Texas students fake GPS signals and take control of an $80 million yacht

One big confusion in the electrical world is people use the same noun for audio "mixers" which are as linear as possible, and RF "mixers" which are as non-linear as possible. They do different things. linear mixers you could say superimpose signals without changing them. Much like pumping up the gain on a CD should not distort the sound on a mic at a "DJ" mixer. nonlinear mixers add and subtract signal frequencies from each other and what came in shouldn't come out at all. in fact a BPSK mod is a kind of balanced mixer, with peculiar TTL compatible (or 3.3v or whatever) levels of course.

You want a 4-port combiner. Most passives have a reciprocal path, a 4-port splitter usually makes a decent 4-port combiner. Think of the gadget that probably splits you cable TV signal in your house. They're electrically and mechanically simple. And relatively cheap. Also they are somewhat lossy. Good luck passively splitting a signal 4 ways with less than 6 dB of loss. And something is warming the resistors in there, so its going to be worse. Conversely yes you combine multiple signals there will be internal loss but the aggregate output will be higher. This is kind of the whole point of a class B or class AB amplifier... what if you took two perfectly good signals, 180 degrees out of phase, and (sorta) mixed them (using baluns), well you get very near twice the power out. Think of putting the whole works in a calorimeter... 4 zero dBm sources will heat it up just as fast combined or separate.

Aviation GPS "around 1.5 GHz" works pretty well despite being feet/inches away from a couple watt radar transponder around 1090 MHz or whatever it is exactly for ADS-B. Physically zorching it sounds unlikely. Distortion to the point of un demod ability is however possible. If you generate enough signal to overload it, attenuators are cheap. High power is expensive. If you're screwing around at "workbench range" you're not going to pay $XXX to generate multiple watts of power so you're not going to need multiple watt rated attenuators to reduce the sig level to something reasonable. Cost scales WAY beyond quadratically, like exponential at microwave freqs for a given tech type. Stuff working around a hundredth of a watt aka 10 dBm is going to be very cheap compared to stuff rated for old fashioned weather radars at kilowatts.

My suggestion is make what amounts to exactly one working satellite. Then make three more.

I don't think you can feed all four digital signals into the same BPSK mod by doing weird things with the clock rate, modulation does not work that way.

What you're building is vaguely reminiscent of a cable TV headend. Both in block diagram and actual wiring. Of course its been a long time since BPSK was cutting edge in CATV. If you think of BPSK as no amplitude modulation and either 180 or 0 degree phase modulation depending on 0 or 1 being input, well, a 256QAM signal is just 16 equally spaced levels of amplitude modulation and 16 equally spaced levels of phase modulation, sorta kinda a grown up cousin of the BPSK modulation in GPS signals. And 256QAM is sort of cutting edge for CATV. Anyway you could do worse than looking at a wiring diagram of a CATV headend WRT mods and digital sources and combiners and such.

Find your local ham radio VHF/UHF microwave club / community. Don't bother with the 160meter low band guys (well, not for this particular individual project, I mean) By the time you're done you'll know quite a bit about RF and might gain a new interesting hobby. Reading several "microwave handbook/project" ham radio books at the library would probably be as good a place to start as any. You could do worse than some chapters of the ARRL handbook to start.

Analog is fun. People will tell you the world is digital, but even their digital ckts are fundamentally analog. And if analog is fun, RF is just magic. A craft not a science at the higher levels.

Let's say a captain enters a course to stay 500nm offshore of a pirate-infested coast (btw, after Somalia, West Africa is now a pirate hotspot[1]). By interfering with the GPS, the ship could be turned imperceptibly towards shore, and after a day be within range of pirate boats. The GPS display on the electronic charts would still show the intended course, because the GPS thinks it's on the right track. And without any land for reference, the captain or crew might not notice. If GPS is the sole position-finder, the fake coordinates would also endanger the reliability of ship-to-ship collision detection such as AIS[2].

With a cell phone or any consumer device, the user has to constantly read the GPS output and then react to it based on the roads or other physical landmarks. You couldn't just "steer" a person to the wrong place by making the GPS believe it's in a different location. And then I can't think of any other "exploit" that you could do with the GPS on a phone.

As mentioned elsewhere in this thread, the solution to the boat navigation problem is to have alternate sources of position info (Loran, GLONASS, etc.). Alternatively, the error introduced by the fake GPS could also be detected by weather info. Any deviation from the dead-reckoning course (heading and speed) can only be accounted for by wind and current. I believe wind and current forecasts are fairly common for all areas of the globe now, so the calculated values could be compared to the expected values and raise an alarm if they are far off.

[1] http://gcaptain.com/tag/piracy/ [2] http://en.wikipedia.org/wiki/Automatic_Identification_System

If I were designing such a system (and I'm sure I've overlooked about a million subtleties), I imagine I would make it so that the GPS corrects the IMU for small perturbations that lie roughly within the IMU's predicted error budget, and that the IMU causes the GPS to be ignored if the GPS coordinates suddenly diverge greatly.

In other words, the IMU is going to be saying something like, "our position is X, to within 5km". If the GPS disagrees with X by 1km or perhaps even 10km, correct the IMU's current state using the GPS. If the GPS disagrees with X by 100km, ignore the GPS until it gets its act together.

Seems like a decent first pass, at least.