User's Manual Part 1

Principles of Operation
P/N 7010-0713
1-3
For information on the current status of the GPS constellation, visit
http://tycho.usno.navy.mil/ or http://www.navcen.uscg.gov/. For
GLONASS, visit http://www.glonass-center.ru/frame.html.
Despite numerous technical differences in the implementation of
these systems, both GPS and GLONASS have three essential
components:
• Space – GPS and GLONASS satellites orbiting approximately
12,000 nautical miles above Earth and are equipped with a clock
and radio. These satellites broadcast digital information
(ephemerides, almanacs, time&frequency corrections, etc.).
• Control – Ground stations located around the Earth that monitor
the satellites and upload data, including clock corrections and
new ephemerides (satellite positions as a function of time), to
ensure the satellites transmit data properly.
• User – The community and military that use GPS/GLONASS
receivers and the corresponding satellites to calculate positions.
Calculating Positions
Once the receiver locks on to a satellite, it starts recording
measurements and receiving the various digital information
(ephemerides, almanac, and so on) the satellites broadcast. To
calculate a position, receivers use the following formula:
Velocity x Time = Distance
Where Velocity is the speed at which radio waves travel (i.e., the
speed of light) and Time is the difference between the signal
transmission time and signal reception time.
To calculate absolute 3-D positions—latitude, longitude, altitude—
the receiver must lock on to four satellites. In a mixed, GPS and
GLONASS scenario, receiver’s must lock onto at least five satellites
to obtain an absolute position.
To provide fault tolerance using only GPS or only GLONASS, the
receiver must lock onto a fifth satellite. Six satellites will provide
fault tolerance in mixed scenarios.