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The paper presents attitude determination results of the "GPS Attitude, Positioning and Profiling Experiment" (GAP) on board the CASSIOPE satellite using real flight data. The GAP payload consists of five minimally modified commercial‐off‐the‐shelf NovAtel OEM4‐G2L receivers that provide dual‐frequency GPS measurements and allow for attitude and orbit determination of the satellite as well as electron density profiling. To the authors' knowledge, the CASSIOPE mission is the first space mission that provides dual‐frequency observations for attitude determination. The data has been analyzed with a GPS attitude determination algorithm originally developed for the analysis of data from the "Flying Laptop" mission. The GPS‐based solution for selected attitude maneuvers is compared to a reference orientation provided by the satellite's star sensors. Furthermore, an analysis of the typical time‐to‐first‐fix (TTFF) for the attitude solution is provided. The advantage of dual‐frequency ambiguity fixing compared to single‐frequency is assessed. [ABSTRACT FROM AUTHOR]

ABSTRACT Measurements of Global Navigation Satellite System (GNSS) receivers are affected by systematic offsets related to group and phase delays of the signal generation and processing chain. The resulting code and phase biases depend on the transmission frequency and the employed signal modulation. Within this study differential code biases (DCBs) of legacy and modernized GNSS signals are derived from pseudodrange observations of a global multi-GNSS receiver network. Global ionosphere maps (GIMs) are employed for the correction of ionospheric path delays. Based on six months of data collected within the Multi-GNSS Experiment (MGEX) of the International GNSS Service (IGS), DCBs for the publicly available signals of GPS, Galileo, and BeiDou have been determined. The quality of the resulting DCB estimates is assessed and compared against group delay parameters transmitted by the GNSS providers as part of the broadcast ephemeris data. Copyright © 2014 Institute of Navigation. [ABSTRACT FROM AUTHOR]

ABSTRACT QZS-1 is the first satellite of the Japanese Quasi-Zenith Satellite System (QZSS) launched in September 2010. Five stations of the Cooperative Network for GIOVE Observation (CONGO) are able to track the QZSS L1, L2, and L5 navigation signals. Together with the GPS observations of these stations, QZS-1 orbit and clock parameters were estimated. Various orbit parameterizations were tested and a sub-meter accuracy of the best orbits could be achieved. Differences of orbits obtained from various linear combinations are on the decimeter level and differential code biases are on the few nanosecond level. Short periodic variations with a peak-to-peak amplitude of about 4 cm were detected in the QZS-1 clock estimates. Three time intervals with different periods of 13.6 - 15.5 minutes could be identified. Three way carrier phase analysis confirmed the presence of these variations. The origin of these periodicities is considered to be an artifact of the time keeping system. Copyright © 2013 Institute of Navigation. [ABSTRACT FROM AUTHOR]

ABSTRACT The passive hydrogen maser of GIOVE-B and the new GPS Block IIF Rubidium clock have demonstrated a superior stability in ground tests. In practice, however, the apparent clock performance for GNSS users is limited by measurement errors and imperfections of the signal chain that affect the clock variance at different time scales. Within this paper, we provide a direct comparison of the apparent clock performance for GIOVE-B and the first Block IIF satellite (SVN62). Periodic errors in the apparent clocks of both satellites are analyzed and an effort is made to separate the impact of orbit determination errors from physical clock or line bias variations. For SVN62 an empirical clock correction model is discussed, which offers a notable reduction of the Allan variance at orbital time scales. Furthermore, triple-frequency observations are used to demonstrate the presence of thermally induced line bias variations and to quantify the resulting inter-frequency clock biases. Copyright © 2012 Institute of Navigation. [ABSTRACT FROM AUTHOR]

The GENESIS mission prepared for launch in 2027 integrates the four space-geodetic techniques on a single spaceborne platform in medium Earth orbit. With its unique observations and alternative tie concepts, the mission aims to contribute to an improved accuracy and homogeneity of future terrestrial reference system realizations. To assess the expected contribution of Global Navigation Satellite System (GNSS) tracking, a comprehensive GNSS coverage analysis is performed based on detailed link-budget simulations, taking into account the best available gain patterns and signal-specific transmit power estimates derived for this work from measurements of a high-gain dish antenna. The benefit of different receiver antenna concepts for the GENESIS spacecraft is assessed, and it is demonstrated that a single-antenna system with either a nadir-looking or side-looking boresight is a viable alternative to the dual-antenna configuration considered in initial mission studies. Compared to terrestrial users and missions in low Earth orbit, GENESIS will collect GNSS signals transmitted at up to two times larger off-boresight angles. Only limited information on the actual transmit antenna phase patterns is presently available in this region, which hampers a quantitative assessment of the expected measurement and orbit determination accuracy. As such, a comprehensive release of manufacturer calibrations is encouraged for all blocks of GPS and Galileo satellites. In parallel, a need for in-flight characterization and calibration of the GNSS transmit antennas for off-boresight angles of up to 30 ∘ using observations of the GENESIS mission itself is expected. The impact of such calibrations on the overall quality of terrestrial reference frame parameters will need to be assessed in comprehensive simulations of global GNSS network solutions with joint processing of terrestrial and GENESIS GNSS observations., Competing Interests: Conflict of interestThe authors have no conflicts of interest that are relevant to the content of this article., (© The Author(s) 2023.)

Gravity fields derived from GPS tracking of the three Swarm satellites have shown artifacts near the geomagnetic equator, where the carrier phase tracking on the L2 frequency is unable to follow rapid ionospheric path delay changes due to a limited tracking loop bandwidth of only 0.25 Hz in the early years of the mission. Based on the knowledge of the loop filter design, an analytical approach is developed to recover the original L2 signal from the observed carrier phase through inversion of the loop transfer function. Precise orbit determination and gravity field solutions are used to assess the quality of the correction. We show that the a posteriori RMS of the ionosphere-free GPS phase observations for a reduced-dynamic orbit determination can be reduced from 3 to 2 mm while keeping up to 7% more data in the outlier screening compared to uncorrected observations. We also show that artifacts in the kinematic orbit and gravity field solution near the geomagnetic equator can be substantially reduced. The analytical correction is able to mitigate the equatorial artifacts. However, the analytical correction is not as successful compared to the down-weighting of problematic GPS data used in earlier studies. In contrast to the weighting approaches, up to 9-10% more kinematic positions can be retained for the heavily disturbed month March 2015 and also stronger signals for gravity field estimation in the equatorial regions are obtained, as can be seen in the reduced error degree variances of the gravity field estimation. The presented approach may also be applied to other low earth orbit missions, provided that the GPS receivers offer a sufficiently high data rate compared to the tracking loop bandwidth, and provided that the basic loop-filter parameters are known., (© The Author(s) 2021.)