[time-nuts] GPS Timing in Interference Environment

Rob Kimberley time.bandit at btinternet.com
Tue Jul 25 08:03:47 UTC 2006


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
>
>
>
>
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