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0 pointsgeenew5y ago0 comments

Reading this again I wasn't as complete as I should have been.

Any introduction of a false timing signal whose time offset from the existing measurements is greater than the local clock's uncertainty can be flagged as false.

If your previous reported satellite time is Sp, previous local time is Lp, local time uncertainty is U, current local time is Lc = Lp + Ld±U, and current reported satellite time is Sc = Sp + Sd, then, roughly speaking,

if Sd > (Ld + U) or Sd < (Ld - U), the reported signal is a spoof.

So the original spoof signal as well as any subsequent spoof signals are subject to the stricter constraints offered by your higher-accuracy local time source.

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sudosysgen5y ago· 2 in thread

Again I don't think you understand what I meant. The spoofed signal will start with an offset of exactly 0.

This offset will progressively increase in a way that is undistinguishable from an offset caused by receiver moving slightly left, for example.

That being said, the main source of error in GPS signals isn't the time-keeping of the receiver, but rather atmospheric interference and rounding errors in computations.

geenewOP5y ago

I understand what you mean. The fact remains that having a local and accurate source of time reduces the ability for an attacker to move a target off course through spoofing.

At each frame the attacker has to add to the error in the victim's position. By limiting how much error the attacker can add at each frame, you limit how much damage the attacker can do.

There are other systems involved here, including accelerometers and gyros, that together with a kalman filter allow the moving system to estimate the state, read values, and output new states based on a combination of those inputs. Less uncertainty in time allows for more precise predictions and more rejection of invalid inputs.

Read the link in my original post if you don't believe me: "If GPS is disrupted or jammed, a CSAC could provide precise time to the GPS receiver to enable rapid recovery or to protect receivers from GPS spoofing, a condition where false GPS signals are broadcast to fool GPS receivers with erroneous information. The hope is that the Soldier wouldn't even know that his GPS is being jammed," Olson said. "

sudosysgen5y ago

There is no error. The error produced is completely indistinguishable from the plane turning left or right.

I mentioned gyros and accelerometers in my first comment. They are the only thing that can at all help with advanced spoofing attacks. They are not sufficient. If they were sufficient, there would be no use for GPS to begin with.

This chip can only help against primitive attacks. It can help if the GPS signal is being jammed with nonsense signals by allowing the navigation system to lock back into the correct signal more quickly.

In the case of a sophisticated attack, the navigation system will not be able to detect the spoofing at all. The error induced by the spoofing will be completely indistinguishable from gyro drift.

That is to say, in the RQ-170 incident, the attacker was limited in the amount of error that could be added each frame by the gyros and accelerometer. Now, the attacker is still limited by the gyros and accelerometer, there is no change to the amount of error that can be added over time.

This chip can help against unsophisticated attacks. It cannot do anything at all for sophisticated attacks like those that allowed the capture of the RQ-170.

Let us have a thought experiment to ascertain this. Imagine a drone with an unphysically perfect clock. The drone receives time from 4 satellites and uses this time delta to calculate the distance from all four satellites.

Now let us imagine an attacker which overpowers this time signal. Initially, the signal is exactly the same as before the jamming. The signal at time t is such that it is exactly equal to that if the drone was turning left at exactly half of the gyro drift rate.

How would you be able to detect that this signal is incorrect? The answer is, it is impossible.

You may claim that the clock will be able to detect errors in the time by the spoofer. However, there is inherent noise in the time signal from GPS due to numerical errors in the predicted orbit of the satellites as well as interference from the ionosphere. The stochastic component of this noise is equal to more or less 3 meters. So the time is error in the date signal from the GPS is already of the order of 1/(100 000 000) seconds, meaning that any clock with better than that is not useful for discriminating against sophisticated attacks (but still useful against unsophisticated attacks).

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