The implementation of frame synchronization software is
straight forward as long as the structure of a frame is
determined. Therefore this subsection will concentrate
the structure of frames of the KITSAT-3 image data.
There are two transmission mode for the KITSAT-3. The
full transmission mode is to transmit the image ‚data
stored in the solid-state mass memories into the ground
station. The transmission is done in a full scale (no
reduction). The sub-sampled transmission mode is to
transmit sub-sampled image data (by the factor of 5) into
the ground station. This mode is used to monitor in real-
time the quality of images which the onboard camera
takes.
In the sub-sampled transmission mode, a data frame is
defined as table 4. A sub-sampled data frame starts with
a synchronization word of 4 bytes. Frame
synchronization software uses this word to define the
starting point of a data frame out of the incoming bit
Stream. Scene number is to distinguish image data when
the image data transmission is performed over multiple
scenes (The KITSAT-3 can take upto 20 scenes at one
time). Frame type is to distinguish whether a data frame
is for the sub-sampled image data frame or other types.
A sub-sampled data frame has the total size of 4327
bytes.
Table 4. Sub-sampled image data frame.
Data type Size
Synchronization word 4 bytes
Scene No. and frame type | 6 bits + 2 bits, i.e. 1 Byte
Frame No. 2 Bytes
Sub-sampled image data 4320 Bytes
Total 4327 Bytes (34616 bits)
In the full transmission mode, there are two types of data
frame. One is a full image data frame and the other is a
telemetry data frame. The structure of each frame is
shown in table 5 and 6.
The total size of a full image data frame or a telemetry
data frame is same as that of a sub-sampled image data
frame. However, in a full image data frame, the size of
image data is 3456 bytes, which corresponds to the
amount of data collected from one line of the KITSAT-3
CCD array. Unlike a sub-sampled image data frame, a
full image data frame and telemetry data frame contain
an error detection code for more accurate transmission.
In a full image data frame, 2 bits are assigned to indicate
the spectral band of an image (RGB). Frame number is
used to distinguish each full image data frame. As a
KITSAT-3 image scene is defined as 3456 pixels by 3456
pixels, 3456 image data frames are combined to create a
scene in a single spectral band.
Table 5. A full image data frame
data type Size
Synchronization word 4 bytes
Scene no. and frame | 6 bits + 2 bits, i.e., 1 byte
type
RGB and Frame No. 2 bits + 12 bits, i.e., 2 bytes
Image data 3456 bytes
Error Detection code | 864 bytes
(1:4) :
Total 4327 bytes (34616 bits)
Table 6. A telemetry data frame
data type size
Synchronization word 4 bytes
Scene No. and frame | 6 bits + 2 bits, i.e., 1 byte
Type
Frame No. 2 bytes
Telemetry Data 3456 bytes
Error Detection code | 864 bytes
(1:4)
88
Total 4327 bytes (34616 bits)
Commercial remote sensing satellites normally have a
frame structure which has image data and telemetry data
in the same frame. However, telemetry data of the
KITSAT-3 is transmitted separately as a telemetry data
frame. This is due to hardware limitations of the KITSAT-3
onboard imaging instrument. The telemetry data sampling
rate of the KITSAT-3 is not high (5~10 Hz) and the amount
of telemetry data for a scene is very small. One telemetry
data frame is enough to cover whole telemetry data for an
image scene.
After the data from the PC interface card are converted
into a appropriate format, they are restored into the hard
disk of a PC. The proper image data are then used for
further processing such as geometric or radiometric
correction.
4. Conclusions
So far, this paper briefly described the development of a
low cost direct archiving system and a frame
synchronization software. The developed system will be
used for the archiving and preprocessing of the KITSAT-3.
The motivation of this development was to design a low
cost ground receiving station for small satellites. The use
of dedicated hardware was therefore abandoned and
dedicated software with commercial PC was used instead.
The techniques used for this development are not very
complicated and some of them are even very simple. This
shows the advantage of using small satellites in the sense
that the ground receiving station can be developed easily.
Compared to other commercial satellites, the transmission
speed for the image data from this satellite is low. This
may have made it easier to develop the low cost direct
archiving and preprocessing system. However, at least
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
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