Novel GALILEO Applications in the LEO Orbit
Svehla, D.¹; Rotacher, M.²; Salomon, C.³; Wickert, J.²; Helm, A.²; Beyerle, G.²; Montenbruck, O.⁴; Ziebart, M.⁵; Dow, J.^6
1Technische Universität München; ²GeoForschungsZentrum; ³Laboratoire Kastler Brossel; ⁴DLR; ⁵University College London; ⁶ESA/ESOC

We proposed to place a geodetic dual-frequency GNSS receiver (GALILEO/GPS/GLONASS) on board the International Space Station to demonstrate the combination of the GNSS receiver with the highly stable frequency of the ACES clock ensemble as a sensor for remote sensing and relativistic geodesy. Although large solar panels reduce the field of view and number of tracked GNSS satellites, we show that due to the ISS inclination (52°), almost identical to that of the GNSS satellites, the number of visible GPS satellites is considerably larger (by a factor of 1.5) compared to CHAMP and GRACE satellites in the polar orbit. On the other hand, by installing GNSS antenna on top of the Columbus module, the near field multipath can almost completely be reduced.

ACES mission on board the Space Station will for the first time introduce so-called relativistic geodesy, i.e., it will serve as a test-bed to demonstrate the estimation of gravitational potential differences between ground stations on the Earth. By making use of the frequency transfer based on the ACES microwave link (MWL) and collocated GNSS receiver, we expect to estimate gravitational potential differences between optical clocks on the ground with an accuracy below 10 cm in terms of geoid heights. Such a novel measurement type can be used to help establishing a unified global height system across national height systems and different continents and complement the current space geodetic missions such as CHAMP, GRACE and GOCE. The purely geometric 3-D terrestrial coordinate system based on GNSS and other space geodetic techniques is fully operational and it has to be complemented with a globally uniform physical height system of similar precision. The current precision level of regional height systems, in terms of gravity potential differences, is on the order of 1 m2/s2 (10 cm) with inconsistencies between these various systems up to several 10 m2/s2 (several meters). Requirement of high precision height control comes from the need to understand, on a global scale, processes such as sea level change, global and coastal dynamics of ocean circulation, ice melting, glacial isostatic adjustment and land subsidence as well the interaction of these processes. Only by means of monitoring in terms of gravity potential changes at the above level of precision the change of ocean level can be understood as a global phenomenon and purely geometric height changes be complemented by information about the associated density or mass changes.

GALILEO will provide navigation signal of higher power and S/N ratio and introduce several new frequencies. That was a driving force to propose coherent reflectometry/radio occultation that would allow for pioneering experiments with interferometric techniques to derive sea surface heights within or below decimetre level in a large swath width, covering large areas of the ocean below the ISS orbit. Beyond reflectometry applications, the used instrument configuration also would allow for GNSS radio occultation applications with several new scientific aspects compared to the current operational missions COSMIC (U.S./Taiwan) and MetOp (EUMETSAT/ESA). Simulation studies show that due to the lower orbit inclination of the ISS, the largest number of radio-occultation events will be observed in the tropics and most significantly at high antenna gain. Hence, the ISS occultation data could be regarded 'orthogonal' to data sets from polar orbiting satellites and will constitute an important complement to the polar data sets. It is expected, that this fact also would improve the potential of coherent reflectometry in tropical regions in contrast to polar orbiting LEO’s. The ACES system on ISS offers the great opportunity for implementing for the first time a large scatterometry antenna or antenna array. Such opportunity would allow for experiments to derive sea surface characteristics on a global scale using GNSS techniques (e.g., altimetric information, wind speed and direction).