P2OD: Real-time Precise Onboard Orbit Determination for LEO Satellites

Satellite orbit determination is a fundamental information for many space missions, several requiring a high level of orbit accuracy. Nowadays the Precise Orbit Determination (POD) is the technique routinely used on ground for computing the orbit of LEO missions, especially when the position of the satellite center of mass has to be known at cm level (e.g.: GRAS, Sentinels, SWARM, GOCE, etc.). The current POD approach must be performed only on ground in post-processing. The orbit accuracy achieved with the POD is typically between 0.1 mm/s to 1 mm/s for the velocity. Despite being in line with mission needs, the post-processing limitation is preventing its use for more advanced applications that require high accuracy in real-time, such as formation flying, autonomous docking and rendezvous, increased spacecraft autonomy, etc. This contribution investigates how to overcome the real-time limitations and shows that real-time Precise Onboard Orbit Determination (P2OD) could be achieved in the near future, bringing the current ground PPP (Precise Point Positioning) and PPP-RTK (Precise Point Positioning -Real Time Kinematic) concept to space users. The target real-time orbit accuracy to be achieved with this approach is 10 cm RMS 3D (1 mm/s for velocity). Different algorithms have already been developed for onboard orbit determination, ranging from a least square approach to Extended or Unscented Kalman Filtering (EKF, UKF). Usually the initial position is provided through a Single Point Positioning (SPP) technique, using only the pseudorange measurements. The SPP solution can be smoothed by fitting a dynamic model. These onboard algorithms allow to reach an orbital velocity accuracy (3D RMS) at m/s or cm/s level ([23], [24], [25], [26] and [27]). Several studies ([28], [29] and [30]) have also investigated the optimization of the force models acting on the satellites and the related parameters for propagating the orbit with the electronics onboard a LEO satellite. The main limitation for such onboard OD algorithms is the lack of precise ephemeris and clock products for the GNSS satellites in real-time. GNSS receivers in space can use techniques and concepts adoptable by GNSS receiver on ground. There are at least two main concepts for real-time high accuracy positioning computation based on GNSS: RTK and PPP. RTK relies on use of ground stations in the proximity of the user receiver, therefore cannot be considered viable from space users. PPP (and its evolution, the PPP-RTK) is based on the broadcast of precise corrections (mainly precise orbits and clocks, but also a precise ionospheric model and, for PPP-RTK, delta phase correction for performing Integer Ambiguity Resolution on the rover) through different communication channels (e.g., via geostationary satellites, GSM, Internet). This study addresses two main types of communication channels for providing precise orbit and clock corrections of the GNSS satellites: the dedicated broadcast and the global broadcast channels. The dedicated broadcast channel assumes that each GNSS satellite (e.g.: GPS or Galileo) broadcasts its own precise corrections for orbits and clocks. Such a condition could be achieved with an improvement of the current broadcasted ephemerides and clocks (via standard navigation message) or via a dedicated communication channel provided by the GNSS satellites that goes on top of the broadcasted navigation message. In this scenario the available bandwidth is normally limited, so the amount of corrections that can be disseminated is limited (allowing to transmit only the corrections for the satellite itself or a small group of nearby satellites) and there is a need for a constant communication contact from the uplink stations to all or a high number of the GNSS satellites. Modern GNSS systems are very close to provide the infrastructure to enable the dedicated broadcast scheme and the present study aims to propose potential strategies to overcome the current limitations. The global broadcast channel relies on geostationary satellites for disseminating the precise corrections for all the GNSS satellites at the same time. Two major cases are considered: SBAS and commercial PPP providers. SBAS is planned to achieve worldwide coverage by 2020, but the dissemination of precise corrections to achieve cm level accuracy is today outside the scope of such systems, mainly because providing the integrity for such an accuracy might be not easy to achieve. Nevertheless, a new service providing high accuracy corrections (initially without integrity) could be implemented using a potentially available spare bandwidth in the SBAS. Nowadays, commercial geostationary satellites are the only way to achieve the dissemination of precise corrections, using commercial PPP service providers that currently cover the entire globe. This work will analyze the implications of using such communication channels on the space receiver architecture. The adopted solution will be suitable for different types of 24h 3-axis stabilized LEO spacecraft, from nano-satellites to large Earth-observing satellites. A novel concept based on the use of multi-antennas will be proposed to overcome the weak communication links above the poles for receiving the corrections from geostationary satellites. The real-time precise orbit determination will be sequentially computed with an EKF-based OD algorithm. The approach will be validated using real data from flying missions. Finally, a potential roadmap for In-Orbit Demonstration (IOD) will be presented.