Precise orbit determination of Haiyang‑2D using onboard BDS‑3 B1C/B2a observations with ambiguity resolution

The Haiyang-2D altimetry mission of China is one of the first Low Earth Orbit(LEO)satellites that can receive new B1C/B2a signals from the BeiDou-3 Navigation Satellite System(BDS-3)for Precise Orbit Determination(POD).In this work,the achievable accuracy of the single-receiver ambiguity resolution for onboard LEO satellites is studied based on the real measurements of new BDS-3 frequencies.Under normal conditions,six BDS-3 satellites on average are visible.However,the multipath of the B1C/B2a code observations presents some patchy patterns that cause near-field variations with an amplitude of approximately 40 cm and deteriorate the ambiguity-fixed rate.By modeling those errors,for the B2a code,a remarkable reduction of 53%in the Root Mean Square(RMS)is achieved at high elevations,along with an increase of 8%in the ambiguity-fixed rates.Additionally,an analysis of the onboard antenna’s phase center offsets reveals that when compared to the solutions with float ambiguities,the estimated values in the antenna’s Z direction in the solutions with fixed ambiguities are notably smaller.The independent validation of the resulting POD using satellite laser ranging at 16 selected high-performance stations shows that the residuals are reduced by a minimum of 15.4%for ambiguity-fixed solutions with an RMS consistency of approximately 2.2 cm.Furthermore,when compared to the DORIS-derived orbits,a 4.3 cm 3D RMS consistency is achieved for the BDS-3-derived orbits,and the along-track bias is reduced from 2.9 to 0.4 cm using ambiguity fixing.

Quasi-4-dimension ionospheric modeling and its application in PPP

Ionospheric delay modeling is not only important for Global Navigation Satellite System(GNSS)based space weather study and monitoring,but also an efficient tool to speed up the convergence time of Precise Point Positioning(PPP).In this study,a novel model,denoted as Quasi-4-Dimension Ionospheric Modeling(Q4DIM)is proposed for wide-area high precision ionospheric delay correction.In Q4DIM,the Line Of Sight(LOS)ionospheric delays from a GNSS station network are divided into different clusters according to not only the location of latitude and longitude,but also satellite elevation and azimuth.Both Global Ionosphere Map(GIM)and Slant Ionospheric Delay(SID)models that are traditionally used for wide-area and regional ionospheric delay modeling,respectively,can be regarded as the special cases of Q4DIM by defining proper grids in latitude,longitude,elevation,and azimuth.Thus,Q4DIM presents a resilient model that is capable for both wide-area coverage and high precision.Four different sets of clusters are defined to illustrate the properties of Q4DIM based on 200 EUREF Permanent Network(EPN)stations.The results indicate that Q4DIM is compatible with the GIM products.Moreover,it is proved that by inducting the elevation and azimuth angle dependent residuals,the precision of the 2-dimensional GIM-like model,i.e.,Q4DIM 2-Dimensional(Q4DIM-2D),is improved from around 1.5 Total Electron Content Units(TECU)to better than 0.5 TECU.In addition,treating Q4DIM as a 4-dimensional matrix in latitude,longitude,elevation,and azimuth,whose sparsity is less than 5%,can result in its feasibility in a bandwidth-sensitive applications,e.g.,satellite-based Precising Point Positioning Real-Time Kinematic(PPP-RTK)service.Finally,the advantages of Q4DIM in PPP over the 2-dimensional models are demonstrated with the one month's data from 30 EPN stations in both high solar activity year 2014 and low solar activity year 2020.