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USA 179


Tyneside, UK
2018 Jul 17
Tuesday, Day 198

Maintained by:






USA-179 Analysis:









VHF Signal Switching

Before the arrival of USA 198 on orbit, USA 179 had sole use of a pair of VHF frequencies providing some sort of communication link through one of its payloads. It is possible that the link represents a replacement for AFSATCOM as carried by both of its predecessors. When USA 198 arrived, the two satellites started to share the workload thereby bringing to an end an interesting period where orbital information could be derived from the switch on/off times of the VHF transmissions.

Geographical Location of VHF Signal Switching

Below is a list of four successive timings of USA 179's VHF transmitter activity. They were made by Richard Flagg in Hawaii and Bob Christy in the UK and cover 2006 November 16 and 17 with the two Hawaiian observations straddling midnight UTC.

On (UTC) Duration Off (UTC)
Pacific Loop17:54:057h 59m 55s01:54:00
European Loop05:41:217h 59m 55s13:41:16

All four switching events occurred at latitude 43°.7 North (+/-0.1 degree). The geometry of the ground track loop means that the longitudes at switch on and switch off were similar - 14° East in the case of Europe and 166° West on the Pacific loop. The orbit is such that the mid-point of each transmission occurred as the satellite reached apogee.

The map shows the satellite location and ground track at the moment it switched on over Europe. Switch off was at the same location but on the southbound leg of the ground track loop nearly eight hours later. A look at the Pacific loop shows a similar track with the switching events occurring NW of Hawai.

The repeating, semi-rigid routine hints at timer-controlled operation with the timer being re-set on ground command and occasionally with errors. There are also occasions where the timer is not corrected every day.

USA 198 Switching - Map

S-band Operations

The S-band transmitter also goes through a switching regime relating to its geographical location but the timing is much less rigid and suggests manual control. It switches from low power to high power a variable number of minutes before/after the VHF switching and usually around 40° North latitude. It probably indicates ground control assessing the condition of the satellite before commencing operations on the orbital pass and again when operational equipment has been switched off before perigee passage.

When the satellite is around apogee, it has been observed to switch to an even higher power output for periods of time.
Copyright © Robert Christy, all rights reserved
Reproduction in whole or in part without permission is prohibited