量化姿態角及訊號傳播之誤差於福爾摩沙衛星三號之軌道求定

Abstract

福爾摩沙衛星三號(FORMOSAT-3/COSMIC, F3/C)發射於2006年4月15日，為台美合作的衛星任務。此任務共有6顆微衛星被發射升空，每顆衛星均裝載2個POD GPS天線。本研究使用GPS 無差分觀測量與利用減動力法與動態法進行LEO軌道的解算，並且將F3/C與GRACE衛星進行GPS觀測量品質分析比較。對於F3/C 與GRACE衛星，其電碼觀測量之多路徑效應的影響分別為P1 (MP1)，0.77 m和0.35 m; P2 (MP2) 1.06 m 和0.57 m; 週波脫落發生的頻率分別為1/29 和 1/84; 後驗單位權標準偏差分別為4 cm 和 1 cm; 動力軌道與動態軌道之差異分別為，10 cm 和2 cm。 本文進行F3/C衛星質量中心(COM)的變化、衛星姿態、GPS天線相位中心的變化和纜線延遲影響之相關性的研究。在本研究中，nominal 姿態給定出的F3/C軌道優於observed 姿態給定的軌道。數值的測試顯示出，為了不破壞軌道的求定，F3/C的COM必須精準地被率定。 兩條不同的30小時軌道弧長被使用於5小時和6小時之軌道重疊分析，而動力軌道與動態軌道之精度幾近相同，且落於2-3 cm的精度。本研究的軌道與UCAR(near real-time)和WHU (post-processed)比較，差異大約為10 cm，其原因為力模式、GPS軌道與GPS時錶改正之不同所致。而本研究的F3/C動態軌道將被使用於地球時變重力場之反衍。利用F3/C GPS資料進行重力場的求定，仔細的選擇GPS資料是必須的。然而由於F3/C共有6顆衛星，其大量的軌道資料量將可以補足GPS資料品質的缺陷。 而另一個評估定位品質的方法，就是利用量化姿態誤差。姿態轉換矩陣主要用於座標框架之間的轉換，而當中之間的軌道精度損失可能發生於不穩定的姿態控制時段。使用時間段DOY 118 to 336, 2008的GPS資料進行評估， 可得F3/C定位精度依序為FM1 (2.72 cm), FM2 (2.62 cm), FM3 (2.37 cm), FM4 (1.90 cm), FM5 (1.70 cm), and FM6 (1.99 cm).

The joint Taiwan-US mission FORMOSAT-3/COSMIC (F3/C) was launched on April 15, 2006. Each of the six satellites is equipped with two precise orbit determination (POD) antennas. The POD antennas of F3/C and GRACE-A satellites are from the same manufacturer, but are installed in different configurations. The LEO satellites are determined from GPS data using undifference carrier-phase measurements by the reduced dynamic and kinematic methods. This study compares the qualities of GPS observables from F3/C and GRACE. Using selected satellites and time spans, the following average values for the satellite F3/C and satellite A of GRACE are obtained: multipath effect on the pseudorange P1 (MP1), 0.77 m and 0.35 m; multipath effect on the pseudorange P2 (MP2), 1.06 m and 0.57 m; occurrence frequency of cycle slip, 1/29 and 1/84; standard error of unit weight, 4 cm and 1 cm; dynamic-kinematic orbit difference, 10 cm and 2 cm. The effects of satellite center of mass (COM) variation, satellite attitude, GPS antenna phase center variation (PCV), and cable delay difference on the F3/C orbit determination are studied. Nominal attitudes estimated from satellite state vectors deliver a better orbit accuracy when compared to observed attitude. Numerical tests show that the F3/C COM must be precisely calibrated in order not to corrupt orbit determination. Based on the analyses of the 5-h and 6-h orbit overlaps of two 30-h arcs, orbit accuracies from the reduced dynamic and kinematic solutions are nearly identical and are at the 2-3 cm level. The mean RMS difference between the orbits from this study and those from UCAR (near real-time) and WHU (post-processed) is about 10 cm, which is largely due to different uses of GPS ephemerides, high-rate GPS clocks and force models. The kinematic orbits of F3/C are expected to be used for recovery of temporal variations in the gravity field. For gravity determination using F3/C GPS data, a careful selection of GPS data is critical. With six satellites in orbit, F3/C’s large amount of GPS data will make up the deficiency in data quality An alternative assessment of the positioning quality is made by propagating attitude error to orbit error. The attitude transformation matrix is responsible for coordinate frame conversions, and a degraded orbit accuracy in the F3/C satellites might occur under an unstable attitude control. This assessment, using GPS data of DOY 118 to 336, 2008, leads to the following 3-D positioning accuracies: 2.72, 2.62, 2.37, 1.90, 1.70, and 1.99 cm for FM1, …, and FM6.