What is the signal?
GPS satellites transmit two low power radio signals, designated L1
and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF
band. The signals travel by line of sight, meaning they will pass
through clouds, glass and plastic but will not go through most solid
objects such as buildings and mountains.
A GPS signal contains three different bits of information — a
pseudorandom code, ephemeris data and almanac data. The pseudorandom
code is simply an I.D. code that identifies which satellite is
transmitting information. You can view this number on your Garmin GPS
unit's satellite page, as it identifies which satellites it's
receiving.
Ephemeris data, which is constantly transmitted by each satellite,
contains important information about the status of the satellite
(healthy or unhealthy), current date and time. This part of the signal
is essential for determining a position.
The almanac data tells the GPS receiver where each GPS satellite
should be at any time throughout the day. Each satellite transmits
almanac data showing the orbital information for that satellite and
for every other satellite in the system.
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