How Does GPS Work? And Why Do You Need To Understand It?

The document provides an overview of the Global Positioning System (GPS), detailing its origins, operation, and the number of satellites in use. It explains how GPS enables location determination through satellite signals and emphasizes the importance of additional navigation tools like paper maps and mobile map applications in areas with no signal. The author, a bestselling author and former Green Beret, shares personal insights on navigating remote areas and the value of GPS technology for safety and communication.

1. WHAT YOU NEED TO KNOW ABOUT GPS

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. GPS currently has 31 active satellites in orbits inclined 55 degrees to the equator.

4. You need to reach 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. The satellites orbit about 20,000 kilometers up and make two revolutions per day. The orbits are designed so that there are always 6 satellites in range from most places on the earth.

6. The GPS receiver gets a signal from each GPS satellite with the exact time the signal is 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 altitude.

7. An easy way to remember which is which for latitude and longitude, think of latitude as a ladder on the globe running east-west. Thus, longitude is vertical.

8. 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 the satellite to determine the exact time.

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. 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. Cellphone GPS units get a fix almost immediately. They use Assisted GPS (A-GPS) off of cell phone towers to improve the time to first fix, and even allow a fix in conditions where the GPS might not otherwise be able to function. As long as you are in range of a tower.

12. Once you are out of cellphone and WiFi coverage, a cellphone GPS unit has to rely on the satellites to provide the ephemeris & almanac data so, like a standard recreational GPS, it takes a minute or so to get a fix from a cold start.

13. Things to consider about GPS: Make sure you have whatever maps you might need for your navigating app pre-loaded in the memory in case you can’t download them— ie you can access the GPS satellites but not wifi for the map tiles.

14. Solar Flares can also interfere with the GPS System. Commercial planes are required to have back up navigation to the GPS system. You should too.

15. 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. See my slideshow on MAPS

16. I carry a SpotX 2 Way Satellite messenging system. While going to Hole in the Rock in Grand Staircase-Escalante National Monument, my clutch began to burn out. 120 miles from the nearest civilization. Luckily, I managed to sustain in third gear, but it made me consider the situation. There are places all over the country, including in the Smoky Mountains, with no cell coverage. I view this as a potentially life-saving investment. It will give me my grid location so that, in fact, I will never be actually “lost”.

17. Location Apps There are several topo map Apps you can get. I’ve used a number over the years but the best one is Gaia. The basic app is free. The premium is definitely worth it. One essential thing is to download the map tiles you want to use beforehand and learn how to use the app.

18. To the left is an example of some of the many maps you can download and use. You can have multiple maps on screen at the same time, adjusting them as you need. In this case, I have the US Forestry Service Map at max along with USFS roads and trails along with public lands. I can bring up hidden layers as needed. Also note the Historic Topo for 1930 which is fun to play with. You also have National Park Service Visitor, topo maps, streets maps, weather and more. There are dozens of different types of maps available to premium members.

19. More Free Information I’ve put all the links to free apps, all gear mentioned and web pages on my web site at www.bobmayer.com Go to the GEAR page There are also free books on my web site, updated daily.

20. New York Times bestselling author, a graduate of West Point and former Green Beret. He commanded a Green Beret A-Team and also served in a variety of other positions. He was a certified instructor at the JFK Special Warfare Center & School which trains Green Berets and also runs the Survival, Evasion, Resistance and Escape (SERE) school. www.bobmayer.com