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Bypassing US GPS Limits For Active Guided Rockets
Kristian von Bengtson writes with a link to a short guest post at Wired with an explanation of how his amateur rocket organization Copenhagen Suborbitals
managed to obtain GPS receivers without U.S. military limits for getting accurate GPS information at altitude. Mostly, the answer is in recent relaxations of the rules themselves, but it was apparently still challenging to obtain non-limited GPS hardware. "I expect they only got the OK to create this software modification for us," von Bengston writes, "since we are clearly a peaceful organization with not sinister objectives – and also in a very limited number of units. Basically removing the limits is a matter of getting into the hardware changing the code or get the manufacturers to do it. Needless to say, diplomacy and trust is the key to unlock this."
Given that it's possible to build your own gps received from scratch anyway this seems little unnecessary. (See http://www.holmea.demon.co.uk/GPS/Main.htm for someone who did) Ok so it's not trivial but it's certainly possible.
The limitations at issue are not accuracy limits. Nowadays there are no real differences in accuracy between military and civilian GPS, since selective availability was turned off years ago. The problem is that civilian GPS firmware prohibits the device from giving a fix if it is above a certain altitude (around 60,000 feet) and moving faster that about mach 1. This makes it useless for midcourse guidance of a rocket, which is the point.
I am a geek attorney, but not your geek attorney unless you've already retained me. This is not legal advice.
Not entirely true. The P/Y code still offers improved accuracy compared to even a non-degraded C/A code due to it being transmitted at 10x the rate of the C/A code. It also allows for dual-frequency operation, permitting ionospheric delay to be corrected for. (There are tricks to using the P/Y code to obtain iono delay even without the decryption keys by cross-correlating the signals on each frequency, but these require a LOT of data collection and processing and I think the receiver has to be stationary.)
That said, modern civilian receivers do such a good job of processing the C/A code that they come close to matching many military receivers which are processing the P/Y code with far older hardware/software algorithms. Systems like WAAS can compensate for a large amount of ionospheric delay even without dual-frequency operation.
Upcoming GPS satellites will permit civilian dual-frequency operation.
retrorocket.o not found, launch anyway?
Is someone with the technical abilities to build a guided missile really going to be deterred by the fact that off the shelf civilian GPS firmware is crippled in this way? The specifications for the GPS system are publicly available and many manufacturers have successfully used them to build GPS receivers, so it can't be rocket science (pun intended). And even if one were to use off the shelf GPS components, how hard would it be to modify the firmware? Firmware is just software stored in some type of read only or flash memory. Would it be that hard to download, inspect and modify it? It seems to me it would be about as hard as removing copy protection from a game.
Yes, it is a substantial deterrent. The limitations are imposed in the lowest-level parts of the GPS receiver, the first stage of data processing at which it is technically feasible to infer speed and altitude. The hardware that runs this code is highly specialized - it's a mixed analog/digital RF ASIC that is designed in hardware to run that specific code, including the limitation. There is little distinction between hardware and firmware at that point, and it is likely that the code responsible for the limitation is not programmable/reprogrammable at all. The sophistication needed to build a rudimentary short-range guided missile is surprisingly basic, and many hobbyists (or terroristically-inclined groups) could do it without too much difficulty, on a five-figure or low-six-figure budget. The GPS limitation significantly hinders the on-target accuracy that could be achieved, since the high speed terminal phase of the flight is where excellent guidance in an absolute reference frame is most important. The sophistication needed to build or microscopically alter a GPS receiver without the limitation is significantly greater (and in an entirely different technical field) than what is needed to build a missile that would benefit from that GPS guidance.
I am a geek attorney, but not your geek attorney unless you've already retained me. This is not legal advice.
Check out RTK systems using the L2 carrier to figure out the ionospheric error. Yes you do need a stationary unit and also a mobile unit. If you want real time corrections you need a wireless link between them to transmit the corrections over and it becomes harder to get rid of the off by 1 error that is often prevalent. More popular is to have a base station that is operational recording raw pseudo range and carrier phase data at a well known position (survey bench mark or from a long initial self survey) and a roving unit also collecting the pseudo range and carrier phase data. Once your surveying is done the data is post processed to provide highly accurate results. The problems with single reverence station RTK solutions like that is that you are limited to a radius of about 10mi (might be km) before the accuracy starts falling off so a better solution is having a CORS network with the ability to create virtual reference stations from "near by" reference stations.
Time to offend someone