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Search and Rescue Synthetic Aperture Radar
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DASS

In September of 1997, a Canadian study revealed that a constellation of mid-earth orbiting (MEO) satellites could be used to augment the existing Cospas-Sarsat system by providing an improved space-based distress alerting and locating capability. NASA, in coordination with the Global Positioning System (GPS) Program Office and Sandia
National Laboratories, has determined that the GPS constellation would be the best and most cost-effective
MEO satellite constellation to host the search and rescue (SAR) instruments. This project is called the Distress
Alerting Satellite System (DASS).

GPS Constellation

The NASA Search and Rescue Mission Office at the Goddard Space Flight Center (GSFC) in Greenbelt, Maryland, under the leadership of Mr. Dave Affens, advocated and supported the DASS Program development to the point where it is expected to become a premier worldwide system for lifesaving.

NASA has committed funds for the development of a proof-of-concept (POC) system for DASS, which includes the funding necessary to modify up to 30 instruments for deployment onboard future GPS satellites, and the installation of a POC DASS ground station at the GSFC.  Funding for development and operation of DASS will be provided or shared among the supporting agencies according to their agreed roles.

DASS is intended to enhance the international Cospas-Sarsat satellite-aided search and rescue (SAR) system by installing 406 MHz SAR instruments on the GPS medium Earth orbit (MEO) navigational satellites and by introducing new ground segment tracking stations and processing algorithms.  DASS is expected to significantly enhance current Cospas-Sarsat operations by providing near-instantaneous detection and location of 406 MHz emergency beacons.

Responsibilities of federal agencies for SAR, Cospas-Sarsat, and DASS are covered in a collage of key references (agreements, understandings and plans); they contain information on DASS, the DASS development effort, and SAR organization.  The primary agency partners involved are the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), the U.S. Air Force (USAF) and the U.S. Coast Guard (USCG).  NASA leads DASS planning, development, and proof-of-concept testing efforts.  Upon transition to an operational system, DASS is expected to be managed and operationally funded by NOAA, the USAF, and the USCG.  While a lot of U.S. effort is going into developing DASS, related international efforts to support DASS and develop compatible MEOSAR systems are also underway.

Initial investigations identified many possible benefits that might be realized from a MEOSAR system, such as the following:

  • near instantaneous global coverage with accurate independent location capability
  • robust beacon-to-satellite communication links
  • high levels of satellite availability
  • resilience against beacon-to-satellite signal obstructions
  • possible SAR services in addition to alerting

Operational DASS will function as a secondary mission aboard Global Positioning System Block III (GPS III) satellites, and when fully deployed will consist of 24 to 27 payloads in medium Earth orbit (MEO).  The GPS constellation arrangement will be such that no less than four DASS-equipped satellites will be visible from anywhere on Earth at any time. For a satellite to be able to relay a distress alert from an emergency beacon, it must be “visible” to that beacon.  It is expected that a fully operational DASS, along with SAR/Galileo and SAR/GLONASS (see International Efforts), will become a component of the Cospas-Sarsat System.

DASS will greatly reduce the time to locate a beacon, which will result in more lives saved and fewer national resources expended.

The DASS concept of operations is to use SAR instruments on GPS satellites, with appropriately located ground processing stations to receive, decode, and locate 406 MHz distress beacons throughout the world.  DASS is considered to be a low-risk simple addition to the GPS constellation.  DASS will be completely compatible with Cospas-Sarsat 406 MHz emergency beacons as defined in the Cospas-Sarsat beacon specification (document C/S T.001).

The system also has the potential of providing additional one-way or two-way, non-vocal digital messaging. This, for example, might let the person in distress know that the alert message has been received, and could help mitigate false alarms by confirming whether an alert is for a distress situation, or was a false alert.  U.S. agencies involved have not made a formal decision regarding a DASS supplemental digital messaging.

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