Partial Decoding of the GPS Extended Prediction Orbit File

Abstract

The paper is concerned with a decoding of the Extended Prediction Orbit data format file for an assisted-GPS web-service via cypher-text only attack. We consider mandatory data content of the file and reveal the changes of this content at different moments of time. The frequency of changes hints on the location of records for current GPS date and satellite orbits information. Comparing the repeating data patterns against reference orbits information, we obtain meaning of data fields of the orbit record for each operational satellite. The partially deciphered GPS almanac data layout is provided as a table within the paper.

Keywords

• assisted gps
• almanac
• extended prediction orbit
• extended ephemeris
• decoding
• data structure
• binary format
• cypher-text only attack