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Homemade GPS Receiver

holmea.demon.co.uk

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Re: Homemade GPS Receiver

#41

Post by Florin_Andrei »

mikeash wrote: post #17
raverbashing wrote: post #9

Earlier quoted context omitted.

Probably not Which is funny, because to know you're higher/faster than allowed you need to calculate that from GPS data In practice, not sure how this is achieved, but for speed this may even be a limitation of the circuit (for example, if you can't compensate for the doppler effect at that speed or has to add some other processing - still, the doppler effect should be small) 1000 knots is still +/- 1.5 Mach (at sea…

I don't think there's a requirement to completely shut down, only a requirement to cut off your data output. You can keep calculating your speed and position, you just stop telling the outside world.

The problem is, COCOM says:

GPS devices should disable themselves when at an altitude above 60,000 feet AND a speed faster than 1,000 knots

Whereas some vendors read it:

GPS devices should disable themselves when at an altitude above 60,000 feet OR a speed faster than 1,000 knots

Which effectively disables your cheap GPS dongle in your home-made stratospheric balloon, which is quite capable of reaching 30 km, above the COCOM altitude spec.

Sometimes it's quite tricky to figure out if a given receiver is an AND or an OR.

Re: Homemade GPS Receiver

#42

Post by Florin_Andrei »

fvox13 wrote: post #11

Sure, he designed it, but I don't think those nice printed PCBs are "homemade". I'd be more impressed if it were through-hole construction. Still, it's pretty cool.

I've designed, drawn, corroded, and soldered entire soundcards, synthesizer blocks, and various systems with microprocessors, back in the day. All by hand.

To quote Linus, it's quite "studly" to operate like this, but the result is big and ugly. Fabbed PCBs are much more compact.

Re: Homemade GPS Receiver

#43

Post by robobrobro »

wglb wrote: post #24
js2 wrote: post #23

Earlier quoted context omitted.

Perhaps the resubmission is due to the May 2013 update on the page.

Oh, likely, but the duplicate detection is based on URL matching, not on newness of content. See the other submission at https://news.ycombinator.com/item?id=5717674 and note that the referenced URL does not have the # at the end that this one does.

I think you meant https://news.ycombinator.com/item?id=3065619. Interesting find.

Re: Homemade GPS Receiver

#44

Post by jff »

joezydeco wrote: post #12
fvox13 wrote: post #11

Sure, he designed it, but I don't think those nice printed PCBs are "homemade". I'd be more impressed if it were through-hole construction. Still, it's pretty cool.

Doubtful you can get an FPGA of that class in a thru-hole package. The dude was smart to shop out the PCB instead of spending weeks trying screen and drill his own.

Not like it's really that hard to solder SMD. I soldered 40-pin SMD all the time with nothing more than a fine-tip soldering iron, a syringe full of flux, and a magnifier or microscope.

Re: Homemade GPS Receiver

#45

Post by jessaustin »

wglb wrote: post #24
js2 wrote: post #23

Earlier quoted context omitted.

Perhaps the resubmission is due to the May 2013 update on the page.

Oh, likely, but the duplicate detection is based on URL matching, not on newness of content. See the other submission at https://news.ycombinator.com/item?id=5717674 and note that the referenced URL does not have the # at the end that this one does.

How about this one:

https://news.ycombinator.com/item?id=5699429

Whatever. I'm glad people are looking at this because it is incredible.

EDIT: I do wonder, however, that the link from 3 days ago only has one point, which is what it had as soon as it was created. Presumably elemeno appended the "#" before he submitted the link, because if he had just submitted the link then the 3-day-ago link would have at least two points. Is automatically adding the hash a thing that people do? Is reposting non-hashed links from several days before with added hashes a thing that people do?

I guess since this version of the link has been upvoted 201 times, elemeno actually has provided a valuable service to HN.

Re: Homemade GPS Receiver

#46

Post by shabble »

Florin_Andrei wrote: post #41
mikeash wrote: post #17

Earlier quoted context omitted.

I don't think there's a requirement to completely shut down, only a requirement to cut off your data output. You can keep calculating your speed and position, you just stop telling the outside world.

