EUROFIX: A Synergism of Navstar/GPS and Loran-C
The Selective Availability in the SPS-mode of Navstar/GPS, which largely degrades the potential accuracy, may be effectively countered by differential techniques. Wide-band (geo-stationary satellites) and narrow-band (terrestrial) DGPS services offer good accuracies and may additionally improve integrity. Unfortunately, it will not increase the availability of GPS and the cost aspects are not yet clear.
The EUROFIX concept integrates Navstar/GPS with Loran-C. Differential-GPS data are phase coded on the Loran-C signals. This narrow-band Loran-C communication link (5..12 bps) transfers a full DGPS data set in about 40..100 seconds. As reported by Kremer et al (Navigation, Spring 1990), the long message time delay causes correlation loss between the DGSP data and the actually measured SAed pseudoranges which in turn introduces range-error differences of 5..12 meter, equivalent to about 15..36 meter horizontal position errors (957). Better results are feasible by temporary storing the measured pseudoranges (track history) in the receiver which makes re-synchronization of the pseudoranges with the delayed DGPS data possible.
The derived DGPS positions are consequently 40..100 seconds old which is generally acceptable for stationary users. Dynamic-user positioning is attainable by applying Loran-C as a dead-reckoning device which is updated by the computed DGPS positions every 40..100 seconds. Sampling the Loran-C signals more towards (+40 µs) the peak of the burst increases the SNR by -12 dB yielding better tracking and data-link performances. Possible errors due to skywaves and ASF are made ineffective by the frequent DGPS updating. Analysis indicate that for the newly proposed North Sea Loran-C chain navigation accuracies of 25 meter (95%) are feasible.
The hybridization of GPS and Loran-C gives a better navigation availability than either of the systems may provide separately. Hybridization of the pseudoranges of both systems also eases RAIM procedures. The infra-structural costs are very modest as many Loran-C stations are operational in large areas of interest.
The EUROFIX user needs a standard-type GPS receiver, a modified Loran-C receiver for the data extraction and a software package to perform the position, velocity and integrity calculations.
KeywordsComposite Signal Message Delay Selective Availability Differential Data Horizontal Position Error
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