[time-nuts] I thought GPS repeated every 12 hours (-2 minutes)

[Bob Camp]

Hi

Multipath on GPS normally requires a couple of things:

1) The satellite you are trying to lock on to needs to be obscured. Being below the 
local horizon is one normal way for this to happen. 

2) The satellite signal needs to be reflected off of something that does not put in enough 
dopler to take it out of whatever the doppler correction “stuff” is happy with 

3) The signal path length has to be close enough that the normal firmware does not reject
the solution. 

4) The reflected signal has to be strong enough to be out of the noise in the normal demodulation
bandwidths. This includes odd things like polarization. 

5) The geometry between the objects (sat included) has to hold long enough for it to contribute to 
a multi second solution. Anything below a few seconds is normally rejected by the GPSDO firmware. 

I’m sure there are a few other qualifiers. Bottom line is that it’s easier to get multi path off of a 
stationary object. A shallow angle fixed geometry setup can give you a problem for quite a while. 

Bob



> On May 24, 2016, at 10:26 PM, David <davidwhess at gmail.com> wrote:
> 
> On Tue, 24 May 2016 16:15:15 -0700, you wrote:
> 
>> 
>> kb8tq at n1k.org said:
>>> The glitches are to narrow (short duration) and far to regular for the
>>> ionosphere to be the issue.  
>> 
>> Is multipath from a large airliner in a landing pattern likely to cause that 
>> sort of problems?
>> 
>> I'm 20+ miles off the end of SFO, but it's common to see large planes going 
>> over and turning to line up for a landing.  On my one-of-these-days list is 
>> to grab the airline location data and see if it correlates with GPS glitches.
> 
> Back when I did a lot of transmitter hunting, I listened to multipath
> from airliners from 2 meters to 23 centimeters.  The 2 meter
> directional antenna I ultimately designed was good enough to not only
> track airlines by their reflected RF, but it could see reflections and
> shadows from nearby objects like street lights and trees which
> ultimately limited outside performance testing.
> 
> As you can calculate from the geometry, the flutter started out fast
> and decreased in frequency until there was a slow null/peak and then
> it reversed.  If the same happened with GPS, then maybe the receiver
> could briefly lock onto the reflection from the plane producing a
> different solution for a short time.
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