[time-nuts] BPSK Receiver & GPS Antenna siting

[David]

I have been thinking about this problem on and off over the last
couple of days.

Would it be better to take the absolute value rather than squaring the
signal?

I might try some tricky but impractical analog sampling and/or
synchronous demodulation recovery method but the Costas loop looks
awfully easy to do without any digital signal processing.

On Thu, 27 Sep 2012 16:45:16 -0700 (PDT), "J. Forster"
<jfor at quikus.com> wrote:

>You cannot put a narrow filter before the squarer for reasons previously
>cited. In a low S/N area, squaring just makes matters worse wrt dynamic
>range and clipping.
>
>-John
>
>==========
>
>> Hi
>>
>> A PLL locks to phase. If the phase switches by 180 degrees, the phase
>> tracking switches signs. There's no way to track that. You either need to
>> double the frequency (and thus eliminate the modulation) or demodulate the
>> signal and lock to the result. If you simply put up a real narrow filter
>> and hit it with continuous 180 degree phase shifts, the output will be
>> nothing at all

>>
>> Bob
>>
>>
>> On Sep 27, 2012, at 7:11 PM, johncroos at aol.com wrote:
>>
>>> Various comments -
>>>
>>> Hal mentioned SNR for the scheme I suggested. A PLL can be a coherent
>>> demodulator of arbitrary
>>> bandwidth. Thus the PLL at the output of the doubler can have a small
>>> bandwidth since at that point
>>> there is no PSK, it having been removed by the doubler. So given a
>>> stable VCXO you can probably get down
>>> to 1 Hz and thereby achieve a good SNR. There is a lot of stuff out
>>> there on phase tracking receivers
>>> that do exactly that. You know the frequency so the loop does not have
>>> to search far and the BW can be increased
>>> for acquisition and closed up for tracking.
>>>
>>> On writing reams of code - my point was that it is not required to used
>>> the admittedly more powerful software
>>> techniques to do this job - I noted that one reason to write reams of
>>> code is for the fun of it, this is after all
>>> a hobby.
>>>
>>> GPS Antenna Siting -
>>>
>>> Lets not make this so hard. Mine is at 6 ft elevation and is blocked to
>>> an elevation angle of 20 to 30 degrees by a house
>>> within 15 ft and a forest of trees. I have room and no restrictions but
>>> I also have severe thunderstorms - so the house
>>> plays lightning protect for the antenna. My T bolt tracks a Rb to better
>>> than 1e-12 over 24  hours with no serious 10 MHz phase bumps
>>> as plotted on a  strip chart recorder.
>>>
>>> Sooooo  -
>>>
>>> Put your antenna at 6 ft in back yard. Start out on a photo tripod - who
>>> is gonna notice?
>>> set up a t bolt at EL=5 AMU=0 Damping = 1.2 and Time Constant = 100 sec.
>>> get the t bolt manual
>>> get Tbolt monitor
>>> get Lady Heather and read all that stuff.
>>>
>>> Run Lady Heather antenna survey (command SAS)  for at least two days -
>>> you get a map of signal level in dBc vs elevation
>>> Reset the Tbolt elevation mask to reject anything that is shown as
>>> blocked using the signal
>>> level map. Likewise experiment with the AMU setting to reject the weak =
>>> poor signals. Mine works good
>>> with AMU all the way up to 10 as fewer good satellites are better than
>>> lots of weak ones.
>>>
>>> The satellites are in high orbits so masking those below 25 degrees is
>>> OK and the AMU sets the acceptable signal
>>> level - at AMU 10 my setup throws out those below about 40 dBc - the
>>> strong guys go up to 50. This is a function
>>> of you antenna performance so some experimentation is required.
>>>
>>> -73 john k6iql