International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004
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Figure 7. Dense GCPs around Tsukuba, Japan
assumed descending orbit when PRISM is operated normally in
the daytime and the ground elevation is assumed to be 0 meters.
The variability of distance between each radiometer at the same
time is more than 10 km in the recurrent orbit.
3.2 Dense GCP to Evaluate the Relative CCD Alignments
First, we evaluate the relative CCD alignment in each telescope
after launching the ALOS because that is the affected geometric
sensor model to process the standard product. At least one GCP
for each CCD is required to evaluate the relative alignment.
Figure 7 illustrates the location of the dense GCPs around
Tsukuba City, in the northern part of Tokyo, Japan. Three lines
(67 to 69) indicate the ALOS paths based on the Reference
System of Planning (RSP). This dense GCPs area cover a 100 x
40 km area with a Skm step, and will be used to evaluate the
relative alignment of CCD as well as the pointing determination
accuracy within 5 seconds, which corresponds to one scene in a
PRISM image. This can also be used to validate the high-level
products such as the generated DEM and ortho-rectified image.
The eastern part of this area (40 x 40km) was established by the
Geographical Survey Institute (GSI) of Japan using 1:2,500-
scale urban planning maps (blue dots in Figure 7). We prepared
the western part of the area (60 x 40km) measured by real-time
differential GPS (DGPS) survey (pink dots in Figure 7). One
GCP is defined by latitude and longitude in the International
Terrestrial Reference Frame 1997 (ITRF 97), height on the
GRS 80 ellipsoid model, and an aerial photograph or available
high-resolution satellite image such as IKONOS as the chip
image.
3.3 Along Path GCP to Evaluate Radiometers Alignments
The alignments between each radiometer are important for
evaluating the geometric sensor model, as well as for internal
orientation to generate DEM using PRISM's stereo pair images.
The simultaneous observations of GCP by each radiometer are
necessary to evaluate the alignment between each radiometer
after ALOS launch. Using the observed images in the same
orbit to reduce the effects of thermal condition variability is
also convenient.
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Figure 8. Along-path GCP in Japan
Figure 8 depicts an along-path GCP used to evaluate the
alignments between each radiometer in Japan: and green, blue
and pink lines indicate RSP 67, 68 and 69. When the nadir-
looking radiometer observes around the Iwate area (red dots in
Figure 8), the forward-looking radiometer can observe around
Tochigi (blue dots), and the backward-looking radiometer can
observe around Tomakomai (sky blue dots) in a descending
orbit. The distance between Tochigi and Tomakomai is 614 km,
which corresponds to the results of Figure 6. We first
considered the Tsukuba area in Figure 8 as one GCP area for
this purpose. However, the backward-looking radiometer is
located over the ocean in this case, so GCPs were moved
slightly north. The Iwate area was originally located in a
mountainous area. We had to prepare GCPs near urban areas.
Unfortunately, as a result, the Tomakomai area has a gap of one
PRISM scene in order to obtain observations at the same time.
The Iwate area was measured by DGPS as 25 x 70 km with a 5
km step, which can also be used as a dense GCP. The
Tomakomai area was measured by DGPS as 10 x 70 km with a
5 km step. The Tochigi area was measured by dual-frequency
static GPS with post processing, and covered 25 x 70 km with a
5 km step.
Photo | shows an example of static GPS measurement. One
point was measured in two hours. These GCP areas can be used
to evaluate the alignment between radiometers and to validate
generated DEMs, pointing determination accuracy, pointing
stability, and attitude control accuracy from 5 to 91 second,
which corresponds to the triple observation mode.
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