Incorporation of LEO GNSS observations into global network solutions

This research focuses on specific aspects of geodesy with the aim of deepening the understanding of Earth's dynamic geodetic parameters and refining the precision and completeness of global network solutions. The work comprises various investigations, each addressing key aspects of geodetic research and contributing to a broader understanding. The central approach involves the integration of observations from Global Navigation Satellite Systems (GNSS) collected by satellite-based receivers aboard Low Earth Orbiters (LEOs) into the determination of a global GNSS network solution. The focus is on the influence of integrating LEO-GNSS observations on resulting parameters such as GNSS and LEO orbits, geodetic parameters, such as Earth's center-of-mass coordinates and Earth rotation parameters, and ground station coordinates. Fundamental to this is the current state of realizing terrestrial reference frames (TRFs). The incorporation of LEO-GNSS observations addresses conceptual deficiencies in TRFs by considering the gravity field. Satellites in low Earth orbits play a crucial role, acting in the role of "Space Ties", and being highly sensitive to the gravitational field. These satellites become integral components for various satellite geodetic techniques, promising a comprehensive and improved representation of Earth's geodetic reference frame. One aspect of the work involves an examination of traditional methods for precise orbit determination (POD) of LEOs using observations from the Global Positioning System (GPS). Traditionally relying on fixed GPS orbits and clock corrections derived from ground-based receivers, this work highlights the potential of integrating LEO-GNSS observations into global network solutions. A comprehensive analysis, including data from LEOs, equipped with dual-frequency GNSS receivers, is methodically scrutinized through a joint least-squares adjustment process, resulting in a combined GNSS-LEO solution. The analysis shows a significant improvement in the observability of the estimated geodetic parameters. Acknowledging the observational advantages of LEO satellites due to their proximity to the Earth's surface, this work emphasizes their complementary role alongside established systems like GPS. The investigation carefully analyzes the potential for further qualitative enhancements in the framework of network solutions when LEOs are seamlessly integrated, particularly in conjunction with multiple GNSS systems

Precise Orbit Determination of CubeSats

CubeSats are faced with some limitations, mainly due to the limited onboard power and the quality of the onboard sensors. These limitations significantly reduce CubeSats' applicability in space missions requiring high orbital accuracy. This thesis first investigates the limitations in the precise orbit determination of CubeSats and next develops algorithms and remedies to reach high orbital and clock accuracies. The outputs would help in increasing CubeSats' applicability in future space missions

PCO and hardware delay calibration for LEO satellite antenna downlinking navigation signals

Augmentation of the Global Navigation Satellite System by low earth orbit (LEO) satellites is a promising approach benefiting from the advantages of LEO satellites. This, however, requires errors and biases in the satellite downlink navigation signals to be calibrated, modeled, or eliminated. This contribution introduces an approach for in-orbit calibration of the phase center offsets (PCOs) and code hardware delays of the LEO downlink navigation signal transmitter/antenna. Using the satellite geometries of Sentinel-3B and Sentinel-6A as examples, the study analyzed the formal precision and bias influences for potential downlink antenna PCOs and hardware delays of LEO satellites under different ground network distributions, and processing periods. It was found that increasing the number of tracking stations and processing periods can improve the formal precision of PCOs and hardware delay. Less than 3.5 mm and 3 cm, respectively, can be achieved with 10 stations and 6 processing days. The bias projections of the real-time LEO satellite orbital and clock errors can reach below 3 mm in such a case. For near-polar LEO satellites, stations in polar areas are essential for strengthening the observation model

Orbit Determination System for Low Earth Orbit Satellites

The IAI/MBT Precise Orbit Determination system for Low Earth Orbit satellites is presented. The system is based on GPS pesudorange and carrier phase measurements and implements the Reduced Dynamics method. The GPS measurements model, the dynamic model, and the least squares orbit determination are discussed. Results are shown for data from the CHAMP satellite and for simulated data from the ROKAR GPS receiver. In both cases the one sigma 3D position and velocity accuracy is about 0.2 m and 0.5 mm/sec respectively

Advances in Measurement and Force Modeling for Improved GNSS-based Precise Orbit Determination of CYGNSS and Sentinel-6 MF

