Grunwaldt, Ludwig (GeoForschungsZentrum Potsdam, D-14473 Potsdam, Germany)
The GeoForschungszentrum (GFZ) Potsdam operates the Tracking, Occultation and Ranging Payload (TOR) on TanDEM-X under an agreement between DLR and GFZ. This experiment comprises a dual-frequency GPS receiver with 2 zenith antennas for Precise Orbit Determination (POD) on choke rings for multi path suppression, 2 Radio Occultation (RO) antennas in velocity and anti-velocity direction and a Laser Retro Reflector (LRR) array on the nadir side of the TanDEM-X spacecraft. The dual-frequency GPS receiver IGORTM (manufactured by BroadReach Engineering, Tempe, AZ) serves as the main instrument for highly precise orbit determination on TanDEM-X. It operates fully autonomously and tracks up to 12 satellites on both GPS frequencies in a semi-codeless mode. The radio occultation antennas allow for the observation of GPS signals from spacecraft close to and even below the Earth limb. The obtained atmospheric excess phase data from such GPS satellites transmitting their signals through the dense layers of the troposphere towards TanDEM-X enable the computation of vertical atmospheric profiles for humidity and temperature in near real-time on a global scale. The LRR array provides an external calibration and validation tool for the GPS-derived precise TanDEM-X orbit products. It reflects ultra-short laser pulses from special ground stations and allowes for an unambiguous two-way ranging between TanDEM-X and the transmitting laser telescope with sub-cm single-shot accuracy.