Consolidating the Precision of Interferometric GNSS-R Ocean Altimetry Using Airborne Experimental Data

Abstract

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.

First Results of a GNSS-R Experiment From a Stratospheric Balloon Over Boreal Forests

“…Newer applications of GNSS-R include wind speed measurements [3], [4], ice altimetry [5], soil moisture, and vegetation determination [6]. Several experiments have been carried out to analyze the performance of different GNSS-R techniques, i.e., conventional GNSS-R (cGNSS-R) (GPS L1 C/A) (e.g., see [7]), interferometric GNSS-R (iGNSS-R) (GPS L1 C/A, P(Y), and M) [8], and reconstructed-code GNSS-R (rGNSS-R) [GPS L1 and L2 P(Y)] [9]. March 25, 2016.…”

The Impact of Inter-Modulation Components on Interferometric GNSS-Reflectometry

“…It allows to extract information from the full spectrum of the transmitted GNSS signals, thus resulting in a sharper auto-correlation function (ACF) and improving the altimetric precision. Results from airborne experiments [15] show at least two-fold improvement in altimetric precision compared to the clean replica method, further theoretical analysis [16][17][18][19] predicts an improvement of the ranging precision by a factor of 2∼3. The modernized GNSS signals, such as GPS L1 signals transmitted by all block IIR-M and later design satellites, Galileo signals transmitted at E1/E6 bands and BeiDou signals transmitted at B1/B3 bands, are composed of several components modulated on to the same carrier to provide different navigation services (e.g., C/A (Coarse Acquisition) code, P (Precise) code and M (Military) code signals at GPS L1 band).…”

SNR Degradation in GNSS-R Measurements Under the Effects of Radio-Frequency Interference

“…One of these applications is GNSS-Reflectometry (GNSS-R), a promising technique in the field of remote sensing, that was first proposed in 1988 for scatterometry applications [6]. Later in 1993, GNSS-R was first suggested for mesoscale ocean altimetry [7], and since then, this technique has been used in the retrieval of many geophysical parameters such as measuring sea surface state [8], sea surface salinity [9], soil moisture [10], and ocean altimetry [11] among others.…”