Global Positioning System (GPS)
A space-based navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.
GNSS vs. GPS
Global Navigation Satellite Systems
- GPS: The US NAVSTAR Global Positioning System is the most well-known constellation, but it is just one part of a broader system.
- GNSS: Global Navigation Satellite System is the overarching term for all satellite navigation systems providing global coverage. Modern survey receivers are GNSS receivers, meaning they track multiple constellations simultaneously to increase accuracy, reliability, and the number of available satellites (especially in urban canyons or under tree canopy).
- GLONASS: The Russian satellite constellation.
- Galileo: The European Union constellation.
- BeiDou (BDS): The Chinese constellation.
GPS Segments
System Architecture
- Space Segment: A constellation of at least 24 satellites (currently often 31 or more active) orbiting the Earth in 6 orbital planes at an altitude of approximately 20,200 km.
- Control Segment: Ground stations worldwide (Master Control Station in Colorado Springs) that monitor and control the satellites, upload navigation data, and synchronize time.
- User Segment: Receivers (civilian and military) that process signals to determine position, velocity, and time.
How GPS Works
GPS receivers calculate their position by timing the signals sent by GPS satellites.
Trilateration
The receiver measures the distance to at least four satellites to determine its 3D position (Latitude, Longitude, Altitude). It requires the precise time the signal left the satellite and the time it arrived at the receiver.
Distance Equation
Where:
Checklist
- : Speed of light ( m/s)
- : Time taken for the signal to travel.
Coordinate Systems
Coordinate Systems
- WGS84 (World Geodetic System 1984): The standard geocentric coordinate system used by GPS. It uses an ellipsoid to approximate the shape of the earth.
- Local Datums: Regional reference systems (e.g., NAD83 in North America, PRS92 in the Philippines) that fit the earth's surface better in a specific area. GPS coordinates often need to be transformed to local datums for surveying use.
GPS Surveying Methods
GPS Surveying Methods
- Static Surveying: Used for high-accuracy control points. Receivers occupy stations for long periods (hours). Baseline accuracy: 1 ppm. Requires post-processing.
- Fast Static: Shorter observation times (minutes) but requires specialized post-processing and good satellite geometry. Useful for densifying control networks.
- Differential GPS (DGPS): A technique to improve positioning by using a stationary reference (base) station at a known location. The base calculates the difference between its known position and the GPS-derived position to produce pseudorange corrections. These corrections are broadcast to moving rovers to greatly reduce atmospheric and orbital errors, improving accuracy to sub-meter levels. DGPS was the precursor to RTK for applications where sub-meter precision is sufficient.
- Real-Time Kinematic (RTK): The current standard for topographic mapping and stakeout.
- Uses a base station (at a known point) and a rover.
- Provides centimeter-level accuracy in real-time.
- Requires a radio link or cellular internet connection (NTRIP) between base and rover to transmit phase corrections continuously.
Sources of Error
GPS Satellite Geometry (DOP) Simulator
Observe how satellite distribution affects Positional Dilution of Precision (PDOP).
ClusteredWidely Spaced
Geometry Quality
Estimated PDOP:1.4
Rating:Excellent
A larger highlighted area in the skyplot indicates better satellite geometry, resulting in a lower (better) PDOP value. Clustered satellites yield high uncertainty in position.
Sources of Error
- Clock Errors: Satellites use highly precise atomic clocks, but receiver clocks are less accurate. This is solved mathematically by tracking a 4th satellite.
- Ephemeris Errors: Orbital position errors (satellite is not exactly where it says it is).
- Ionospheric and Tropospheric Delays: Signal slows down passing through the atmosphere (refraction). Dual-frequency receivers can largely correct for ionospheric delay by comparing two different frequencies (L1 and L2).
- Multipath: Signal bounces off surfaces (buildings, trees, ground) before reaching the receiver, increasing the travel time and causing large positional jumps.
- Dilution of Precision (DOP): Geometric arrangement of satellites.
- GDOP: Geometric Dilution of Precision (Overall).
- PDOP: Position Dilution of Precision (3D Position).
- HDOP: Horizontal Dilution of Precision (2D Position).
- VDOP: Vertical Dilution of Precision (Height).
Low DOP values (below 4) indicate better geometry and accuracy. High DOP means satellites are clustered together in one part of the sky, leading to poor intersecting geometry for trilateration.
Key Takeaways
- Trilateration: Principle of GPS positioning using distances based on time of flight ().
- 4 Satellites: Minimum required for a 3D fix and receiver clock correction ().
- GNSS: The general term encompassing GPS, GLONASS, Galileo, etc.
- RTK: Real-Time Kinematic, cm-level accuracy using base and rover with a data link.
- DOP: Dilution of Precision (Geometry factor). Lower is better (spread out satellites).
- Multipath: Error caused by reflected signals near the receiver.