When DASS first becomes operational, the space segment will be hosted aboard GPS III satellites that will orbit the Earth at altitudes of about 20,200 km. The constellation will consist of 24 to 27 operational satellites and up to three on-orbit spares spread across either 3 or 6 orbital planes equally distributed in longitude and inclined about 55 degrees from the equator. The geometry is such that every point on the Earth will be visible by at least 4 satellites at all times, with a minimum elevation angle of 5º. The operational DASS system will include 406 MHz repeaters (transponders) on all GPS satellites. The operational transponders will use downlinks in the 1544-1545 MHz band (L-Band). DASS proof-of-concept and demonstration will be conducted using transponders with S-band downlinks.
DASS transponders will be capable of relaying data from both standard and location protocol beacons (see emergency beacons). Future beacon designs will be supported if they comply with Cospas-Sarsat document C/S T.001 (406 MHz beacon specification).
DASS will require equipment on the ground for satellite and payload control, to include turning instruments on and off, reconfiguring instruments, monitoring payload health, and other actions as necessary. NASA operates this equipment at the Goddard Space Flight Center.
High Frequency Antenna for Satellite Transponder Control
The DASS space segment will be implemented in two stages: proof-of-concept/demonstration; and operational.
Proof-of-Concept and Demonstration Space Segment
Proof-of-concept (POC) and demonstration DASS payloads will operate aboard nine GPS Block IIR and all Block IIF satellites. They will use an existing GPS capability to minimize impact to the satellites prior to full fielding of DASS. Beacon signals, without any processing aboard the satellites, will be relayed to the MEOLUT through an existing S-band antenna (the future operational link will be L-band, a frequency internationally-recognized frequency for this purpose).
Operational Space Segment
The operational DASS satellite payload will continue to function as a repeater for 406 MHz beacon signals, but will convert all incoming signals to the L-band (1544.8-1545.0 MHz ) for rebroadcast. Canada will likely provide the DASS transponders for the GPS III satellites.
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