Benefit of GOCE gravity gradients at the lowered orbit on the global gravity field model
20/09/2016 | 08:30 | Session 2: Model Development
Author(s): Hu Wu, Jürgen Müller and Phillip Brieden
Hu Wu, Jürgen Müller and Phillip Brieden
Since the nominal and extended mission phases completed in July 2012, ESA (European Space Agency) decided to lower the satellite’s orbit to further dig the potential of the GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission. In this study, a thorough analysis of the gravity gradients at the lowered orbit is carried out, and their effects on the global gravity field recovery are discussed.
To increase the sensitivity of the gradiometer to the gravity signal, the GOCE satellite was gradually lowered from the initial altitude of 259 km to 229 km in four steps. The first three maneuvers took place in August 2012, November 2012, and February 2013, and lowered the satellite by 8.6 km, 15 km and 20 km from its designed orbit. In-between the maneuvers, the satellite collected two 61-day cycles of data. After another 70-day cycle, ESA performed a fourth and final maneuver in May 2013, and lowered the orbit by another 10 km, which started a 143-day repeat cycle with 56-day sub-cycle. Five groups of gravity gradients are then obtained from mission phases with different satellite altitudes. Each group contains about two months’ observations. More specifically, the duration of these five groups are: March to April 2012 (altitude: 259 km), September to October 2012 (altitude: 250.4 km), December 2012 to January 2013 (altitude: 244 km), March to April 2013 (altitude: 239 km) and July to August 2013 (altitude: 229 km).
The spectral behavior of the gravity gradients in each group is studied. And then the gravity field models that are independently recovered from each of the five groups of gravity gradients are compared. Finally, to sum up all the improvements achieved by lowering the orbit, a gravity field model is recovered from all the gradients after the first lowering of the orbit.