**1. Introduction**

Satellite radar altimetry and tide gauges have been used to monitor large-scale global sea surface heights (SSH) in the past several decades; these have contributed much to Earth sciences [1,2]. However, their spatial and temporal resolutions cannot meet the requirements for probing mesoscale features in the ocean height. To solve this problem, global navigation satellites system-reflectometry (GNSS-R) was proposed as a multi-static radar means with the prospect of providing additional high-density SSH measurements [3]. Essentially, the performance of this technique relies on the accuracy of the relative path delay between the direct and reflected signals. As GNSS signals are not dedicated for altimetry, the precision of GNSS-R code-level altimetry is restricted by their smaller bandwidth and lower transmitted power [4].

Many experiments have been performed on different platforms to test the precision and accuracy of GNSS-R code-level altimetry. The results of a bridge-based experiment showed that global positioning system (GPS) C/A code and P-code provided the water surface reflector height with accuracies of 3 and 0.3 m, respectively [5]. The experiment was enhanced; its results indicated a significant improvement in GNSS-R altimetric performance with 7.5 cm uncertainty [6]. The feasibility of code-level altimetry based on BDS B1I signals using coastal GNSS-R setups was also verified [7]. The first airborne GNSS-R ocean altimetry experiment was performed in 2002, with results showing that the root-meansquare residual height was at the meter level for GPS C/A code and decimeter level for

**Citation:** Gao, F.; Xu, T.; Meng, X.; Wang, N.; He, Y.; Ning, B. A Coastal Experiment for GNSS-R Code-Level Altimetry Using BDS-3 New Civil Signals. *Remote Sens.* **2021**, *13*, 1378. https://doi.org/10.3390/rs13071378

Academic Editors: Hugo Carreno-Luengo, Dallas Masters and Chun-Liang Lin

Received: 15 February 2021 Accepted: 31 March 2021 Published: 3 April 2021

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P-code [8,9]. Another airborne experiment was conducted to investigate the performance of code-delay altimetry using clean-replica and interferometric approaches based on GPS L1 signals [10]. GNSS-R airborne GPS L5 signals were also used for altimetry analysis; precision within meter and sub-meter levels was achieved [11]. Apart from the cases for the lower-altitude of the receivers, an accuracy of two to three meters can be achieved for space-borne GNSS-R code-level ocean altimetry, based on GPS C/A and BDS code, using the data from TDS-1 and CYGNSS missions [12–14]. In addition, by analyzing the signals reflected from the lake surface, the accuracy can be reached at the sub-meter level on board [15].

In 2020, China finished constructing its BDS-3, which can transmit B1C and B2a civil code signals with wider bandwidths at center frequencies of 1575.42 and 1176.45 MHz, respectively [16]. Hence, it can be expected that the precision of GNSS-R code-level altimetry can be improved by using the new BDS civil codes. In order to demonstrate the potential of BDS B1C and B2a signals for GNSS-R altimetry, we performed a static coastal experiment on a trestle bridge. The raw intermediate frequency (IF) data produced by GNSS-R setups and other precise auxiliary measurements obtained by geodetic GNSS setups, radar altimeter, and electronic total station were collected. These data were postprocessed to solve the SSH every second continuously for over eight hours.

The remainder of this paper is organized as follows. In Section 2, we briefly review the characteristics of the two new BDS-3 civil signals and the basic principle of our work. In Section 3, details of our coastal GNSS-R altimetry experiment and the setups that were used are described. In Section 4, we analyze the solutions and evaluate their accuracy by comparing them with the measurements of the radar altimeter. Finally, the main results are summarized and the problems that remain unsolved in this work are discussed.
