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Show HN: Building a GPS receiver

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Re: Show HN: Building a GPS receiver

#121

Post by codyd51 »

michaelt wrote: post #54

Great project, thanks for posting it! It just so happens I've got an RTL-SDR, a GPS receiver that outputs raw pseudoranges, and a signal splitter that lets me put the signal from one antenna into two receivers. So if you like I can get the pseudoranges out of a commercial GPS receiver, and the raw signal from an RTL-SDR at the same time, which might help you pinpoint your last bit of location inaccuracy. Would you be…

Thank you very much for your thoughtful offer! I won't be able to readily work on side projects after starting my new role next week, and am content to consider this project complete for the time being. However, what you've proposed does sound interesting and fun. I'm going to go ahead and shoot an email to the address listed in your profile.

Re: Show HN: Building a GPS receiver

#122

Post by codyd51 »

throw0101c wrote: post #33

Standford has/had a course that is available online on GPS/GNSS and a lot of the nitty-gritty details: * https://www.youtube.com/playlist?list=PLGvhNIiu1ubyEOJga50LJ...

Thank you for linking this, this course is phenomenal! Several of the videos were immensely helpful in my GPS journey.

Re: Show HN: Building a GPS receiver

#123

Post by codyd51 »

NovemberWhiskey wrote: post #53

It's not entirely clear to me from the write-up, but it seems some of the problems that the author had with the "tracker" come from attempting to do carrier phase synchronization (with the Costas loop) before any kind of clock recovery.

Whoa, thank you! I don't know much about RF and learned ad-hoc for this project, and it seems perfectly plausible to me that someone knowledgeable would be able to look at this and identify a root cause - I certainly didn't do anything special for clock recovery, and based on the name I would have blindly assumed that synchronising the carrier wave _would_ be tantamount to recovering the satellite clock. I haven't researched clock recovery yet, and will do so. Once again, thank you!

Re: Show HN: Building a GPS receiver

#125

Post by nayuki »

ck2 wrote: post #50

If you really want to see all the gps-like services out there and have android, you must play with the open-source GPStest https://github.com/barbeau/gpstest I've been fascinated for years how badly GPS does altitude (mean-sea-level) in the USA they had to build an augmentation system for airplanes for altitude (WAAS)

Awesome app you mentioned. So far, I was only using the lesser app of "GPS Status" for Android: https://play.google.com/store/apps/details?id=com.eclipsim.g... .

Re: Show HN: Building a GPS receiver

#127

Post by AdamJacobMuller »

codyd51 wrote: post #11
AdamJacobMuller wrote: post #5

Cool article. Whenever I see "from scratch," I'm always curious to see how from scratch the author actually means so I'll admit I was a bit disappointed to see that the hardware was just RTL-SDR. Still, the protocol decoding was very interesting and the result is great. > GPS was launched in 1978, which was 45 years ago at time of writing. Five billion people are currently under 40 years old, so well over half the wo…

Hi! Yes, "from scratch" is definitely always a bit of a funny term. I also implemented the receiver in Python, which is quite far from "scratch" =). What I mean by it in this context is that I'm taking a piece of hardware that knows nothing about GPS, and just has the ability to sample the EM field, and building up a receiver from there. Re. slow TTFF, or time-to-first-(position)-fix on older hardware, this essential…

Please don't take the "from scratch" bit as criticism. It was just amusing trivia :)

Ah, the almanac part I completely forgot about, that makes a lot of sense, I read that part but forgot how it USED to be done when we couldn't just throw cycles at it.

Re: Show HN: Building a GPS receiver

#128

Post by simplyluke »

kube-system wrote: post #49
OmarShehata wrote: post #36

Amazing! I also had exactly the same experience that led me to research this a few years ago, realizing that: - GPS works even in airplane mode (while on a literal airplane) - It works without cell service, or wifi, or anything - The United States of America controls the GPS constellation, and they can (and have!) turned off GPS off certain regions at will when necessary (which has prompted other countries to launch…

> - GPS works even in airplane mode (while on a literal airplane) - It works without cell service, or wifi, or anything Maybe this is a conception that some people have when their first experiences of using GPS was on a smartphone? But my first couple GPS receivers were standalone devices without any sort of data connection, so it seems obvious to me that GPS doesn't require data.

A My only exposure to GPS without data came from outdoor sports where GPS watches and PLBs are common, as are offline mapping applications. I don't think it's obvious to most casual users of GPS technology today that it is entirely independent from data connectivity.

Re: Show HN: Building a GPS receiver

#130

Post by noman-land »

codyd51 wrote: post #117
noman-land wrote: post #6

I really love how this article is paced in real time from the first person as a learning adventure. Even down to the search terms used and the inner monologue. This is my absolute favorite kind of tutorial because you're not just being taught to fish, you're being shown how go about sourcing the parts to built your own fishing machinery.

Thank you for sharing this! It is sort of a vulnerable thing to write, because it reveals the shapes and bounds of my own ignorance, and the path I took to learning. I am really grateful to hear that you appreciated the approach.

No, this is great. This is extremely rare for people to do, even rarer for people to do well. You have a gift! Keep going.
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