Sea Tel ST144-21, Installation And Operation Manual

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ST144-21 C/Ku-Band TVRO Maintenance and Troubleshooting
13-11
13.6.3.1.8.
•
BDE Modem Rx Port Failure (Not receiving at 460.0MHz) or
BDE Receive or ADE Transmit (RF M&C):
•
ADE Modem Tx Port Failure (Not transmitting at 460.0MHz)
o
Install Spectrum Analyzer in line with the Rx IF coax path.
1. If 465.0MHz Transmit Beacon in present, the fault is the BDE
modem.
2. If 465.0MHz Transmit Beacon is NOT present, the fault is with
the ADE modem.
13.6.3.1.9.
•
BDE Modem Tx Port Failure (Not transmitting at 447.5MHz) or
BDE Transmit Or ADE Receive (Radio M&C):
•
ADE Modem Rx Port Failure (Not receiving at 447.5MHz)
o
Install Spectrum Analyzer in line with the Rx IF coax path.
1. If 465.0MHz Transmit Beacon in present, the fault is the BDE
modem.
2. If 465.0MHz Transmit Beacon is NOT present, the fault is with
the ADE modem.
13.7. Troubleshooting
13.7.1.
The antenna system is mounted on a three axis stabilization assembly that provides free motion with 3
degrees of freedom. This assembly allows the inertia of the antenna system to hold the antenna pointed
motionless in inertial space while the ship rolls, pitches and yaws beneath the assembly. Three low friction
torque motors attached to each of the three free axes of the assembly provide the required force to
overcome the disturbing torque imposed on the antenna system by cable restraints, bearing friction and small
air currents within the radome. These motors are also used to re-position the antenna in azimuth and
elevation.
Theory Of Stabilization Operation
The Pedestal Control Unit (PCU) uses inputs from the level cage sensors to calculate the amount of torque
required in each axis to keep the antenna pointed /-0.2 degrees. The primary sensor input for each
loop is the rate sensor mounted in the Level Cage Assembly. This sensor reports all motion of the antenna to
the PCU. The PCU immediately responds by applying a torque in the opposite direction to the disturbance to
bring the antenna back to its desired position. Both the instantaneous output of the rate sensor (Velocity
Error) and the integrated output of the rate sensor (Position Error) are used to achieve the high pointing
accuracy specification.
The calculated torque commands are converted to a 5 volt differential analog signal by a Digital to Analog
converter (D/A) and sent to each of three Brush-Less Servo Amplifiers. These amplifiers provide the proper
drive polarities and commutation required to operate the Brush-Less DC Servo Motors in torque mode. The
Torque acting on the mass of the antenna cause it to move, restoring the rate sensors to their original
position, and closing the control loop.
Since the rate sensors only monitor motion and not absolute position, a second input is required in each axis
as a long term reference to keep the antenna from slowly drifting in position. The Level and Cross Level
reference is provided by a two axis tilt sensor in the level cage assembly. The Azimuth reference is provided
by combining the ships gyro compass input and the antenna relative position.
13.7.2.
Refer to the System Block Diagram in the Drawings section of this manual. The feed has a 24VDC motor to
rotate the body of the OMT to optimize the linear polarization angle of the LNBs to the polarization angle of
the signal coming from the targeted satellite. The 24VDC motor is remotely controlled by the ACU (Manual
OR Auto-Polarization) through the PCU and Shielded Polang Relay Assy.
ST-21 Series Dual C-Band OR Quad Ku-Band TVRO RF Flow
Two fixed frequency C-Band LNBs and one Quad Ku-Band LNBs are installed. Both C-Band polarizations (H &
V) are routed to J1 of the coax switches mounted on a C/Ku Switch Panel. Both Ku-Low Band polarizations
(H&V) are routed to J2 of the coax switches. The coax switches are controlled from the ACU MODE –
TRACKING Band Selection through the PCU and the Shielded Polang Relay Assy. The ACU band selection will
route either the C-Band, OR the Ku-Low Band, signals through two of the channels of the rotary joint. The
other two channels of the coax rotary joint are the (un-switched) Ku-High Band outputs of the Quad Ku LNB.