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Comment by blhack | Hacker News Reader

If anybody was going to be outraged by this, wouldn't it be the FAA and the Us Military?

The presumption is that more expensive equipment based on GPS (aircraft navigation, etc) have the appropriate "filters" in place to prevent interference.

If LightSquared is to be believed, the problem is that commodity GPS makers did not take adequate precautions against interference.

There's probably a grain of truth to both positions. Dealing with RF interference is something of a black art, so given that LightSquared hasn't actually deployed their network yet, it's not surprising that existing GPS devices don't filter out those signals perfectly. At the same time, LightSquared seems to have assumed that they could avoid interfering with the GPS band to a greater degree than they were ultimately able to deliver.

Of course, it's also a question of money - from the GPS manufacturers' perspective, they will have to assume all the costs while LightSquared derives the benefits. From LightSquared's perspective, they paid to acquire their spectrum while the GPS manufacturers get access to government infrastructure for free and are effectively blocking access to a significant chunk of spectrum just to reduce their per-unit costs.

Seriously, we drop bombs targeted using this technology. I can't imagine that the military would let anything diminish the accuracy of existing systems.

I dont think (and hope) it's that simple to disarm the GPS on guidance missles. Else what's stopping our "enemies" from just building these transmition towers near their bases?

It is that simple, which is why the bombs just use GPS to get close to the target, then dead reckon[1] the rest of the way.

[1]http://en.wikipedia.org/wiki/Dead_reckoning

Military GPS operates at a different frequency.

I believe that military GPS units utilize both the L1 band (civilian) and L2 band (military only) to be able to calculate ionosphere delays.

Sure they are. The enemy might send a guided missile through the window of the office next to the general's. What a disaster!

Fun fact: (most) civilian GPS receivers stop working when they're moving faster than 515m/s, are more than 18,000m above sea level, or are accelerating faster than 4g. (39.2m/s^2)

If you're thinking that these operational constraints sound a lot like the performance envelope of a guided missile, then gosh, you're right. IIRC, unlocked GPS recievers are classified as MTCR category II dual-use missile related components, with the usual suite of import restrictions.

EDIT: Looked it up[1] for... some reason, and yep, category II, item 11, section A.3:

  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.

So, in fact, it's just the speed restriction. The numbers I have are probably from some American arms-control law, not a MTCR restriction.

1: http://www.mtcr.info/english/MTCR-April2011-Technical-Annex....

It seems that the cost of fabbing your own gps chip would be peanuts compared to the cost of even one missile. Given the raw GPS data its pretty trivial to do all location processing in software.

Due to my current job, I've had to become familiar with my country's export restrictions on some tech. I've since thought it would be great fun to start a string of projects with the goal of meeting those requirements that make them export-restricted. If nothing else, I know they are (at least mildly) challenging goals, and concrete specs to meet so that I know I've accomplished the goals.

You wouldn't need to fab your own chip. Plenty of FPGAs would work. In fact, you could make do with a decent speed ADC and a bit of software.

It should be noted that there are numerous civilian GPS receivers that do not have the altitude limit or have it much higher than 18km. Many are purpose built for high altitude balloon payloads.

I was researching GPS receivers for an high altitude balloon and most receivers stop working at more than 18km OR at 500m/s.

Some manufacturers misinterpreted the restriction. Instead of OR it should be AND.

If you look around high altitude balloon forums, they usually maintain a list of good gps chips(stop working at 500m/s AND 18km instead of 500m/s OR 18km).

    *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.*

I find this one slightly annoying. So no matter what, you can't connect an electronically steerable antenna to a GPS receiver and export the resulting device?

Sure you can export it. Category II items aren't banned, they're just controlled.

So you can export it, it'll just be really expensive to do so, and they won't want you to sell them to terrorists.

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