Geoid Determination for Precise Satellite Height Measurements

Abstract

Physical height systems are directly related to the Earth’s gravity field and its local variations. Heights on Earth’s surface are naturally referred to the local mean sea level (MSL), which is called the geoid, and the reference level of countries physical height systems is the local geoid. Positions on the Earth are nowadays conveniently measured by Global Navigation Satellite System (GNSS) positional technology. However, heights measured by GNSS refer to a reference ellipsoid, which is a mathematical model of the Earth, and not to the local sea level (geoid). The discrepancy between the geoid and the reference ellipsoid is ±100 m globally. Precise gravity field modelling and accurate knowledge of the gravity field is paramount in order to accurately and precisely determine the height difference between the geoid and the reference ellipsoid.
National height systems are based on the local MSL and discrepancies are common between neighbouring countries, due to different references and lack of standardisation. The International Height Reference System (IHRS) and its realisation The International Height Reference Frame (IHFR) have been defined as a global unified system for physical heights to provide a globally consistent framework for height measurements.
This PhD thesis concerns various aspects related to geoid determination. These are related to collection of gravity data, maintenance of gravity networks, computation of (quasi)geoids for physical heights systems, and connection between local height systems and a global unified physical height system.
Different regions with varying and rugged topography have been studied for geoid modelling. The terrain has a significant impact on the local gravity field and needs to be well represented and handled properly. Modifications of the Stokes’ kernel have also played a central part. The Nordic and Baltic region has been the main study region. Geoid solutions have been computed for a new official geoid solution of Denmark, a gravimetric quasigeoid solution as part of the Baltic Sea Chart Datum 2000 under the FAMOS project, and a gravimetric quasigeoid for the Nordic Geodetic Commission (NKG) region. In connection with the latter a simple tilt correction indicated the possibility of significant tilts the levelling networks of Denmark and Latvia. Preliminary computation of the new official geoid of Greenland has been initiated, and there are visible changes when comparing to the current official geoid from 2016. Further, new quantum gravity data in the region of Nuuk show a significant impact on the geoid in the region, especially over the fjords.
The connection of the Danish, Faroese and Greenlandic height systems, respectively, to the IHRF was established and the height differences between the reference levels was determined. The IHRF coordinates were also inferred at permanent GNSS stations in the countries to densify the IHRF in the countries.
Precise geoid determination depends on high­quality gravity data which in turn depends on a well­established absolute gravity network. Quality control of old gravity data sources, which form a part of the Danish land gravity network, was conducted, with the conclusion of no systematic trends on a national level. However, some of the small islands seem to have local biases. New absolute gravity stations have been added to the absolute gravity network by absolute gravimetry and established stations are periodically measured.
Finally, airborne gravity campaigns in Namibia and Malaysia where conducted and finalised. Especially Namibia was lacking gravity measurements in large parts of the country. The campaigns were followed by computation of geoid models with inclusion of the new airborne gravity data for improvement of the countries height systems. The new gravity data will be included in future Global Gravity Models of the Earth.

Date of Award	2024
Original language	English
Awarding Institution	DTU Space, Technical University of Denmark
Supervisor	René Forsberg (Supervisor), Gabriel Strykowski (Supervisor) & Tim Enzlberger Jensen (Supervisor)