What is a positioning system?
· Land-based Tri-Angulation.
· Satellite based ranging.
· Differential position (correction) – use of radio waves to correct GPS positions.
· Use of dead reckoning.
Types of Positioning Systems:
¨ Land-based – use of radio towers and receivers.
What is GPS?
§ Navigational system that is satellite based and operated by the Department of Defense.
§ Became fully operational in 1995.
§ Designed for precise military navigation.
§ Consists of 3 segments – Space, Control, and User.
Space Segment:
q 24 NAVSTAR satellites. 21 are in use, 3 are spares.
q Each satellite will orbit the earth every 12 hours.
q There are 6 orbital paths, each with 4 satellites.
q The arrangement of satellites is called a constellation.
q Receivers can only use satellite signals that are above the horizon.
q All satellites are rigged with transmitters, receivers, and atomic clocks.
q Atomic clocks are powered by cesium and rubidium. (2 of each)
Control Segment:
v A network of monitoring stations makes up the control segment.
v There are 4 monitoring stations worldwide.
v Master station is located in Colorado Springs at the Air Force Base.
User Segment:
Ø Made up of GPS and receivers.
Ø Civilians can only receive.
Satellite Ranging:
· GPS determines its position by measuring its distance from several satellites.
· Each satellite must transmit its position and accurate time signal.
· Ranging is when a receiver uses the time delay to calculate its distance.
· GPS is 3 dimensional, plus it incorporates time. It needs 4 satellites.
· Small errors in the GPS receiver’s clock causes distance errors. This is called pseudoranging.
Signal Details:
· GPS uses 2 radio signals on separate L bands.
· Signals are coded.
· L1 uses the C/A (course acquisition) and P (precision) code.
· L2 uses only the P code and is encrypted to be used by the military. This becomes the Y code. This process is referred to as ‘anti-spoofing’.
· Both L1 and L2 signals produce PPS or precise positioning service. Only the military and selected agencies have access to PPS.
· Civilian access is called SPS or standard positioning service (L1).
Identifying satellites:
¨ Each satellites L1 signal C/A code consists of 3 items of data: location, time, and condition.
¨ Satellites are identified by their SVN (space vehicle number) or PRN (pseudorandom noise code).
¨ All satellites transmit an almanac of information in their constellation.
Accuracy:
§ There are a potential of 5 error sources: clocks, orbits, atmosphere, receivers, or multi-path errors.
§ Clock errors of 1/1000 of a second will produce an error of 186.3 miles.
§ The Department of Defense adjusts the clocks.
§ Satellite orbits are affected by: gravitational forces, solar radiation pressure, position, or speed.
§ Multiple path errors occur when signals bounce off of objects in their path.
Selective Availability:
q Commonly called S/A. This is what the DoD does so that civilians can not gain precise positioning.
q They will introduce digital noise into the satellite clocks. This is called dithering.
q Dithering will limit the signal use by hostile forces. S/A errors will not affect the accuracy of the military receivers.
q GPS accuracy is limited to within 100 meters horizontally and 60 meters vertically.
q Differential correction or DGPS is commonly used to compensate for dithering.
q Differential correction uses real-time correction. Four common sources of real time correction are; Coast Guard, FM signals, Base station equipment, or satellite based differential correction.
q Coast Guard signals are limited. Are broadcast using radio beacons. Begin to fade after 300 miles.
GLONASS
v Glonass is the Russian government equivalent to NAVSTAR GPS. They also utilize 24 satellites. It is possible to access both systems with special receivers.
Accuracy Terms:
Ø Geometric Dilution of Precision (GDOP) is the quality of satellite geometry and magnitude of error.
Ø GDOP is affected by: Time (TDOP), Position (PDOP), horizon (HDOP), or vertical (VDOP).
Precision Ag Unit 2 Page # 1