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This paper revises the precision of altimetric measurements made with signals of the Global Navigation Satellite Systems (GNSS) reflected (GNSS-R) off the sea surface. In particular, we investigate the performance of two different GNSS-R techniques, referred to here as the clean-replica and interferometric approaches. The former has been used in GNSS-R campaigns since the late 1990s, while the latter has only been tested once, in 2010, from an 18-m-high bridge in static conditions and estuary waters. In 2011, we conducted an airborne experiment over the Baltic Sea at 3-km altitude to test the interferometric concept in dynamic and rougher conditions. The campaign also flew a clean-replica GNSS-R instrument with the purpose of comparing both approaches. We have analyzed with detail the data sets to extract and validate models of the noise present in both techniques. After predicting the noise models and verifying these with aircraft data, we used them to obtain the precision of altimetric measurements and to extrapolate the performance analysis to spaceborne scenarios. The main conclusions are that the suggested noise model agrees with measured data and that the GNSS-R interferometric technique is at least two times better in precision than a technique based on using a clean replica of the publicly available GPS code. This represents a factor of at least four times finer along-track resolution. A precision of 22 cm in 65-km along-track averaging should be achievable using near-nadir interferometric GNSS-R observations from a low earth orbiter.

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Reflectometry using Global Navigation Satellite System's signals (GNSS-R) of opportunity was originally conceived in the early 1990 s for mesoscale altimetry, and since then, many studies have shown its applicability to other remote sensing applications such as sea state determination, soil moisture, vegetation, snow monitoring, etc. In December 2012, the Phase A studies of ESA's PAssive Reflectometry and Interferometry System In-orbit Demonstration (PARIS IoD) mission concluded. In conventional GNSS-R (cGNSS-R), the satellite navigation signals scattered over the Earth's surface are cross-correlated with a locally generated replica of the transmitted ones shifted in frequency ( ), and in delay ( τ). However, in PARIS, a different technique called interferometric GNSS-R (iGNSS-R) is used, which allows the use of the whole signal's bandwidth, and improve the altimetry precision, despite the large bandwidth signals' codes being not publically available. This is achieved by using the direct signal collected by a directive antenna, instead of the locally generated replica. This study presents a methodology to optimize the configuration of a generic iGNSS-R altimeter, and evaluate its performance. The methodology presented is then particularized to a PARIS IoD-like case.

[1] This paper evaluates the usage of reflected GPS signals for Earth observations to study changes of sea level and sea-ice in remote sensing. In a coastal setup, ∼670 m above Disko Bay (Greenland), signals with different carriers L1 and L2 were recorded. A method is presented that analyses the interferometric phase between the reflected and the direct signals and derives the height of the reflecting surface. The analysis includes a ray tracing and an estimation of signal coherence. It is shown that coherent reflections are related to sea-ice coverage. Absolute heights are derived with a time interval of ∼30 min. The altimetric results show semidiurnal tides that are validated using the AODTM-5 tide model. The residual height has a mean of 9.7 cm for L1 and 22.9 cm for L2. The dispersion is not significant but a significant tropospheric bias is detected with an error of up to 20 cm.