5 ms·

The idea they can have this type of interference low to the ground in a landing situation (when the reflected power is high) is a total failure on behalf of boe

The idea they can have this type of interference low to the ground in a landing situation (when the reflected power is high) is a total failure on behalf of boeing. I can't even fully get my head around the physics that would get you an issue with even normal filtering, much less what once should be able to do on an airplane with high fixed costs and safety of life factors. Amazing they can't do GPS as a cross check on indicated altitude over a terrain model.

vel0city 5y ago
> Amazing they can't do GPS as a cross check on indicated altitude over a terrain model. I mean they could, but they skimped out on just getting a halfway decent filter for frequencies 200MHz away from their signals what are the odds they'd pay for that.
- tjohns 5y ago
  No, they really can't. At least not until GBAS is rolled out. GPS is notoriously unreliable for altitude data. GPS altitude is used for LPV approaches, provided you also have a WAAS correction signal, but those require higher weather minimums than the Cat II/III approaches that you'd use a radio altimeter for.
kube-system 5y ago
1. This isn't just a Boeing problem. Not only does the FAA have these concerns for other jets, they've also prohibited some other jets from landing at some airports in some conditions because of it. 2. Boeing doesn't get to decide to just cross check against GPS. The FAA has rules about the equipment that can be used for various types of landings.
p_l 5y ago
GPS is incapable of getting even close to the resolution required from radar altimeter, and altitude ranging in GPS was always worse than its circular error probable in 2d - and the best accepted dynamic, unaided GPS deviation is big enough to show the same result between correct landing, missed landing, and a fiery crash. The problem is that radar altimeter is very precise device that for best precision needs to use all of its 200MHz bandwidth on transmit. And there are no methods to recognize that as part of the return in its own 200MHz band (or in the guard band) a random CDMA-encoded signal radiated outside of allocated spectrum (happens) reflected of random crap around the airport and gets interpreted as valid data. This is an analog system where the analog characteristics are core to how it works, so the methods that allow tight channel spacing in communications tech do not work for radar altimeters
chihuahua 5y ago
I don't know much about aviation, but I wouldn't want to rely on GPS for anything important. It's easy to jam and not always reliable.

vel0city 5y ago

> Amazing they can't do GPS as a cross check on indicated altitude over a terrain model. I mean they could, but they skimped out on just getting a halfway decent filter for frequencies 200MHz away from their signals what are the odds they'd pay for that.

tjohns 5y ago
No, they really can't. At least not until GBAS is rolled out. GPS is notoriously unreliable for altitude data. GPS altitude is used for LPV approaches, provided you also have a WAAS correction signal, but those require higher weather minimums than the Cat II/III approaches that you'd use a radio altimeter for.

kube-system 5y ago

1. This isn't just a Boeing problem. Not only does the FAA have these concerns for other jets, they've also prohibited some other jets from landing at some airports in some conditions because of it. 2. Boeing doesn't get to decide to just cross check against GPS. The FAA has rules about the equipment that can be used for various types of landings.

p_l 5y ago

GPS is incapable of getting even close to the resolution required from radar altimeter, and altitude ranging in GPS was always worse than its circular error probable in 2d - and the best accepted dynamic, unaided GPS deviation is big enough to show the same result between correct landing, missed landing, and a fiery crash. The problem is that radar altimeter is very precise device that for best precision needs to use all of its 200MHz bandwidth on transmit. And there are no methods to recognize that as part of the return in its own 200MHz band (or in the guard band) a random CDMA-encoded signal radiated outside of allocated spectrum (happens) reflected of random crap around the airport and gets interpreted as valid data. This is an analog system where the analog characteristics are core to how it works, so the methods that allow tight channel spacing in communications tech do not work for radar altimeters

chihuahua 5y ago

I don't know much about aviation, but I wouldn't want to rely on GPS for anything important. It's easy to jam and not always reliable.