Zarya - Soviet, Russian and International Spaceflight
carousel image
 
USA 179


Tyneside, UK
2018 Jan 20
Saturday, Day 20

Maintained by:






USA-179 Analysis:









USA 179 and its Role

USA 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.

USA 179 patchThe 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 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. The other two satellites in the constellation are USA 137 and USA 198. 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.

Transmission Frequencies

Freq (MHz) Satellite Mission Signal Type Notes
251.700 USA 179
(SDS-3 F-3)

2004-034A
28384
Third generation Satellite Data System government/military communications satellite. AM - 4.8 kHz data tone One of a pair of transmissions from a secondary payload on US classified satellite in HEO - Switched on and off to co-ordinate coverage with USA-198 - Prior to 2008 Mar 14, USA-179 was operating alone and was providing near 8 hours' coverage per pass at this frequency - Also transmits at 256.375 MHz and 2242.50 MHz
256.375 USA 179
(SDS-3 F-3)

2004-034A
28384
Third generation Satellite Data System government/military communications satellite. AM - 4.8 kHz data tone One of a pair of transmissions from a secondary payload on US classified satellite in HEO - Switched on and off to co-ordinate coverage with USA-198 - Prior to 2008 Mar 14, USA-179 was operating alone and was providing near 8 hours' coverage per pass at this frequency - Also transmits at 251.700 MHz and 2242.50 MHz
2242.503 USA 179
(SDS-3 F-3)

2004-034A
28384
Third generation Satellite Data System government/military communications satellite. CW carrier plus side-bands - strong Part of a constellation of three satellites in highly eccentric orbit (HEO), switches to low power when over the southern hemisphere - Also transmits at 251.700 MHz and 256.375 MHz

VHF Radio Observations - Europe

The 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 USA 125 and USA 137 but the principal reason for it, nameley shared duty with another satellite, was not there.

During 2005 October, Ivan Artner and Bob Christy made observations of the switch on and off times of what was then an unknown transmitter VHF transmitter. The conclusion reached was that it 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 4-5 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 from Mike McCants's "classified.tle" to the fore. It was the only one to fit in with transmissions detected from across the north pole. Back-tracking 90027 to the USA 179 launch date put the orbit plane near to the location of USA 179's when it was seen 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 Observations

From 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 Timings

What 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'.

Purpose of the VHF Transmissions

USA 179 is a multi-tasking satellite and the VHF transmissions come from a payload that has its own radio transmission/reception requirements. The only other known current VHF payloads on an HEO were the AFSATCOM transmitters on USA 125 and USA 137 - the other two satellites in the same constellation. It is tempting to speculate that USA 179 and USA 198 VHF channels represent an alternative to AFSATCOM for some particular users.

Impact of USA 198's Arrival

In 2008, USA 198, an SDS-3 satellite, joined the SDS constellation as a replacement for USA 125, an older SDS-2 type. It too carries the VHF payload. Initial tests of the USA 198 transmitter were at 251.275 MHz and 258.800 MHz but on March 14, it switched to using the same 251.700 Mhz and 256.375 MHz pair as USA 179. Between them, the two satellites provide 24 hour coverage of northern regions where USA 179 on its own was only able to give 16 hours of service. Transmissions switch between the two satellites so that the two are never received simultaneously when in the same region of sky.

Copyright © Robert Christy, all rights reserved
Reproduction in whole or in part without permission is prohibited