Homemade GPS Receiver | Hacker News Reader

Homemade GPS Receiver

aholme.co.uk

106 pointssly01055 comments

If anyone is interested in GPS, we're looking for a software engineer ready to learn about it. (Full Time, Austin)

Totally off topic but I'm seeing a lot of hiring going on in Texas. What is going on down there?

Tax incentives, lower cost of living, and pro business local governments are driving larger companies to relocate headquarters or major research ops to Texas en-mass.

Please don't take this the wrong way, but Texas is a pretty terrible place to live:

http://pastebin.com/Vt6vNgeS

http://web.archive.org/web/20150401050939/https://www.texast...

Sounds like a great place to live, if you're privileged and don't care about the environment or the poor.

Or, perhaps you move there because you care about the environment or the poor, and that's how change happens.

Gonna need a whole lot more people to move there before any change occurs.

Well you need some way to pay for the tax subsidies.

Are those environmental stats per capita or area? I ask only because it's by far the largest state in the CONUS, which makes me wonder if that doesn't play a contributing role.

Not to mention a crazy governor who believes military exercises are a prelude to Obama taking over his state.

It's a big tech hub outside of silicon valley that's arguably more fun and undeniably less expensive

Right after CA, Texas is the highest GDP state. It's also has beautiful clouds and sunsets.

Google Fiber? (In Austin at least)

Texas has always had a huge tech industry presence, particularly the telecom industry: http://en.wikipedia.org/wiki/Telecom_Corridor (and there's also a lot of tech industry here outside the Corridor proper, too)

Personally, I love living and working in the suburbs of Dallas, and I wouldn't want to live anywhere else.

The Texas economy has, until a few months ago, been booming due to shale oil & gas development. Add in no income tax and warm weather with young folks doing sporty things and it's fairly attractive.

Austin has a somewhat robust semiconductor focus, with Freescale, AMD, Intel, and Apple present. Some low-power IOT type tech including Silicon Labs (Energy Micro), and Ambiq in addition to afore-mentioned Freescale. There are a few DOD/spook type shops, and some financial techs, such as Visa, Paypal, Netspend, etc.

That sounds interesting, can you post more details? Experience, compensation, requirements, description, etc?

Isocline - Austin, TX - Software Engineer for High Performance Computing and Modeling

We are looking for two people - one interested in neural networks and one interested in GPS.

We are developing microchips that yield a 10-1000x improvement in performance & energy-efficiency compared to digital ASICs, GPUs, and FPGAs. We are a bootstrapped company and are fully funded through mid 2016. Patents pending.

C++ experience is required!

$70K – $150K Salary

0.5% – 1.2% Equity

Full Job Description: https://angel.co/isocline/jobs/38767-software-engineer

Company website: http://isoclineengineering.com/

US citizens only? (since it's a defense company)

"The best fix so far was ±1 metres at a very open location using 12 satellites; but accuracy is typically ±5 metres in poorer locations with fewer satellites."

That seems very good accuracy. The off-the-shelf GPS I have (https://www.adafruit.com/products/746) is always >= ±5m, even with 360 degree view of the sky.

(the specs for my GPS claim < 3 meters, and also say "(all GPS technology has about 3m accuracy)").

Commercial-grade technology has about 3m accuracy, but different methods can yield down to 10cm accuracy:

https://en.wikipedia.org/wiki/Global_Positioning_System#Prec...

And then there's a whole segment of survey-grade equipment that can get within a centimeter or two:

http://water.usgs.gov/osw/gps/

That MediaTek chip is capable of better. Do you have WAAS (SBAS) enabled? 5m is about what you'd expect without the improved orbit/clock/ionosphere corrections.

It also pays off to use a good antenna, or at least put a ground plane beneath a mediocre one to block multipath reflections from the surrounding ground objects.

I just used the default settings. Didn't know about WAAS - thanks for pointing me to it.

From reading some forum posts, it seems you have to explicitly enable WAAS - I'll give that a try and see how much it helps.

A while back I was interested in building a DIY GPS after watching Lost. I day-dreamed about being stranded somewhere remote and building myself a GPS out of spare parts like MacGyver or something.

After finding this article, I realized that would be an impossible task without an engineering degree.

Has anyone found a more "For Dummies" version of this?

I don't know if "For Dummies" is possible for building a GPS system from first principles.

