The DASS ground system will be composed of local user terminals (MEOLUTs) and the existing Cospas-Sarsat mission control center (MCC) network. The principal functions of a MEOLUT are to receive and process the satellite signals, calculate 406 MHz beacon locations, and forward this information to an MCC associated with the MEOLUT. The MCC network will distribute the alerts to other MCCs or SAR points of contact, validate the alert data, filter redundant data, and perform other actions as necessary.
The DASS MEOLUTs will provide independent distress beacon location information using a combination of time difference of arrival (TDOA) and frequency difference of arrival (FDOA) techniques. Because the MEOSAR system will be completely compatible with all Cospas-Sarsat 406 MHz beacon message protocols, it will also provide location information available from the message content of location protocol beacons. In such instances location information could be provided without the need for TDOA/FDOA processing, and could be available even if only one satellite provided simultaneous visibility of the beacon and the MEOLUT.
DASS Coverage with MEOLUTs in Hawaii and Maryland
As in the space segment, the DASS ground segment will be implemented in two phases.
Proof-of-Concept and Demonstration Ground Segment
A four-antenna POC and demonstration DASS ground station (aka the Developmental Local User Terminal or DLUT) at NASA GSFC was completed in early 2006 and is being used for POC activities. All antennas can track signals at both S-band and L-band, to allow visibility both of POC DASS signals and of Cospas-Sarsat L-band signals from operational LEOSAR and GEOSAR satellites. The ground station will be used to verify and characterize the DASS concept after sufficient POC payloads are on orbit to allow simultaneous four-satellite visibility. Within the U.S., distress alert data will not be distributed to operational SAR services during the POC, but will be transmitted during the D&E so that operational assessment can be performed.
Operational Ground Segment
Two appropriately located MEOLUTs will cover the North American and most of the South American continents, including the surrounding waters of the Pacific and Atlantic Oceans and the Caribbean Sea. The MEOLUT at GSFC is expected to be used as an operational MEOLUT for a short period after DASS is declared initially operational. The operational MEOLUTs are planned for installation in Hawaii and the Eastern United States, with the Eastern MEOLUT allowing the GSFC ground station to resume its role as a full-time research, development, and test facility. These MEOLUTs will be capable of processing signals from GLONASS and Galileo satellites as well (see International Efforts). The operational MEOLUTs will be connected to the existing Cospas-Sarsat ground infrastructure. MEOLUTs and MCCs will use commercial communications to forward information to directly or indirectly to SAR personnel.
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