USA-179 BackgroundThe Satellite and its RoleUSA-179 is probably an SDS-3 type satellite. It goes under the alternative name NROL 1 as a payload operated by the US National Reconnaissance Office. USA-179 was launched from Launch Complex 36A at the Cape Canaveral Air Force Station 2004 August 31. The launch was delayed several times over the period of one week - initially by technical issues and then by the weather. The launch vehicle was the 63rd, and last, Atlas IIAS. The satellite is multi-tasking but its major function is reportedly to act as a radio-relay for data collected US military and intelligence-gathering satellites in low orbit. It is probably operating as part of a constellation of three satellites including two other classified objects. Orbital elements for USA-179 can be found on the Web in Mike McCants's "classified.tle" file as catalogue number 90027. The other two satellites in the constellation are 90025 and 90028 - both known for their transmission of AFSATCOM data signals. As at 2007 July 7, USA-179's orbit was 1,177 x 39,169 kilometres at 62.8 degrees inclination with an orbital period of 717.6 minutes. USA-179 transmission frequencies are:
VHF Radio Observations - EuropeThe two VHF transmissions were first reported publicly by the late Ivan Artner. He noted that they were present over Europe for long periods but were not present all of the time. They seemed to switch on and off rather than fade as would be expected if the satellite was rising or setting at the horizon. The source was not identifiable. The pattern of switch on/off seemed similar to the AFSATCOM transmissions from 90025 and 90028 but the principal reason for it - another satellite on the same frequency was not there. During 2005 October, Ivan Artner and Bob Christy made observations of the switch on and off times and came to the conclusion that the transmitter was probably in HEO. It was fairly certain that both frequencies were from the same satellite because the switching seemed to be simultaneous. They noted that the times got earlier by about five minutes per day but did not discern any other pattern. Bob Christy was able to track the satellite across the north pole on the Pacific Ocean loop of the orbit. A look for candidates among satellites in HEO brought object 90027 to the fore. It was the only one to fit in with the over-the-pole transmissions. Back-tracking 90027 to the USA-179 launch date put the orbit plane near to the location of USA-179's when it was tracked in its transfer orbit by visual observers. Later, during 2006 June, Peter Wakelin made a series of timed observations of the VHF switching. When the timings were analysed, they revealed that transmissions normally lasted for 7h 59m 55s +/-1s, though it was not true on all occasions. Another feature was that switch on/off time drifted earlier each day but not at a constant rate - it changed slowly. Occasionally there was a significant 'reverse' jump in the rate between two days, and then it settled back into its regime of slow change. After a few weeks, it was clear that event times were driven by the satellite's orbital period. This was confirmed when the period between two days' switch on/off turned out to be twice the orbital period derived from 90027's visual orbital data, and that when visual observation indicated a manoeuvre, the radio timings changed in sych. Pacific Ocean ObservationsFrom Europe, the over-the pole observations showed rising and settting, indicating that the transmitter was being switched while below the horizon. During 2006 November and December, Richard Flagg of Hawaii monitored transmissions on the Pacific orbital loop. His observations showed that the transmitter behaviour echoed the European experience. He saw the same 7h 59m 55s transmission period and the transmissions sat half way between successive European ones. Significance and Consistency of the TimingsWhat had been discovered was 'back door' into measuring USA-179's orbital period/mean motion. The VHF transmissions are from a secondary payload aboard the satellite. USA -179's operators appear to be feeding orbital data to the controllers of the secondary paylod so it can be operated in synchronisation with the satellite orbit. The sequence ot times is not particularly tidy - the correlation is not perfect from day to day. Occasional, switching occurs early or late, thereby causing variations in the actual length of the transmission. However, it obeys the 7h 59m 55s rule on the significant majority of occasions. If readings are taken daily, then it is possible to connect them with a best-fit line that reveals the value of 'n' - the Mean Motion. Because the orbit is close to 63.4 degrees inclination, the rate of rotation of the line of apsides is practically zero. This means that the nodal orbital period inferred from the switching times can be assumed to be the same as the anomalistic period - hence a straightforward calculation of the value of 'n'. Further AnalysisThe left hand menu leads to additional analyses, plus notes on the transmissions and the orbit. |
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