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Differential GPS Explained
The Radio Technical Commission for Maritime Services (RTCM), a nonprofit scientific and educational organization that serves all aspects of maritime radio communications, radio navigation, and related technologies, defined the differential data protocol for relaying GPS correction messages from a base station to a field user. Its Special Committee 104 (RTCM SC-104) format recommendations define the correction message format. Each correction message includes data about the station position and health, satellite constellation health, and the correction to be applied. Using real-time differential corrections allows navigation to within one to two meters of any location depending on the service and the GPS receiver.
Satellite Differential Services
Another method for obtaining real-time differential correction data in the field is by using geostationary satellites. This system obtains corrections from more than one reference station. Reference stations collect the base station GPS data and relay this data in RTCM SC-104 format to a Network Control Center, which sends the information to a geostationary satellite for verification. The verified information is sent to the roving GPS receiver to ensure it obtains GPS positions in real time.
The Wide Area Augmentation System, or WAAS, is being developed by the Federal Aviation Administration (FAA) to provide precision guidance to aircraft at airports and airstrips that currently lack these capabilities, using a system of satellites and ground stations that provide GPS signal corrections. Although not yet approved for aviation use, it is available to civilian users. WAAS is broadcast from geostationary satellites so the signal is often available in areas where other DGPS sources are not available. Two commercial satellite differential service providers, Thales Survey LandStar (formerly Racal LandStar) and OmniSTAR Inc., use a control hub where reference station data is checked, formatted, and uploaded to a geostationary satellite for rebroadcasting to subscribers.
DGPS radio beacon systems operate in many parts of the world. These stations - part of a large network that covers coastal areas, navigable rivers, and, more recently, inland agricultural areas - are used for marine navigation. However, these beacons have a range of a few hundred kilometers inland and can provide free real-time differential accuracy in the one-meter range, depending on the GPS receiver and the distance from the radio beacon.
Reprocessing Real-Time Data
Some GPS manufacturers provide software that can correct GPS data that was collected in real time. This is important for GIS data integrity. When collecting real-time data, the line of sight to the satellites can be blocked or a satellite can be so low on the horizon that it provides only a weak signal, which causes spikes in the data. Reprocessing real-time data removes these spikes and allows real-time data that has been used in the field for navigation or viewing purposes to be made more reliable before it is added to a GIS.
Postprocessing Correction
Differentially correcting GPS data by postprocessing uses a base GPS receiver that logs positions at a known location and a rover GPS receiver that collects positions in the field. The files from the base and rover are transferred to the office processing software, which computes corrected positions for the rover's file. This resulting corrected file can be viewed in or exported to a GIS.
There are many permanent GPS base stations currently operating throughout the world that provide the data necessary for differentially correcting GPS. Depending on the technology preferred by the base station owner, this data can be downloaded from the Internet or via a bulletin board system (BBS). Because base station data is consistent (i.e., with no gaps due to multipath errors) and very reliable because base stations usually run 24 hours, seven days a week, it is ideal for many GIS and mapping applications. Sources of base station data for postprocessing fall into four categories - public sources, commercial sources, Web-based services, and base station ownership. Before purchasing a GPS receiver, it is best to identify the source of base station data.
Public sources - Government agencies worldwide collect and store base data. However, laws regarding public access to government data vary from country to country as well as between government agencies in the same country. Agencies that collect differential data have legitimate concerns, such as legal liability and cost recovery, that affect decisions regarding offering this data to the public.
Commercial sources - Some consulting firms and universities collect base data. Generally, this data can be purchased at per hour or daily rates. Information on these services can be found by browsing the Internet, by calling local base station distributors, or by talking to a local GPS sales representative. This can often be the most cost-effective way to obtain data for postprocessing.
Web-based services - This is an easy and economical way to process GPS data. GPS data is submitted to a service with some processing criteria specified. The GPS data is processed and returned. This approach is excellent for use with large field crews or when there isn't time to train GPS users in processing techniques and requirements.
Base station ownership - This is the most flexible way to obtain base data for postprocessing but it has additional setup costs because two GPS receivers must be purchased and managed. If large amounts of data will be collected, the investment is often worthwhile.
To attain accuracy levels on the order of one to 10 meters, differential correction is essential. The three main methods currently used for ensuring data accuracy are real-time differential correction, reprocessing real-time data, and postprocessing. Each method will achieve similar levels of accuracy, so the decision regarding which technique is appropriate will depend on factors such as project specifications, the end use of the data, and the sources available for differential correction.
For more information on GPS and GIS, see Integrating GIS and the Global Positioning System. This book by Karen Steede-Terry is published by Esri Press and available online at .