Page 8 - 卫星导航2021年第1-2合期

Research Summary
Volume 2, Issue 1-2, July 2021
PRIDE PPP-AR II to assess multi-GNSS phase biases
Ambiguity Resolution in Precise Point Positioning (PPP-AR) is important to achieving high-
precision positioning in wide areas. The International GNSS (Global Navigation Satellite System)
Service (IGS) and some other academic organizations have begun to provide phase bias products to
enable PPP-AR, such as the integer-clock like products by Centre National d’ Etudes Spatials (CNES),
Wuhan University (WUM) and the Center for Orbit Determination in Europe (CODE), as well as
the Uncalibrated Phase Delay (UPD) products by School of Geodesy and Geomatics (SGG). To
evaluate these disparate products, we carry out Global Positioning System (GPS)/Galileo Navigation
Satellite System (Galileo) and BeiDou Navigation Satellite System (BDS-only) PPP-AR using 30 days
of data in 2019. In general, over 70% and 80% of GPS and Galileo ambiguity residuals after wide-lane
phase bias corrections fall in ± 0.1 cycles, in contrast to less than 50% for BeiDou Navigation Satellite
(Regional) System (BDS-2); moreover, around 90% of GPS/Galileo narrow-lane ambiguity residuals
are within ± 0.1 cycles, while the percentage drops to about 55% in the case of BDS products. GPS/
Galileo daily PPP-AR can usually achieve a positioning precision of 2, 2 and 6 mm for the east, north
and up components, respectively, for all phase bias products except those based on German Research
Centre for Geosciences (GBM) rapid satellite orbits and clocks. Due to the insufficient number
of BDS satellites during 2019, the BDS phase bias products perform worse than the GPS/Galileo
products in terms of ambiguity fixing rates and daily positioning precisions. BDS-2 daily positions
can only reach a precision of about 10 mm in the horizontal and 20 mm in the vertical components,
which can be slightly improved after PPP-AR. However, for the year of 2020, BDS-2/BDS-3 PPP-AR
achieves about 50% better precisions for all three coordinate components.
Related article: Assessing IGS GPS/Galileo/BDS-2/BDS-3 phase bias products with PRIDE
PPP-AR (doi: 10.1186/ s43020-021-00049-9)
Modified Precise Point Positioning
Precise Point Positioning (PPP), initially developed for analysis of Global Positing System (GPS) data
from large geodetic networks, gradually represents an effective tool for positioning, timing, remote
sensing of atmospheric water vapor, and monitoring of Earth’s ionospheric Total Electron Content
(TEC). In some previous studies it has been implicitly assumed that the receiver code biases stay
constant over time when formulating the functional model of PPP. In this contribution, the authors
show that this assumption is not always valid, and neglecting this fact can result in a degraded PPP
performance, especially for Slant TEC (STEC) retrieval and timing. For this reason, the authors
consider modifying PPP by extending its functional model to account for time-varying receiver code
biases at two frequencies and beyond. With the Modified PPP (MPPP) model, the temporal variations
of the receiver code biases turn out to be estimable and their adverse impacts on PPP parameters,
such as ambiguity parameters, receiver clock offsets and ionospheric delays, are thus mitigated. They
confirm this by undertaking numerical tests based on real dual-frequency GPS data from a set of
continuously operating reference stations deployed globally. The results imply that the variations of
receiver code biases exhibit a correlation with the ambient temperature. With the application of the
modified functional model to ionosphere delay retrieval and timing, improved performance has been
achieved in comparison to customary PPP which ignores the receive code bias variations.
Related article: Functional model modification of precise point positioning considering the
time-varying code biases of receiver (doi: 10.1186/ s43020-021-00040-4)