SARSAT MEO 406-MHz Systems on GPS Satellites

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jimh
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SARSAT MEO 406-MHz Systems on GPS Satellites

Postby jimh » Fri Dec 15, 2017 11:11 am

The United States Air Force Global Positioning System (GPS) satellites have been launched with a SARSAT 406-MHz distress beacon receiver and S-band downlink transmitter as part of an initial experiment in medium Earth orbit (MEO) satellite reception of 406-MHz distress signals. This experimental system is referred to as the GPS-DASS, for GPS Distress Alerting Satellite System.

The first GPS satellite to carry GPS-DASS was said to have launched in January 2001 (and was probably GPS IIR-7 also known GPS SVN-54, launched January 30, 2001). A complete listing of 20 active GPS-DASS satellites is given below.

The GPS-DASS system was considered experimental. The downlink uses S-band (2,226-MHz). The present-day operational SARSAT MEO satellites use an L-band downlink (1544 to 1545 MHz). The COPAS-SARSAT organization characterizes the GPS-DASS project as follows:

GPS/DASS satellites are S-band satellites. They are viewed as experimental payloads and cannot therefore be considered for long-term MEOSAR operations. Their operational use on a temporary basis for the MEOSAR EOC [Experimental Operating Capability] and IOC (Initial Operating Capability) is authorized, however.


According the COPAS-SARSAT, "the USA plans to carry Canada-supplied L-band SAR repeaters on 24 GPS satellites beginning with the launch of the ninth GPS Block III satellite, anticipated for deployment as early as 2020." On that basis, I would infer that at this moment there are no SARSAT 406-MHz L-band transponders in orbit on GPS satellites. Also, considering that the first GPS III series satellite has still not been launched as of December 2017, it seems optimistic to expect that by 2020 the GPS III series would be launching its ninth satellite. The GPS Block IIIA satellites were originally scheduled to begin launching in 2014, but the first launch is now planned for May 2018, according to Wikipedia.

Cf.: https://en.wikipedia.org/wiki/GPS_Block_IIIA

Also, that same article suggests that the SARSAT transponders won't be added until the second block of GPS III series, which may mean not until the very end of the Block IIIA production, which would suggest a launch date in 2022 or later. That means it will more than four years before a GPS satellite will be operational in the MEOSAR system.

The GPS-DASS system's use of S-band downlink may be a problem for ongoing operational status; the ground terminals being build for COPAS-SARSAT MEO satellites all seem to be focused on L-band terminals. However, the USA has ground terminals for S-band, so when the GPS satellite orbits bring them in range of the USA ground terminals, the S-band downlinks can be used for relay of SARSAT signals.

jimh
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Re: SARSAT MEO 406-MHz Systems on GPS Satellites

Postby jimh » Fri Dec 15, 2017 12:31 pm

The best reference I can find for current MEO satellite status is at

Current Space Segment Status and SAR Payloads
http://www.cospas-sarsat.int/en/current-space-segment-status-and-sar-payloads

The above publication shows 20 GPS-DASS satellites in orbit with operational status "Initial Operational Capability."

It took some digging into reference documents to figure out the numbering convention of the SARSAT satellite designators. For GPS, the conversion is to subtract 300 from the SARSAT number to get the PRN number. This yields the identities of the present in-orbit GPS satellites having GPS-DASS transponders as those with PRNs shown below.

GPS Satellites in Orbit with GPS-DASS transponders:

jimh
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Re: SARSAT MEO 406-MHz Systems on GPS Satellites

Postby jimh » Mon Dec 24, 2018 12:13 pm

A recent public presentation about GPS III modernization indicates that the SAR-SAT payload with L-band downlink will not be available until the 11th GPS III satellite is launched, which is expected for sometime around October 2025. More discussion in a separate thread about GPS III.