GPS uses signals from satellites to determine location on Earth. It requires signals from 3 satellites for a 2D location (without altitude) or 4 satellites for a 3D location. GPS satellites orbit Earth every 12 hours and there are usually 6 in view from any location. The GPS receiver in a device calculates distance to satellites using signal travel time to determine the user's position, latitude, longitude, and altitude. Factors like satellite positions, atmospheric conditions, and clock errors can impact accuracy, which is typically within 5 meters under ideal conditions. Cell phones use assisted GPS to get fixes faster through cell towers when in range. Understanding maps, backup navigation, power sources, and solar flare impacts is important when using GPS.

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.
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 out of
there 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”.
Also peace of mind as my family can get
hold of me any time and I can update
them on my progress.

16. Download the contour map for your area for free, then
print it out, or order the map sheets.
A link for USGS free downloadable topo maps i son my
web site in the pop up.
I also have a series of National Geographic area
specific maps for various National Forests and Parks
Map

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 on my web site, 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 signal) and you learn
how to use the app.
They also send interesting email updates on various
outdoor activities that are very informative from outdoor

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 constantly update free, downloadable
slideshows like this on my web site for
preparation and survival and other topics.
Use Your Camera on this QR Code

20. More Free Information
Since Slideshare no longer supports links,
I’ve put all the links to free apps, gear and web
pages on my web site at
www.bobmayer.com
Use the pop up from the following image there and
scroll through for what you want:

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

22. AMAZON

25. 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