallysman TW4721) to measure snow height in three 90-min campaigns. More recently, Strandberg and Haas (2020) assessed the antenna embedded in a mobile tablet device (Samsung Galaxy Tab A), for water-level monitoring in a 36-h session. However, the short duration of evaluation in the limited periods reported above raises questions about the long-term stability and durability of low-cost hardware options for continuous operation in harsh weather conditions.
Now turning to software, open-source options include WAVPY (Fabra et al. 2017), an object-oriented library developed in Python language for simulations in GNSS-R. Although aimed at more advanced observables, such as delay-Doppler maps, it could presumably be used for simulating multipath SNR as well. Nievinski and Larson (2014c) developed an open-source GNSS-R simulator in MATLAB/Octave capable of providing SNR, carrier phase and pseudorange under multipath reception conditions. The tool allows one to investigate the behaviour of observations at a user-specified case, which is useful for site planning. The simulator is also used as the physically based forward modelling step embedded in the statistical inversion of SNR measurements collected in the field (Tabibi et al. 2017). An open-source software package for processing SNR measurements is MATLAB is described by Roesler and Larson (2018), including the translation of RINEX files, mapping of reflection zones and reflector height estimation. It allows researchers interested in getting to know SNR-based GNSS-R to get started without implementing code from scratch Martin et al. (2020; Zhang et al. (2021).