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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?

  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 height) or 4 satellites for a 3D fix.
  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 (latitude), north-south (longitude) 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. If the GPS receiver uses the signal from a fourth satellite it can solve an equation that lets it determine the exact time, without needing an atomic clock.
  8. 8. If the receiver is only able to get 3 satellites, you can still get your position, but it will be less accurate. The GPS receiver can get an approximate position by making the assumption that you are at mean sea level. If you really are at mean sea level, the position will be reasonably accurate. However if you are at altitude (such as in the mountains), the 2-D fix could be hundreds of meters off.
  9. 9. 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.
  10. 10. 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.
  11. 11. 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.
  12. 12. 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.
  13. 13. 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. You can use GPS to mark geocaching locations. 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
  14. 14. Solar Flares can also interfere with the GPS System. Commercial planes are required to have back up navigation to the GPS system.
  15. 15. 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.
  16. 16. Apps are very useful; some can be life-saving. Some caveats on relying on Apps: Your phone needs to be charged. Your phone could be damaged in an emergency situation. You need to have your phone with you. Some Apps require you be able to access a network. I have all these location apps on my iPhone.
  17. 17. After downloading any App, make sure you register and add contacts as required. It’s too late to do it when you need it. All Apps listed are FREE, unless otherwise noted. My listing of these apps doesn’t indicate endorsement, but I’m doing this, along with product descriptions, to give you access and information. I recommend you have a physical copies of my two books, one for preparing, the other one for the actual emergencies, on hand.
  18. 18. Family Locator-GPS Tracker for Android. GPS Tracker for Android. Family Locator simplifies life in the digital world by making it easy to stay connected to the people who matter most. With Life360 you can: • Create your own groups, called “Circles,” of loved ones, friends, teammates -- whoever matters most and chat with them in Family Locator for FREE. • View the real-time location of Circle Members on a private family map that’s only visible on Family Locator • Receive real-time alerts when Circle Members arrive at or leave destinations (Eliminate disruptive “Where are you?” texts) • Track stolen phones or lost phones
  19. 19. Emergency Alert System for Apple. Alert, track and notify loved ones as well as 911 in an emergency situation. Silent Beacon provides you with the peace of mind that comes from knowing that you and your loved ones are able to instantly connect. The emergency alert system for the digital age.
  20. 20. Road ID eCrumbs App (Apple and Android) for Apple and Android. This is one I use when off by myself running, biking or hiking. It’s a way of alerting people by email and text if something happens to me and I stop moving for a certain period of time (mine is set for 5 minutes). The all new Road ID App is a great tool for runners, cyclists, hikers, and walkers. With amazing features like eCrumb Tracking, a Stationary Alert, and a custom Lock Screen creator, the Road ID App is your perfect training partner. With the ability to track your workouts in real time, your friends and family can stay better connected whenever you head outdoors...delivering peace of mind like never before.
  21. 21. Compass An interesting tidbit for Apple Compass: it has a second page that is essentially a level and gives you accurate readings on inclines. Google Earth (Apple) (Android)
  22. 22. Print Book
  23. 23. The book on the left is how you prepare NOW. The book on the right, is your guide to surviving an emergency or catastrophe given you’ve prepared. The handbook in the center is a discounted, distilled version of both books with the most important basics - click on cover for any. Print Book
  24. 24. I highly recommend getting at least one print copy of the survival manual. During an emergency you can’t count on an eBook. It’s pocket-sized— shown next to my trusty, old Ranger Handbook. I keep one in the glove compartment of each car, in every Grab-n-Go bag and also give them as gifts— the gift of life. Click on image for print version Print Book
  25. 25. 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 seventy 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 book below available HERE