SeaSpace TeraScan 1.5M, Operation And Maintenance Manual

Page: 103 / 232

TeraScan 1.5m Polar Satellite Tracking Antenna O & M Manual
87
Section 6: How it Works
6.1 S
ATELLITE
T
RACKING
Once the antenna has been powered up, calibrated, and aligned, it runs unattended. Satellite passes are
scheduled to be acquired (either manually or automatically). When it is time to acquire the data, the TeraScan
program
antenna is executed, with the antenna, telemetry, start time, and duration passed to the program. The
tracking sequence is stored in the file
TRACKSxx
(
xx
= chain number) in the directory
$PASSDIR/antenna
.
The tracking sequence consists of a fixed time series of azimuth, elevation, and cross-level commands. The time
between each command is usually 0.5 seconds. The sequence is constructed to minimize the axis acceleration
in all three axes. The antenna positioner is a three-axis positioner (elevation over cross-level over azimuth)
driven by servo-controlled DC torque motors.
The antenna is pointed at the first position in the tracking sequence prior to the beginning of the pass. As the
real-time moment occurs, the command from the tracking sequence is sent to the antenna controller (PCU). The
antenna controller (PCU) drives the tracking positioner to point the dish. This is program track mode.
As data is acquired by the feed, it is amplified by the LNA and downconverted to a lower frequency (usually
centered at 137.5 MHz) by the downconverter. The signal is then sent down the coaxial cable to the receiver
located at the computer. The band received by the feed (L-Band or S-Band) can automatically be selected on the
feed.
When the satellite pass is over and the tracking sequence has been completed, the antenna is "stowed". The
stowing is optional and is selected in the file: $PASSDIR/antenna/antenna.calib.
6.2 S
ERIAL
C
OMMUNICATIONS WITH THE
A
NTENNA
To output each tracking command to the antenna, the following sequence occurs (see drawing 70909030,
page 37 for an illustration of hardware components discussed):
1. The command is issued on the computer by the antenna program to the serial port attached to the antenna as
defined in the file
$PASSDIR/ system.config
or
$PASSDIR/config/ system.config
, setting "antenna.device".
Baud rate: 9600, no polarity, 8 data bits, 1 stop bit.
2. The serial port used is usually one of the ports on a serial multiplexer card in the computer chassis.
3. The serial data is output from this port to an RS232-to-RS422 converter. The converter may be an external
component, part in the process chassis, or a part of the HR300 receiver.
4. The serial command is sent to the PCU.
5. The PCU executes the command and issues a reply. The reply travels the same path as the command in reverse
and is sent to the
antenna program.