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The FAA has announced that GPS jamming will only affect aircraft above 5000 feet, not above 50 feet that was mistakenly reported. But I'm still confused by you

The FAA has announced that GPS jamming will only affect aircraft above 5000 feet, not above 50 feet that was mistakenly reported. But I'm still confused by your explanation about how GPS jamming can avoid affecting areas below 5000 feet. I can imagine two ways that the jamming could work: (1) You're jamming the GPS receiver. An analogy would be shining a laser pointer into a photographer's camera. You're overwhelming the receiver so he can't receive the signal, but you are not actually modifying the original signal. This seems to fit your explanation. (2) You're blocking or modifying the actual GPS signal. The analogy here would be spreading out a huge curtain in the sky at an altitude of 100,000 feet to block out the sun. Aircraft flying at 50,000 feet won't get any sunlight. But neither will people on the ground. Your explanation does not seem to work in this case. You can't block sunlight at 100,000 feet without also blocking it for people at ground level. What am I missing here?

mikeash 10y ago
Your #2 isn't possible without actually spreading something out in the sky, and blocking a lot more than just GPS. "Jamming" refers to #1. You broadcast a signal on the same frequency with a greater strength, preventing the receiver from being able to pick out the original. For a ground-based jammer, if you're at some distance from it then the lower you are the less jamming you'll receive because stuff gets in the way. At some point the strength of the jamming signal isn't enough to prevent you from picking out the real signal.
_kst_ 10y ago
Perhaps they're just saying that any spot below 5000 feet AGL is subject to jamming, not that all points above that level will be jammed. (I have no idea how any of this would work.)
detaro 10y ago
some ideas: The jamming sender(s) could be flying as well. If you only send upwards from a flying plane, you obviously won't jam receivers below it. and/or the areas where you can receive it below 5000ft are inside restricted airspace. There is a lot of that around China Lake, but I don't know if they just can't tell you about issues in there. I'd expect so though, given that they do all kinds of exercises in there that all would need an extra warning otherwise. (EDIT: but the sizes/the angle of the cone don't look as they'd work out for a sender on the ground, unless they added massive safety margins) Or they are doing something clever with multiple senders, and below 5000 you can't receive the signals overlapping in such a way that actual jamming happens. Not sure if that would work in GPS frequency ranges though.
- chipperyman573 10y ago
  >Plane at 5000ft obviously can send signals only forwards/upwards. Why? Can't you put a sender on the base of a plane?
  - detaro 10y ago
    bad wording on my part, meant that if you put a sender that only sends upwards on a plane at 5000ft, it it obviously won't be received below it. Edited the comment.
chockablock 10y ago
> The FAA has announced that GPS jamming will only affect aircraft above 5000 feet Where did they announce this? The NOTAM linked from the parent article is still on the FAA website and clearly says GPS may not be available between 50 Ft and 40,000 Ft. (NB I am not confusing FL 50 for 50 Ft) https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_GPS_Flight_Advisory.pdf https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_...
- mysterypie 10y ago
  My apologies, you are correct; the jamming will affect aircraft above 50 feet. I was reading an article on Gizmodo[1] that originally said 50ft, then they "corrected" it to 5000ft[2], and then corrected it again to 50ft. Serves me right for relying on Gizmodo rather than reading the official FAA site. [1] http://gizmodo.com/faa-warns-of-gps-outages-this-month-during-mysterious-t-1780866590 http://gizmodo.com/faa-warns-of-gps-outages-this-month-durin... [2] http://www.govtslaves.info/faa-warns-of-gps-outages-this-month-during-mysterious-tests-on-the-west-coast/ http://www.govtslaves.info/faa-warns-of-gps-outages-this-mon... [this is someone's capture of the incorrect Gizmodo article where they said 5,000 ft]
  - ridgeguy 10y ago
    I'm curious - If I fly a GPS-dependent drone above 50 feet within the jamming footprint, do you think it would degrade or fail? If I were near the area, I might try the experiment.

mikeash 10y ago

Your #2 isn't possible without actually spreading something out in the sky, and blocking a lot more than just GPS. "Jamming" refers to #1. You broadcast a signal on the same frequency with a greater strength, preventing the receiver from being able to pick out the original. For a ground-based jammer, if you're at some distance from it then the lower you are the less jamming you'll receive because stuff gets in the way. At some point the strength of the jamming signal isn't enough to prevent you from picking out the real signal.

_kst_ 10y ago

Perhaps they're just saying that any spot below 5000 feet AGL is subject to jamming, not that all points above that level will be jammed. (I have no idea how any of this would work.)

detaro 10y ago

some ideas: The jamming sender(s) could be flying as well. If you only send upwards from a flying plane, you obviously won't jam receivers below it. and/or the areas where you can receive it below 5000ft are inside restricted airspace. There is a lot of that around China Lake, but I don't know if they just can't tell you about issues in there. I'd expect so though, given that they do all kinds of exercises in there that all would need an extra warning otherwise. (EDIT: but the sizes/the angle of the cone don't look as they'd work out for a sender on the ground, unless they added massive safety margins) Or they are doing something clever with multiple senders, and below 5000 you can't receive the signals overlapping in such a way that actual jamming happens. Not sure if that would work in GPS frequency ranges though.

chipperyman573 10y ago
>Plane at 5000ft obviously can send signals only forwards/upwards. Why? Can't you put a sender on the base of a plane?
- detaro 10y ago
  bad wording on my part, meant that if you put a sender that only sends upwards on a plane at 5000ft, it it obviously won't be received below it. Edited the comment.

chockablock 10y ago

> The FAA has announced that GPS jamming will only affect aircraft above 5000 feet Where did they announce this? The NOTAM linked from the parent article is still on the FAA website and clearly says GPS may not be available between 50 Ft and 40,000 Ft. (NB I am not confusing FL 50 for 50 Ft) https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_GPS_Flight_Advisory.pdf https://www.faasafety.gov/files/notices/2016/Jun/CHLK_16-08_...

mysterypie 10y ago
My apologies, you are correct; the jamming will affect aircraft above 50 feet. I was reading an article on Gizmodo[1] that originally said 50ft, then they "corrected" it to 5000ft[2], and then corrected it again to 50ft. Serves me right for relying on Gizmodo rather than reading the official FAA site. [1] http://gizmodo.com/faa-warns-of-gps-outages-this-month-during-mysterious-t-1780866590 http://gizmodo.com/faa-warns-of-gps-outages-this-month-durin... [2] http://www.govtslaves.info/faa-warns-of-gps-outages-this-month-during-mysterious-tests-on-the-west-coast/ http://www.govtslaves.info/faa-warns-of-gps-outages-this-mon... [this is someone's capture of the incorrect Gizmodo article where they said 5,000 ft]
- ridgeguy 10y ago
  I'm curious - If I fly a GPS-dependent drone above 50 feet within the jamming footprint, do you think it would degrade or fail? If I were near the area, I might try the experiment.