ABSTRACT The German Antarctic Receiving Station (GARS) O’Higgins at the northern tip of the Antarctic Peninsula is a dual purpose facility for earth observation and has existed for more than 20 years. It serves as a satellite ground station for payload data downlink and telecommanding of remote sensing satellites as well as a geodetic observatory for global reference systems and global change. Both applications use the same 9 m diameter radio antenna. Major outcomes of this usage are summarised in this paper. The satellite ground station O’Higgins (OHG) is part of the global ground station network of the German Remote Sensing Data Centre (DFD) operated by the German Aerospace Centre (DLR). It was established in 1991 to provide remote sensing data downlink support within the missions of the European Remote Sensing Satellites ERS-1 and ERS-2. These missions provided valuable insights into the changes of the Antarctic ice shield. Especially after the failure of the on-board data recorder, OHG became an essential downlink station for ERS-2 real-time data transmission. Since 2010, OHG is manned during the entire year, specifically to support the TanDEM-X mission. OHG is a main dump station for payload data, monitoring and telecommanding of the German TerraSAR-X and TanDEM-X satellites. For space geodesy and astrometry the radio antenna O’Higgins significantly improves coverage over the southern hemisphere and plays an essential role within the global Very Long Baseline Interferometry (VLBI) network. In particular the determination of the Earth Orientation Parameters (EOP) and the sky coverage of the International Celestial Reference Frame (ICRF) benefit from the location at a high southern latitude. Further, the resolution of VLBI images of active galactic nuclei (AGN), cosmic radio sources defining the ICRF, improves significantly when O’Higgins is included in the network. The various geodetic instrumentation and the long time series at O’Higgins allow a reliable determination of crustal motions. VLBI station velocities, continuous GNSS measurements and campaign-wise absolute gravity measurements consistently document a vertical rate of about 5 mm/a. This crustal uplift is interpreted as an elastic rebound due to ice loss as a consequence of the ice shelf disintegration in the Prince Gustav Channel in the late 1990s. The outstanding location on the Antarctic continent and its year-around operation make GARS O’Higgins in future increasingly attractive for polar orbiting satellite missions and a vitally important station for the global VLBI network. Future plans call for the development of an observatory for environmentally relevant research. That means that the portfolio of the station will be expanded including the expansion of the infrastructure and the construction and operation of new scientific instruments suitable for long-term measurements and satellite ground truthing.

The most important scientific instrument of the station is the 9-m antenna system which is used for the reception of satellite data, telecommanding of satellites and also for geodetic radiotelescope measurements. The antenna has been designed for use in extreme Antarctic conditions. With this antenna system a wide range of Earth observation data of European and German satellite missions (ERS-1/-2, TerraSAR-X, TanDEM-X) are recorded since 1991. These data provide important insights into the climate and environmental changes at the Antarctic Peninsula since 20 years now.

Electronic beam steering via antenna arrays has the potential of replacing the mechanical steering at satellite ground stations. In this work, we exploit the benefits of antenna arrays at satellite ground stations combined with beamforming and direction-of-arrival (DOA) estimation techniques in combating interferences and compensating for erroneous knowledge of the spacecraft position. First, we propose two array geometries for the reception of payload data from lower earth orbiting (LEO) satellites at ground stations. Then considering a scenario where there exist cross-orbit spacecraft, the minimum variance distortionless response (MVDR) beamformer is employed to achieve the goal of the interference suppression and the reception of the desired signals. Moreover, by taking into account the potential errors in the a priori knowledge of the spacecraft flight path at lower elevation angles, we investigate the electronic tracking as a local refinement to compensate for these errors by resorting to DOA estimation techniques. Finally, numerical results demonstrate the advantages of employing antenna arrays in combination with signal processing techniques at satellite ground stations.

In this work, we propose electronic beam steering via antenna arrays as a substitute for large parabolic antennas at satellite ground stations. We concentrate on two array geometries, faceted arrays and hemispherical arrays. A thorough analysis is carried out of the radiation characteristics, the array size, as well as the antenna element distribution and spacing. Moreover, in order to fulfill the requirement of the array design, that is, to achieve a higher gain at low elevation angles where the longer spacecraft to ground station distance leads to a larger range loss, we propose to adjust the number of active antenna elements, i.e., some antenna elements are turned on while others are turned off according to the required level of antenna gain. This also contributes to a concept of an optimized array design for this specific application. In the simulations, the array optimization for both array geometries is further investigated and realized with a realistic ephemeris incorporated. The numerical results support the proposal of replacing large reflector antennas by electronic beam steering via antenna arrays at satellite ground stations.

This paper recalls the essential elements of the joint TerraSAR-X and TanDEM-X ground segment. It elaborates on some topics which are usually not in the primary focus from a pure SAR technical point of view, e.g. the flight formation. Both commissioning and early routine phase results from operating the joint TerraSAR-X and TanDEM-X ground segment are given.

