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SYNTHETIC APERTURE RADAR (TOPSAR)
is an airborne interferometric radar system that can acquire topographic
data in 10-km wide swaths with a single pass of the aircraft. Generally,
cloud cover does not affect the system, and the aircraft collects data
at a speed of 550 knots at a height of 8,500 m above ground level. TOPSAR
normally has a data spacing of 10 m horizontally and 0.1 m vertically,
with vertical accuracy of about 0.5 m. Thus, TOPSAR has lower resolution
and accuracy than ALTM, but it can more rapidly survey large cloudy regions.
experimenting with c-band (radar wavelength of 5.7 cm) TOPSAR to acquire
digital elevation models of coastal zones and integrating them with topographic
data from ALTM surveys. In June 1996, we acquired
imagery along Galveston Island and Bolivar Peninsula.
images to enlarge
1. NASA's Dryden Flight Research Center and the Jet Propulsion
Laboratory (JPL) operate the DC-8 Airborne Laboratory and the
Two separate radar antennas allow topographic data to be collected
in a single pass of the aircraft. The difference in arrival
times of the return signal at the two antennas can be related
to the relative height of the target because the positions of
the antennas are known. During nominal operation, one antenna
transmits while both receive. In "ping-pong" mode,
however, both antennas transmit and receive. The phase noise
increases in ping-pong mode, but the vertical resolution also
increases roughly by a factor of 2. Only the ping-pong mode
is capable of resolving the major topographic features in the
Texas coastal zone.
3. TOPSAR digital terrain model (DTM) of Bolivar Peninsula acquired
in June 1996 by NASA/JPL. Horizontal resolution is 10 m . Approximately
1 million data points describe this barrier spit, which has
a total relief of only about 5 m. The view, looking northeast,
shows the southwest portion of the peninsula, which is 16 km
This is a TOPSAR shaded-relief topographic image of the same
portion of Bolivar Peninsula. Despite TOPSAR's low resolution
and high RMS, many of the cultural and geographic features visible
in the ALTM image are discernible in
the TOPSAR DTM. Dunes, beach ridges, elevated roadbeds, and
other features with as little as 1 m of relief are identifiable.
This contoured surface depicts the differences between the ALTM
and TOPSAR elevations. We averaged ALTM elevations over 10 x
10 m squares and then subtracted the corresponding TOPSAR elevation.
The contoured residuals show that TOPSAR elevations tend to
be 0 to 1 m above the ALTM elevations along the Gulf shoreline
but drop to 1 to 3 m below the ALTM in center of the peninsula.
This down-range departure of the TPSAR from the ALTM is consistent
throughout the study area and suggests a systematic error in
6. This plot compares topographic measurements across an undeveloped
portion of the peninsula (C-C'). (see fig.
5 for location). The beach profile extends from the waterline
across the beach, foredune, back dunes, and the barrier flat.
We conducted a rapid-static GPS survey to measure this beach
profile, then sorted the ALTM data for laser points that fell
within +/- 2 m of the transect line. The land surface is covered
by a variety of grasses and low shrubs that grow up to 0.5 m
in height. The scatter in ALTM topography therefore reflects
height error, real topographic variations within +/- 2 m of
the beach profile line, and random reflections of vegetation.
more information, please contact Jeff Paine