It seems related to an upgrade on that Collins equipment, in the Receiver Autonomous Integrity Monitoring, and quality of nav data sent to ADS-B equipment, which is downlinked from the aircraft to air traffic control.
The Multi Mode Receivers aren't strictly GPS units - they also get nav signals from terrestrial VOR, DME, and ILS stations. DME - DME fixes are qite precise, but not as precise as GPS positioning.
There's also an altitude restriction on affected aircraft: they aren't permitted into airspace using Reduced Vertical Separation Minimums, where tighter navigation and monitoring restrictions apply.
Wow that could have affected WAAS GPS approaches for aircraft which is rather critical stuff. (WAAS approaches allow for airplanes to descend to much lower altitudes without ground visibility. Basically the GPS equivalent of the Instrument Landing System.)
And Mode S has serious implications for TCAS systems (preventing air to air collisions.)
Extremely bad to be messing with those frequencies. Potentially catastrophic.
The GPS jamming map linked to in the article[1] discusses this somewhat, in the "About the data" box:
- ADS-B messages include position information from Global Navigation Satellite Systems (GNSS), like GPS, Galileo, GLONASS, BeiDou, etc.
- It is not possible to directly measure GNSS interference, but we do calculate the NIC (Navigation integrity category) for ADS-B messages.
- The NIC value encodes the quality and consistency of navigational data received by the aircraft.
- Poor NIC values alone might indicate a problem with an aircraft’s equipment or unfavorable positioning. However, when observed in multiple aircraft in close proximity during the same time frame, it suggests the presence of a radio signal interfering with normal GNSS operation.
They have a GPS beacon. The ADS-B system is exactly that. It just stopped transmitting. Apparently it can be manually turned off from the cockpit. Or it malfunctioned.
What I don't understand is how this is supposed to work when the point of using the more precise GPS-based ADS-B positional reporting is for controllers to be able to pack airplanes closer together. If they do this, there will literally not be space in the sky to spread out existing air traffic to radar-based separation requirements.
It was just a few weeks ago that there was a US midwest and eastern GPS timing precision error that prevented commercial jet's ADS-B from accepting it and causing them to give up throwing errors. ref: https://hackaday.com/2019/06/09/gps-and-ads-b-problems-cause...
> As part of the ADS-B messages we receive from each aircraft, the Navigation integrity category (NIC) encodes the quality and consistency of navigational data received by the aircraft. The NIC value informs how certain the aircraft is of its position by providing a radius of uncertainty.
> Poor NIC values alone might indicate a problem with an aircraft’s equipment or unfavorable positioning. However, when observed in multiple aircraft in close proximity during the same time frame, it suggests the presence of a radio signal interfering with normal GNSS operation.
A single observer can't really say for certain that jamming is happening; you need a distributed sample from multiple different sensors over a period of time to have reasonably high confidence.
> Even if an aircraft isn't broadcasting its own position via ADS-B, if it's using Mode S its position can be multilaterated if you have a cooperating network of receivers (which is what FR24, ADS-B Exchange, etc. have).
I am a total noob in this area (and in general) but from my cursory understanding of Mode S, you would still have to have an active transponder on the aircraft. Wouldn't a military aircraft that wants to hide it's location not output any radio signals (except narrowband directed signals if those are practical?).
What is MLAT in this case? The planes transmit their location in the ADS-B message. They are filtering a lot of redundant message receptions from multiple receivers. I would assume the messages also include altitude and heading information, so locations can be predicted between message receptions.
A lot of comments seem to be about spoofing. Remember that there are multiple humans in the loop -- experienced pilots and experienced air traffic controllers -- and multiple levels of redundancy. Pilots have access to barometric altitude, radar altitude, traditional VHF navaids, TCAS-issued resolution advisories (based on directional antennas, not GPS)... in addition to GPS. Air traffic controllers have primary radar and secondary radar in addition to ADS-B information.
You could probably cause a lot more trouble by buying an aviation radio off the shelf and just chatting on the frequency or issuing fake clearances if you want to be a mass murderer.
Ultimately the system comes down to many systems working together, not one stream of commands between two computers.
So I'm a little skeptical on that front because military aircraft are still not that often broadcasting ADSB even during routine flights, at least in my (also close to the border) region. In theory the Air Force was supposed to have completed ADSB installation on their fleet last year but they blew the deadline pretty bad on even installing transponders, and of course they still reserve the right to disable them during military operations.
Maybe with the data we can figure out what portion of military flights are included?
For the helicopter training flights that I notice most often, it's still rare to see one that broadcasts ADSB, probably <10%. C-130s usually don't either here but it's more often, maybe more like 25%. Perhaps for other categories of aircraft they've installed more transponders. But in the city where I live, even passive mode-C MLAT is probably around 50% success on tracking military flights for ADSB Exchange. FlightAware might have better coverage for mode-C. mode-C can't contribute to this GPS reliability data anyway but it illustrates that even C-130 pattern practice is sometimes "stealth" from a radio perspective due to the low installation rates for ADSB and difficulty of good mode-C coverage.
The paper linked elsewhere (https://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Liu_...) mentioned issues with military training flights resulting in spurious low-NIC cases but unfortunately doesn't quantify it. With the way the AF rollout has gone it probably depends on the specific installation, command, and aircraft type.
In the border region specifically we would tend to expect the majority of non-military flights to be civilian CBP aircraft that aren't performing unusual maneuvers. CBP has a somewhat complicated and limited authority to disable ADS-B that I don't know the contours of, I'm not sure how often they do so on their larger (non-sUAS) aircraft. Involvement of the Air National Guard in the Texas area might complicate the analysis though.
The issue with the oceanic flights is that there are no receiving stations for the ADS-B messages, as these stations are ground based. Therefore, there is no way to receive the position reports to feed into the flight data network.
They could also be switching the transponder to Mode 3 (equivalent to civilian Mode C) which is not ADS-B compatible but would still show up on normal normal civil radar scopes.
Or they can use some military only modes. Some of those could in theory be shown on civil scopes, but I'm not sure they actually are, while others like mode 4 or 5 cannot. But they could show up on the UK's military scopes, given nato intercompatibility.
Another theoretical option would be for the air force planes to be using mode-S, but having disabled mode-S extended squitter (which means the planes don't periodically broadcast their location, but simply only reply when interrogated by radar). I would expect ADS-B receiver to pick up on these periodic interrogations though. I'm not sure if interrogation responses are directional, but even if so, some receivers would probably be in the general direction of the radar site.
Its not really clear which. Outside of wartime, normal US Air Force policy is to fully integrate into the civil ATC system, except in special military use/training areas or certain special missions. This integration just makes things safer for everybody.
MLAT is multilateration, where they use the difference in time of arrival to multiple receivers with known locations to estimate the positions of planes that doesn't transmit their location using ADS-B but only use older Mode S. This is mostly small or military planes.
The Multi Mode Receivers aren't strictly GPS units - they also get nav signals from terrestrial VOR, DME, and ILS stations. DME - DME fixes are qite precise, but not as precise as GPS positioning.
There's also an altitude restriction on affected aircraft: they aren't permitted into airspace using Reduced Vertical Separation Minimums, where tighter navigation and monitoring restrictions apply.
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