Fast Direct-Y GPS Acquisitions with Inaccurate Time

Ken Heckroth, Keith Scherrer and John Nielson

Abstract:	It has become evident that GPS must undergo evolutionary changes in order to satisfy military requirements of the 21st century. The military must improve the anti-jam performance of the system to ensure its availability for precision delivery of weapons and general navigation functions in the battle field. A key component to this strategy is reducing the military's reliance on C/A code. Legacy GPS receivers typically acquire C/A code and hand-over to P(Y) code. In order to achieve a great deal more jamming and spoofing immunity, it is desirable to be able to directly acquire Y-code, or perform direct-Y (DY) code acquisitions. Most currently fielded GPS receivers have a few deficiencies which must be remedied before robust DY acquisition in the face of jamming can be achieved. Since the Y-code is essentially infinite in length (versus 1 ms C/A code), the frequency and code phase error must either be reduced considerably, or the resultant uncertainties must be searched more quickly. Traditionally GPS receivers require very accurate time in order to be able to perform direct Y-code acquisitions within a short time. Many platforms, especially airframes that use the MILSTD- 1553/1760 interface can only provide time to an accuracy of +/-10 ms. This deficiency has required many existing weapons to continue to use C/A code to acquire Y-code. The new generation Rockwell Collins Selective Ability Anti-spoofing Module (SAASM) incorporates an Application Specific Integrated Circuit (ASIC) known as the Panther which builds on the Rockwell Collins ACE and Nighthawk ASICs that are in many legacy products. Furthermore, the Panther also incorporates thousands of correlators such that a receiver can achieve DY very rapidly with the stated amount of time error. This ASIC also includes 24 tracking channels which can be used for numerous applications especially those that require simultaneous all-in-view tracking of both GPS frequencies and provides a growth path for addressing future threat scenarios. The new SAASM is being integrated into the next generation NavStrike and other Rockwell Collins receivers. The first application of the new NavStrike is JDAM.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	573 - 578
Cite this article:	Heckroth, Ken, Scherrer, Keith, Nielson, John, "Fast Direct-Y GPS Acquisitions with Inaccurate Time," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 573-578.
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