Improving the Positioning Accuracy of Satellite-Borne GNSS-R Specular Reflection Point on Sea Surface Based on the Ocean Tidal Correction Positioning Method  
[PERSON]  
1 Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, 104 Youyi Road, Beijing 100094, China 1,2  
[PERSON]  
1 Qian Xuesen Laboratory of Space Technology, China Academy of Space Technology, 104 Youyi Road, Beijing 100094, China 1  
GNSS-R altimetry has the advantages of a rich signal source, low payload cost, all-weather, all-day time, low power consumption, and so forth. GNSS-R can acquire multiple sea surface reflection signals at the same time based on multi-channel receiver. Combined with multiple satellites network, GNSS-R can achieve high-spatial-resolution global sea surface altimetry. To take advantage of the high resolution and realize the application, the accuracy of GNSS-R satellite sea surface altimetry needs to reach cm level, which is comparable to SSH obtained by satellite radar altimeter. As one of the main error sources in GNSS-R altimetry, the positioning error of the specular reflection point restricts the improvement of the altimetry accuracy [14; 15]. The specular reflection point is the point which minimizes the distance across which the GNSS satellite signal reaches the receiver through reflection on the reflection surface [16]. It is the datum point and reference center of the GNSS-R signal reflection geometry and relevant parameters. The positioning error of the specular reflection point affects the sea surface altimetry accuracy, which is directly related to the path error, on the reference datum [17; 18; 19; 20; 21]. In 1997, [PERSON] proposed a two-step method to search the position of the specular reflection point on the reference ellipsoid [15]. In 2009, [PERSON] proposed the minimum path length (MPL) method which determined the specular reflection point on the WGS84 ellipsoid as an optimization problem [5]. In 2012, [PERSON] proposed the osculating sphere method which provided a simpler way to search the specular reflection point on the ellipsoid [6; 7]. In 2018, [PERSON] proposed a latitude dependent unit difference positioning method that satisfied Snell's Law on the WGS84 ellipsoid, this method is more accurate and more computationally efficient than MPL method [10]. The signal reflection surface of the GNSS-R altimetry is the instantaneous sea surface. However, among the reflection reference surfaces selected by the current specular reflection point geometric positioning methods, the one which is the closest to the instantaneous sea surface is the earth ellipsoid surface. The elevation difference between the ellipsoid surface and the instantaneous sea surface, sea level height (SSH), is not considered in the positioning, resulting in non-ignorable positioning error of specular reflection points [13; 15; 22] (see Figure 1). SSH can be decomposed into static-state elevation and time-varying elevation: 1) The former is the elevation difference between geoid and the reference ellipsoid surface--the geoid undulation. It is determined by the earth's own gravity, with low time variation. 2) The latter is the real-time change of SSH from geoid to the instantaneous sea surface caused by tides, winds, ground rotations, mesoscale eddies, currents, tsunami and other external dynamics, ie, dynamic topography, which varies with time. The static-state elevation error of the reflection reference surface is the basic error source for the specular reflection point positioning. We have modified this in a previous study by proposing the gravity field-normal projection reflection reference surface combination correction method [13]. Figure 1: GNSS-R reflection reference surfaces and the corresponding specular reflection points. Where \(S\) and \(S^{\prime}\) are the specular reflection points on geoid and ocean tidal surface, respectively. The purple arrow represents the positioning correction by the ocean tidal correction positioning (OTCP) method.