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Field Service Procedure – Replacement PCU Kit, ST88, ST94 & ST144
Page 7 of 22
Document No
137738 Rev A
Copyright © Sea Tel, Inc 2012 - The information contained in this document is proprietary to Sea
Tel, Inc. This document may not be reproduced or distributed in any form without prior written
consent of Sea Tel, Inc.
12. Drift:
If a rate sensor is “drifting”, its nominal voltage output has changed from the correct 2.5VDC. This will cause error in the
axis control loop and cause the antenna to move. It is more common to see this drift in the azimuth axis as the CL and EL
axis both have a long term reference, provided by the tilt sensor. However, should the rate sensors drift be large enough,
it can over ‐ compensate the tilt sensor and the system will drive into one of the CL or EL end stops.
To verify if the system is drifting in azimuth, turn tracking off and monitor the relative position. Under static conditions,
when the vessels heading is unchanged, the relative count should remain stationary. If the relative value begins to
change from its current position, the azimuth rate sensor is drifting, introducing error into the axis control loop.
If the rate sensors are checked as OK, a ships gyro can induce drift. This will introduce loop error into the PCU control
loop and cause the antenna to drift. This can be verified by checking the heading displayed on the DAC against the ships
actual heading. The two headings should be the same. If there is a difference, enable Sat Ref Mode to temporarily bypass
the ships gyro to prove the fault. If enabling Sat Ref Mode does prevent drift, verify the heading source.
13. Further Diagnostics:
If any of the above checks have indicated a problem, one or more of the axis control loops have incorrect loop errors. The
most likely cause of this is a defective rate sensor. To rectify this, replace the motion platform PCB and repeat the checks
above, to verify the antenna now functions correctly.
Should the problem persist, there are other possibilities that can produce the same results. These can be bad
connections between the motion platform PCB and PCU motherboard or the PCU itself.
14. 400MHz Modem LED Indicators:
For diagnostic purposes, the 400MHz FSK Modem Assemblies have an LED Indicator (located on the bottom left hand
side of the Enclosure for BDE modems and directly underneath the Rotary Joint port on the 09 Series PCU). By observing
the amount of amber colored flashes during power up, the modems configuration may be established. You can also
verify the communications link between above decks and below decks modems themselves. Refer to the below list for
an explanation of the different LED states.

Upon power up, the modems’ LED will flash amber. The number of flashes indicates the dash number configuration
of the modem. Refer to the configuration on page 8 of this document for the appropriate dash number
configuration for your modem assembly.

Solid green indicates dual channel communications lock between modems (i.e. there is enough signal being
received to establish communications).

Red and Green alternating LEDs indicates a single channel failure (i.e. there is low RSSI signal strength on one
channel).

A flashing Red LED indicates no communication between both modems (2 failed channels), (i.e. there is low RSSI
signal strength on both channels).

Solid RED ‐ Modem fault detection, hardware or software failure of the modem.

Solid ORANGE ‐ Software update to the Modem in progress.