high precision measurement on the absolute localization accuracy of ...
HIGH PRECISION MEASUREMENT ON THE ABSOLUTE LOCALIZATION ACCURACY OF TERRASAR-X Ulrich Balss1, Xiao Ying Cong2, Ramon Brcic1, Moritz Rexer2, Christian Minet1, Helko Breit1, Michael Eineder1, and Thomas Fritz1 1
Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR), D-82230 Wessling, Germany 2
Technical University Munich (TUM), D-80333 Munich, Germany location accuracy of just a few millimeters seems to be feasible. Here, we present our measurement results.
ABSTRACT The German SAR (synthetic aperture radar) satellites TerraSAR-X (TSX-1) and TanDEM-X (TDX-1), launched in June 2007 and June 2010 respectively, provide an unprecedented geometric accuracy. Previous studies showed an absolute pixel localization for both sensors at the centimeter level [4] [5] [6]. However, recent measurements show that in range, under extraordinary good conditions, a location accuracy of even a few millimeters seems to be attainable. While on a long-term scale, we observed a slow variation of subsequent measurements; on a short-term scale, they coincided to within a few millimeters. The measurement series will be continued. The cause of the long-term variation is the subject of current investigation. Index Terms— Synthetic aperture radar, TerraSAR-X, absolute localization accuracy, imaging geodesy
2. MEASUREMENT METHOD Radar systems indirectly measure geometric distances by means of the travelling time of radar pulses from the radar transmitter to ground and back to the radar receiver. Usually, the conversion from travelling time to geometric distance refers to the vacuum velocity of light. However, electrons in the ionosphere, dry air and water vapor in the troposphere introduce additional signal delays which have to be taken into account. At the level of geometric accuracy of the TerraSAR-X mission, geodynamic effects like solid earth tides and continental drift shift the true position of a ground target by several centimeters over the course of a day or years, respectively [1]. In order to verify the pixel localization accuracy of TSX1 and TDX-1, the radar range and azimuth times of corner
1. INTRODUCTION The German SAR satellites TSX-1 and TDX-1, launched in June 2007 and June 2010 respectively, outperform former civilian space-borne radar sensors in terms of geometric accuracy. Since 2010, each satellite supports the TerraSARX mission equally. In contrast, the bistatic interferometric TanDEM-X mission uses both satellites’ resources at once. With the high geometric accuracy of both satellites, the geolocation of ground targets has to accurately consider signal propagation effects and geodynamic effects such as solid earth tides and the continental drift which were formerly negligible [1] [2] [3]. Previous studies which took these effects carefully into account showed an absolute pixel localization at the centimeter level for TSX-1 and TDX-1 [4] [5] [6]. However, recent measurements show that in range, under extraordinary good conditions, a
978-1-4673-1159-5/12/$31.00 ©2012 IEEE
Figure 1: Corner reflector at the geodetic observatory Wettzell, Germany.
1625
IGARSS 2012
Figure 2: Close-up of the Wettzell corner reflector imaged by TerraSAR-X. reflectors in focused SAR images are compared with their expected values obtained from precise GPS measurements of their positions and estimated propagation delays. The conversion of the spatial GPS coordinates into expected radar time coordinates is based on zero Doppler equations [7] and orbit interpolation of the satellite’s position during acquisition. Our recent measurement series is based on a trihedral corner reflector with 1.5 meters edge length which we installed at the geodetic observatory at Wettzell, Germany (see Figure 1 and Figure 2). Thus, we benefit from the very close distance (about 240 meters) to the local EUREF [8] GPS reference station. The ground position of the corner reflector which is defined by its phase center (see Figure 1, point number 401), is known very precisely (
Remote Sensing Technology Institute (IMF), German Aerospace Center (DLR), D-82230 Wessling, Germany 2
Technical University Munich (TUM), D-80333 Munich, Germany location accuracy of just a few millimeters seems to be feasible. Here, we present our measurement results.
ABSTRACT The German SAR (synthetic aperture radar) satellites TerraSAR-X (TSX-1) and TanDEM-X (TDX-1), launched in June 2007 and June 2010 respectively, provide an unprecedented geometric accuracy. Previous studies showed an absolute pixel localization for both sensors at the centimeter level [4] [5] [6]. However, recent measurements show that in range, under extraordinary good conditions, a location accuracy of even a few millimeters seems to be attainable. While on a long-term scale, we observed a slow variation of subsequent measurements; on a short-term scale, they coincided to within a few millimeters. The measurement series will be continued. The cause of the long-term variation is the subject of current investigation. Index Terms— Synthetic aperture radar, TerraSAR-X, absolute localization accuracy, imaging geodesy
2. MEASUREMENT METHOD Radar systems indirectly measure geometric distances by means of the travelling time of radar pulses from the radar transmitter to ground and back to the radar receiver. Usually, the conversion from travelling time to geometric distance refers to the vacuum velocity of light. However, electrons in the ionosphere, dry air and water vapor in the troposphere introduce additional signal delays which have to be taken into account. At the level of geometric accuracy of the TerraSAR-X mission, geodynamic effects like solid earth tides and continental drift shift the true position of a ground target by several centimeters over the course of a day or years, respectively [1]. In order to verify the pixel localization accuracy of TSX1 and TDX-1, the radar range and azimuth times of corner
1. INTRODUCTION The German SAR satellites TSX-1 and TDX-1, launched in June 2007 and June 2010 respectively, outperform former civilian space-borne radar sensors in terms of geometric accuracy. Since 2010, each satellite supports the TerraSARX mission equally. In contrast, the bistatic interferometric TanDEM-X mission uses both satellites’ resources at once. With the high geometric accuracy of both satellites, the geolocation of ground targets has to accurately consider signal propagation effects and geodynamic effects such as solid earth tides and the continental drift which were formerly negligible [1] [2] [3]. Previous studies which took these effects carefully into account showed an absolute pixel localization at the centimeter level for TSX-1 and TDX-1 [4] [5] [6]. However, recent measurements show that in range, under extraordinary good conditions, a
978-1-4673-1159-5/12/$31.00 ©2012 IEEE
Figure 1: Corner reflector at the geodetic observatory Wettzell, Germany.
1625
IGARSS 2012
Figure 2: Close-up of the Wettzell corner reflector imaged by TerraSAR-X. reflectors in focused SAR images are compared with their expected values obtained from precise GPS measurements of their positions and estimated propagation delays. The conversion of the spatial GPS coordinates into expected radar time coordinates is based on zero Doppler equations [7] and orbit interpolation of the satellite’s position during acquisition. Our recent measurement series is based on a trihedral corner reflector with 1.5 meters edge length which we installed at the geodetic observatory at Wettzell, Germany (see Figure 1 and Figure 2). Thus, we benefit from the very close distance (about 240 meters) to the local EUREF [8] GPS reference station. The ground position of the corner reflector which is defined by its phase center (see Figure 1, point number 401), is known very precisely (