The problem is, COCOM says: GPS devices should disable themselves when at an altitude above 60,000 feet AND a speed faster than 1,000 knots Whereas some vendors read it: GPS devices should disable themselves when at an altitude above 60,000 feet OR a speed faster than 1,000 knots Which effectively disables your cheap GPS dongle in your home-made stratospheric balloon, which is quite capable of reaching 30 km, above t…

You could maybe beg/steal/borrow a GPS simulator[1] and test your candidate receivers with it. Best do it in a Faraday cage though, unless you want to upset some Very Serious Important Things[2], and (likely government) owners thereof.

Probably way out of budget range for hobby stuff though

[1] One is the WelNav GS720: http://www.t-e.com.tw/GS-720.pdf

[2] A Simple Demonstration that the Global Positioning System (GPS) is Vulnerable to Spoofing: http://www.ne.anl.gov/capabilities/vat/pdfs/GPS-Spoofing-%28...

Re: Homemade GPS Receiver

#47

Post by mikeash »

Florin_Andrei wrote: post #39
mikeash wrote: post #30

Earlier quoted context omitted.

Fun fact, the Boeing 747 features a sextant port for convenient celestial navigation in flight.

Wow. Was it ever used?

Can't find much information about it, but what I did find says that it got significant use in the early years of the 747. It makes sense, as the 747 long predates civilian GPS, and other electronic navigation techniques were not completely reliable for long flights. I don't know that there were many radio beacons available for long over-water flights, and pilots probably liked cross-checking their INS.

While searching, I was reminded of another interesting fact, that the SR-71 had a fully automated celestial navigation system that could get still a position fix on the ground in the middle of the day, and was more accurate than an INS on long flights.

Re: Homemade GPS Receiver

#48

Post by jessaustin »

jessaustin wrote: post #45
wglb wrote: post #24

Earlier quoted context omitted.

Oh, likely, but the duplicate detection is based on URL matching, not on newness of content. See the other submission at https://news.ycombinator.com/item?id=5717674 and note that the referenced URL does not have the # at the end that this one does.

How about this one: https://news.ycombinator.com/item?id=5699429 Whatever. I'm glad people are looking at this because it is incredible. EDIT: I do wonder, however, that the link from 3 days ago only has one point, which is what it had as soon as it was created. Presumably elemeno appended the "#" before he submitted the link, because if he had just submitted the link then the 3-day-ago link would have at least two p…

Just to close the circle, this one also has a hash at the end:

https://news.ycombinator.com/item?id=5721006

Re: Homemade GPS Receiver

#49

Post by mikeash »

Florin_Andrei wrote: post #38
rwg wrote: post #33

Earlier quoted context omitted.

Some of the relativistic error (time dilation) is taken care of on the satellites themselves by tweaking the 10.23 MHz frequency generators to run at 10.22999999543 MHz. The author actually mentions a third source of relativistic error without saying it's relativistic error: the Sagnac effect. ("Originally, by not transforming satellite positions from earth-centred-earth-fixed (ECEF) to earth-centred-inertial (ECI) c…

The Sagnac skew would still occur in a non-relativistic universe. All that's needed is a finite speed for light. However, yes, relativity further complicates it.

It's interesting to consider that GPS as a concept simply wouldn't work at all if the speed of light were infinite.

Re: Homemade GPS Receiver

#50

Post by hatcravat »

raverbashing wrote: post #9
mherdeg wrote: post #8

Hmm. I don't see any mention of whether this design complies with the COCOM Limits, an agreement that GPS devices should disable themselves when at an altitude above 60,000 feet and a speed faster than 1,000 knots.

Probably not Which is funny, because to know you're higher/faster than allowed you need to calculate that from GPS data In practice, not sure how this is achieved, but for speed this may even be a limitation of the circuit (for example, if you can't compensate for the doppler effect at that speed or has to add some other processing - still, the doppler effect should be small) 1000 knots is still +/- 1.5 Mach (at sea…

Doppler shift due to observer speed will almost always (i.e. for anything other than an actual ballistic missile, and even then only a missile traveling west) be less than the doppler shift due to satellite speed for a stationary observer. So, artificially restricting the doppler spread isn't an effective method of implementing the COCOM restrictions. It would also make PLL lock-up more difficult.

Concorde is old enough that its operators probably never went to the trouble of doing the paperwork to have a GPS integrated with the rest of the NAV system. For its (relatively) short flights, an internal navigation system plus VHF or LF radionav works adequately (and there were very few, if any, GPS/WAAS approaches certified by the time of its retirement).

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