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Interference between GPS Installations ?

 
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philip4242
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Joined: May 26, 2004
Posts: 123

PostPosted: Wed Jul 28, 2004 11:52 pm    Post subject: Interference between GPS Installations ? Reply with quote

I wasn't certain whether to post this here or in the Holux or RoyalTek forums, as the're both involved.

I have a Medion PPC-based GPS system installed in my car, with the power adaptor connected to a permanently live connection - so the RoyalTek GPS mouse always has a supply to it.

I've recently bought a Toshiba E740 PPC (with 802.11b wireless LAN) and just bought a Holux GM-270 CF GPS card for it.

(The intention is to get the Toshiba and Holux kit up and running - then sell the Medion kit.)

Anyhow, I've installed the supplied GPS Viewer utility on the Toshiba PPC and fitted the CF GPS card.
The 'Scan' facility found the GPS card on COM4 and pressing 'Open GPS' lights the LED on the card solidly and starts to show text messages in the window, so the unit appears to be 'talking' OK to the PPC.

However, I've so far failed to see any satellites at all. The only changes I've seen on the status screen are the numbers at the 'north pole' of the globe, and near the bottom left of the screen changing every few seconds.
It was still the same with the external aerial connected and positioned on the dashboard of the car, near where the RoyalTek GPS mouse sits and gives a good signal normally.

To check I wasn't parked in a genuinely poor signal area, I connected the Medion kit up and was somewhat relieved to find it also showed no satellites.

So I moved the car down the road to an area where I've definitely seen 7 or 8 satellites (yes - I know the number of satellites visible at any one time at any location can vary !) - still no satellites on either GPS.

Then I powered down the Toshiba PPC prior to driving back home to start looking at forums for help with this problem.

The Medion PPC then showed 4, then 5, then 7 satellites !

Powering on the Toshiba PPC and pressing 'Open GPS' caused the Medion PPC to lose its lock again - power the Toshiba PPC off - 7 satellites on the Medion PPC again !

So the Toshiba PPC GPS appears to interfere with the Medion PPC GPS.

Inspiration strikes !!

I power down the Medion PPC's RoyalTek mouse !!

Does the Toshiba PPC GPS then show lots of satellites ? - Does it f*** !!

I've done soft resets on the Toshiba PPC and cold starts on the CF card and waited and waited until I'm blue in the face - all to no avail.

In case anyone thinks to ask, the wireless LAN card on the Toshiba PPC has been turned off all the time during this saga, so it's not that causing the interference.

So the actual question I now have is - Even though I've removed the power from the RoyalTek GPS mouse, does the internal battery within it keep the GPS receiver powered up (thus keeping on generating the interfering signal) or does the battery solely support the internal memory of the GPS mouse ?

Or do I have a Holux card with a faulty receiver ?

The 2 receivers are now at opposite sides of the car, so the only way I can get them further apart (since the RoyalTek GPS mouse is semi-permanently installed) is by going walkabout with the Toshiba PPC - but I'm not going to do that at this time of night !

Any ideas, anyone ?
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Dave
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Joined: Sep 10, 2003
Posts: 6460
Location: UK

PostPosted: Thu Jul 29, 2004 11:33 am    Post subject: Reply with quote

Ok, it sounds like you've carried out quite an exhaustive test. I'll quote some specifications just in case you're not aware.

1) New GPS's can take up to 12.5 mins to download the complete almanac and ephemeris table, so you may find inital TTFF is slow, or even longer than 12.5 mins if it doesn't have a good view of the sky. I doubt this is your issue because you have had fixes, correct ?

2) The RoyalTek will not be powered when you unplug the cable, all the backup battery/capacitor does is to retain the settings in the chip, that's all. There's very minimal power going to the GPS and this won't cause any interference.

3) GPS's can't actually interfere with each other, they don't do any transmitting out of the antenna, all they are doing is sitting in passive mode, blindly downloading the data it's receiving, then sending it along the cable.

Now what I suspect is happening is the Toshiba is interferring with the Medion, and vice versa. These being cheap(er) Pocket PC's may not be shielded all that well. You may also find the cables aren't shielded that well and outputting interference that is knocking the signal out.

Does the Holux work in the Toshiba if the RoyalTek/Medion is unpowered ? If no, then you could have an intermittent problem with the card, or you could be finding that the processor on the Toshiba isn't shielded properly and as it's close to the CF slot, it could be creating RF interference to the Holux card which is blanking out the data. Similar instances occurred on Navman 3000 Sleeves because the SiRF chip on the GPS was in-fact too close and un-shielded to the iPAQ's processor.

Hope this helps.
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philip4242
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Joined: May 26, 2004
Posts: 123

PostPosted: Thu Jul 29, 2004 1:42 pm    Post subject: Reply with quote

Thanks for the response, Dave.

I've now spent some time with the Toshiba PPC and CF card in the garden, well away from the car and the Medion PPC.

With the external aerial connected, I've now seen several satellites, got the correct time of day downloaded and seen my latitude and longitude displayed, although the display still shows 'no fix'.

Without the external aerial connected, I don't see any satellites, although I only waited a couple of minutes with the setup like this.

As regards to GPS receivers being passive, I realise that they don't transmit (as such), but most radio receivers have a local oscillator to mix with the incoming signal to convert the received signal down to a lower frequency (a superhet design).

I'm suspicious that this (interference between the 2 local oscillators) is the root of the problem, as disabling the CF card, but leaving the Toshiba PPC powered up allows the Medion PPC to get a good lock. Pressing 'Open GPS' on the Toshiba PPC causes the failure again. Press 'Close GPS' and the problem goes away.

I haven't had chance to do any more testing in the car yet, as I'm about to set off for London, where I was hoping to use the Toshiba PPC and CF card to be fully mobile while sight-seeing in the City, but I haven't had chance to load the navigation software and maps onto it.

Oh well ! Back to pocket maps !
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ckuethe
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Joined: Aug 08, 2004
Posts: 18
Location: ECEF X/Y/Z: -1514741/-3483783/5106784 m

PostPosted: Mon Aug 09, 2004 12:08 am    Post subject: Reply with quote

I've done a bunch of testing with my Rayming TN200 sitting right on top of my Garmin eTrex and have never seen anything like interference. Usually the problem is doing this while not standing in a big forest or obscuring the antennas with my body.
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