GPS: Accessing the System
The GPS is accessed for a multitude of reasons, ranging from recreational hiking to commercial ocean navigation. Access methods for positioning using GPS reflect this diversity. For example, someone who is precisely locating a static point for ground surveying purposes might have stricter accuracy requirements but less time pressure than a vehicle needing navigation support while moving at sixty miles an hour.
This section examines the two most commmon positioning modes, autonomous and differential. Actual field positioning methods may be a refinement of these modes, but rely on one of these two basic concepts.
- Autonomous Positioning
- Differential Positioning
- Differential Correction Sources
- Other Positioning Methods
Autonomous Positioning
Autonomous positioning, also referred to as point positioning, uses a single receiver and at least three satellites to determine location instantaneously. It is the simplest mode, and does not require any post-processing or additional equipment; the resulting cooridnate value on the receiver is the position of your location. Almost all receivers offer this type of positioning. Expected accuracy is around 10 to 20 meters (with Selective Availability or SA turned off; for the Federal Aviation Administration's discussion on SA, click here). Specific receivers may use special techniques to achieve greater accuracy.
Fig 1. Point Positioning
Differential Positioning
Differential positioning uses multiple receivers to increase the positioning accuracy. One receiver remains stationary on a site with precisely known coordinates. The other receiver takes readings at unknown locations. If the receivers use the same satellites for positioning, the errors and biases should be close to the same. The receiver at the known location then compares its GPS-determined location with its known location. These satellite distance errors at the known location are then used to correct the coordinates determined independently by the roving receiver. This correction may be done real-time or afterwards (post-processing), assuming the correction information is precisely maintained. Accuracy using this method may reach the sub-centimeter level.
Fig 2. Differential Positioning
Differential Correction Sources
Differential data for correction may be obtained from third-party sources or obtained as part of the data collection process. If you are setting up your own base station you must locate it on a precisely known position, such as monuments in the High Accuracy Reference Network available across Wisconsin and in other states. If you are not setting up your own base station, several third party sources are increasingly available across the country (and around the world), and can be very inexpensive. In the United State the Continuously Operating Reference Station (CORS) network provides high accuracy correction information for post-processing and real-time, and is available over the internet for free. The Digital GPS (DGPS) system run by the U.S. Coast Guard broadcasts differential information for real-time positioning at near one-meter levels; it does however require a beacon receiver connected to the GPS receiver. Both CORS and DGPS operate using land-based networks. A correction service using both land-based control stations and geostationary satellites called the Wide Area Augmentation System or WAAS is also in development, and allows sub-three meter level accuracy on WAAS-equipped handheld receivers for no additional fee. The system is geared towards air navigation and is not yet complete, although it is available to civilians with WAAS-enabled receivers.
See the following web sites for more information on these services:
- CORS: http://www.ngs.noaa.gov/CORS/
- DGPS: http://www.navcen.uscg.gov/dgps/Default.htm
- WAAS: http://gps.faa.gov/Programs/index.htm
Other Positioning Methods
Positioning methods vary substantially, and a discussion of all of them is beyond the scope of this introduction. For example, methods must account for whether positioning needs to be done on a moving target (e.g. navigation) versus static positions versus stop-and-go rapid surveying. Different satellite signal components may also be used for positioning to increase the accuracy of positioning. Suffice to say, the positioning mode, along with the equipment required, must to be matched up to the needs (and constraints) of the particular project.
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