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Differential Carrier GPS (Survey)
- All carrier-phase tracking is differential, requiring both a reference and remote receiver tracking carrier phases at the same time.
- In order to correctly estimate the number of carrier wavelengths at the reference and remote receivers, they must be close enough to insure that the ionospheric delay difference is less than a carrier wavelength. This usually means that carrier-phase GPS measurements must be taken with a remote and reference station within about 30 kilometers of each other.
- Special software is required to process carrier-phase differential measurements. Newer techniques such as Real-Time-Kinematic (RTK) processing allow for centimeter relative positioning with a moving remote receiver.
Differential Carrier-Phase Positioning refer page
Common Mode Time Transfer
- When time information is transferred from one site to another, differential techniques can result in time transfers of around 10 ns over baselines as long as 2000 km.
Common-Mode Time Transfer
GPS Techniques and Project Costs
- Receiver costs vary depending on capabilities. Small civil SPS receivers can be purchased for under $200, some can accept differential corrections. Receivers that can store files for post-procesing with base station files cost more ($2000-5000). Receivers that can act as DGPS reference receivers (computing and providing correction data) and carrier phase tracking receivers (and two are often required) can cost many thousands of dollars ($5,000 to $40,000). Military PPS receivers may cost more or be difficult to obtain.
- Other costs include the cost of multiple receivers when needed, post-processing software, and the cost of specially trained personnel.
- Project tasks can often be categorized by required accuracies which will determine equipment cost.
- Low-cost, single-receiver SPS projects (100 meter accuracy)
- Medium-cost, differential SPS code Positioning (1-10 meter accuracy)
- High-cost, single-receiver PPS projects (20 meter accuracy)
- High-cost, differential carrier phase surveys (1 mm to 1 cm accuracy)
GPS Applications, Costs, and Signals
all about global positioning system page: 1,2,3,4,5,6,7,8.9.10,11,12