6 - Global Positioning System

GPS

Global navigational satellite system created by Department of Defense. Uses about 30 satellites positioned about 20.200km above Earth orbiting very 12 hours.

Maximum accuracy = 1 cm (more like 20-500cm (.98-16ft))

Needs 4-7 satellites

How gps works

• Triangulation.

• Distance = velocity * travel time.

• Radio waves travel at the speed of light ≈ 300,000 km/sec.

• GPS relies on accurate time keeping: up to 10-9 seconds.

• Adding one more satellite reduces error.

Triangulation

Method used by GPS to determine position by measuring distances from multiple satellites.

PDOP (Positional dilution of precision)

a measure of the quality of satellite geometry in determining GPS accuracy.

PDOP ≤ 4: under 1m. PDOP = 4-6: acceptable accuracy (open area).

PDOP ≤ 8: under forest canopy

Ionosphere

the uppermost part of the atmosphere, ionized by solar radiation. It may cause transmission delays and can affect GPS accuracy.

L1 and L2 signals

Two frequencies (L1:1575.42 MHz, L2:1227.60 MHz) used by GPS for different types of receivers.

Consumer - GPS and mapping-grade GPS receive L1 signals.

Survey-grade GPS and military GPS capable of receiving both L1 and L2 (dual frequency receivers)

Satellite error

Inaccuracy of atomic clock onboard satellites or drifting from predicted orbit.

Multipath error

Error caused by signals reflecting off obstructions before reaching GPS receiver. Cannot be corrected

Receiver error

Error caused by clock or internal noise in the GPS receiver.

Selective availability (SA)

Scrambling of GPS signals to limit precision to about 100m before May 1, 2000.

Post-processing

Adjusting GPS data using simultaneous data from a known location. Raw GPS data from a base station was stored and later used to adjust raw rover GPS files. (Better than WAAS)

Differential GPS (DGPS)

Real-time correction of GPS data using data from both satellites and reference stations.

Using a beacon receiver: A separate box that receives correction data from a base station (beacon) and then sends them to the GPS.

WAAS (Wide Area Augmentation System) (real time)

a system that provides correction data from a set of ground stations all over the US uploaded to the geo-stationary satellite, which transmits the data down to the local GPS, which figures out which correction data are applicable to its current location.

Point averaging

Spatial analysis technique used to calculate the average value of a specific attribute within a defined radius.

Measurement offsets

Measuring distance and direction from a known location to an object of interest.

External antenna

A taller antenna that can improve GPS accuracy by reducing PDOP.

Signal reception

Positioning the GPS receiver to face the direction with the greatest number of satellites.

Rainfall

Moisture can reduce GPS accuracy, so data collection should be avoided in forested environments during or after rainfall.