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How Does GPS Really Work?

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Understanding how GPS works will give you insight into how effective it is, how accurate and how much you rely on it. Why does your car get your location faster at certain times? How many satellites are there? Does your cell phone GPS need reception for the GPS to work?

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How Does GPS Really Work?

  1. 1. HOW DOES GPS WORK? The Procastinator’s Survival Guide
  2. 2. GPS is the Global Positioning System, also called NavStar. It was built by the US Military and has been operational since 1995. Most GPS receivers utilize a combination of both NavStar and the Russian GLONOSS satellites for best results.
  3. 3. GPS currently has 31 active satellites in orbits inclined 55 degrees to the equator.
  4. 4. You need 3 GPS satellites for a 2D fix (i.e. no altitude) or 4 satellites for a 3D fix. This could be significant if you are in the mountains at higher altitudes, making it less accurate.
  5. 5. The satellites orbit about 20,000 kilometers up and make two orbits per day. The orbits are designed so that there are always 6 satellites in range from most places on the earth.
  6. 6. The GPS receiver gets a signal from each GPS satellite with the exact time the signals are sent. By subtracting the time the signal was transmitted from the time it was received, the GPS can tell how far it is from each satellite. The receiver knows the exact position in the sky of the satellites. Given the travel time of the GPS signals from three satellites and their exact position, the GPS receiver can determine your position in three dimensions - east-west (longitude), north-south (latitude) and
  7. 7. To calculate the time the GPS signals took to arrive, the GPS receiver needs to know the time very accurately. GPS satellites have atomic clocks that keep precise time, however it's not feasible to equip a GPS receiver with an atomic clock. The GPS receiver uses the signal from satellite to determine the exact time.
  8. 8. When you switch your GPS on, the time-to- first-fix depends on how long it is since you last used the GPS. If the GPS has been recently used you should get a fix almost immediately. If it hasn't, put the GPS outside with a clear view of the sky. If you have a GPS in a vehicle, it's better to wait for the unit to get a fix before driving off. Receiving data from a satellite takes 30 seconds. If you drive in an area with tall buildings or other obstructions it may take longer to get the data from four satellites, needed for the first fix.
  9. 9. The accuracy of the position your GPS reports is influenced by a number of factors, such as the positions of the satellites in the sky, atmospheric effects, satellite clock errors, etc. GPS units often show on the screen an accuracy figure. Under ideal conditions, this usually is within 5 meters.
  10. 10. Cellphone GPS units get a fix almost immediately. They use Assisted GPS (A-GPS) to improve the time to first fix, and even allow a fix in conditions where the GPS might not otherwise be able to function.
  11. 11. Once you are out cellphone and WiFi coverage, a cellphone GPS unit has to rely on the satellites to provide the ephemeris & almanac date so, like a standard recreational GPS, it takes a minute or so to get a fix from a cold start.
  12. 12. Things to consider about GPS: Make sure you have whatever maps you might need pre-loaded in the memory in case you can’t download them. Remember, GPS works only as long as the power works. It also only works as long as the satellites are transmitting. Always back up with paper maps and know how to navigate
  13. 13. Solar Flares can also interfere with the GPS System. Commercial planes are required to have back up navigation to the GPS system.
  14. 14. For information on where to get free topographic maps and basic map reading, please use the link at the end to my other survival slideshares.
  15. 15. Print Book Free slideshows on survival, click on image
  16. 16. Information is the most important tool you can arm yourself with. I wrote this book specifically for those who’ve been uncertain about how to proceed. The Procastinator’s Survival Guide
  17. 17. New York Times bestselling author, graduate of West Point, former Green Beret, and feeder of two yellow Labs, most famously Cool Gus. He’s had over 75 books published, including the #1 bestselling series Time Patrol, Area 51, Atlantis, and the Green Berets. Born in the Bronx and having traveled the world he now lives peacefully with his wife and labs. Sort of. Free books below available HERE www.bobmayer.com

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