Instrumentation & Measurement Magazine 25-5 - 31

The number of visible satellites and the quality of the
observation data are insufficient to meet requirements in
mountains, cities, and other circumstances. Therefore, a single
system cannot guarantee the continuity, stability, and
reliability of high-precision time comparisons. Multi-GNSS integration
can largely solve the problems caused by the single
system because of the multi-GNSS compatibility and interoperability.
The Galileo system completely suspended service
on July 12, 2019, at 5:50 AM (Beijing time), and then resumed
operation after 117 hours on July 17, 2019, at 3:00 AM (Beijing
time). However, compared with the other three navigation systems,
some satellite navigation messages are still missing in
the Galileo system. Therefore, the multi-GNSS fusion PPP time
comparison will become the prevailing trend for UTC calculation.
At the same time, multi-GNSS integration can provide
more stable and reliable service for users.
At present, BDS-3 not only broadcasts BDS-2 compatible
signals B1I and B3I, but also the new signals of B1C (1572.42
MHz), B2a (1176.45 MHz) and the other signals, which were
designed with novel modulation methods to enhance compatibility
and interoperability with other navigation systems.
These include GPS (L1 and L5) signals, Galileo (E1 and E5a)
signals, Satellite-Based Augmentation System (SBAS) (L1 and
L5) signals and Quasi-Zenith Satellite System (QZSS) (L1 and
L5) signals. Currently, the BDS-3 precision clock products are
released by the analysis center of IGS based on B1I and B3I observations,
and some research papers of time comparison with
BDS-3 B1I and B3I signals have been published [6]. Therefore,
based on the new signal of BDS-3, the BDS-3 PPP time comparison
experimental research was carried out, and it has certain
reference value for the application of new frequency.
In this paper, the pseudo-range and carrier phase measurement
of the BDS-3 new signal (B1C, B2a) at an interval of 30 s
are used. The high precision orbit, clock and DCB products are
published by Geo Forschungs Zentrum Potsdam (GFZ), one
of the IGS analysis centers. Experiments based on the BDS-3
new signal PPP and multi-GNSS (GPS+Galileo+BDS-3) fusion
PPP time comparison between the National Time Service Centre
of Chinese Academy of Sciences (NTSC) and the Institute
of Photonics and Electronics Academy of Sciences of the Czech
Republic (TP) are carried out, and time comparison results are
analysed and studied.
Basic Principle and Method of BDS-3 PPP
The BDS receiver connects the local time-frequency source as
reference, and receives BDS space signals. Its link is shown
in Fig. 1 [7]. In the BDS PPP calculation, the non-differential
pseudo-range and phase observation data are collected by receiver,
and its observation equation can be described in (1) as
follows [8], [9]:
 P ,ri r     
Lri,  r     i r ,1


c dt dTs MF ZWD Ii r ,1
c dt dTs MF ZWD I N ,  i
+


r
r
August 2022
i ri
where the subscriptirepresents the BDS carrier frequency
(i=B1C,B2a). The parameters are defined as:
IEEE Instrumentation & Measurement Magazine
i
(1)
Fig. 1. The method of BDS space signal receiver.
Pr,i/Lr,i
: The pseudo-range and carrier phase observations,
respectively.
ρr
: The geometric distance from the satellite to the receiver
antenna.
dtr
: The clock bias of receiver, which is the time difference between
the receiver time and BDS time.
dTs
: The clock bias of satellite, which is the time difference the
between satellite time and the BDS time.
MF: The mapping function related to satellite elevation angle.
ZWD: The zenith tropospheric wet delay.
I r ,1
andf1
μi
: The slant ionospheric delay at frequencyf1
=1575.42MHz
=f1
2/fi
2), fi
points.
λi
,
: The frequency dependent ionospheric delay amplification
factor(μi
: The wavelength of the carrier frequency at frequencyi.
Nr,i: The integer ambiguity at frequencyi.
εi/ζi
: The pseudo-range/phase measurement noise.
c: The speed of light (299792458 m/s).
Given that the precision clock products were released by
GFZ, which are based on B1I and B3I signal observations, the
pseudo-range at the new signal should be corrected with differential
code bias (DCB) during data processing.
For the B1C and B2a users, the dual-frequency ionosphericfree
combined pseudo-range is used to correct the influence of
ionosphere, and the calculation method is shown in (2):
22
P

r IF
, 22
12
1 1 22
ff

f P fP

(2)
31
is the frequency value at other frequency

Instrumentation & Measurement Magazine 25-5

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