[time-nuts] Frequencies used for GPS

[Jim Lux]

On 10/19/11 6:25 PM, David Kirkby wrote:
> Can anyone tell me the transmission frequencies used for GPS?
> Wikipedia says 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal), but
> I'm confused about what the L2 signal is. Is this only of use to the
> military since the data is encrypted, or can consumer based GPS
> systems use this frequency?
>
> My reason for wanting to know is so I can design an antenna for GPS,
> but it wont be used for military purposes, so I've no idea whether I
> need to worry about the performance at the lower frequency or not.
>
> As always with Wikipedia, one can't be sure of the accuracy of the
> information. I'm guessing someone here will know precise details about
> the frequencies used.
>
> Also, what sort of polarisation is used?
>
> If anyone can tell me the desirable characteristics of an antenna for
> GPS, I would like to know. The antenna will be mounted outside and
> needs to be waterproof, so I'm not looking to fit the antenna in a
> small package.
>

most GPS antennas are helices of one sort or another (quadrifilar is 
popluar) and helices are broad band enough that designing for one gets 
you all frequencies.

There are also a fair number of microstrip patch antennas out there, but 
they tend to need stacked patches to get L1, L2, and L5


Even without the P(Y) code or using L2C, you can do carrier phase 
comparison between L1 and L2 to get better performance: the second 
frequency lets you compensate for ionospheric effects, which is 
important if you're looking for sub-1-meter kinds of accuracies.

L5 is coming along, and will eventually replace L2.  L2 is in spectrum 
that is not as well "protected" as L5.  There are currently two 
satellites radiating L2 (and I think the WAAS signal is also on  L5 as 
well as L1)

If you're building one for yourself for amusement or a project, there's 
lots of do it yourself projects out there.  If you need better 
performance or a more rigorous process, then you should get yourself a 
copy of the GPS ICDs: http://www.gps.gov/technical/icwg/

which provides the formal description of all the signals, incident flux, 
polarization, etc.