hard disk of a PC are fully used with the archived image
data, an (low cost) external memory device such as an
exabyte tape driver is used to store image data. The next
section will describe the development of this PC interface
card in detail.
After the incoming data are recorded in a hard disk of a
PC, they need further processing. They contain not only
image data but also auxiliary data and frame
header/footer. A proper image data and auxiliary data
need to be extracted and restored in a pre-defined format.
This process is called a frame synchronization procedure.
In some ground receiving stations for commercial remote
sensing satellites, this procedure is done by a dedicated
hardware. However, as emphasized earlier, the speed of
the KITSAT-3 eliminates the need of a dedicated
hardware. Frame synchronization can be done by
software.
The use of dedicated software for frame synchronization
reduces the cost of the total receiving station. This
approach also has many other advantages compared to
the conventional systems: the system can be easily
maintained and error-handling is simple; the system can
be easily modified to handle image data from other
satellite as the frame synchronization is done through
software; the system upgrade can be easily done with
the fast development of computer hardware technologies.
In the next section, the development of this frame
synchronization software will be described.
After frame synchronization, image data are stored in a
hard disk of a PC. This image data is “raw” image of the
KITSAT-3. Further preprocessing, such as radiometric
correction and/or geometric correction, can be performed
as requested (see Lee et al.). The raw image data are
displayed using a Moving Window Display so that
operator can validate the quality of incoming image data.
After frame synchronization, image data is properly
recovered and stored into the hard disk of a PC. To these
image data , further processes such as geometric or
radiometric correction can be applied. In figure 1, high
speed recording, frame synchronization, and moving
window display processes are performed at the same
time as the antenna receives the signal from the satellite
(on-line). External archiving is performed when the
microprocessor of a PC is idle (off-line).
3. The development and Implementation of the
KITSAT-3 Direct Archiving and Preprocessing
System
3.1. The direct archiving recorder - development of a
PC interface card
As explained earlier, the problem of developing a
recorder using PC can be simplified as designing a PC
interface card and developing a software to control I/O
operation of a PC. The development of software is simple
as there are many built-in I/O routines. This subsection
will concentrate the development of a PC interface card.
86
The followings are the definition of the input and output of
the PC interface card.
Table 2. The definition of input and output for the PC
interface card
Input A: Two serial input (I, Q channels) with a speed of 3
Mbps.
Input B: One serial input(l, Q combined)
Clock A: A clock signal synchronous to the |, Q channel
input
Clock B : A clock signal synchronous to the |, Q
combined input
Output A : A word of 16 bits (After combining | and Q
channels to a meaningful word)
Output B : A word of 8 bits
The input signal is created from the KITSAT-3 receiver
(Le., the down converter and bit synchronizer of the
KITSAT-3 receiving station) as shown in figure 1. It is
assumed that there are two possibilities of input signal.
One possibility is to have two serial input streams with the
speed of 3 Mbps. As the KITSAT-3 uses Quadratic Phase
Shift Keying (QPSK) scheme, the output signal from the
receiver can be two serial input of | and Q channels. The
other possibility is to have one serial input stream. In this
case, the KITSAT-3 receiver combines | and Q channels
internally. When the input to the interface card is two
serial streams, the output from the interface card is a 16-
bit parallel word. A “read” command stores a 16-bit word
into the hard disk of a PC. When input is one serial
stream, the output is a 8-bit parallel word.
The incoming clock signal is assumed to be synchronous
to the input streams. However, in order to ensure the
operation of the interface card when the KITSAT-3
receiver cannot provide accurate clock signal, an internal
clock generator is also included. A block diagram for the
PC interface card is shown in figure 2.
Let's consider one input stream first. As the KITSAT-3
uses a scrambling algorithm for transmission, the input
stream needs to be de-scrambled. The de-scrambling
algorithm is dependent on the scrambling algorithm used
onboard. After de-scrambling, the serial stream is
converted into a parallel word using a First-In-First-Out
memory device. For this, a FIFO IC device is used. One of
the most suitable devices for this purpose is the IDT72103
FIFO memory chip. This can handle fast input signals and
convert them into parallel words with varying bit-width.
This can store the parallel words of upto 4Kbytes in its
internal memories. This also provides various flags
indicating the status of its internal memories.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
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