Geodetic applications of Low Earth Orbit (LEO) satellites requires accurate satellite orbits. Instead of using onboard Global Navigation Satellite System observations, this contribution treats the LEO satellite constellation independently, using Inter-Satellite Links and the measurements of different ground networks. Due to geopolitical and geographical reasons, a ground station network cannot be well distributed. We compute the impact of different ground networks (i.e., global networks with different numbers of stations and regional networks in different areas and latitudes) on LEO satellite orbit determination with and without the inter-satellite links. The results are based on a simulated constellation of 90 LEO satellites. We find that the orbits determined using a high latitude network is worse than using a middle or low latitude network. This is because the high latitude network has a poorer geometry even if the availability of satellite measurements is higher than for the other two cases. Also, adding more stations in a regional network shows almost no improvements on the satellite orbits if the number of stations is more than 16. With the help of ISL observations, however, the satellite orbits determined with a small regional network can reach the same accuracy as that with the global network of 60 stations. Furthermore, satellite biases can be well estimated (less than 0.6 mm) and have nearly no impact on satellite orbits. It does thus not matter if they are not physically calibrated for estimating precise orbits.
<p>Since 2010s, many companies such as SpaceX, OneWeb, Amazon and Samsung showed their interests to launch hundreds and even thousands of low Earth orbit (LEO) satellites for global internet service. Due to their unique characteristics compared to medium Earth orbit (MEO) and geostationary Earth orbit (GEO) satellites, these LEO mega-constellations soon draw much attention from the scientific community. Studies from constellation design, to applications such as positioning, ionosphere modelling and gravity recovery are investigated by many researchers.</p><p>Orbit determination is a key to many applications. Traditionally, onboard Global Navigation Satellite System (GNSS) receivers are used to determine LEO satellite orbits. However, with thousands of satellites in space in the future, an independent system without relying on GNSS is worth to be studied. Since these LEO satellites are intended for internet service, connections between the satellites and to the ground are available by nature. But how would the distribution of a station network affect the orbit accuracy? How many stations would be sufficient to determine a precise orbit? Besides observations from ground stations, inter-satellite link (ISL) is also proposed and implemented by many current GNSSs. It already showed its potential to improve the orbits. Could this technique also be applied to the orbit determination of LEO satellites?</p><p>This simulation study investigates the influence of ground station distribution to orbit determination, as well as the benefit from ISL observations. By using a constellation with 60 LEO satellites, we show that for regional station networks, a high latitude network leads to worse orbit accuracy than a middle or low latitude network. With the help of ISL observations, orbit errors reach the same level as a global station network. We further investigate the influence of different number of stations contained in the network. The results prove that although increasing the station number could improve orbits, the improvement is minimal when the global network contains more than 16 stations. While for a regional network, even with 60 stations, the orbit errors are 1.5 times larger than for a small global network with 6 stations. This proves that the ground station distribution is more important than the number of observations. Furthermore, if the ISL technique is adopted, even a regional station network with 16 stations could be sufficient to determine an accurate orbit.</p>
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