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I've been surfing the web for info regarding the GPS carrier
frequencies for you guys wanting to design and build your own
antennas. Though it sounds like a bit of a daunting task, the
following may help.
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GPS Signals in detail
Carriers
The GPS satellites transmit signals on two carrier frequencies. The L1
carrier is 1575.42 MHz and carries both the status message and a
pseudo-random code for timing.
The L2 carrier is 1227.60 MHz and is used for the more precise military
pseudo-random code.
Pseudo-Random Codes
There are two types of pseudo-random code (see tutorial for explanation
of pseudo random codes in general). The first pseudo-random code is
called the C/A (Coarse Acquisition) code. It modulates the L1 carrier. It
repeats every 1023 bits and modulates at a 1MHz rate. Each satellite
has a unique pseudo-random code. The C/A code is the basis for civilian
GPS use.
The second pseudo-random code is called the P (Precise) code. It
repeats on a seven day cycle and modulates both the L1 and L2 carriers
at a 10MHz rate. This code is intended for military users and can be
encrypted. When it's encrypted it's called "Y" code. Since P code is
more complicated than C/A it's more difficult for receivers to acquire.
That's why many military receivers start by acquiring the C/A code first
and then move on to P code.
Navigation Message
There is a low frequency signal added to the L1 codes that gives
information about the satellite's orbits, their clock corrections and other
system status.
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I copied this from a Trimble site which
has a very well written tutorial on GPS, with pictures and
animation for us "reading comprehension challenged" at
"http://www.trimble.com/gps/"
It's full of general info about the system and how it works
and they have made it digestible.
I'm sure there's other company's sites out there too, this
is just the first one I hit via Yahoo. Good luck with your
antenna design.
Dan Schaefer
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