GPS altitude
Official PPRuNe Chaplain
Joined: Apr 2001
Posts: 3,498
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From: Witnesham, Suffolk
Especially when (as happened to us in the USA last week, several times) your GPS suddenly declares "poor satellite coverage" and shows you stationary, pointing North.
I think it's all these GPS jamming trials the military are doing.
I think it's all these GPS jamming trials the military are doing.
Joined: Jun 2003
Posts: 13,787
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From: EuroGA.org
Send Clowns
Thank you for the correction. I had just seen Honeywell sell it along with the KLN94B and the rest of their stuff...
All agreed with - except the bit about the ADF being OK if one can hear the ident. As far as the system failure modes go, this definitely isn't true for the ADF, isn't true for the VOR and isn't true for the DME as implemented in products in current usage.
I could give you the registration of an aircraft which I used to fly where you can check out all the above. Obviously I won't because the school concerned might complain about loss of rental business !!! But ADFs that ident and point somewhere off are feasible, VORs likewise, and I did most of my IMC Rating with a DME which would read perfectly plausible figures (e.g. "3.6" when a few miles away from the beacon) but which were completely wrong. The instructor knew this, and would read out the correct figure off a £150 GPS from Milletts where he put the beacon in as a waypoint
No single device or system is infailible, which is why I always fly with GPS/VOR/DME/ADF concurrently, and use the lot.
I think GPS altitude is handy for gross error checking of the altimeter.
Of all the systems above, the only one which has the capability to check itself is GPS. There is no way to verify the signal from an NDB; it is just a LW carrier with some morse AM on top of it. Same with the VOR - the receiver is supposed to put up the flag when the signal strength is below a certain value, but that may not happen on typical GA planes which are used for VFR and which haven't had the kit checked with a VOR test set for years, if ever. One could do clever signal processing on it but I don't think anyone does. The DME could be verified (I believe the data has a CRC on it) but evidently there are DMEs that do no checking; they just display any old number. Whereas a GPS can do various cross-checks which would pick up all types of crude jamming. I don't think the very old designs (which covers a lot of handhelds on today's market) do that though.
Thank you for the correction. I had just seen Honeywell sell it along with the KLN94B and the rest of their stuff...
All agreed with - except the bit about the ADF being OK if one can hear the ident. As far as the system failure modes go, this definitely isn't true for the ADF, isn't true for the VOR and isn't true for the DME as implemented in products in current usage.
I could give you the registration of an aircraft which I used to fly where you can check out all the above. Obviously I won't because the school concerned might complain about loss of rental business !!! But ADFs that ident and point somewhere off are feasible, VORs likewise, and I did most of my IMC Rating with a DME which would read perfectly plausible figures (e.g. "3.6" when a few miles away from the beacon) but which were completely wrong. The instructor knew this, and would read out the correct figure off a £150 GPS from Milletts where he put the beacon in as a waypoint
No single device or system is infailible, which is why I always fly with GPS/VOR/DME/ADF concurrently, and use the lot.
I think GPS altitude is handy for gross error checking of the altimeter.
Of all the systems above, the only one which has the capability to check itself is GPS. There is no way to verify the signal from an NDB; it is just a LW carrier with some morse AM on top of it. Same with the VOR - the receiver is supposed to put up the flag when the signal strength is below a certain value, but that may not happen on typical GA planes which are used for VFR and which haven't had the kit checked with a VOR test set for years, if ever. One could do clever signal processing on it but I don't think anyone does. The DME could be verified (I believe the data has a CRC on it) but evidently there are DMEs that do no checking; they just display any old number. Whereas a GPS can do various cross-checks which would pick up all types of crude jamming. I don't think the very old designs (which covers a lot of handhelds on today's market) do that though.
Joined: Apr 2001
Posts: 231
Likes: 0
From: London UK
Clowns
...is exactly why GPS is so good. If you are using a RAIM incorperated unit (all approach approved GPSs are) then any likely outages will be notified. Also, should the unit just die (jamming or whatever) then it will also mention it to you.
What could possibly be better about having 'known errors'? How do you know what errors you ADF has? Variable coastal errors, day/night errors, pointing at thunderstorms, dip and airframe refraction - the list is horrifying. I can recall a good number of approaches using NDBs all over the world that were absolutely awful.
I recently demonstrated consecutive approaches at Edinburgh (in Day VMC) using the NDB and then a GPS based version of the same approach. The comparison was remarkable - the NDB weaved left and right of the c/line by up to 3/4 mile (coastal errors) and the latter part of the approach is ded reckoning since you are overflying the station. At MDA we were in a position to make a landing but not a lot more.
The GPS sensed approach also included vertical guidance from the BARO system and was effectively an exercise in monitoring. At the end of it we were foot-perfect and in the slot. I know what was safer in every way - the GPS approach - but I cannot understand the desire to continue using NDBs when GPS is available. It does not make sense - the GPS is a perfect non-precision approach aid as it stands. More significantly, why should I have to put what amounts to a $4k AM radio in my aircraft to make it 'Airways'?
NDBs can be lethal is poor weather - the stats bear it out. The ident is virtually meaningless.
when it goes, it all goes
What could possibly be better about having 'known errors'? How do you know what errors you ADF has? Variable coastal errors, day/night errors, pointing at thunderstorms, dip and airframe refraction - the list is horrifying. I can recall a good number of approaches using NDBs all over the world that were absolutely awful.
I recently demonstrated consecutive approaches at Edinburgh (in Day VMC) using the NDB and then a GPS based version of the same approach. The comparison was remarkable - the NDB weaved left and right of the c/line by up to 3/4 mile (coastal errors) and the latter part of the approach is ded reckoning since you are overflying the station. At MDA we were in a position to make a landing but not a lot more.
The GPS sensed approach also included vertical guidance from the BARO system and was effectively an exercise in monitoring. At the end of it we were foot-perfect and in the slot. I know what was safer in every way - the GPS approach - but I cannot understand the desire to continue using NDBs when GPS is available. It does not make sense - the GPS is a perfect non-precision approach aid as it stands. More significantly, why should I have to put what amounts to a $4k AM radio in my aircraft to make it 'Airways'?
NDBs can be lethal is poor weather - the stats bear it out. The ident is virtually meaningless.
