# Improving Low Earth Orbit (LEO) Prediction with Accelerometer Data
## 1 Introduction  
More and more Low Earth Orbit (LEO) satellites have been launched to explore various phenomena on Earth [1; 2], e.g., ocean altimetry [3; 4], climate change, and Earth mass change [5]. In recent years, many research plans or commercial projects depending on a large number of LEO satellites have been proposed with the expectation of providing real time services on a global scale [6; 7; 8]. Meanwhile, the idea of a LEO enhanced Global Navigation Satellite System (LeGNSS) [9; 10], where LEO satellites will transmit navigation signals to be used as navigation satellites, was put forward with such a large number of LEO satellites. With such a huge demand of real time services and different kinds of real time applications, especially for the real time Precise Point Positioning (PPP) of LeGNSS, real time centimeter level LEO orbits are the prerequisite for real time users. Near real time LEO orbits are provided for the fields of satellite occultation for meteorological purposes [11; 12] and the short-latency monitoring of continental, ocean, and atmospheric mass variations [13; 14]. The European Space Agency (ESA) has deployed an operational system for routine Earth Observation named Copernicussince 2014, which is designed to support a sustainable European information network by monitoring, recording, and analyzing environmental data and events around the globe [15]. The Copernicus program consists of six different families of satellites being the first three missions Sentinel -1, -2, and -3. These missions have the requirements of 8-10 cm in less than 30 min for Near Real Time (NRT) applications to 2-3 cm in less than one month for Non-Time Critical (NTC) applications [16]. The orbit accuracy of Sentinel-2A NRT products is better than 5 cm, which is better than the requirement of 10 cm as well as better than the accuracy of previous NRT LEO orbits studies [17; 18]. Though the accuracy of the NRT orbit is on the centimeter level, the latency of about 30 min is not acceptable for real time positioning service of LeGNSS. Thus, predicted LEO orbit with centimeter precision should be investigated. In this study, we try to find a proper way to conduct the prediction of LEO orbits with an orbit precision on the centimeter level for real time applications, especially for the case of LeGNSS. In order to obtain such highly accurate predicted LEO orbit for precise real time applications, precise initial conditions (position and velocity at reference time) as well as the dynamic parameters of LEO satellites used for LEO orbit prediction are required as fast as possible via LEO Precise Orbit Determination (POD). Thus, standard GNSS navigation message information is not adequate for LEO POD since the qualities of GNSS orbits and clocks are not good enough. The precision of the LEO orbit is usually at the decimeter or meter level by using the standard GNSS navigation message [12; 19]. Under this circumstance, precise GNSS products should be used for LEO POD in order to get precise initial conditions and dynamic parameters of LEO satellites. Due to the Real Time Pilot Project (RTPP) launched by the International GNSS Service (IGS) in 2007 and officially operated since 2012 [20], real time precise satellite orbit and clock correction products can be obtained with Real Time Service (RTS). Currently, there are also many commercial providers, which deliver real time GPS/GNSS orbit and clock correction products, e.g., Veripos, magicGNSS, etc. Obviously, RTS can be applied to LEO POD both in the kinematic and dynamic mode. However, most onboard LEO receivers have not yet used these corrections, as it might not yet be mature enough for space applications. An alternative way is to conduct LEO POD at the analysis center with the RTS on the ground with powerful computational resources. As for the POD mode, though either kinematic or dynamic POD can achieve centimeter level orbit precision, we prefer to choose the dynamic method since the orbit of this method is consecutive and stable while that of the kinematic method is discrete.