When adapting GPS sensor technology from an aviation environment to a space environment, the search window for a satellite’s frequency and code phase is greatly increased. This problem is also magnified when multiple antennas are used. A new algorithm is required to meet the demands of acquiring satellites in a space environment.

NASA’s Goddard Space Flight Center developed Fast Acquisition hardware capable of taking a snapshot of the GPS RF spectrum and calculating a precise position of a single satellite. This effectively reduces the search window to a tiny fraction of its entirety. The position can then be fed to the traditional aviation method and be found very quickly, as several searches over the complete search window can he completed in less than one second.

The new invention creates an algorithm to optimize use of the Fast Acquisition hardware. This algorithm also optimizes the use of the tracking channels based on data returned from the Fast Acquisition hardware.

In order to optimize the hardware, the algorithm breaks down a search for a single satellite into smaller, more manageable searches that the hardware is capable of handling. The algorithm searches across multiple antennas and must also account for the destructive interference and loss of power during a navigation bit transition in Weak Mode operation.

The primary use of this algorithm is in the acquisition of satellites under the conditions where almanac, ephemeris, and position data may not be available. This algorithm, in conjunction with the Fast Acquisition hardware, can be used to very quickly scan the entire sky for every satellite’s Doppler frequency and code phase in a fraction of the time it would take a traditional GPS. These results can then be passed to the baseband for initializing of tracking channels.

A secondary use of this algorithm is when the sensor has been fully initialized or has a valid navigation solution. Both of these conditions refer to when the sensor knows where it is (or thinks it knows) and has a list of satellites that are visible, with their approximate frequency and code phase. The sensor can use this information to constrain frequency that the Fast Acquisition hardware has to search, resulting in reduced search time. This use is exceptionally valuable when using the Fast Acquisition hardware’s weak mode.

This work was done by Stephen Vickers and Mike Vukas of Honeywell International, Inc. for Johnson Space Center. For further information, contact the JSC Technology Transfer Office at (281) 483-3809. MSC-25772-1

NASA Tech Briefs Magazine

This article first appeared in the March, 2015 issue of NASA Tech Briefs Magazine.

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