Otdfbt

GPS satellites - almost all satellites, really - need to have the capacity to be remotely "disabled" as part of their end-of-life protocol. Satellites degrade over time like all things do (especially things exposed to the orbital environment) and they're also steadily rendered obsolete. To avoid old satellites cluttering up the most useful orbits and becoming a navigational hazard for their successors, satellites are normally boosted upwards into a graveyard orbit (an orbit nobody is actively using, where it can drift in peace) or downward into a disposal orbit (where the satellite will eventually deorbit and disintegrate in the atmosphere). They will also be passivated, removing volatiles like propellant and power storage to minimize the risks of a future collision.

Given the operating parameters of satellites, end-of-life is a one-way trip. Once it begins its graveyard or disposal burn, it won't have sufficient fuel to return to its intended orbit, and once it's passivated, it literally lacks the motive power to be able to restore itself. All your hackers have to do is convincingly mimic signals from the ground controller telling the GPS network to retire itself, and it will be irreparable. As a bonus, since satellites EOL at different times, you could retire satellites one by one to prove you're serious.

According to their Orbital Debris Mitigation Standard Practices (pdf) the US government requires end-of-life disposal for all missions, whether civilian or military. I don't know what methods GPS satellites specifically use, or if they were exempted for some reason.

This is from the Wikipedia entry for GPS, regarding its control segment:

The flight paths of the satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein Atoll, Ascension Island, Diego Garcia, Colorado Springs, Colorado and Cape Canaveral, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC. The tracking information is sent to the Air Force Space Command MCS at Schriever Air Force Base 25 km (16 mi) ESE of Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the U.S. Air Force. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein, Ascension Island, Diego Garcia, and Cape Canaveral). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite's internal orbital model. The updates are created by a Kalman filter that uses inputs from the ground monitoring stations, space weather information, and various other inputs.

Satellite maneuvers are not precise by GPS standards—so to change a satellite's orbit, the satellite must be marked unhealthy, so receivers don't use it. After the satellite maneuver, engineers track the new orbit from the ground, upload the new ephemeris, and mark the satellite healthy again.

So all you need is access to a GPS engineer's workstation (which can maybe be obtained remotely) and someone's login (so many ways to get one) and you can start spreading chaos. Merely changing a few satellites' datetime to random values will cause the system to stop working until someone fixes it.

You may be wondering how secure the air force base must be. Consider that GPS security is probably less critical for world safety than intercontinental nuke security. Consider the baseline for the security of those things (it's a miracle we're not all dead by now). You might be able to sneak in with a pizza box and a pizza delivery uniform.

It may be that the GPS operation console runs on floppies just like the nuke consoles do. If so, switch the GPS floppies with malicious ones and see as the operstors upload bad instructions to the satellites.

GPS transmissions come to Earth very attenuated, and hence are very sensitive to jamming (radio blabbering) and spoofing (fake messages impersonating sattelites). A radio station switched to the appropriate frequency would deny a large area.

The catch is that it automatically broadcasts the jamming source location. A missile could literally lock on to the jamming signal.

Luckily, we're all carrying small radio transmitters. A sufficiently successful mass-deployed cell phone virus could turn all infected devices into jammers or spoofers, effectively denying GPS in all populated areas. Be warned that there may be extra safeguards against changing the radio frequency, so owning the Operating System may not be enough and the solution will change with cell phone models. On the other hand, you don't need to cover all models.

To cover unpopulated areas such as oceans, you'd need a massive drone army.

Certainly -- if you're the United States Government.

They temporarily turned off the "dithering" of the system at various times (including during the first Gulf War, due to shortage of mil-spec GPS devices) and did so permanently in the late 1990s (I don't recall the exact year, and it doesn't really matter here). They could, in theory, turn it back on at any time, limiting civilian GPS accuracy to around 50 meters error as it was originally.

Or, they could ("temporarily") turn the entire system off, if there were a situation that made it seem like a good idea (say, advance notice of a bunch of GPS-guided Bad Things being launched at American targets).

Now, given that it's possible for the folks who built and operate the system, I'd say it's very much possible for sufficiently skilled and motivated hackers to gain access to the control systems and either disable the entire system, or disrupt it enough that (at least for a while) it's more dangerous to use it than to ignore it (say, for holding course in an airliner on a trans-Polar flight).

As with information on how to build a nuclear bomb, it's probably best to hide the details of exactly how it would be done -- write it as a "hacking" scene the way other novels and movies generally do.

"Maybe":

GPS, and the other services, satellites contain communications and state control interfaces because they are designed to be able to be controlled from ground stations.

One of the key functionalities of those state controls are what level of encryption is used to broadcast their signals - the US's GPS is first and foremost a military service, that was opened up to civilian use as a secondary functionality to gain more usage out of the expense of putting them in orbit.

During times of war the network can be modified to revoke that open civil usage and rely on more heavily encrypted signals that only official military receiver units would be able to use or understand.

So we clearly have vectors to carry out various attacks, with multiple options to the "end result" of the attack.

• Limit or remove Civil Use


• Disrupt official military use with a more complex attack


• Potentially destroy the array entirely by placing all/most satellites in unrecoverable modes and require physical replacements or repair.

As far as carrying out the attack, that would require a very complex and skill/resource intensive operation. [Possibly made far easier with advanced insider knowledge. As with any computer system, there is potentially a shockingly trivial exploit that has been overlooked]

Simply flipping modes to restrict Civil usage [the majority of GPS units/receivers] would likely be detected and corrected in minutes, but repeated attacks may take some time to completely correct and prevent being disrupted over and over again. [Maintaining spotty GPS with it flipping on and off for a full 24 hours like this might be stretching it]

An exploit to attack each satellite's station keeping systems probably has the highest potential to bring the system down and keep it from being restored in minutes.

All GPS satellites have two-way ground communications, they need semi-continuous (the longest maximum no check-in time is measured in months) ground telemetry updates. You wouldn't need to hack the satellites, individually or collectively, you'd just need to corrupt that data and crash everything. By which I mean literally crash it, hot metal falling from the sky and no more GPS until new satellites can be launched. There are at least two major obstacles to this approach, first you have to hack the US Air Force's 50^th Space Wing to plant the bad data, that will not be an easy mission. The second barrier may be even higher; you have to get all 8 separate blocks of satellites all with different programming, design specifications and check-in schedules to request and upload that bad data within a window of only a few minutes because as soon as the first satellite hits atmosphere someone will almost certainly start to make manual corrections.

So can you do it? Maybe. But It would require good timing and/or a very rare timing interface event that could be exploited and you'd really need to disable ongoing communication afterwards or timely intervention by ground control is going to mess you up.

Okay, so they can in theory be hacked. That's already been explored enough there is no need to delve any deeper, but they are one of the world's most secure networks because they are intrinsically so simple. They only need to accept a few commands such that securing all of your vectors of attack is much easier than most systems you'll otherwise encounter.

That said, there are ways to mess with a GPS network that don't involve directly hacking it. During operation desert shield, Iraqi e-warfare specialist found that they could not decrypt GPS signals to feed the false data they wanted, but they could recorded the encrypted transmission, and replay it later to fool American Aircraft into crashing by making guidance systems believe satellites were in the wrong spot at the wrong time. I doubt military grade systems still suffer from this limitation, but I would not be surprised if a number of civilian devices could still be fooled in this way.