Undifferenced and uncombined GNSS approach with integer ambiguity resolution for absolute and relative POD of LEO satellites in formation flying

Abstract

Absolute or relative precise orbit determination (POD) is an essential prerequisite for different low earth orbit (LEO) missions. For the POD of LEO satellites, the onboard global navigation satellite system (GNSS) is an important reliance. Classical absolute POD is usually based on an ionosphere-free (IF) combination, which causes a waste of observation information and is not flexible in multi-frequency scenarios. Integer ambiguity resolution (IAR) can only be achieved with external GNSS satellite phase bias (SPB) products, which is difficult to achieve for real-time missions. Concerning relative POD, the double-differenced (DD) with IAR is the most classical method. However, the differencing process amplifies observation noise and loses the opportunity to impose dynamic constraints on some eliminated parameters. In this contribution, based on the undifferenced and uncombined (UDUC) observations, a new model that can be used for both absolute and relative POD is proposed. In this model, the ambiguities of common-view satellites are constructed into DD form, thus IAR can be achieved without any external SPB products. Working with the UDUC observations, multi-frequency scenarios can be easily extended, and residuals can be separated for each frequency. In addition, with precise products, both the absolute and relative orbits can be derived, thus can be beneficial for absolute and relative LEO POD. Based on onboard GPS observations of TH02-2A and TH02-2B missions, the performance of the UDUC POD model with DD ambiguity was evaluated. With the UDUC algorithm and IAR, the proposed model presented a consistency of 3-4 cm in 3D with the reference orbits, and the orbit difference was reduced by 32.7% and 20.1% for TH02-2A and TH02-2B compared with the classical IF POD, respectively. In addition, the relative orbit of the two satellites derived from the proposed model showed a consistency of 1.1-1.3 mm, which proved the feasibility of the UDUC POD model with DD ambiguity for formation flying missions.