software computes baselines using simultaneous measurement data from two or more GPS receivers. The baselines represent a three-dimensional line drawn between the two points occupied by each pair of GPS antennas. The post-processed measurements allow more precise positioning, because most GPS errors affect each receiver nearly equally, and therefore can be cancelled out in the calculations. Differential GPS measurements can also be computed in real time by some GPS receivers if they receive a correction signal using a separate radio receiver, for example in Real Time Kinematic (RTK) surveying or navigation. The improvement of GPS positioning doesn't require simultaneous measurements of two or more receivers in any case, but can also be done by special use of a single device. In the 1990s when even handheld receivers were quite expensive, some methods of quasi-differential GPS were developed, using the receiver in quick turns of positions or loops of 3-10 survey points. See also RTCM SC-104 - a standard for transferring dGPS data to a GPS receiver Assisted GPS (A-GPS) - System used primarily in GPS-equipped cellular devices to improve start-up performance GNSS augmentation GNSS enhancement References External links SiReNT information page US NDGPS fact sheet USCG Navigation Center National DGPS system USCG coverage maps Canadian Coast Guard DGPS information (English) Canadian Coast Guard DGPS information (French) Product Survey on RTK DGPS receivers for (mainly) hydrographic use DGPS Decoding Software Useful DGPS Links, Databases and Resources Worldwide database of IALA DGPS Reference stations on an interactive map Geomatics engineering Global Positioning System Wireless locating