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We built a new GPS receiver engine

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Re: We built a new GPS receiver engine

#101

Post by joshvm »

durkie wrote: post #81
joshvm wrote: post #79

Earlier quoted context omitted.

This is the best reference I've found so far on the SR-71 system (the NAS-14V2): https://airandspace.si.edu/webimages/collections/full/NAS-14... That's some kind of flight/operational manual for the astroinertial navigation system (ANS), pretty detailed. Quite amazing that it's been declassified - you can see the link is from the Smithsonian Air and Space Museum. It also discusses details of how missions are loaded a…

Very cool! Can you expand on the idea that it's just a single IR photodiode? This would get rid of visible atmospheric light, but wouldn't there still be residual IR light from the sun (whatever isn't absorbed by the atmosphere) interfering with collecting the light from stars?

From the sibling comment seems like I had the right idea. The main restriction I was thinking of is what technology was used at the time. The SR 71 used film for imaging, and the CCD array was developed later on and probably wouldn't have enough sensitivity even if there was an early prototype. You'd also need an InGaS(?) detector for SWIR and I'm not sure when they first came out.

For this application you want as sensitive as you can get, since you want to detect minute differences in background intensity. That points to a big ole photodiode.

Hence why I suggested that they use an off axis sensor to do online dark subtraction. Locally if there was no star, the diodes should give you the same intensity. If a star enters the field of view of one, but not the other, then you have a detection. It's more complicated than that apparently, but the idea is sound.

From the patent it actually looks like they use some kind of optical chopper and a single diode. A chopper is a disc with slits cut out, in the fold of view. If there's a star that doesn't fill the field of view, the chopper will give you a periodic signal as it rotates. If the sky is empty then you'll get roughly the same signal all the way round. (from a quick skim)

Re: We built a new GPS receiver engine

#102

Post by mianos »

wwkeyboard wrote: post #100
cptskippy wrote: post #58

Earlier quoted context omitted.

It seems silly in today's world where anyone can roll a custom SoC and have it spun out by any number of Fabs over the weekend, but 30 years ago custom chips were pipe-dreams for all but the most advanced tech firms. The $2 microcontroller you use to turn your lamp on has more processing power than a desktop computer from the early 80s. By putting artificial constraints on off-the-shelf GPS technology, it forced bad…

I thought SA was removed because we discovered ways around it. Specifically differential GPS(tl;dr is that if you have a known point on the ground you can take a measurement there and get the SA offset, this is widely used to compensate for atmospheric error in GPS).

Although WAAS and other similar techniques can be used to improve the accuracy there are many impracticalities of using secondary sources. The reason stated was to encourage civilian uses of the better accuracy. The other federal departments agreed there was more to be gained from civilian use than lost by making the more accurate signal than lost but limiting its use.

Re: We built a new GPS receiver engine

#103

Post by PaulHoule »

gsich wrote: post #99
PaulHoule wrote: post #22

Earlier quoted context omitted.

I dunno. Back in the day when "selective availability" was turned on they added "brown noise" with strong low frequency components that would defeat attempts to use temporal averaging to approve GPS accuracy. I remember setting a receiver up outside and looking at the tracks go in circles like a spirograph.

Selective availability is off since almost 2 decades.

That's how long ago it was.

Re: We built a new GPS receiver engine

#104

Post by RealityVoid »

ohazi wrote: post #82
bjt2n3904 wrote: post #80

Earlier quoted context omitted.

According to Google, the F22 can do 1500 mph. Looks like civillian GPS is still good as long as you keep it under Mach 1.8 at sea level ;) ICBMs on the other hand, reportedly go ten times the speed of an F22.

Neither of which would be using a civilian GPS receiver?

Well, not now they wouldn't.

I fail to see what is so hard to believe that an actor _could_ use civilian parts to get military capabilities. Yes, sure the are points about performance and reliability and what not but I can't shake the feeling that at least some of those points are exaggerated or marketing speak and the re is obviously a point where these components are good enough to be dangerous.

Re: We built a new GPS receiver engine

#106

Post by kbaker »

at_a_remove wrote: post #49
Accujack wrote: post #30

Earlier quoted context omitted.

RTK gps is better and quite affordable now. Look it up, you can use two receivers costing maybe $15 each for it.

I've just started looking into RTK GPS. My state has a Trimble network that I might use. My application is, weirdly, something a lot like the waypoints of old Garmin GPS handhelds for hikers, only more accurate. I need to be able to lay down a series of points as I drive (perhaps around 30 mph or so) to map out a street network. However, as I look around trying to find something I can just drop into my car and pull t…

Well, $265 (+antenna $60), but I purchased an Emlid Reach M+ [1] for drone RTK experiments, and it has been pretty simple to use... comes with a really nice UI on the device and internal storage in the unit. Also their website has good documentation, which is helpful since there is a lot to learn about how to do RTK.

Found a local NTRIP caster (within 10 km, lucky I am in range of a public RTK2go one) and the Reach uses the mobile hotspot on my phone to connect to the caster. After a couple seconds to minutes, it settles down to a 10-12 mm accurate fix in lat, lon, alt, and is kinematic so it maintains the accuracy as I am flying the drone around. It records the whole thing internally, or you can have various output modes like standard NMEA over serial, or bluetooth to your phone.