Precise orbit determination (POD) based on global navigation satellite systems (GNSS) tracking is fundamental to many space-based geodesy missions. The research presented here develops and implements improvements to the models and methods for two missions: CYGNSS, a lowcost constellation of small satellites, and Sentinel-6 Michael Freilich (MF), the current reference global ocean altimeter mission. The orbit solutions are improved though the advancement of the measurement models, dynamic force models, and solution strategies. CYGNSS is a constellation of eight small satellites designed to use reflected GNSS signals for retrieval of ocean surface winds. The navigation requirements to achieve this primary mission are quite loose, allowing the project to use simple point positioning, with a single-frequency GPS receiver, to support mission orbit needs. Research presented here demonstrates that orbits with 3-D positioning accuracy better than 10 cm can be achieved, with an iterative solution strategy that includes calibration of the antenna, use of combined code and carrier GRAPHIC (GRoup And PHase Ionosphere Correction) observables, and correction of a timing difference between code and carrier measurements. The process is validated using comparable data from the GRACE (Gravity Recovery and Climate Experiment) mission, for which high precision reference orbits are available. To support stringent POD requirements, Sentinel-6 MF is equipped with multiple tracking instruments: a TriG GPS receiver, a pair of redundant PODRIX GNSS (GPS + Galileo) receivers, a satellite laser retroreflector, and a Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) receiver. The first study develops an improved dynamic solar radiation pressure model. Compared to the previously used macromodel, this results in more consistent estimates of drag and solar scale parameters throughout changes in the orientation of the sun relative to the orbit plane (beta angle). The second study improves the measurement model by extending the new GPS IIIA transmitter antenna calibration out to boresight angles of 14-17 degrees, which are not observed by ground-based receivers, but are quite important for receivers in low Earth orbit. Implementation of this extension produces solutions that incorporate GPS IIIA measurements with statistics consistent with older satellite families. Finally, applying lessons learned from the previous studies, orbit solutions are generated from all available Sentinel-6 MF GNSS tracking data. This multi-receiver/GNSS configuration with two independent receivers and constellations (GPS + Galileo) revealed a range bias effect in the TriG GNSS observations that can be calibrated. Processing the calibrated TriG and PODRIX observations separately results in highly accurate orbit solutions, which are both consistent with one-way satellite laser ranging (SLR) residuals at the level of 6.9 mm rms. When processed together, the TriG plus PODRIX multi-GNSS solutions produced the most accurate orbit solutions with one-way SLR residual rms of 6.8 mm</p

Integer Ambiguity Resolution for Multi-GNSS and Multi-Signal Raw Phase Observations

