• Application of the Global Positioning System (GPS) to Space Shuttle Navigation

      Nilsen, Peter W.; Axiomatic (International Foundation for Telemetering, 1977-10)
      The present baseline Space Shuttle navigation system is comprised of several separate subsystems: TDRSS doppler ranging, TACAN (Tactical Air Navigation), MSBLS (Microwave Scanning Beam Landing System), and ground tracking. With the advent of the DOD development of the GPS, it may be possible to replace several of these subsystems with an on-board GPS navigation system. However, the Shuttle signal dynamics and environmental considerations impose unique and severe requirements for a GPS navigation system. Consequently, a preliminary study has been conducted to define requirements and configure candidate GPS systems for the Shuttle navigation function. Three configurations have been considered: an early test/demonstration system, which could be flown on early OFTs (Orbital Flight Tests), an interim system having greater capability but still not having full operational capability, and, finally, a fully operational system. A description of the test/demonstration and interim systems and the results of performance analyses are given. These preliminary results indicate that a GPS navigation accuracy greater than that obtained from the baseline system can be obtained for the orbital and de-orbit phases of the Shuttle mission.
    • Double Quadriphase Modulation/Demodulation Technique for Three-Channel Communication Link

      Alem, Waddah K.; Axiomatic (International Foundation for Telemetering, 1977-10)
      A modulation technique for a three-channel communication link is introduced. The structure of the modulator is such as to form an unbalanced quadriphase signal wherein the high rate data stream is bi-phase modulated on the in-phase carrier component, while the sum of the two lower rate signals is bi-phase modulated on the quadrature component of the same carrier. The sum of the two lower signals is, in turn, formed by modulating with the respective data streams the in-phase and the quadrature components of a square wave subcarrier. At the demodulator, the tracking of the carrier and the subcarrier is performed by two independent Costas loops. The demodulation of the high data rate signal is carried out after establishing the carrier reference signal, while the lower rate signals are demodulated after the subcarrier loop recovers the subcarrier. In this paper, the performance of the two loops is analyzed and the expressions for the tracking errors are derived. Finally, a numerical example pertaining to the Space Shuttle-to-TDRS Ku-band link is presented for illustration.