GPS spoofing and IRS
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GPS spoofing and IRS
The latest OPS Alert from the OPS Group mentions a new type of spoofing around LLBG FIR with a few reports from crews. One of them reports IRS affected .
How come IRS can be affected by GPS Jamming/spoofing ? I thought IRS systems were totally independent from GNSS sources.
Crew says: “Lots of jamming as usual. Spoofing was new for us. All systems including IRS were confused about location. All VMC. HDG and vectors to final from ATC.”
IRS is independent, the FMS isn't as it has a GNSS input. Bear in mind that when aligning the IRS it's commonplace to insert your GNSS derived present position so if that's wrong the IRS will question the Latitude.
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Thanks , in the meantime I found this explanation in another bulletin :
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Intresting times...
we have discovered, in many aircraft the IRS now uses GPS signals to provide updated position information. In these cases, it seems that the avionics assume that the (fake) GPS signal is correct, and the IRS has failed. The aircraft clock also shows a false time.
Intresting times...
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If the FMS is seeing different positions from IRS and GPS, it doesn't know which is true. To make a determination, it uses all sorts of probabilistic algorithms based on how stable the positions are compared to their own histories, how they differ from each other and which one is assigned a heavier baseline weight, etc.
For example, in the CRJ I could bring up a screen that shows each nav sensor and its position relative to the final FMS computed position. The GPS'es were always at 0.0 or 0.1 NM, while the DME's and IRS'es could be multiple miles off. So from that, you could deduce that the GPS'es are weighed far more heavily in their input than the other sensors.
Then from that, it's easy to see that if the GPS is spoofed into thinking it's an a wrong, but *stable and consistent*, wrong position (instead of jumping around randomly, in a way that the FMS would decide that something wrong is clearly happening and flag the GPS position as faulty), it could consider that position good and flag all the other ones as faulty.
For example, in the CRJ I could bring up a screen that shows each nav sensor and its position relative to the final FMS computed position. The GPS'es were always at 0.0 or 0.1 NM, while the DME's and IRS'es could be multiple miles off. So from that, you could deduce that the GPS'es are weighed far more heavily in their input than the other sensors.
Then from that, it's easy to see that if the GPS is spoofed into thinking it's an a wrong, but *stable and consistent*, wrong position (instead of jumping around randomly, in a way that the FMS would decide that something wrong is clearly happening and flag the GPS position as faulty), it could consider that position good and flag all the other ones as faulty.
Consider that a GPS nav unit, sitting still on the ground will report zero velocity based on time stamps issued from satellites over ten thousand miles away going almost ten thousand miles an hour. Many will think, of course, I can see it is not moving, without noticing the speed of the surface of the Earth beneath their feet due to rotation has also been cancelled out. The orbital planes of satellites aren't tied to Earth's rotation.
That ability pushes it to top position for accuracy except where a nation-state level of involvement in undermining its operation.
Using phased array antennas to actively track the GPS satellites would help - this is how Starlink manages its system. However, commercial users don't often operate in hostile environments and a quick search found no devices on the market, but did find tests and research.
See https://www.linkedin.com/pulse/overv...-curtis-arnold for Starlink detail of how narrow the beam is.
That ability pushes it to top position for accuracy except where a nation-state level of involvement in undermining its operation.
Using phased array antennas to actively track the GPS satellites would help - this is how Starlink manages its system. However, commercial users don't often operate in hostile environments and a quick search found no devices on the market, but did find tests and research.
See https://www.linkedin.com/pulse/overv...-curtis-arnold for Starlink detail of how narrow the beam is.
That’s always been the case and is known by crews…..you can deselect ( inhibit ) the GPS on the FMC’s easily and they’ll come back to the IRS position mix or radio updated position mix…..might take a few minutes to come back though depending how far they drifted. In the mean time advising ATC and using basic modes.
The clocks will go crazy perhaps until the crew reset them to manual against their watch then the FMC’s will show correct time predictions again. LNAV and VNAV will be available as long as ANP is ok for the stage of flight, look ahead terrain monitoring and ADSB reporting won’t be. No biggie.
Last edited by ACMS; 25th Oct 2023 at 13:56.
There's a difference between jamming and spoofing. Loss of GPS signal due to jamming will mean that the FMS will revert to using a mix of the GNADIRS positions( together with VOR/DME) to produce an accurate FMS(aircraft) position. Spoofing means that the GNSS signal is still there but inaccurate so the GNADIRS pass their hybrid GPIRS posn to the FMS for use. The FMS is biased toward this GPIRS posn. If the FMS can get a DME/DME fix then Mr Kalman and his filter will no doubt do a bit of head scratching and bias the FMS position based on what it thinks is the most accurate data. As for the time, surely one looks at one's Breitling?
There's a difference between jamming and spoofing. Loss of GPS signal due to jamming will mean that the FMS will revert to using a mix of the GNADIRS positions( together with VOR/DME) to produce an accurate FMS(aircraft) position. Spoofing means that the GNSS signal is still there but inaccurate so the GNADIRS pass their hybrid GPIRS posn to the FMS for use. The FMS is biased toward this GPIRS posn. If the FMS can get a DME/DME fix then Mr Kalman and his filter will no doubt do a bit of head scratching and bias the FMS position based on what it thinks is the most accurate data. As for the time, surely one looks at one's Breitling?
Question: are the GPS augmented? SBAS? If so, it is conceivable that the broadcast "corrections" are spoofed, perhaps with a stronger signal.
