Receiver Performance for an Enhanced DGPS Data Channel
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The Coast Guard currently operates a maritime differential GPS service consisting of two control centers and over 85 remote broadcast sites. This service broadcasts GPS correction information on marine radiobeacon frequencies to improve the accuracy and integrity of GPS. The existing system provides differential corrections over a medium frequency carrier using minimum shift keying (MSK) as the modulation method. MSK is a version of the Continuous Phase Frequency Shift Keying (CPFSK) modulation technique that is "spectrally compact," meaning that it is a narrow band modulation scheme. In a binary signaling channel, the two instantaneous frequencies for this modulation method are chosen in such a way so as to produce orthogonal signaling with a minimum modulation index. Current DGPS corrections are transmitted at a relatively low data rate, with message structures designed in an era when Selective Availability was in full operation. Greater demands for accuracy coupled with current operations in a "post SA" environment have prompted a reexamination of the DGPS data and signal structure, with an eye towards improving information rate while minimizing legacy user impact.
A two-phased plan for a new generation of DGPS capability can be envisioned. In the first phase (near-term) new ionospheric messages would be introduced to allow greater DGPS accuracy at larger distances from the beacons. This capability could support both double (LI/L2) and triple (L1/L2/L5) frequency operation. This phase requires only the definition of the new message type(s) and the commitment of receiver manufacturers to implement the usage of the new data. In the second phase (intermediate future) a new signal would come on line to support RTK using two and three frequencies and homeland security messaging. This signal would have the capacity to send 500 bps or so without disrupting the legacy signal or legacy receiver performance.
This new signal could be one of the new modulation techniques that we have been investigating; phase trellis overlay and orthogonal frequency division multiplexing.
Preliminary examinations of both of these techniques have shown the potential for increased bandwidth usage (ION NTM Jan. 2004), the effects on legacy receiver performance through a modulator test-bed (ION AM June 2004), and some effects of an actual transmitter (including antenna and coupler) on the signal (ION GNSS Sept 2004). The current paper describes recent investigations into the architecture of the receivers for these modulation methods including details of the demodulation and decoding methods. We also establish receiver performance measures and present preliminary performance results.
- Reprinted with permission from The Institute of Navigation (http://ion.org/) and The Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, (pp. 788-800). Fairfax, VA: The Institute of Navigation.
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