GNSS data processing strategies and antenna phase center calibration techniques
Typ
Examensarbete för masterexamen
Program
Publicerad
2022
Författare
Abdalla, Meaad
Johansson, Frida
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
GNSS-related applications have increased dramatically in recent decades, raising the need
for more precise results. Thus, enhancements for GNSS’s different segments are vital to
achieving higher accuracy. The receiver-related biases are prominent errors that affect
GNSS measurements and results, particularly errors associated with the antennas and
their phase-center. In addition, identifying the correct antenna measuring point is difficult
since phase-centers are electrical characteristics that change with frequency, elevation, and
azimuth angles. Therefore, antenna calibration is indispensable to mitigate phase-center related errors. Multiple approaches are currently employed to calibrate GNSS antennas,
specifically model-specific and antenna-specific. Although these techniques have consid erably contributed to GNSS accuracy, they face limitations related to the respective site
specifications.
Using the internationally available (ONSA & ONS1) and regional (OTT 1 through 6)
GNSS stations at the Onsala Space Observatory (OSO), the impact of different calibra tion methods on GNSS accuracy has been investigated. Station-specific corrections were
also considered in the study. Various constraints and cut-off elevation angles were exam ined and modified to reduce the influence of other error sources. Data from 2019 -2021
for GPS, Galileo, and GLONASS constellations, were collected and processed using the
GipsyX v.1.7 software, which uses the Precise Point Positioning technique. During the
processing, a period of 30 hours, instead of the typical 24 hours, of observations was
used to guarantee higher stability for the Kalman filter. In addition to the coordinate
estimations, the analysis included the tropospheric zenith delay (ZTD) and tropospheric
gradients. The estimated delay parameters were verified and compared with estimations
from other measurement techniques such as Very Long Baseline Interferometry (VLBI)
and water vapor radiometer (WVR). Temporary stations were established at four accu rate geodetic checkpoints from March-May 2022 to be used to derive a station-specific
calibration matrix for the international stations at OSO. It was found that supported by
other measurement techniques, the examined constraints and elevation cut-off values im pact the accuracy of GNSS results. Model-specific calibration values were compared with
VLBI & WVR results. However, minor differences were detected when comparing model
and antenna-specific calibration methods.
Beskrivning
Ämne/nyckelord
GNSS, Zenith Total Delay, Tropospheric Gradients, Antenna Phase-Center, GipsyX , GPS, Precise Point Positioning, Onsala Space Observatory