2 Physical Principles and Technical Aspects of Remote Sensing
37
over the snow- and ice-free land surfaces. Over water the signal decorrelates
completely, over glaciers and snow-covered areas the degree of coherence was
reduced due to surface melt, but still sufficient for calculating the interferogram. One colour cycle (one fringe) in the interferogram represents a phase
shift of 2 1f between the two images. This corresponds to a differential motion of 21.7 cm on a horizontal surface. As derived from the distance of 29
m between the antenna positions of the two images and the wavelength, one
fringe corresponds to an altitude difference of 550 m for a non-moving target. Because only one interferogram was available, the topographic phase was
calculated from digital elevation data in order to generate the motion map
of the glacier terminus. Only the motion component in direction of the radar
beam (range) can be obtained by means of interferometry. Therefore the direction of motion on the glacier was derived from flowlines in optical and
SAR images.
2.6 Summary of Current and Future Earth Observation
Missions
Many earth observation instruments are presently flown on satellites or are
due to be launched in the next few years. A selection of un-manned earth
observation missions is listed in Appendix 20.1 and sensor characteristics are
specified in Appendix 20.2. Comprehensive information on satellites and sensors is provided by Kramer (1996). The majority of the satellites has nearpolar orbits, with the lowest altitudes (around 250 km) for photographic
camera missions, and typical orbital altitudes between 700 km and 900 km
for the other missions. Geostationary satellite series, such as GOES and Meteosat, are imaging the earth disk in close time intervals (typically 1/2 hour)
from an altitude of 35800 km, but are not covering the high latitudes. Important experimental earth observation missions have been flown on manned
spacecrafts, including the NASA Space Shuttle and the Russian space station
MIR. Of particular interest for the development of SAR applications were the
Spaceborne Radar Laboratory Missions SRL-1 and SRL-2 in 1994 with the
multi-frequency (L-, C-, X-band) polarimetric SIR-C/X-SAR.
Sensors with medium spatial resolution (1-5 km) on polar orbiting and
geostationary satellites, such as AVHRR on POES (NOAA), VISSR on GOES,
and MVIRI on Meteosat, enable global imaging with high temporal resolution. Data from various of these sensors can be received ill real time with
reasonable technical efforts, making them very useful tools for operational
meteorology and hydrology. There is also a broad choice of high resolution
optical sensors, ranging from photographic cameras to various types of multispectral scanners. Data reception for these sensors is usually restricted to
special ground stations. Some of the agencies (e.g. SPOT Image) offer possibilities for fast delivery. Cameras are flown on a number Russian satellites
in low orbits; the spatial resolution and areal coverage varies with the or-
37
over the snow- and ice-free land surfaces. Over water the signal decorrelates
completely, over glaciers and snow-covered areas the degree of coherence was
reduced due to surface melt, but still sufficient for calculating the interferogram. One colour cycle (one fringe) in the interferogram represents a phase
shift of 2 1f between the two images. This corresponds to a differential motion of 21.7 cm on a horizontal surface. As derived from the distance of 29
m between the antenna positions of the two images and the wavelength, one
fringe corresponds to an altitude difference of 550 m for a non-moving target. Because only one interferogram was available, the topographic phase was
calculated from digital elevation data in order to generate the motion map
of the glacier terminus. Only the motion component in direction of the radar
beam (range) can be obtained by means of interferometry. Therefore the direction of motion on the glacier was derived from flowlines in optical and
SAR images.
2.6 Summary of Current and Future Earth Observation
Missions
Many earth observation instruments are presently flown on satellites or are
due to be launched in the next few years. A selection of un-manned earth
observation missions is listed in Appendix 20.1 and sensor characteristics are
specified in Appendix 20.2. Comprehensive information on satellites and sensors is provided by Kramer (1996). The majority of the satellites has nearpolar orbits, with the lowest altitudes (around 250 km) for photographic
camera missions, and typical orbital altitudes between 700 km and 900 km
for the other missions. Geostationary satellite series, such as GOES and Meteosat, are imaging the earth disk in close time intervals (typically 1/2 hour)
from an altitude of 35800 km, but are not covering the high latitudes. Important experimental earth observation missions have been flown on manned
spacecrafts, including the NASA Space Shuttle and the Russian space station
MIR. Of particular interest for the development of SAR applications were the
Spaceborne Radar Laboratory Missions SRL-1 and SRL-2 in 1994 with the
multi-frequency (L-, C-, X-band) polarimetric SIR-C/X-SAR.
Sensors with medium spatial resolution (1-5 km) on polar orbiting and
geostationary satellites, such as AVHRR on POES (NOAA), VISSR on GOES,
and MVIRI on Meteosat, enable global imaging with high temporal resolution. Data from various of these sensors can be received ill real time with
reasonable technical efforts, making them very useful tools for operational
meteorology and hydrology. There is also a broad choice of high resolution
optical sensors, ranging from photographic cameras to various types of multispectral scanners. Data reception for these sensors is usually restricted to
special ground stations. Some of the agencies (e.g. SPOT Image) offer possibilities for fast delivery. Cameras are flown on a number Russian satellites
in low orbits; the spatial resolution and areal coverage varies with the or-