# Stabilising the Orbital Reference Frame for Ice Cap and Sea Level Observation and Modelling: CRYOSAT and Jason-2

Lead Research Organisation: University College London
Department Name: Civil Environmental and Geomatic Eng

### Abstract

There is a pressing need to quantify the exchange of mass between the world's oceans and polar ice caps, and this can only be achieved by measuring how their volumes are changing. Currently circa 50% of the observed sea level rise of 1.8 mm yr-1 cannot be explained. The required measurements can only be made effectively from space using satellites, and several missions are either in space now, or are about to be deployed to attack this problem. In simple terms the sea and ice topography, and how it changes, can be inferred by measuring ranges from the satellites to the surface, and then subtracting the ranges from the position of the satellites in a geocentric reference frame. The satellite position is calculated by the process of orbit determination, which requires mathematical modelling of the forces acting on the satellites. Errors in the satellite orbit map directly into errors in the inferred topography. Both the orbit determination process and the modelling of the time evolution of the sea and ice changes rely upon a 'reference frame' - put simply this is a list of coordinates and velocities of the tracking stations used to observe how the satellites move in space. Velocities are needed because the tracking stations are sited on tectonic plates, all of which are in continuous motion. As these kind of analyses model geophysical effects that last decades this motion of the tracking stations must be known accurately. In turn, the methods used to calculate the station positions (coordinates) and velocities are linked to the orbit determination process - so once again, errors in the orbit estimates create problems. Orbital accuracy in the satellite radial direction of around 1 cm is required to reduce the uncertainty in the target geophysical parameters. We believe this can be achieved by accurate modelling of the satellite forces. The principal problems here are satellite surface forces caused by solar radiation pressure, thermal effects and forces caused by radiation reflected and emitted by the Earth (termed albedo effects), as well as atmospheric drag effects. These forces, particularly the earth radiation effects, have very strong seasonal and latitudinal characteristics which, if not modelled appropriately, appear as seasonal and latitudinal variations in the inferred sea and ice topography. The PI and his group have developed a suite of software utilities to attack these force modelling problems that are recognised as the leading techniques in the world for dealing with complex, realistic models of the spacecraft response to its environment. The group has been invited to participate in several international experiments that involve modelling complexity that has never been attempted before, and this proposal seeks to extend the group's techniques and apply them to current missions to achieve the 1 cm goal. Failure to address this problem of systematic biases in the satellite orbits would seriously undermine any attempt to constrain climate change models on the basis of the estimated mass exchanges.

### ORCID iD

Marek Ziebart (Principal Investigator)

### Publications

10 25 50

Description Advanced methods to calculate the forces acting on near Earth operating satellites - the new ideas have now resulted in a European Space Agency contract to develop orbit calculation methods for the new European version of GPS called Galileo
Exploitation Route On going research programmes with NASA and the European Space Agency - very active and exciting work
Sectors Aerospace

Defence and Marine

Environment

Description An impact case study was submitted to the REF 2014 based partially upon this work, which has been used (and is very much an on-going research and development activity between UCL and NASA) in the measurement of changes in global mean sea level from space
First Year Of Impact 2014
Sector Aerospace, Defence and Marine,Environment
Impact Types Societal

Description Track custody at GEO
Amount \$250,000 (USD)
Organisation US Air Force European Office of Air Force Research and Development
Sector Public
Country United Kingdom
Start 03/2017
End 03/2018

Title enhanced spacecraft force modelling algorithms and code
Description Numerically stabilised and automated spacecraft force modelling tools which can be deployed on a variety of platforms
Type Of Material Improvements to research infrastructure
Year Produced 2017
Provided To Others? No
Impact The improvements to our force modelling capability have resulted in UCL being asked to contribute to next generation modelling approaches to the International Terrestrial Reference Frame - this is essentially the global coordinate system used to measure earthquakes, tectonics and global sea level change

Description DSTL
Organisation Defence Science & Technology Laboratory (DSTL)
Country United Kingdom
Sector Public
PI Contribution The application of our ongoing work on surface forces for space vehicles and space debris to understanding the way in which counter measures disperse from incoming ballistic missiles, with a view to developing next generation radar systems
Collaborator Contribution Modelling and analysis of counter measure dispersion and trajectories
Impact Modelling and analysis of counter measure dispersion and trajectories
Start Year 2014

Description European Office of Air Force Research and Development - track custody at GEO research programme
Organisation European Office of Aerospace Research & Development (EOARD)
Country United Kingdom
Sector Public
PI Contribution Ideas and models resulting from the ongoing study into orbit dynamics are about to be applied to the behaviour of geostationary satellites
Collaborator Contribution EOARD, along with DSTL, carried out facilitated discussions about problems related to understanding the behaviour of geostationary satellites on orbit
Impact Funding from EOARD to carry out a substantive programme of research
Start Year 2017

Description NATO lecture tour
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact I gave an invited lecture series to several NATO countries (Germany, Turkey, Italy and the USA), paid for by NATO and building directly on several research projects funded by UK research councils
Year(s) Of Engagement Activity 2016

Description Research visit, Goddard Space Flight Centre, NASA, USA
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited visit to NASA Goddard Space Flight Centre to discuss future research programmes with UCL for the next five years
Year(s) Of Engagement Activity 2016