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?

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

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.
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

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

15. I carry a SpotX 2 Way Satellite
messenging system along with
GPS locator.
I’ve gone many places where
there is no cell phone
coverage. There are locations
all over the country, including in
the Smoky Mountains, with no
cell coverage. I view this as a
potentially life-saving
investment.
Also peace of mind as my
family can get hold of me any
time and I can update them on
my progress.

16. For information on where to get free
topographic maps and basic map
reading, please use the link below or at
the end to my other survival
slideshares.

17. Location Apps
There are several topo map Apps you can get.
I’ve used a number over the years but the best
one I’ve found is Gaia. The basic app is free and
then there are two levels of membership. The
premium, which is discounted 20% via my
affiliate link, is $32 for a year but for the number
and types of maps you get, it is definitely worth it.
One useful thing to using any map app is to
download the map tiles you want to use
beforehand (when you’re out in the wilds with no

18. This is 2 Gaia maps, one using Apple Carplay
and the other my iPad. I use different layers and
scales when Jeeping.

19. More Free Information
I constantly update free, downloadable
slideshows like this on my web site for
preparation and survival and other
topics.
FREE SLIDESHOWS
Also, I conduct Area Study workshops
for those interested in properly
preparing for their specific
circumstances.

20. AMAZON

21. AMAZON

22. AMAZON
This book walks you through your personal situation,
your home, and your Area of Operations.

24. New York Times bestselling author, is a graduate of West
Point and former Green Beret. He’s had over 80 books
published, including the #1 bestselling series Green Berets,
Time Patrol, Area 51, and Atlantis. He’s sold over 5 million
books. He was born in the Bronx and has traveled the world.
He’s lived on an island off the east coast, an island off the
west coast, in the Rocky Mountains, the Smoky Mountains
and other places, including time in East Asia studying martial
arts.
He was an instructor and course developer/writer for years
at the JFK Special Warfare Center and School which trains
Green Berets and also runs the SERE school:
Survival, Evasion, Resistance and Escape.
www.bobmayer.com