Medium-Frequency Data Link for Differential NAVSTAR/GPS Broadcasts

摘要

Differential GPS must communicate differential corrections to civilian users of the Global Positioning System. Modulation of existing marine radiobeacons can provide the needed communication link for DGPS, provided the operation of existing radiobeacon direction finding equipment is maintained ad adequate DGPS range is provided in the presence of atmospheric noise and skywave fading. Existing DF and ADF units were studied through a manufacturers' survey, and a radiobeacon modulation scheme has been proposed that maintains reliable DF and a ADF operation. MSK modulation with a frequency offset of 325 Hz is recommended, with a power offset of -3dB. This scheme limits ADF bearing errors to under 3 deg based on analysis of the modulation energy entering the effective passbands of the various ADF designs. CCir data on atmospheric noise and multipath were studies and a rate 1/2 convolutional code with constraint length 6 is recommended to provide a DGPS bit error probability of less than 0.00001. Reliable DGPS range for 10 nautical mile beacons will be 100 km, and for 30 nm beacons, 195 km. Discussion of interleaving to control atmospheric burst noise is included.