If you want to really learn by building, I recommend "Fundamentals of GPS Receivers: A Hardware Approach" by Dan Doberstein, and building the analog GPS system described in the first few chapters.

If you simply want to play with GPS, I'm sure there are many breakout modules from Adafruit et al.

GPS system from first principles.

To be fair, that would probably be navigation by naturally observable indicators such as the rising or setting sun, shadows over the day, locally / seasonally reliable stars for your area, and knowledge of local fauna/wave patterns.

After getting interested in sailing again (after 20 years) last year through multihull vessels, I started reading about the evolution of multihulls in the Pacific and then began looking at traditional navigation techniques. Following that, I was wondering for fun whether it would be possible to plot one's position given a rough time, rough position guestimate, and perfectly measured horizonal inclination, lens property and visible to 'the naked eye' star positions (ie. visible to the naked CCD at evening time = high ISO, noise and short shutter speed to failitate clarity in rough seas). My conclusion was yes, it's possible, with sub-GPS accuracy but accuracy which would be good enough to get you reliably between certain locations in certain seasons even on a sailboat.

I was surprised to read that some ICBMs allegedly used astronomy and pre-loaded landform navigation to avoid issues with jamming, and that fat databases of precise naked-eye type visible star locations are widely available, indeed so is software for reducing them to identities from an image input.

The SR-71 had an automated celestial navigation system that could get a lock in daylight: http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Astro-...

(The big disadvantage is that it can't see through clouds, of course)

Didn't it fly above the clouds?

It was a surveillance aircraft, so it would need line of sight with the ground as well, which might require flying under cloud cover. Also, having a gyroscope component, I'm guessing that it would need to calibrate while stationary, on the runway.

It didn't fly under cloud cover. It flew Mach 3.3, which it couldn't do in dense air. Also, flying at 80,000 feet makes it much more difficult to shoot down.

The author got a lot further than my attempt! [0] I was attempting to make a Python-based GPS signal generator for later use in testing a pure VHDL/FPGA (no embedded CPU) receiver.

[0]: https://github.com/jevinskie/jevps

I would be bit careful. GPS is military technology and commercial versions must have limitations, to be non-usable in rockets (max speed, max altitude...).

Says the USA, thankfully the author is not based there. That being said, the USA does seem to think many of its laws apply all over the globe.

He might want to visit the USA some time.

Incorrect.

The relevant agreement is the Missile Technology Control Regime, an informal partnership between 34 countries. http://en.wikipedia.org/wiki/Missile_Technology_Control_Regi...

Unlocked GPS receivers are classified as dual-use missile-related components under category II, item 11, section A.3: http://www.mtcr.info/english/MTCR-April2011-Technical-Annex....

  b. Designed or modified for airborne applications and having any of the following: 
    1. Capable of providing navigation information at speeds in excess of 600 m/s; 
    2. Employing decryption, designed or modified for military or 
    governmental services, to gain access to GNSS secure signal/data; or
    3. Being specially designed to employ anti-jam features (e.g. null steering 
    antenna or electronically steerable antenna) to function in an 
    environment of active or passive countermeasures.

Huh, I should have checked that first. How well do you think you could argue that since the treaty did not specify what reference frame you were supposed to use to calculate the 600 m/s limit, you decided to use the reference frame of the receiver and therefore no limit is required because the speed is always 0? I would have thought GPS people most of all would have understood this problem \s.

My crystal ball tells me that the judge would find your argument very amusing, but not substantive.

Unfortunately many receivers have a (speed > X || altitude > Y) check when, IIRC, the law only requires (I don't see the altitude part in your quote) (speed > X && altitude > Y). I think the altitude limit was 20 km or 25 km. Anyways, it is a pain for people who want to put a receiver on a high altitude balloon. I was able to... find... a modified firmware for my SIRF receiver but I have been to afraid to flash it. Besides, I don't even know if the receiver has the "bad" logic. I've been jonesing to make a SDR GPS simulator that generates a signal that I can feed directly into the antenna port on the receiver to test high altitude functionality.

