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Xia et al. Satell Navig (2021) 2:8 Page 3 of 19

BDS satellite visibility reached 30. In general, the Eastern Hemisphere had bet-
Te introduction of BDS-3 has greatly improved the ser- ter satellite visibility than the Western Hemisphere due
vice scope of BDS, and the number of satellites in orbit is to the presence of IGSO and GEO satellites.
46 (i.e., 16 BDS-2 satellites and 30 BDS-3 satellites). From We calculated the nominal number of the visible sat-
a global service perspective, the new system with 27 ellites for BDS, GPS, Galileo and GLObal Navigation
MEO satellites makes a considerable number of satellites Satellite System (GLONASS) in the UK territory with a
visible to users. We employed the precise satellite orbit grid cell of 0.5° × 0.5°, and the elevation cutof angle 5°, as
product of July 23, 2020 provided by German Research shown in Fig. 3. One can see that BDS has the best satel-
Centre for Geosciences (GFZ) to draw the 24-h ground lite visibility, followed by GPS. Te maximum of 16 BDS
tracks of BDS satellites, as shown in Fig. 1a, which con- satellites can be observed over the UK territory.
tains 43 BDS satellites (no C18, C59 and C61). It can be Furthermore, we calculated the GDOP values for the
seen that BDS MEO satellites cover most global areas BDS satellites at the above three moments using the
except the polar regions and are densely distributed. Fig- same grid division and the same elevation cutof angle.
ure 1b depicts the nominal sky plot of the satellites at the Figure 4 illustrates the GDOP distribution maps. Te
static data collection site in Nottingham, central England. nominal GDOP values in the entire UK territory do not
As many as 38 BDS satellites were visible in one day. In exceed 2, which is very satisfactory. Moreover, the GDOP
addition to all the existing 27 MEO satellites, 10 IGSO values worldwide are within 3. Like the satellite visibility,
satellites and 1 GEO satellite (C05) were observed. How- the GDOP values in the Eastern Hemisphere are mostly
ever, due to the 55° inclination of MEO and IGSO satel- lower than those in the Western Hemisphere.
lite orbits, it is difcult for BDS satellites to cover most For the UK area, Fig. 5 shows the distribution of the
of the northern sky of the site, which limits the PNT ser- GDOP values for BDS, GPS, Galileo and GLONASS. In
vices of BDS in higher latitudes (Meng et al. 2004). In the general, the nominal spatial geometric distribution of
future, this situation can be improved by increasing the BDS satellites is also the most ideal with the maximum
inclination of IGSO satellites (Yang et al. 2020). GDOP less than 2.2, which is consistent with the results
For a more intuitive view of the instantaneous number in Fig. 4. Te above results suggest BDS is favorable for
of visible BDS satellites on the Earth, we divided the Earth the location-based services on a global scale. In the next
surface into 5° × 2° grid cells and calculated the nominal step, we will use the measured GNSS data with a smart-
number of visible satellites with elevation angles above phone to evaluate BDS ranging precision and positioning
5° in each grid cell. Te satellite visibility maps from the performance in the UK territory.
visual angle of Britain at GPS time 15:00:00, 16:00:00 and
17:00:00 are shown in Fig. 2. As can be seen from the GNSS data collection
fgure, the number of visible BDS satellites with eleva- Our experiments were conducted in the local afternoon
tion angles above 5° in the UK was from 9 to 15 during of July 23, 2020 in Wollaton Park in Nottingham, UK.
the two-hour period. Globally, at least 6 satellites were A Huawei Mate 20 smartphone was used to collect the
observed, and the maximum number of visible satellites static and kinematic observations of BDS/GPS/Galileo/

Fig. 1 a Ground tracks of BDS satellites and b the sky plot at an observation site in Nottingham, UK on July 23, 2020