In the early days of GPS, reference stations were
established by private companies who had big projects demanding high accuracy - groups
like surveyors or oil drilling operations. And that is still a very common approach. You
buy a reference receiver and set up a communication link with your roving receivers.
But now there are enough public agencies transmitting corrections that you might be
able to get them for free!
The United States Coast Guard and other international agencies are establishing
reference stations all over the place, especially around popular harbors and waterways.
These stations often transmit on the radio beacons that are already in place for radio
direction finding (usually in the 300kHz range).
Anyone in the area can receive these corrections and radically improve the accuracy of
their GPS measurements. Most ships already have radios capable of tuning the direction
finding beacons, so adding DGPS will be quite easy.
Many new GPS receivers are being designed to accept corrections, and some are even
equipped with built-in radio receivers.
On to the Next Section
