Also, signal strength will decrease, and the correction information itself becomes less accurate farther from the base station receiver. Commercial satellite providers are the second common source for DGPS signals. These organizations will establish GPS base stations at various locations in their geographic region of interest. Most of these service providers, however, require a subscription fee for usage. Some earlier generation DGPS receivers required separate antennas for GPS signals and differential correction signals.
Most modern receivers combine all antennas into one single antenna unit.
An RTK system will attempt to count the number of wavelengths of the carrier frequency radio signal between the satellite and receiver, thereby achieving accuracies of less than 1 foot. GPS users should be aware that manufacturers use one of several methods to report the accuracy of a GPS receiver. Though it is rarely the intent of the manufacturer to be deceptive, an uneducated user can easily be misled. Another related accuracy specification is 2 drms or twice the distance rms. This reporting information is particularly important when comparing GPS receivers.