Observation of BDS-2 IGSO/MEOs yaw-attitude behavior during eclipse seasons

The yaw mode history of BDS-2 IGSO/MEOs since 2016 is inferred from reverse kinematic precise point positioning. Experimental results show that C06 (IGSO-1) and C14 (MEO-6) satellites have abandoned the orbit-normal (ON) attitude mode in favor of continuous yaw-steering (CYS) mode since March and September 2017, respectively. BDS C13 (IGSO-6), launched in March 2016, and C16 (IGSO-7) always adopt the CYS mode during eclipse seasons. The majority of BDS-2 IGSO/MEOs still experience attitude switches between nominal and ON mode. Most of the attitude switches from nominal to ON mode take place when the sun elevation angle above the orbital plane ( β angle) decreases below 4° (| β| < 4^∘ ) . A few switches also occur for | β| slightly above 4°. However, most of the attitude switches from ON to nominal mode are undertaken when | β| increases to above 4°, but a few switches with | β| just below 4° happen. The exact switch condition between the two attitude modes is presented in this study. For BDS-2 IGSO/MEOs using the CYS mode, the yaw-attitude model previously established by the Wuhan University (indicated by WHU model) can basically reproduce their yaw maneuvers. However, reverse midnight-turn maneuvers occasionally occur for C13 and C14 for β angles falling into the range (0^∘ ,0.14^∘ ) . This discrepancy in the form of a reversal in the yaw direction during the noon-turn maneuvers is first observed for C13 and C14 when the β angle is in the range of ( - 0.14^∘ ,0^∘ ) . The mismodeling of the satellites attitudes during reverse yaw maneuvers significantly degrades the performance of BDS precise orbit determination (POD). The phase observation residuals extracted from BDS POD reach 40 cm, which thereby leads to the misidentification of a substantial number of observations around orbital midnight and noon points as outliers when the WHU model is applied in our experiments. The WHU model is modified to reproduce the reverse yaw maneuvers of satellites in the post-processing BDS POD. The derived phase residuals decrease to normal levels, and the clock solutions become smoother relative to solutions employing the WHU model.