# Navigation in GEO, HEO, and Lunar Trajectory Using Multi-GNSS Sidelobe Signals
Moreover, it shows the multi-GNSSSSS performance of global space and specific space missions, with only mainlobe signals involved. Simulation results showed that the multi-constellation, multi-frequencySSV improved signal availability significantly over any single constellation. However, with only mainlobe signals included, there was still signal outage for the geostationary orbit (GEO) and high elliptical orbit (HEO) spacecraft, let alone the lunar trajectory. Getting sidelobe signals involved is a way of improving the SSV performance. The main reasons that limited the use of sidelobe signals are (1) the relatively lower carrier to noise ratio of sidelobe signals than that of mainlobe signals and (2) the unpublished transmitter antenna pattern varying with satellite types and signals. The contribution of sidelobe signals to SSV performance has been extensively demonstrated through simulation works [14; 15; 16; 17]. Moreover, several experiments have been made to test the characteristics of sidelobe signals. In 2002, the GPS experiment on the AO-40 spacecraft demonstrated that GPS sidelobe signals with low powers can be tracked [18]. Later in 2015 and 2017, the experimental results of magnetospheric multi-scale (MMS) and geostationary operational environmental satellite (GOES-R) further proved that sidelobe signals constitute the majority of the received signals. With sidelobe signals included, increased navigation performance can be realized [19; 20; 21]. Meanwhile, experiments have been made to study GPS antenna patterns; this published antenna pattern data provides a great chance to investigate the contribution of sidelobe signals for space users [22; 23]. Regarding high-sensitivity receiver design, [24; 25] show that spaceborne receivers are able to track signals as low as 5-8 dB-Hz, making sure that sidelobe signals can be used even at lunar distance. The above-mentioned studies show that multi-GNSS, multi-frequency sidelobe signals with the high-sensitivity spaceborne receiver will improve the navigation performance of space vehicles. GPS/Galileo combined navigation performance in GEO and HEO are investigated in [26; 27]. In addition, the CE-5T1 spacecraft flight mission results show that, with GPS/GLONASS combination, single point positioning (SPP) accuracy improved by 22.7% compared with GPS-only [28]. Regarding the latest BDS system, BDS-3 navigation performance in super synchronous transfer orbit was investigated in [29], with only mainlobe signals included. In [30], with the 3D antenna pattern, SSV characteristics of new BDS-3 signals were demonstrated. It revealed that, with sidelobe signals included, signal availability and GDOP could be greatly improved. Nevertheless, signal outages can still happen for a lunar mission with BDS-3 signal only. Current studies concerning BDS-3 are mainly focused on the SSV characteristics. These characteristics show a great potential of SSV navigation performance with BDS-3 sidelobe signals. In our paper, to give a better understanding of BDS-3 SSV real-time navigation performance, we intend to take a step further by simulating pseudorange observations of GNSS satellites and spacecraft. Based on the experiment results, the observation noises vary with the received signal power. Specifically, instead of distinguishing sidelobe signals by the beamwidth, the published GPS antenna patterns are used to evaluate performances of all the signals that are beyond receiver sensitivity. Moreover, with over 100 GNSS satellites that can offer positioning, navigation, timing (PNT) service, contribution of multi-GNSS to SSV navigation performance was studied; signal visibility and GDOP of each constellation were also presented. In particular, the importance of BDS-3 non-MEO satellites to SSV was demonstrated.