IEEE Instrumentation & Measurement - September 2023 - 42

Research on Single-Frequency
PPP-B2b Time Transfer
Zaimin He, Lan Li, Runzhi Zhang, Juan Hou, Gongwei Xiao, Wei Guang,
Jihai Zhang, and Xiangyi He
T
ime transfer based on global navigation satellite system
(GNSS) using precise point positioning (PPP)
is a critical technique in universal time coordinated
(UTC) calculation. PPP relies on precise satellite orbit and
satellite clock offset products obtained through network solutions.
Since August 2020, the Beidou global navigation satellite
system (BDS-3) has provided GNSS users with the PPP product
using PPP-B2b signal through three geostationary earth
orbit (GEO) satellites instead of a network in the Asia-Pacific
area. Network interruption will cause PPP reconvergence and
terminate the time transfer, which hinders the application of
PPP time transfer. The real-time PPP-B2b product broadcast by
GEO satellites can be considered to solve the problem of interruption
caused by network fluctuation in the field of real-time
PPP. To promote the application of real-time PPP time comparison
in UTC calculation and develop PPP-B2b application
in time transfer, single-frequency (SF) PPP-B2b time transfer
is investigated. Meanwhile, the treatment of ionospheric
delay of SF PPP-B2b is crucial, and a GRAPHIC SF PPP-B2b
model is proposed and validated in order to keep PPP-B2b network-independent.
Data from two international atomic time
(TAI) time-keeping laboratories and two international GNSS
service (IGS) stations were used to analyze the accuracy in
SF time comparison applications using the PPP-B2b product
from common clock difference (CCD) and long-baseline time
links. Results show that the SF PPP-B2b CCD sequences can be
concentrated within 0.2 ns. The long-baseline time difference
between SF time transfer using PPP-B2b product and dual-frequency
(DF) time transfer using the final product fluctuated
within 2 ns. In addition, the standard deviation (STD) values
of PPP-B2b time comparison are mainly distributed at 0.5 ns.
Meanwhile, only BDS and BDS + GPS can achieve similar SF
PPP-B2b time transfer accuracy.
Background
The high-precision time scale can be generated and maintained
by the time-frequency system, and the high-precision
time transfer is an indispensable part of the time-frequency
system. Presently, GNSS time transfer mainly includes
42
common-view (CV), all-in-view (AV), and GNSS carrier
phase. Compared with CV and AV based on low-precision
pseudorange observations, GNSS carrier phase time transfer
has higher time transfer accuracy. It is realized by means of
PPP [1] which uses a single GNSS receiver to obtain precise
absolute coordinates. The GeoForschungZentrum (GFZ)
analysis center product GBM, whose accuracy can reach
3 cm, can be obtained with a two-day delay. The STD value
of post-process PPP time transfer using the GBM product can
reach 0.3 ns [2]. Since 2013, the IGS has provided users with
the precise satellite orbit and satellite clock offset products
required by PPP for free via the networked transport of Radio
Technical Commission for Maritime Services (RTCM) via
internet protocol (NTRIP). Ge et al. analyzed real-time PPP
time transfer [3]. Their results showed that real-time PPP
time transfer could achieve a 0.5 ns level. However, the realtime
PPP via the NTRIP protocol can be performed only in
smooth network environments. And network interruption
causes instability in PPP time transfer relying on IGS products.
Since August 2020, precise products called the PPP-B2b
product have been available through three GEO satellites instead
of NTRIP protocol [4]. Presently, the research on the
PPP-B2b product focuses on product quality, positioning,
and DF time transfer [5],[6]. The DF time comparison using
the PPP-B2b product can reach an accuracy of sub-nanosecond.
The price of the SF receiver has attracted more and
more researchers to study SF PPP time transfer. However, the
PPP-B2b time transfer only verifies the feasibility of dualfrequency
receiver, while the accuracy of time transfer of
single-frequency receiver is still blank. The advantages of SF
PPP-B2b time transfer receiver in cost and stability are beginning
to be realized. The treatment of PPP ionospheric delay
is very tricky. For dual-frequency, the effect of ionospheric
delay can be eliminated by the frequency combination, while
single-frequency cannot directly eliminate the effect of ionospheric
delay. Also, further PPP solution of the PPP-B2b
product is independent of a network, so that the external input
of ionosphere is isolated. An analysis of performance of
SF PPP-B2b time transfer is required.
IEEE Instrumentation & Measurement Magazine
1094-6969/23/$25.00©2023IEEE
September 2023
IEEE Instrumentation & Measurement - September 2023

Table of Contents for the Digital Edition of IEEE Instrumentation & Measurement - September 2023
