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Recieving GPS In High Radiation Fields... (Among Other Unfavorable Situations.)

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Patrick

Thu Apr 10 2014, 01:55AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im investigating the use of GPS for unmanned flying systems, monitoring 500kV AC lines and Fukushima tank farms, and the Chernobyl sarcophagus, as well as Hanford, Bhopal, love canal, salton sea and similar sites.

but im skeptical about GPS, its usefulness when its working i dont doubt, but the situations when it wont work could be of profound importance when human life is at stake.

so my question: when within 3 - 100 feet of a ultra high voltage power line, like a 500kV transmission line, would the delicate and sensitive GPS signals get drowned out around the EM/RFI noise of an HV wire?

and next would the same be true about gamma, beta, alpha and x-ray fields, either "light" (not being insulting) fields all the way up to high intensity particulate and emissive fields like a fully open weapon core or reactor core?

2Spoons

Thu Apr 10 2014, 05:01AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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The spreading codes used in GPS signalling are designed to get around the noise issue. In the situation you describe you should have a good, wide sky view and therefore a good "strong"signal. GPS occupies a band roughly 6MHz wide centred on 1.5GHz. I would think this was well away from the sort of noise you'd get on HV lines.
The hard radiation I'm not so sure about - but i would have thought the effect would be to upset the silicon directly - so it could affect microcontrollers as well as GPs recievers.

Shrad

Thu Apr 10 2014, 06:51AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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I'd say the same

You wouldn't harm the GPS signal in itself but your infrastructure, which would either suffer from unwanted logic triggering, gain modification, avalanche effects or inductive effects

Dr. Slack

Thu Apr 10 2014, 07:43AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
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What you're worried about is a standard problem known in receiver engineering as 'blocking', where a strong signal upsets the receiver so it can't receive the wanted one.

In your case, the collosal difference in frequency between the wanted signal (1 ish GHz) and the unwanted (50Hz) means that the GPS antenna is certainly going to be a good enough filter to effectively eliminate any 50Hz at the reciver input. Where you might be less lucky is with the higher frequency effects of any corona etc.

The other way into a receiver is through wiring, unsheilded areas etc, and again the low frequency should mean that coupling into any of those things should be very inefficient.

You could try recreating the sorts of field gradients you expect next to a power line with an NST on the bench, and see it it malfunctions in that field, though that's not going to help you with RF unique to high voltage lines.

There is a systematic way to harden a receiver against strong signals outside your band
a) low-pass filter data and power lines in/out of a sheilded container round the radio
b) band-pass filter the antenna connection with a tight filter around your wanted frequency (should be possible to buy standard GPS filters)

If that doesn't work because there's interference in-band, then you've done all you can with a commercial grade receiver.

Shrad

Thu Apr 10 2014, 12:55PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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I'd use a tesla coil with a massive topload to eliminate breakout and maximize radiated energy, all inside a faraday cage, for testing against corona

I'd use a range of x-ray and electron emitters for some soft radiation testing

I'd try to focus an x-ray beam at a hafnium target to generate harder radiation, but that's not my field ^^

Patrick

Thu Apr 10 2014, 05:02PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Dr. Slack wrote ...

Where you might be less lucky is with the higher frequency effects of any corona etc.

i should have been more clear, having seen electrical linemen crawl out of a helo, wearing woven metal clothing and bond on to a live 500kV line, its the non-50/60Hz stuff i worry about.

looks like ill need a coiler's help...

BigBad

Thu Apr 10 2014, 08:42PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Dunno, you'd have to try it; it would depend on how close you are to sources of corona and the receiver you're using. At 100 feet you're unlikely to have problems (otherwise GPS would fail under power lines, but so far as I know it rarely does), at 3 feet, probably quite likely, at least sometimes.

If you do get problems directional antennas would help and some receivers will be better than others.

Radioactivity; silicon doesn't like that at all; under hard radiation your computers WILL crash.

Patrick

Thu Apr 10 2014, 10:16PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

BigBad wrote ...

Radioactivity; silicon doesn't like that at all; under hard radiation your computers WILL crash.

I know some makers like ST, sell hardened silicon meant for space and martial use, so is that how Casini and other solar traveling machines are built?

So, are there functional limitations on "hardened" silicon? Or just a higher price tag?

A centimeter of lead won't be enough, and more than a cm, means my drone won't fly due to " lead brick-ification."

2Spoons

Fri Apr 11 2014, 01:52AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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From memory some of the older silicon processes are more robust under radiation because the structures are larger : things like gate capacitances are higher so more more energy is needed to cause trouble. If you run a geiger counter on board you would have forewarning of problems.

On GPS interference: one often overlooked source (I know because I fell into this trap myself) is signals at the IF frequency of the reciever. This can fool the AGC into thinking there's lots of gps signal, so it winds back the gain - and the real gps signal gets lost. In my case the IF was at 98MHz, unfortunately a harmonic of my micro clock. I fixed it by picking a crystal with harmonics more than 6MHz from the IF.

BigBad

Fri Apr 11 2014, 04:10PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Patrick wrote ...

So, are there functional limitations on "hardened" silicon? Or just a higher price tag?

Yup and higher price tag.

It's not an area I've looked at much but IRC the radiation tends to do stuff like randomly short out transistors temporarily. They usually will recover provided the power supply doesn't blow the thing up, but the logic will be giving false results in the meantime.

To make things more robust you can do things like run multiple copies of hardware and compare them; or even just calculate the same thing twice and compare the results; with care you don't always need rad-hard parts. Some guys I know slightly online run standard non rad-hard components in orbit. They do get some upsets, but their system is designed to deal with it and return to stability. If you go that route you'd need to test it with a radiation source.

Whatever you do though, there's going to be a limit to how much radiation you can handle.

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