Informal notes
Global positioning system overview
by Cindy Petterson
September 12, 2001

A story on the vulnerabilities of the Global Positioning System (GPS) was eerily reported early September 11–hours before terrorist attacks on America. According to the Transportation Department, GPS systems used by ships, airplanes and trains are vulnerable to "spoofing" and interference by enemies (Wald, 2001). Matthew L. Wald went on to say, due to the low strength of the radio signals, a counterfeit signal could confuse and blank out receivers and satellites could become targets in the event of war (2001).

The following overview on Global Positioning Systems, and its components is gleaned from the Department of Geography at The University of Colorado at Boulder (Dana, 1994).

GPS Overview:

  • GPS or Global Positioning Systems are a system of satellite signals sent to special receivers
  • There are four satellite signals used to compute latitude, longitude, velocity and a time component offset by a receiver clock
  • The U.S. Department of Defense (DOD) developed, funded and controls GPS. Civilians are allowed to use GPS with more limitations than the military counterparts
GPS Satellites:
  • There are 24 GPS satellites or space vehicles orbiting the earth every 12 hours
  • The satellites are spaced 60 degrees apart and on a 55 degree incline to the equatorial plane
  • There are always five to eight space vehicles visible from any point on earth

GPS Receivers:

  • According to the Federal Radionavigation Plan, there are two types of receivers, those used by the U.S. Government agencies and civilians
  • Precise Positioning Service (PPS)–used by U.S. government authorized users who are equipped with cryptographic equipment
  • Standard Positioning Service (SPS)–used by civilians with no restrictions but intentionally less accuracy
  • The PPS systems are accurate to 22 to 27.7 meters of accuracy while SPS is accurate to 100 to 156 meters of accuracy
  • SPS and navigation code signals are transmitted in microwave signals at 1575.42 MHz and PPS signals are sent at 1227.60 MHz

GPS Master Control:

  • Schriever Air force Base in Colorado is the master control over all of the satellite orbits
  • The GPS Master Control has tracking stations throughout the world

GPS Uses:

  • Navigation–this is the primary function of GPS in the air, on the water and on the ground
  • Positioning–by precise location monitoring, surveying and plate tectonics are possible using GPS
  • Time Dissemination–telecommunications and astronomical observations are some of the uses for precise time signalling
  • Other Research–projects have included atmospheric measurement

Sources of Error:

  • Noise–created from the transmission as well as the receiver
  • Bias–the Department of Defense uses intentional degradation of the signal for civilian use
  • Blunders–the most significant source of errors are due to computer and human error in the magnitude of up to hundreds of kilometers

Sources:

Dana, Peter H. Global Positioning System Overview, The Geographer’s Craft Project, Department of Geography, The University of Colorado at Boulder, c1999.

Engelhardt, Jim. "What’s e-commerce have to do with GIS?" Geo Info Systems; Cleveland, Jan 2000.

GPS Points Cops in Direction of Stolen Car. The Toronto Sun c2001 Sun Media Corporation.
August 16.

Kennedy, Randy. "Hail a Cab, Read a Commercial" The New York Times. August 26, 2001.

Report Cautions on Plane Track System, Associated Press, September 11, 2001.

Technology Briefing. Milwaukee Journal Sentinel, c2001 Journal Sentinel Inc. August 21.

TV Must-Know Info. The Toronto Sun c2001 Sun Media Corporation. (broadcast of David Letterman on October 26, 1999). August 14.

Wald, Matthew L. "Report cites Vulnerabilities of Global Navigation Systems" The New York Times. September 11, 2001.

 

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