Session B1: Atmospheric Effects, GNSS Remote Sensing, and Scientific Applications

Verification of Orbit and Clock Determination for Navigation Message of Regional Navigation Satellite System
Jaeuk Park, Bu-Gyeom Kim, and Changdon Kee, Department of Aerospace Engineering and SNU-IAMD, Seoul National University
Location: Beacon B

Navigation satellite systems have emerged as a key positioning, navigation, and timing (PNT) infrastructure for modern world throughout various fields. Based on their coverage, they can be divided into two main groups: global and regional system. Global navigation satellite systems (GNSS) such as GPS, Galileo, and BeiDou provide PNT service throughout the world. These are mainly composed of about 30 navigation satellites in medium earth orbit (MEO) with the orbital period of 12 hours, which can assure efficient global coverage. Unlike GNSS, regional navigation satellite system (RNSS) such as Quasi-Zenith Satellite System and (QZSS), Navigation Indian Constellation (NavIC) provide PNT service for the certain region only. In addition, the Republic of Korea plans to build its own RNSS called Korean Positioning System (KPS) by 2035. These mostly comprise navigation satellites in geostationary orbit (GEO) and inclined geosynchronous orbit (IGSO) with the orbital period of 24 hours, which can provide PNT service over the certain region even with the relatively small number of satellites.
All navigation satellite systems use pseudo-distance measurements, time difference between signal transmission and reception, for user positioning. Since the time difference is based on two imperfectly synchronized clocks, accurate satellite position and clock offset must be known for user positioning. As a result, orbit and clock determination is an essential part of navigation satellite system in which the orbit and clock variation of satellites are seamlessly observed, predicted and broadcasted to the user through the navigation message. The error in orbit and clock information of navigation message acts as an error source for standalone users. The ground segment of navigation satellite systems commonly comprise monitor stations (MS) which track satellites and collect measurements, master control stations (MCS) which perform orbit and clock determination with the measurements, and uplink stations (US) which upload the navigation message including predicted satellite orbit and clock. As the performance of orbit and clock determination is influenced by the distribution of monitor stations, it is crucial to secure a suitable network of the stations. Ground segments are mostly composed of a dozen of monitor stations with highly secure sensors.
Orbit and clock determination for navigation message are basically estimation problem to enhance knowledge of the a priori satellite orbit and clock based on new observations, given the generally available approximate information. Although there may be slight differences, orbit and clock determination generally comprises three parts: observation models, force models, and estimation methods. Firstly, observation models are about describing the measurements as a function of position of satellites and users, and other parameters. Furthermore, force models account for the orbital motion of satellites with various perturbations Finally, estimation methods are related to methodology to attain optimal estimates from the difference between the observed and modeled measurements. Various estimation methods are mainly divided into batch and sequential estimation methods which are usually operated in the manner of post processing and real-time processing, respectively.
As efforts to contribute to future development of KPS, there have been many simulation studies on orbit and clock determination for system with the name of Korean regional navigation satellite system (KRNSS) with various candidates of satellite and station distribution. However, previous studies on KRNSS mainly focused on orbit and clock estimation without prediction, which is actually needed for navigation message generation. Furthermore, they were mostly based on batch least square estimation (BLS) methods which are less suitable for real-time application than the sequential filters. For more realistic simulation, we constructed orbit and clock determination tool based on extended Kalman filter (EKF), suitable for real-time application, and analyzed signal in space user range error (SIS URE) based on the prediction results in our previous work with SIS URE results of 1 m level in one standard deviation for over all 24-hour prediction. In this study, we seek to verify the simulation tool based on real observation data from QZSS, similar system to KRNSS, provided by international GNSS service (IGS) network.
For observation models for orbit and clock determination, smoothed ionosphere-free pseudorange measurements are utilized in this study both for simulation and real data. For force models, simulated orbit data from orbit propagation model are used for measurement simulation and the propagation model is also used for filter processing. The orbit propagation model account for perturbations including geopotential, lunisolar gravity, solar radiation pressure, tidal effect, and general relativistic effect. In addition, atomic clock models are used for both measurement simulation and filter processing.
For estimation method, we implemented extended Kalman filter in this study as mentioned earlier. EKF conducts measurement and time update with reference satellite trajectory propagated every epoch. This can lead to improvement in accuracy compared to the method with preassigned reference trajectory despite of increase in burden of computation. States to be estimated include position, velocity, clock bias, clock drift, and solar radiation pressure coefficients of satellites and clock bias, clock drift, and zenith tropospheric delay of monitor stations. After estimation with measurements, states are predicted by time update only.
In this study, we will use 1 GEO and 3 IGSO satellites of QZSS and 14 stations of IGS network of orbit and clock determination for both simulation and real data. As a mean to assess performance of navigation message, we will analyze SIS URE based on prediction results as conducted in our previous work. By comparing analysis results of simulation and real data, we expect to verify the orbit and clock determination tool for KRNSS navigation message from our previous work.