MTCR is an informal agreement, not a treaty. Its provisions are actually implemented by laws in the member nations-- in the US, that's the International Traffic In Arms Regulations Act. (ITAR) ITAR Category XV, Section c:

  Global Positioning System (GPS) receiving equipment specifically 
  designed, modified or configured for military use; or GPS receiving 
  equipment with any of the following characteristics:
    1. Designed for encryption or decryption (e.g., Y-Code) of GPS 
       precise positioning service (PPS) signals;
    2. Designed for producing navigation results above 60,000 feet 
       altitude and at 1,000 knots velocity or greater;

So the US limit isn't even in metric units! Consulting my psychic powers, since I haven't bothered to dig up the laws for each member nation, I predict that each nation implements MTCR slightly differently, some using the word AND, some using the word OR. Since a GPS manufacturer wants to sell worldwide, they would have to use the strictest limits possible.

Aren't those limitations just in the USA? I'd guess that someone in .co.uk should be able to do what ever they want, especially as a non-commercial project.

Yes, but if you violate ITAR, you'd probably want to avoid transiting the US in the future. We're pretty militant about enforcing our laws beyond what you'd consider sovereign boundaries.

Came for this. This allows someone to build GPS-guided missiles, artillery, etc.

The laws need to be changed, because consumer-programmable things like FPGAs and SBCs like BBB/RPi have made weaponizing with even commercially-available, off-the-shelf products super easy.

For instance, building gps-powered drones with thermite self destruct and an explosive/incendiary payload is fairly straight forward and extremely dangerous with little ability to investigate the violator. ICBMs and fast-flying missiles are much harder.

Don't even need FPGAs; you could do it all in software and use very light software defined radio hardware.

You need to poll many frequencies at once, very quickly, in order to make a missile that can respond fast enough.

Maybe it's possible with a very wide SDR, but I've not seen one where it was.

http://www.gnss-sdr.org/ claims to be able to work with RTLSDR dongles (cost ~$10 and provides 3,2 MHz Bandwidth. Original article's "homemade GPS" has 2,5 MHz BW).

Also: http://www.rtl-sdr.com/tag/gps/

Doing it, and doing it fast enough to guide a missile at > mach 2 are different things entirely. The rate that you have to constantly be calculating the spin (if any) and the vector and speed is incredible. OTS 5-10hz refresh will not cut it.

That said OTC stuff is so dangerous now that I think we should roll back the laws to allow home lab/hackerspace science to evolve. I would love to launch a rocket from weather balloons, but it would stand little chance of getting anywhere with OTS GPS gear.

I'd agree with you. I'm probably mistaken that in-software SDR can decode GPS fast enough. I would argue though that ASIC manufacturing costs are coming down fast enough that you could ship out your hardware design for a GPS system without ITAR altitude/speed limitations and have a finished product back in your hands in 7-10 business days.

I'm not sure how you think changing laws would protect anyone, anymore than they currently do?

Sure lots of things can be used for both good/evil.

Personally I think it's a fascinating project.

I'm saying the laws need to be changed because it doesn't protect anyone. Better to just make it legal and work to protect us in other ways.

Aha sorry, yes definitely. I wanted to get a thermal sensor a while ago and would have to fill-in paper work to say it wouldn't be used for malicious purposes (I didn't end up getting it because it was too expensive anyway), I'm really not sure the idea of such bureaucracy.

I'm convinced Vogons run the government.

If 1000 lines of Verilog code (or a little bit of patching in a commercial receiver's firmware, for that matter) is the only thing missing from the terrorists having a working cruise missile, then we're screwed anyway...

Personally I'd love to see more of the advanced features (such as carrier phase measurements and also multiple antenna support) being available in affordable hardware. Preferably in an open system, being open source, providing schematic diagrams, having open APIs and allow access to raw measurement data.

Ublox LEA-6T + RTKLIB not enough?

RTKLIB doesn't do well with 'challenging' environments like tree canopy induced non-LOS multipath, etc. It would be nice to have access to some deeper internals (doppler bucket size, carrier phase PLL info, etc) to be able to separate outlier per-satellite epochs from good ones.

Selective Availability was turned off in 2000 (15 years ago today) after the US government demonstrated that they could limit accuracy over any region they chose if they ever need to[1]. Most GPS receivers now implement crippled behavior at certain altitudes and speeds, but nobody assumes this to be a dependable defense.

1: "Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened." from http://www.faa.gov/about/office_org/headquarters_offices/ato...

There's more than just SA at work here.

There's still speed and altitude limits in force via ITAR (arms trafficking regulations) that GPS manufacturers are not allowed to report GPS data. I think it something like 512 m/s and 18000M.

Reply on news.ycombinator.com