Fort Wayne incident
06-30-2018, 08:06 AM
#41
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Originally Posted by 501D22G
Sorry to nitpick, but if you are correcting him, it's trilateration, not triangulation.
I had no idea that was even a word before your post. I appreciate the nitpick as I've now learned something new.
06-30-2018, 08:17 AM
#42
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Originally Posted by rickair7777
Non FBW is not immune to interference, just that the interference would affect other systems such as what you described. But at least you can grab the controls and right the ship. A hypothetical EM interference which caused a FBW aircraft control to go hard-over might not be recoverable.
06-30-2018, 08:21 AM
#43
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Originally Posted by NormalAbnormal
It's been a while since I saw a technical presentation on GPS; but I think a synchronized atomic clock is required in the receiver in order to use only 3 satellites (absolute time measurement as you describe above). However, atomic clocks don't lend themselves to portability. If you use a good relative clock, to do differential time measurements, then you need 4 satellites (4 degrees of freedom) and you don't need to know how long the signal took to reach you. You can infer it by looking at the relative delays between the 4 satellite signals. This is how the vast majority of GPS receivers work. I'm pretty sure the ground based calibration sites do have their own synchronized atomic clock.
06-30-2018, 09:22 AM
#44
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To everyone who mentioned harmonics and all the fun little R.F. facts: you warmed my heart! I love that stuff.
Also when I flew the dash, we had a problem with false smoke warnings. Turns out it was cell phones. So ya, interference. By cellular devices.
Did we crash, no. Did we divert and cancel flights, yup. All because some prick couldn’t follow the rules. But hey, pax know better than any of us
Also when I flew the dash, we had a problem with false smoke warnings. Turns out it was cell phones. So ya, interference. By cellular devices.
Did we crash, no. Did we divert and cancel flights, yup. All because some prick couldn’t follow the rules. But hey, pax know better than any of us
06-30-2018, 02:23 PM
#45
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Originally Posted by Excargodog
I suppose anything is possible, but I have a rather vivid memory of flying in the back seat of a FBW fighter and watching ball lightning dance along the canopy. OK, perhaps mil-spec stuff is better EMP protected than civilian aircraft, but after watching that it sort of begged credibility that a cellphone signal could cause problems. Someone hacking the onboard computer in an Airbus on the other hand....
EM interference from man-made sources is going to be about frequency and waveform, and potentially digital packet interference. Near infinite potential variations, like the old analogy about a 10,000 monkeys with typewriters.
06-30-2018, 08:49 PM
#46
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Originally Posted by N1234
This is correct. Most aviation receivers require line of sight to 5 satellites. 3 for position incl alt. 1 for time reference and 1 for RAIM.
06-30-2018, 09:16 PM
#47
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Originally Posted by JamesNoBrakes
Not my understanding. My understanding is that it's a differential math equation with 4 equations and 4 unknowns, solving for 1 time that satisfies all equations. Sats traveling thousands of miles/hr are essentially "dumb", sending out their atomic time. Receiver gets atomic time from 4 different sats, but all the times are different due to the difference in distance. Receiver must solve for distances and time in those 4 equations, pseudo-ranging. This is 3d position. Receiver knows what time it is where it is located with no delay/distance error because it's constantly solving this equation.
The satellite transmits its own position and accurate time (atomic clock).
The receiver itself compares the signal delay from the satellites and interprets it as distance from the reported satellite position.
But it essentially compensates for time inaccuracy.
In the old days submarines could get away with just two satellites by (1) having an atomic clock onboard and (2) making the simplifying assumption to be on the surface.
06-30-2018, 09:23 PM
#48
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Originally Posted by N1234
The satellite transmits its own position and accurate time (atomic clock).
06-30-2018, 11:32 PM
#49
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Originally Posted by JamesNoBrakes
That's where I think the issue is, they are traveling somewhere like 10k mph, right? They are not transmitting their "GPS" position, they are transmitting time, it's all about the time. The receiver doesn't know the satellite position. If it did, it could easily calculate it's own position without time.
06-30-2018, 11:32 PM
#50
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Originally Posted by JamesNoBrakes
That's where I think the issue is, they are traveling somewhere like 10k mph, right? They are not transmitting their "GPS" position, they are transmitting time, it's all about the time. The receiver doesn't know the satellite position. If it did, it could easily calculate it's own position without time.
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