## 1 Introduction  
Spaceborne synthetic aperture radar (SAR) antenna pattern modulation is the main error source affecting the SAR image radiation precision. Therefore, antenna pattern measurement is a major aspect of spaceborne SAR radiation calibration [1]. The spaceborne SAR antenna pattern is usually tested at the ground stage, however, it is impossible to fully achieve far-field conditions or to completely simulate the space environment on the ground. It is necessary to perform on-orbit testing of the antenna pattern in practical applications. The main factor that determines the accuracy of the antenna pattern correction is the measurement accuracy of the antenna pattern shape and the alignment accuracy between the SAR data and the antenna pattern. Therefore, the on-orbit measurement of the antenna pattern of the spaceborne SAR is divided into two aspects: the shape of the antenna pattern measurement and antenna beam pointing measurement.  
Since the launch of SEASAT, the first spaceborne SAR system in the United States in 1978, various spacefaring nations have launched their own SAR satellites, such as SIR-A, SIR-B, ERS-1, ERS-2, JERS-1,and RADARSAT-1. During the 21 st century, SAR satellites have been launched more frequently, including ENVISAT, ALOS, SAR-Lupe, COSMO-SkyMed, TerraSAR-X, Radarsat-2, TanDEM-X and so on [2]. In order to quantify the SAR data, these spaceborne SARs have been tested on the antenna pattern on-orbit. Scientists have developed a complete series of on-orbit test methods to test the shape of antenna patterns, such as the standard reflector method [3], ground receiver method [4; 5], ground transmitter method [6; 7], and tropical rainforest uniform target method [8; 9; 10; 11; 12; 13; 14]. As the number of beams increased, TerraSAR-X developed the antenna pattern measurement and verification method based on a mathematical model [15; 16]. As For the antenna beam pointing measurement, little mention was made in the early days of spaceborne SAR. The DLR designed and implemented a calibration method for TerraSAR-X to measure the antenna beam pointing via transmitting a special kind of antenna beam with a notch pattern, and the accuracy in range direction was better than 0.008\({}^{\circ}\)[16]. The beam pointing in the azimuth direction could be determined likewise by notch patterns now operated in the azimuth direction and measured by deployed ground receivers, with accuracy better than the required 0.002\({}^{\circ}\)[17].  
However, the beam pointing measurement method via the notch beam requires a specifically designed operating mode, and this makes most of the antenna energy unable to be emitted, as it will lead to the antenna overheating. There is a risk of burning down the antenna device, which is unacceptable. In this paper, we propose a high-precision measurement technique for the azimuth direction of the SAR antenna beam based on the ground receivers, which solves the above problems while achieving an accuracy of 0.002\({}^{\circ}\). Since no special adjustments are required for satellites and payloads, this method is more widely applicable. This paper firstly introduces the principle of the new measurement method in Section 2, and then describes the specific algorithm in Section 3. In Section 4, we analyze the accuracy of our method. With the experiment data of the GaoFen-3 SAR satellite, the antenna beam azimuth direction measurement results are given in Section 5. Finally, In Section 6, we illustrate the effectiveness of the method.