ographic obtained ptan(8/2)/2. Similarly, the resolving power z is
given by p/nc. Therefore, with substitution,
corresponding accuracy would be 10-40 arcseconds.
Usually, attitude requirements are not that stringent because
differential attitude 1s derived from the radar ranges.
ograph is R > (2d tan B/2)/nc (2b)
] resolving Recently, methods have been proposed to use SAR
ME For example, a CCD camera with a 28 mm lens and a CCD interferometry for topographic height determination. In
te ; 3 ; x :
Kd T av array of dimension 30 mm by 30 mm with 4K by 4K CCD this case, pairs of SAR images are used which have been
T , elements. When used at a flying height of 1000 m, R> 0.25 acquired from the same sensor flown on parallel flight lines
Ol image at approximately the same flying height. Instead of using a
(Lillesand m. Increasing the focal length or the number of CCD
elements per mm, will result in increased resolution and
increased storage rates. For the example given above, the
storage rate 1s 1.6 MB per second at an aircraft speed of 100
radargrammetric approach (Leberl, 1990) for height
determination, the phase differences of the SAR image pairs
are used to produce interferograms which form the basis for
he distance
the spatial
say qe knots and 50% overlap. This is the limit for current hard nan ; WACH J a: NUN radar
ble. The ANSE T S mier meson why Hs a EE a Ei
prised in BED: hizve.nou been used in arbore applications, with the flight path parameters to resolve the terrain height
> line pairs i imi i i i
lity T the If the aircraft flight speed 1s 100 knots, and the shutter speed HAE a T 3 a She m Single (mass resolution
contribute is 1/250 seconds, the smearing caused by aircraft movement required positional accuracies would be about 2-4 m. Due to
ver of a without any compensation is 0.1 m at the ground level. This the use of dual ranges, the attitude accuracy requirements
black and vil degrade ihe spatialeresolution of Senso systems along would be reduced if relative heights were required. The
power of the flight direction, but can be minimized by using high = determination of absolute terrain heights to 2 m would
nm to 100 Keuacy velocity :Guiput/of.dhe: zeareferencingosensors, require attitude accuracies of about 10-40 arcseconds. SAR
> a number Because CCD cameras do not have the geometrical properties systems need position for georeferencing, and precise
oorer than of aerial photographs, both attitude and position rare needed velocity for motion compensation in real time. Attitude is
fot seoreferencing: pushbroom Scanners and linear array required in radar interferometry but may be optional and
systems. Whether or not motion compensation is radargrammetry.
(1) necessary, depends on the accuracy requirements of a specific
application. 3.4 Summary of Results
s) for the t
hotograph 33 SAR Accuracy Requirements Table 2 summarizes the highest georeferencing requirements
wer of 50 for the three types of remote sensing systems discussed
s 0.1 mor Synthetic-aperture radar (SAR) is different from the sensors above. It indicates that they are ordered in terms of
ements are discussed so far because it requires high precision velocity increasing complexity with respect to the georeferencing
ond to 15 input in real time to compensate for aircraft motion. In requirements. While accurate positioning is usually
individual particular, the SAR imagery is realized as a result of an sufficient for photogrammetric sensors, position and
accumulation of several "looks' which are acquired while the attitude 1s required for pushbroom scanners and CCD frame
aircraft is in motion. The accumulated signal is used to imagers, and position, attitude, and velocity is needed for
used in a minimize the inherent speckle of radar imagery. Since the SAR systems.
istribution velocity requirements are very stringent, it is often
th of the overlooked that the final accuracy of the radar map depends Georeferencing Accuracy
1g can be heavily on the georeferencing accuracy. Currently, Required
nt attitude georeferencing is done by interpolating between GPS ground Tvpe. of
; will take control and by matching visually identified features. AAW Position lAttiiude Velocit
ixing the M
even high Design specifications will be taken from the STAR-1 system Photo-Camera
because it is currently the only system that operates : " 40"
commercially. The T bed non of the STAR.1 (low flying A Poke:
Systems system include a nominal flying altitude of about 10 km to altitude)
provide swath coverage and high fuel efficiency. Operating nus ax
in the X-band of the electromagnetic spectrum, the STAR-1 Digital 0.25-1.0 m 1'-3 1-2 cm/s
ifluencing System can map on either side of the aircraft with a 90? Scanning
CD frame orentation to the flight path. The corresponding azimuth Systems
has been resolutions of the wide swath (WS) and high resolution (HR) (CCD)
5 (CCDs). modes are 12 m and 6 m, respectively. The radar may use up
nsiderably lo seven looks in azimuth by one in range per swath in both SAR Systems 2-4m (10"-40") | 0.02-0.05
ng factor modes. It is designed for a maximum ground speed of 350 cm/s
Xtographic knots. The pulse width is 30 microseconds and is operated at
lem based a pulse repetition frequency (PRF) of 1200 Hz with a
ft iss maximum duty cycle of 0.036 (repeat time). Table 2: Imaging Sensor Accuracies
Correspondingly, velocity measurements with an accuracy
(2a) of 0.0002-0.0005 m/sec are required to precisely correct the
multi-look averaging process. 4. GEOREFERENCING MODEL
e sensors,
array, the While positional accuracies of current radar maps are at the Georeferencing describes a series of transformations
era field- level of 15-20 m, georeferencing requirements should be necessary to obtain coordinates in a chosen mapping system
mately be based upon a 6 m resolution and a seven look sampling. (m) from the output of a remote sensing device in the body
This implies that accuracy requirements in position are about frame (b) of the aircraft. The major steps in this
24 m. If independent attitude is needed for each swath, the
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