[time-nuts] Homemade GPS Receiver

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Fri Sep 26 11:25:58 EDT 2014

On 26 September 2014 13:52, Azelio Boriani <azelio.boriani at gmail.com> wrote:
> From gpsinformation.net:
> In the frequency allocation filing the L1 C/A power is listed as 25.6
> Watts.  The Antenna gain is listed at 13 dBi.  Thus, based on the
> frequency allocation  filing, the power would be about 500 Watts (27
> dBW).
> Now, the free space path loss from 21000 km is about 182 dB.  Take the
> 500 Watts (27 dBW) and subtract the free space path loss (27 - 182)
> and you get  -155 dBW. The end of life spec is -160 dBW, which leaves
> a 5 dB margin.

I have not checked your figures, but assuming they are right, the -155
dBW would be based on a receiver antenna with 0 dBi gain, since that
is what the free space path loss assumes. Any lossless antenna will
have a peak gain of > 0 dBi.

> And if you really get into it, you'll discover ALL of the following
> represent the same approximate signal strength for GPS on the face of
> the earth (m stands for milliwatts and m2 stands for meters squared):
> -160 dBW, -130 dBm, -135 dBW/m2, -105 dBm/m2, -223 dBW/Hz, -163
> dBW/MHz, -193 dBm/Hz, -198 dBW/m2/Hz, -138 dBW/m2/MHz

As I say, without stating the properties of the receiving antenna,
absolute power levels are not a sensible unit.


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