Hacker News Reader

Comment by jkot | Hacker News Reader

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...).

View on HN

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.

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.

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.

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.

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.

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.

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.

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.

He might want to visit the USA some time.

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.

Reply on news.ycombinator.com