The continuous modernisation of existing Global Navigation Satellite Systems (GNSS) and the development of new systems with a multitude of different carrier frequencies and a variety of signal modulations creates a true multi-GNSS and multi-signal environment available today. Still most precise GNSS processing strategies rely on dual-frequency measurements only by applying the Ionosphere-Free (IF) Linear Combination (LC) of GNSS observables and therefore do not benefit from the available multi-signal environment. While in this processing approach the first order effect of the ionospheric delay can be eliminated almost completely, the formation of linear combinations of GNSS observables leads to a noise increase for the resulting observations and a loss of some of the physical characteristics of the original signals, like the integer nature of the carrier phase ambiguity. In order to benefit from the multi-GNSS and multi-signal environment available today, the scientific analyses and precise applications presented in this work are based on the raw observation processing approach, which makes use of the original (raw) observations without forming any linear combinations or differences of GNSS observables. This processing strategy provides the flexibility to make use of all or a selection of available multi-GNSS and multi-signal raw observations, which are jointly processed in a single adjustment as there is no inherent limitation on the number of usable signals. The renunciation of linear combinations and observation differences preserves the physical characteristics of individual signals and implies that multi-signal biases and ionospheric delays need to be properly determined or corrected in the parameter estimation process. The raw observation processing approach is used in this work to jointly process measurements from up to three different GNSS, including eleven signals tracked on up to eight different carrier frequencies in one single adjustment. The bias handling for multi-GNSS and multi-signal applications is analysed with a focus on physically meaningful parameter estimates to demonstrate the benefits of handling clock offset parameters, multi-signal code biases and ionospheric delay estimates in a physically meaningful and consistent way. In this context, receiver-specific multi-GNSS and multisignal biases are analysed and calibrated by the use of a GNSS signal simulator. The disadvantages of eliminating physical characteristics due to the formation of linear combinations of observations or commonly used parameter estimation strategies are demonstrated and discussed. The carrier phase Integer Ambiguity Resolution (IAR) approach developed and implemented in the course of this work is based on the joint processing of multi-GNSS and multi-signal raw observations without forming any linear combinations or observation differences. Details of the implemented IAR approach are described and the performance is analysed for available carrier signal frequencies of different GNSS. Achieved results are compared to the conventional IAR approach based on IF linear combinations and the so called Widelane (WL) and Narrowlane (NL) ambiguities. In addition, the resolution of inter-system integer ambiguities is analysed for common GNSS signal frequencies. The performance of the implemented IAR approach is demonstrated and analysed by the joint Precise Orbit Determination (POD) of multi-GNSS satellites based on fixed multi-frequency carrier phase ambiguities. The improvement of the satellite orbit and clock quality by fixing raw observation ambiguities confirms the successful implementation of the IAR approach based on raw observation processing. Multi-GNSS satellite orbits and clock offsets determined with this approach are compared to results generated with the conventional IF linear combination processing approach and independent external products. This comparison demonstrates an at least equivalent performance of the implemented IAR approach based on raw observation processing. In addition, the fixed raw observation ambiguities are used to investigate and discuss characteristics of multi-GNSS and multi-frequency phase biases

Prediction and ephemeris fitting of LEO navigation satellites orbits computed at the antenna phase center

Nowadays, Low Earth Orbit (LEO) satellites are proposed to augment the Positioning, Navigation and Timing (PNT) service of the GNSS satellites by directly transmitting navigation signals. In such cases, the users eventually need the orbits at the Antenna Phase Center (APC) of the antenna broadcasting navigation signals toward the Earth instead of those at the satellite Center of Mass (CoM). Using real attitudes of Sentinel satellites and simulated attitudes of different source types with enlarged instabilities, the influences of the attitude instability on the prediction and ephemeris fitting of the APC orbits are studied. It was found that different scenarios of attitude stabilities could lead to prediction degradations with a 3D RMS from a few millimeters to more than 4 cm. The study also showed that the ephemeris fitting errors of the APCs are not significantly impacted, considering both the real attitudes of Sentinel-6A and the simulated attitude instabilities

The potential of LEO mega-constellations in aiding GNSS to enable positioning in challenging environments

Signals from the emerging Low Earth Orbit (LEO) satellites from mega-constellations that broadcast internet, such as Starlink (Space X), OneWeb, Iridium etc., also known as “signals of opportunity” (SOP), can potentially aid positioning. These LEO satellites are approximately 20 times closer to Earth compared to the GNSS medium-earth orbit (MEO) satellites – with 300-1500km altitudes, and 90-120 minutes orbital periods. Hence, LEO satellites provide a new navigation space infrastructure with much stronger signal power than GNSS signals. This makes these LEO signals more resilient to interference and available in deep attenuation settings. In challenging environments, with limited GNSS observations that may not allow positioning, such as in urban canyons, bushland, or bottom of mining pits, integrating LEO signals with the available GNSS observations can enable positioning. Moreover, the corresponding high speed of LEO satellites enables faster satellite geometry change, and hereby significantly shortens the convergence time for precise point positioning (PPP). In this contribution, the positioning from LEO Doppler shift time variation integrated with GNSS and two challenges in positioning using LEO will be briefly discussed. For positioning, the orbits of LEO satellites and their clock behaviour must be known. In addition, unlike GNSS satellites, LEO satellites are not equipped with atomic clocks, and typically use ultra-stable oscillators (USOs) or oven-controlled crystal oscillators (OCXOs), nor are they tightly time-synchronised with each other. The estimation and prediction of these orbits and clock errors and drift are discussed