Section 2: Surveying with GPS
Anchor: #i1000084Survey Background Information
All GPS surveying techniques are based upon interferometric observations of radio signals from a network of orbiting satellites. These signals are processed to compute station positions by trilateration: the positions of the satellites and computed ranges are used to determine the antenna position.
These positions are computed in an Earth-centered Earth-Fixed (ECEF) Cartesian coordinate (x, y, z) system, which can be converted to geodetic curvilinear coordinates (latitude, longitude, and ellipsoidal height). With the addition of a geoid height model, orthometric heights can be computed.
Anchor: #i1000099Accuracy of a GPS Survey
The accuracy of a GPS survey is dependent upon many complex, interactive factors, including:
- observation technique used, e.g., static vs. kinematic, code vs. phase, etc.
- amount and quality of data acquired
- GPS signal strength and continuity
- ionospheric and tropospheric conditions
- station site stability, obstructions, and multipath
- satellite orbit used, e.g., predicted vs. precise orbits
- satellite geometry, described by the dilution of precision (DOP)
- network design, e.g., baseline length and orientation
- processing methods used, e.g., double vs. triple differencing, etc.
Error Sources in a GPS Survey
Error sources in a GPS survey include the following:
- reference position errors - coordinate, monument stability, crustal motion
- antenna position errors - equipment setup, phase center variation and offsets
- satellite position errors - orbit ephemeris errors
- timing errors - satellite or receiver clock errors
- signal path errors - atmospheric delay and refraction, multipath
- signal recording errors - receiver noise, cycle-slips
- human errors - field or office blunders
- computing errors - processing and statistical modeling errors.
Operational Procedures
Identify and minimized all errors by redundancy, analysis, and careful operational procedures including:
- the repetition of measurements under independent conditions
- make redundant ties to multiple, high-accuracy control stations
- ensure geodetic-grade instrumentation, field procedures, and office procedures are used
- ensure processing with the most accurate station coordinates,
satellite ephemerides, and atmospheric and antenna models available.
CAUTION: Be aware that these procedures cannot disclose all problems.