Frozen positions

FlightAware ADS-B Flight Tracking
1

Since a couple of days ago, I noticed that in my Skyaware map, aircraft approaching the Mexico City’s airport just freeze their positions a couple of miles before landing, and after about 20-30 seconds they “jump” to a new position close to the airport. (I’m about 10 miles away from the airport just under the aproach path).

I don’t think there is any new building blocking my receiver sight to that point.

The weird thing here is that although the position is not updated during that lapse, the altitude keeps constantly updating, which makes me think I’m receiving messages from aircraft continously.

Any thoughts?

Edit:

Just dumped some port 30003 data and found that while the aircraft is “freezed” I don’t get any “3” messages from that particular aircraft (containing alt, lat & lon), but only “1”, “7” or “8” (7’s had the altitude data too).

How can I get some type of messages and not the others?

While such aircraft is freezed in the map, I still see movement (and “3” messages) from the remaining aircraft. It seems that the freezed aircraft stops sending “3” messages as they approach the airport, but still sends type “7” messages.

2

There is GPS jamming in the area.

https://gpsjam.org/?lat=21.70076&lon=-95.25252&z=5.1&date=2022-09-19

This command will give you more detailed data for a certain aircraft:

view1090-fa --no-interactive --show-only HEXID
3

Thank you. So this GPS jamming is due to interference from other sources (traffic, cwll phones, etc) or an intentional attack?

From the map at gpsjam.org it seems the affected area is much larger than where the aircraft stops reporting (my site is on the red zone, but aircraft above me report continuosly until they are close to the airport).

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I wonder if aircraft stops sending messages if they can’t get a posiion or if it’s my receiver not getting messages from that area (I always get other types of messages, only type “3” are not received during the “blackout” period).
.

From what I see from port 30003, my conclusion is aircraft doesn’t sends any type “3” msgs if it is not able to get its own position, right?

In this log (filtered for type “3” messages) I see it stopped broadcasting for 84 seconds and when it resumed the first 2 messages had no positions at all.

MSG,3,1,1,0D0B10,1,2022/09/22,16:54:39.822,2022/09/22,16:54:39.943,8175,19.40034,-99.13342,0,0,0
MSG,3,1,1,0D0B10,1,2022/09/22,16:54:40.372,2022/09/22,16:54:40.422,8150,19.40055,-99.13313,0,0,0
MSG,3,1,1,0D0B10,1,2022/09/22,16:54:40.822,2022/09/22,16:54:41.012,8150,19.40067,-99.13283,0,0,0

MSG,3,1,1,0D0B10,1,2022/09/22,16:56:04.993,2022/09/22,16:56:05.254,7150,0,0,0
MSG,3,1,1,0D0B10,1,2022/09/22,16:56:05.497,2022/09/22,16:56:05.639,7150,0,0,
MSG,3,1,1,0D0B10,1,2022/09/22,16:56:05.956,2022/09/22,16:56:06.162,7150,19.42802,-99.08415,0,0,
MSG,3,1,1,0D0B10,1,2022/09/22,16:56:06.494,2022/09/22,16:56:06.815,7125,19.42818,-99.08383,0,0,

4

The resolution on the gpsjam map is limited, one would think this is obvious by the size of the hexagons?

No no, this doesn’t happen by accident.
It’s usually aimed against drones with explosives on them or similar.
It’s not something that just happens, the government is more than likely responsible.

Use this command on the console of your receiver:

view1090-fa --no-interactive --show-only 0D0B10

to get a better understanding of message types. (If you don’t have shell access because it’s provided by FA you’ll need another linux computer or VM and then get the data via network from port 30005 using readsb for example, the using viewadsb instead of view1090-fa)
SBS message types have nothing to do with what the plane is actually sending, it’s a different protocol that doesn’t give you access to the raw data.

You conclusion is still correct, the planes lose their GPS reception so the transponder stops sending the position.
They still know their position using an inerial reference.
https://en.wikipedia.org/wiki/Inertial_measurement_unit
https://en.wikipedia.org/wiki/Inertial_navigation_system

At least for a couple minutes after losing GPS, this is more than precise enough to send an ADS-B position but the specification says ADS-B positions must be based only on the received GPS position.
Don’t ask me why but that’s the standard.

5

Very likely, planes stop sending positions exactly when they reach the street where the National Palace (Mexican White House equivalent) is. About a couple of miles south from it.

6

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