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Shi et al. Satell Navig (2021) 2:5 Page 2 of 13

Geostationary Transfer Orbit (GTO) spacecrafts which conclusions about GTO may not be applicable for the
have +Z (nadir) and −Z (zenith) antenna. Zentgraf launch scenario of a typical GEO spacecraft via SSTO. To
et al. (2010) studied the performance in GEO using the evaluate the feasibility of autonomous navigation using
GPS/Galileo, while the receiver has +Z and +X anten- the signals from the opposite of the earth in the GEO
nas. Liu et al. (2016) analyzed the data from the GNSS launch process (Shijian-20), this paper will evaluate the
receiver (only tracking GPS and GLONASS signal) on autonomous navigation performance in the launch pro-
the Chang’E-5T spacecraft and verifed the validity of cess including SSTO and GEO. It also demonstrates how
GNSS based on the orbit determination during the lunar the GEO/IGSO navigation satellites of BDS improve the
exploration. Tese results showed that a combined navi- navigation performance when considering the latest BDS,
gation constellation can efectively increase the number GPS, Galileo and GLONASS satellites in 2020. Obvi-
of available satellites and improve the positioning accu- ously, the physical visibility between the spacecraft and
racy. However, previous researchers usually focused only the GNSS satellites and the minimum received power
on the GPS-Galileo or the GPS-GLONASS combined directly determine the feasibility of autonomous naviga-
system, which consists of Medium Earth Orbit (MEO) tion, the Position Dilution of Precision (PDOP), which
satellites. Palmerini (2014) pointed out that under the measures the positioning accuracy, and the Doppler shift
combination of dual navigation systems, the signals needed in signal processing module. Terefore, the simu-
obtained are usually extremely weak and have a short lation results will be presented in the following section.
duration for the receiver at very high altitude so that the
receiver needs to work in snapshot mode and the opera- Simulation models and assumptions
tion is highly dependent on its software algorithm and Background
hardware resources. Due to a limited number of visible Entering GEO via SSTO follows the following steps:
satellites with double GNSSs, most researches focused frstly, CZ-5 Y3 is launched into a circular orbit with
on the method of orbit fltering or satellite selection algo- a height of about 193 km, and ignite at perigee into
rithm to analyze the autonomous navigation of GEO or SSTO with apogee of about 68,017 km. After reach-
High Elliptic Orbit (HEO) spacecraft for improving its ing the apogee of SSTO, the Shijian-20 engine is fred
accuracy (Lorga et al. 2010; Zou et al. 2019). Wang (2019) to elevate the perigee altitude to the synchronous orbit
also developed GNSS receiver based on GPS and BDS height and decrease the orbit inclination angle to about
and focused on the processing of navigation signals, e.g. 0°. Ten it decelerates at the perigee and performs sev-
fast acquisition and tracking of high sensitivity signals. eral orbit maneuvers to the target GEO. Due to the low
In 2020, with the completion of BeiDou Navigation Sat- speed at apogee, the fuel required for Shijian-20 at SSTO
ellite System with Global Coverage (BDS-3), there are apogee is less than that required at GTO apogee which
four GNSSs with approximately 122 satellites (opera- means it has a longer orbital lifetime. Te SSTO and last
tional). In addition to the traditional 27 MEO satellites, orbit maneuvers on synchronous orbit are two impor-
BDS also has 10 Inclined Geo-Synchronous Orbit (IGSO) tant stages in the launch process. Tus, these scenarios
satellites and 9 GEO satellites (CSNO 2019). Compared were chosen for analysis in this paper. Figure 1 shows the
with the traditional constellation composed of MEO launch process of Shijian-20 via SSTO.
satellites only, the unique satellite composition of BDS
constellation can greatly increase the number of visible GNSS satellites
satellites. According to the latest Two-Line Element (TLE) data on
With the construction of global satellite communi- the related website database and the BDS ofcial docu-
cation/meteorology constellation in the future, many ments BDS, GPS, Galileo and GLONASS constellations
GEO satellites need to be launched and maintained are established (CSNO 2019; Kelso 2020), while the BDS
while autonomous navigation based on multi-GNSSs is satellites decommissioned is not considered. Te constel-
a key issue. In the recent China GEO (Shijian-20) satel- lation elements are listed in Table 1. Note that the BDS
lite launch mission by CZ-5(Y-3) in December 2019, IGSO and GEO satellites are not evenly distributed in
Super-Synchronous Transfer Orbit (SSTO) was used for longitudes, and they are all distributed between 60° E and
the orbit maneuver. Compared with GTO, the apogee 160° E, which means that some synchronous orbit satel-
altitude of SSTO is more than 36,000 km while the satel- lites will be covered by very few or none BDS IGSO or
lite altitude higher than 20,000 km is for nearly 90% of GEO satellites depending on their longitude.
the time and higher than 36,000 km is for approximately
75% of the time. It means that for most of the time, the Physical visibility and received power
receiver can only use +Z antenna to receive leak signals In practice, the spacecraft always uses more than one
from the opposite of the Earth. Terefore, the previous antenna to provide navigation service (Zentgraf et al.