[time-nuts] GPS Timing in Interference Environment

[Normand Martel]

First of all, welcome in our little group of timing
enthusiasts Faisal! You'll see that we often find the
best jams in small pots!!!

My toughts are more on Rob's interference problems
rater on your problem, (since i have no idea of CW
interference effects on timing CPS receivers)...

But my personal tought is that you must take great
care on the type of antenna you'll use on your system.
As Rob said, the important thing to care about is the
INPUT filter (the one right behind the antenna element
and BEFORE any preamplifier). Using a GPS (or ANY) 
receiver in presence of strong RF fields requires high
quality (passive) filtering!

And for Rob's problem, that's a patent case of the
importance of GOOD filtering, not only for the GPS
receiver, but also for the transmitter.

Rob's DAB transmitter has a 500kW ERP, that's +87dBm!
On the other side, GPS signals are in the -130dBm
range, that's an over 217 dB of difference!!! I'm even
surprised that the system worked considering that
1575.42MHz 2nd sumharmonic (787.71MHz) is INSIDE the
TV channel's bandwidth! (783.25-789.25).

So don't hesitate to invest in a high quality antenna!

Good luck in your project and don't hesitate to give
us some news about it!

73 de Normand VE2UM
Montreal, Qc. Canada


--- Rob Kimberley <time.bandit at btinternet.com> wrote:

> Faisal,
> 
> There is probably some documented evidence available
> by Googling, but from 
> my own personal experience, I can tell you about an
> instance which happened 
> about 10 years ago while selling Datum GPS timing
> products in the UK.
> 
> The customer, Crown Castle, was running all of the
> BBC TV and Radio 
> transmitter network in the UK, and had a problem at
> a site near Oxford with 
> GPS reception. They had bought equipment from
> another supplier for a new DAB 
> network which was not working reliably on that site.
> GPS reception was 
> intermittent, and was insufficient to provide good
> disciplining of the 
> internal OCXO in the unit, and therefore the DAB
> installation was not being 
> synchronised properly.
> 
> I brought in one of the new Datum units to try
> (9390-6000), which like the 
> competitor used a Trimble engine and matching
> antenna. We had the same 
> problems. On further investigation one of the TV
> transmitter frequencies 
> being used (actually the second harmonic) was very
> close to the 1575 MHz of 
> the GPS L1. We discussed ways of maybe shielding the
> antenna in some way 
> (not really practical), but eventually after
> discussing with Datum we 
> managed to fix the problem by upgrading the antennae
> to then new Bullet II 
> (HE?) antenna which had a three pole filter rather
> than the single pole 
> filter that was in the original antennae. Both the
> competitors and the Datum 
> product worked fine after the upgrade.
> 
> For the record, the transmitter ERP was 500KW, the
> centre frequency of the 
> channel in question was 786MHz (Vision at 783.25 and
> Sound at 789.25), and 
> the control cabins and GPS installation were
> probably about 50 - 100 yards 
> away from the base of the TV antenna tower. GPS
> antenna were mounted on the 
> roof (single storey) of the control cabins
> 
> Hope this of interest.
> 
> Regards
> 
> Rob Kimberley
> 
> 
> ----- Original Message ----- 
> From: "Phaysal Khan" <mortal_f at yahoo.com>
> To: <time-nuts at febo.com>
> Sent: Tuesday, July 25, 2006 4:33 AM
> Subject: [time-nuts] GPS Timing in Interference
> Environment
> 
> 
> > Hi,
> >  I am new in this group, and wanted to discuss
> something abt GPS timing. I 
> > am working on GPS system & signals from timing
> point of view. I want to 
> > see Effects of Interference(CW) on GPS timing
> receivers. At this stage I 
> > am mainly interested on what will happen to the
> timing receiver itself 
> > when effected by CW interference and the next
> stage will be to find out 
> > how this effect could be mitigated.
> >
> > What I intend to do now, is to compare the 1 PPS
> pulse from two different 
> > rcvrs ( I will be using Motorola M12+ Oncore as
> the subject rcvr and Leica 
> > CRS1000 as the reference.
> >
> > Do you ppl have any suggestions?
> >
> > Regards
> >
> > Faisal
> >
> >
> >
> >
> > __________________________________________________
> > Do You Yahoo!?
> > Tired of spam?  Yahoo! Mail has the best spam
> protection around
> > http://mail.yahoo.com
> > _______________________________________________
> > time-nuts mailing list
> > time-nuts at febo.com
> >
>
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> > 
> 
> 
> 
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
>
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> 


__________________________________________________
Do You Yahoo!?
Tired of spam?  Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com