6 - Global Positioning System

What is GPS?

• Global navigational satellite system created by Department of Defense.

• Uses a constellation of 30 satellites.

• Satellites are positioned about 20,200 km above the Earth and complete an orbit every 12 hours.

• Maximum accuracy = 1 cm (more like 30-500cm (.98-16ft))

• Needs 4 to 7 satelights for a reading.

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.

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

• PDOP = positional dilution of precision ≥ 1.

  • PDOP ≤ 4: under 1m.

  • PDOP = 4-6: acceptable accuracy (open area).

  • PDOP ≤ 8: under forest canopy.

L1 and L2 signals

• Signals transmitted in 2 frequencies:

  • L1 (1575.42 MHz)

  • L2 (1227.60 MHz)

• 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)

Type of errors in GPS

• Satellite error:

  • Inaccuracy of atomic clock onboard satellites

  • Satellite drifting from its predicted orbit

• Atmosphere:

  • Radio signals travel at speed of light only in a vacuum.

  • Ionosphere (the uppermost part of the atmosphere, ionized by solar radiation) may cause transmission delays

• Multipath error:

  • signals reflecting off obstructions before reaching GPS

  • cannot be corrected

• Receiver error:

  • from clock or internal noise

• Selective availability (SA):

  • Before May 1, 2000, signals were scrambled to limit precision to about 100 m

Adjustment

• Post-processing: adjusted from simultaneous data from a known location.

  • Raw GPS data from a base station was stored and later used to adjust raw rover GPS files, using a post-processing software.

  • Both rover and base receivers must collect data at the same time and must have at least 4 satellites in common.

  • Note: multipath error cannot be corrected.

• Differential GPS (real time): DGPS accepts data from both satellites and a reference station.

  • Using a beacon receiver: A separate box that receives correction data from a base station (beacon) and then sends them to the GPS. The US Coast Guard manages 86 transmitters.

  • WAAS: Wide Area Augmentation System: correction data from a set of ground stations all over the US are 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.

Improving GPS measurements

• Point averaging: a spatial analysis technique used to calculate the average value of a specific attribute within a defined radius or distance around a point. It involves aggregating the attribute values of neighboring points within the specified radius and calculating their average. This technique is commonly used in various applications, such as environmental monitoring, population density analysis, and resource management.

• Measurement offsets: measure distance and direction from a known location to an object of interest.

• External antenna: 4-m height lowers PDOP compared to 2-m height.

• Signal reception: facing the direction of the greatest number of satellites.

• Rainfall: moisture reduces GPS accuracy. Avoid collecting data in a forested environment during or immediately after rainfall.