how does differential mode gps work i understand that the new gps boxes now have an option known as differential ready apparently land based beacons tranmit gps correction information to your gps receiver with differential option installed how does this system work what frequency is used for the land based beacons thanks in advance charlie thompson here s a rough sketch of how the system works a reference station with a very exactly known position computes the errors in the incoming gps signals these errors are due to several factors including atmospheric distortion sa selective availability time dithering etc the reference unit contains complex computational equipment to back out the errors in its position since it knows where it is already it then transmits these corrections on a broadcast which is available to any number of relatively local receivers if the receivers are nearby km and are using the same satellites as the reference unit the errors should be very similar for the reference unit and the receiver unit thus the receiver unit may apply the corrections calculated by the reference unit the us coast guard is currently as far as i know installing a series of coastline transmitters for differential gps these stations will use existing radio towers i believe the frequency is to be approximately khz there are many other private corporations offering dgps signals on different frequencies for example pinpoint offers correction signals and receiver units using an fm broadcast system which has stations all across the us the correction codes are usually transmitted using the rtcm format advertised accuracies espouse to meter errors john p baker my opinions are my own i don t know johns hopkins university anyone else who wants them anyway applied physics laboratory laurel md bakerjp aplcomm jhuapl edu