Also, I have used it to record data internally then post-process it for PPK using the county's publically-available RINEX files [2] and Emlid's RTKLIB fork [3].

[1] https://emlid.com/reach/ [2] https://geodesy.noaa.gov/CORS/standard1.shtml [3] https://docs.emlid.com/reach/common/tutorials/gps-post-proce...

Re: We built a new GPS receiver engine

#107

Post by e-_pusher »

willis936 wrote: post #27
rrmm wrote: post #17

Earlier quoted context omitted.

The jitter can be a really important parameter. for example in pdf: https://processors.wiki.ti.com/images/f/f1/TI_GPS_PPS_Timing...

My masters thesis is on jitter measurement techniques in 25 Gbps baseband signals :) Jitter can be tracked and filtered in PPS signals. The related parameter, wander, is interesting to me. Phase noise above 10 Hz is jitter. Phase noise below 10 Hz is wander. Why? ¯\_(ツ)_/¯ Allen variation is the typical metric for wander.

Fascinating! Do you know of any good textbooks or application notes that go into jitter measurement techniques?

I found this note from Keysight, is this is a good place to start?

http://literature.cdn.keysight.com/litweb/pdf/5988-6254EN.pd...

Re: We built a new GPS receiver engine

#108

Post by willis936 »

e-_pusher wrote: post #107
willis936 wrote: post #27

Earlier quoted context omitted.

My masters thesis is on jitter measurement techniques in 25 Gbps baseband signals :) Jitter can be tracked and filtered in PPS signals. The related parameter, wander, is interesting to me. Phase noise above 10 Hz is jitter. Phase noise below 10 Hz is wander. Why? ¯\_(ツ)_/¯ Allen variation is the typical metric for wander.

Fascinating! Do you know of any good textbooks or application notes that go into jitter measurement techniques? I found this note from Keysight, is this is a good place to start? http://literature.cdn.keysight.com/litweb/pdf/5988-6254EN.pd...

That note is interesting, but a bit dense and most of it is standard specific implementation details that aren't too important without first understanding why the standards were written the way they were. That question isn't usually well documented, so a bottom-up understanding of the testing is valuable imo.

If you want to become a time-nut, then check out time-nuts[0]. They know their stuff and will be the most valuable resource on topics related to GPS timing: wander, allen deviation, and jitter of various sources (quartz oscillator vs rubidium, etc.)

I will say that the big test and measurement (T&M) players (Keysight, Tektronix, Rhode&Schwartz) have been the main drivers of T&M research. It's a strange field where academic work and commercial interests line up, so all of the academics work for commercial companies. They'll happily share their results and preferred approaches (except for the secret parts) in white papers and app notes. I don't think there is a good centralized repository of such documents, so it may require some hunting. Knowing keywords might be helpful. If you're mainly interested in jitter then hot things to search for are "jitter", "jitter decomposition", "TIE", "wander", and "dual-Dirac" (just off the top of my head, I'm sure the rabbit hole goes deeper if you start digging into more recent work).

I was in T&M for 7 years. I'm not currently but likely will be in the future. This[1] is the bible I learned on. It's $120 new and not too easy to find free (most of the people who would find it interesting can afford it, ie not in curriculum). Most of my learning was through osmosis over the first few years, then I taught myself the rest. I only used the book as a reference after the first year, but I wouldn't say I knew it all until maybe 5 years in (I was a student at the time too). When I flipped through it towards the end of my time I found it was a good teaching source. I suffer from the curse of knowledge, but I think if you have EE chops then this book will make perfect sense and even help build the intuition necessary to be a good test engineer.

The book is all practical T&M techniques for high-speed SERDES, so wander is outside of the scope of the book. There are other bits in the book besides jitter too (lots of tasty stuff on encodings and impedance).

0. http://www.leapsecond.com/time-nuts.htm

1. https://books.google.com/books/about/An_Engineer_s_Guide_to_...

Re: We built a new GPS receiver engine

#109

Post by brokenmachine »

dfox wrote: post #78
GordonS wrote: post #21

Earlier quoted context omitted.

I believe "strong" encryption is still viewed as munitions in the US, or at least it used to be.

Another interesting class of dual-use technology are fast power switching elements. And both modern SMPSs and Class D audio amplifiers are worryingly near to what would have to be considered a “munition”.

What are the rules for munitions-class switching power supplies and Class D amps?

A CPU switches a hell of a lot faster than either of those, although with much less current.

Re: We built a new GPS receiver engine

#110

Post by a_zaydak »

wwkeyboard wrote: post #100
cptskippy wrote: post #58

Earlier quoted context omitted.

It seems silly in today's world where anyone can roll a custom SoC and have it spun out by any number of Fabs over the weekend, but 30 years ago custom chips were pipe-dreams for all but the most advanced tech firms. The $2 microcontroller you use to turn your lamp on has more processing power than a desktop computer from the early 80s. By putting artificial constraints on off-the-shelf GPS technology, it forced bad…

I thought SA was removed because we discovered ways around it. Specifically differential GPS(tl;dr is that if you have a known point on the ground you can take a measurement there and get the SA offset, this is widely used to compensate for atmospheric error in GPS).

SA is illegal (turned off) in 2000 by Bill Clinton.
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