Page 5 - 卫星导航2021年第1-2合期

Volume 2, Issue 1-2, July 2021 Research Summary

Original Articles
BDS: Space-time references
The BeiDou Navigation Satellite System (BDS) is essentially a precise time measurement and time
synchronization system for a large-scale space near the Earth. General relativity is the basic theoretical
framework for the information processing in the master control station of BDS. The reference systems
used in BDS are considered in the conceptions of relativistic space-time reference systems, the basic
space reference of BDS is BeiDou Coordinate System (BDCS), and the time standard is the BDS
Time (BDT). BDCS and BDT are the realizations of the Geocentric Terrestrial Reference System
(GTRS) and the Terrestrial Time (TT) for BDS, respectively. The station coordinates in the BDCS
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are consistent with those in ITRF2014 at the cm level and the difference in scale is about 1.1×10 .
The time deviation of BDT relative to International Atomic Time（TAI） is less than 50 ns and the
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frequency deviation is less than 2×10 . The Geocentric Celestial Reference System (GCRS) and the
solar Barycentric Celestial Reference System (BCRS) are also involved in the operation of BDS. The
observation models for time synchronization and precise orbit determination are established within
the GCRS framework. The coordinate transformation between BDCS and GCRS is consistent with
the International Earth Rotation and Reference Systems Service (IERS). In the autonomous operation
mode without the support of the ground master control station, EOP is obtained by means of long-
term prediction and on-board observation. The observation models for the on-board astrometry
should be established within the BCRS framework.
Related article: The space–time references of BeiDou Navigation Satellite System (doi:
10.1186/ s43020-021-00044-0)

Reference frame: CGCS2000 maintenance
China Geodetic Coordinate System 2000 (CGCS2000) has been applied for 20 years since it was
released as the national reference frame in 2008. The topics on how to obtain the positions of all
Global Navigation Satellite System (GNSS) stations in the CGCS2000 frame and what optimal
strategies to be adopted for better position results of regional GNSS networks in the ITRF associated
with the GNSS observations are discussed by the authors. Some strategies of dealing with GNSS
observations for obtaining accurate results in related ITRFs and rigorous methods for transformation
the positions derived from the ITRFs at various epochs to the CGCS2000 are recommended based on
their theory analysis and accumulated experience in practical CGCS2000 application. Results indicate
that the accuracy of the transformed coordinate has been greatly improved. This has significance for
updating the national reference frame of CGCS2000 in future.
Related article: Update China geodetic coordinate frame considering plate motion (doi:
10.1186/s43020-020-00032-w)

High orbit autonomous navigation: Performance improvement brought by BDS-3
Autonomous navigation using leakage signal is the technology of receiving navigation satellite signals
from the back of the Earth, can provide long-lasting and high-precision autonomous navigation
services for medium and high orbit spacecraft such as MEO and GEO. It can provide a feasible
way for the expansion of GNSS services from ground and low orbit to medium or high orbit. With
completed and started providing full-scale global services of the BeiDou-3 Navigation Satellite System,
more than 120 satellites in orbit will be able to provide navigation services in the world, which can
significantly increase the number of navigation satellite signals from the back of the Earth that can
be received by medium and high orbit spacecraft and significantly improve the performance. In this
paper, taking the launch process of SJ-20 satellite in December 2019 as an example, the performance
evaluation of medium and high orbit spacecraft using multi-GNSS leakage signals is carried out,
including the number of visible satellites, geometric dilution of precision, carrier to noise ratio, etc.
The analysis results show that the unique satellite constellation of MEO + GEO + IGSO in BDS can
significantly improve the performance of autonomous navigation using leakage signal of medium
and high orbit spacecraft, including available time of positioning service and geometric dilution of
precision, especially for medium and high orbit spacecraft which Sub-Satellite Point is outside the
Asia-Pacific region. Due to the performance improvement brought by BDS-3, the technology of
autonomous navigation using leakage signal has become one of the feasible schemes for autonomous
navigation of all kinds of medium and high orbit spacecraft.
Related article: Performance evaluation of multi-GNSSs navigation in super synchronous
transfer orbit and geostationary earth orbit (doi: 10.1186/ s43020-021-00036-0)