This paper describes DLR's Ground Station Network as setup and operated for the TanDEM-X mission payload data reception. It lists the technical layout and operational characteristics of each station. The Station Monitoring and Control System which has been successfully qualified during the TanDEM-X commissioning phase is introduced. Software design aspects are discussed high level. Key features of the Station Monitoring and Control System are provided.

A main goal of GARS O’Higgins was to support international, European and German missions of Earth observation Satellites, especially ERS1, ERS2, JERS, LANDSAT, and others. The constraints of these missions were fulfilled as well by the equipment components as the operational procedures.

In 2011 the Antarctic Research Station GARS O'Higgins will have been in operation for 20 years. To commemorate this occasion, the German Aerospace Center (DLR), the Chilean Antarctic Institute (INACH) and the Federal Agency for Cartography and Geodesy (BKG) have organized a three-day symposium to take place in Punta Arenas, Chile. The purpose of this event is to stimulate an interdisciplinary exchange in the international community of Antarctic scientists and to improve the networking of research activities with a view to the future use of the GARS O'Higgins station.

Wideband SAR systems such as TerraSAR-X allow estimation of the absolute interferometric phase without resorting to error prone phase unwrapping. This is achieved through the delta-k technique that exploits frequency diversity within the range bandwidth to simulate a SAR system with a much longer carrier wavelength. This benefits all interferometric applications including DEM generation and land surface motion determination. Here we present the results of an ESA study (21318/07/NL/HE) into using delta-k absolute phase estimation for DEM generation and PSI (Persistent Scatterer Interferometry). Using TerraSAR-X data, examples from a delta-k DEM generation system are shown which avoid the errors induced by conventional phase unwrapping. For PSI, the possibilities of absolute phase estimation for a single PS are explored in theory and examples where wideband estimation is compared to conventional PSI processing for a stack of acquisitions over Paris.

This paper describes selected areas in which the TerraSAR-X ground segment had to be extended in order to incorporate the TanDEM-X mission, namely flight dynamics, instrument operations and receiving stations and addresses their testing.

This paper presents results from the synthetic aperture radar (SAR) system performance characterization, optimization, and verification as carried out during the TerraSAR-X commissioning phase. Starting from the acquisition geometry and instrument performance, fundamental acquisition parameters such as elevation beam definition, range timing, receiving gain, and block adaptive quantization setting are presented. The verification of the key performance parameters—ambiguities, impulse-response function, noise, and radiometric resolution—is discussed. ScanSAR and Spotlight particularities are described.

Wideband SAR systems such as TerraSAR-X allow estimation of the absolute interferometric phase without resorting to error prone phase unwrapping. This is achieved through the delta-k technique that exploits frequency diversity within the range bandwidth to simulate a SAR system with a much longer carrier wavelength. This benefits all interferometric applications including DEM generation and land surface motion determination. Here we present the results of an ESA study (21318/07/NL/HE) into using delta-k absolute phase estimation for DEM generation and PSI (Persistent Scatterer Interferometry). Using TerraSARX data, examples from a delta-k DEM generation system are shown which avoid the errors induced by conventional phase unwrapping. For PSI, the possibilities of absolute phase estimation for a single PS are explored in theory and examples where wideband estimation is compared to conventional PSI processing for a stack of acquisitions over Paris.

The paper presents selected results from the TerraSAR-X Commissioning Phase from instrument performance, SAR system performance and command generation.

After the successful participation in the Shuttle missions SIR-C/X-SAR and SRTM, the first national SAR mission TerraSAR-X opened a new era in the German Space Programme and provided a major push for our R&D activities on high resolution X-band SAR based on active phased array T/R module technology. The TerraSAR-X mission spacecraft TSX has been successful launched from Baikonur on June 15, 2007 and since day 5 of mission the TerraSAR-X ground segment is providing superior high resolution SAR images for purposes of scientific observation of the Earth for a period of at least five years after the commissioning phase. At the same time it is designed to satisfy the steadily growing demand of the private sector for remote sensing data in the commercial market. In this spirit, the proposal to add a second, almost identical spacecraft, TDX, to the TSX spacecraft and to fly the two satellites in a closely controlled tandem formation building a single-pass SARinterferometer with adjustable baselines in across- and in alongtrack directions was born. With typical across-track baselines of 250-500 m Digital Elevation Models (DEMs) according to the High Resolution Terrain Information (HRTI)-3 standard will be generated. This synchronized, mainly bistatic operation of TSX and TDX is called the TanDEM-X mission. Like TerraSAR-X the TanDEM-X mission is being implemented in a public-private partnership between the German Aerospace Centre (DLR) and the EADS Astrium GmbH. The launch of the TDX spacecraft is planned for 2009.