Gravity Field Improvement Prospects from the STEP Mission

The Satellite Test of the Equivalence Principle (STEP) mission is under study by ESA and NASA as a 50/50 shared future mission in the discipline area “Fundamental Physics.” The main goal of this mission is the test of the weak equivalence principle by measuring any difference in the rate of fall of test masses within an Earth-orbiting satellite with an accuracy of one part in 1017 of the total gravitational acceleration. To achieve this level of accuracy the measurements will be carried out in a “drag-free” environment by means of a number of extremely accurate differential accelerometers based on state-of-the-art SQUID (Superconducting QUantum Interference Device) technology developed at Stanford University. The spacecraft will carry onboard a Global Positioning System (GPS) receiver to be used for real-time navigation as well as precise orbit determination by means of post-processing of the data at ground facilities. A useful data byproduct of the accelerometer measurements are the gravity gradients at altitude. Along with the GPS tracking data they can be used to improve the present accuracy and resolution of the Earth’s gravity field to degree and order ~80. The simulation study presented here assesses the contribution of the GPS tracking data alone, in an improved determination of the gravitational field of the Earth. The accuracy of the GPS measurements and the extent of the supporting ground-tracking network reflect present-day capabilities.