ADF dip
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ADF dip
ADF dip (in the turn in a hold) is mentioned in some places, but not in the following
PPSC ATPL notes
Trevor Thom vol 5
IFR Principles & Practice
Can anyone explain either about dip
or
where to find mention of it in a good publication
???
PPSC ATPL notes
Trevor Thom vol 5
IFR Principles & Practice
Can anyone explain either about dip
or
where to find mention of it in a good publication
???
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Dip is caused because the ADF loop antenna is tilted from the horizontal. The modern ADF loop antenna is optimised to pick up horizontal magnetic fields, when tilted it picks up vertical components which produce bearing errors. Night effect can cause similar problems when sky-wave signals are picked up by vertical parts of the antenna.
[ 08 January 2002: Message edited by: Noggin ]</p>
[ 08 January 2002: Message edited by: Noggin ]</p>
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Basically yes. The transmission is vertically polarised giving rise to a Vertical E field and a Horizontal M field in phase with the former.
Perhaps I could have simplified things by saying the ADF works on the basis of looking for a signal Null. At this point the emf induced in one side of the antenna is equal and opposite to the emf generated in the opposite side and thus creates a nul. If we rotate either the signal field or the antenna, the emf induced is not equal and thus does not cancel, the ADF servo then drives the goniometer and ADF needle to a false null ( Dip or Night effect)
Admiralty Handbook of Wireless Telegraphy VolII 1938.
Perhaps I could have simplified things by saying the ADF works on the basis of looking for a signal Null. At this point the emf induced in one side of the antenna is equal and opposite to the emf generated in the opposite side and thus creates a nul. If we rotate either the signal field or the antenna, the emf induced is not equal and thus does not cancel, the ADF servo then drives the goniometer and ADF needle to a false null ( Dip or Night effect)
Admiralty Handbook of Wireless Telegraphy VolII 1938.
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Is night effect not the increased multi-path effects experienced by the ADF receiver due to the change in the tropospheric propagation of the original NDB signal. Additionally is it not also the case that the ADF receiver will also receive signals from normally out of range NDB's on the same frequency during the changed atmospheric conditions experienced at night.
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Night effect can be a combination of both, but the primary cause is due to polarity change of the signal when reflected from the ionosphere. Ground and sky wave arrive at different times, this is of no consequence to the ADF receiver but the angle of incidence of the skywave is. Other transmissions within the pass band, not necessarily NDBs, as few are on the same frequency(synchronous transmissions), can add to the problems. Multipath may also occur to the groundwave, called terrain effect.
Dip and Night effect are both in their simplest form, due to a rotation of the incoming signal relative to the antenna.
Dip and Night effect are both in their simplest form, due to a rotation of the incoming signal relative to the antenna.
Yes - dip error certainly exists but seems to feature more prominently on light aircraft installations.
If you are flying a hold, for example, the amount of "lead" when on the inbound turn when turning through a heading which is at 60 degrees to the axis is 7 degrees (assuming negligible wind effect). On may light aircraft the 7 degree lead is cancelled out by "dip error" so if when you turn through said heading the RBI shows a relative bearing of +/- 60 degrees then the turn should connect up. If you have the luxury of an RMI then the desired inbound track would be displayed.
If you are flying a hold, for example, the amount of "lead" when on the inbound turn when turning through a heading which is at 60 degrees to the axis is 7 degrees (assuming negligible wind effect). On may light aircraft the 7 degree lead is cancelled out by "dip error" so if when you turn through said heading the RBI shows a relative bearing of +/- 60 degrees then the turn should connect up. If you have the luxury of an RMI then the desired inbound track would be displayed.
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Surely if both the ground waves and the skywaves arrive at the ADF receiver then there will no doubt be a phase difference leading to an erroneous interpretation by the ADF. (Not too dissimilar to ghosting on a TV set.) The ADF is a phase interpreting instrument hence the presence of a number of phases of the same original signal can only lead to inaccurate readings.
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The ADF is the least accurate of the nav aids due to many errors it is subject to, when discussing ADF errors one must also factor in such errors as precipitation static, electrical static such as st. elmos fire, mountain/valley errors, sea/land errors and on and on go the reasons for the ADF being inaccurate.
Several years ago I was required to write the U.S. Commercial rotorcraft exams for a U.S. licence and to my surprise there were seven ADF time and distance / bearing change questions on the exam. Can anyone tell me what advantage knowing the answers to such inaccurate methods would give me?
Of course it is nice to be able to chat about the errors that are inherent in the ADF but at the end of the conversation we still are left with the same result...the ADF is an aid for cloud breaking procedures when flying IFR and is without doubt a true non precision aid. Knowing this very simple fact we also must be aware that it will soon go the way of the radio range method of navigation.
As a closing comment I defy anyone that is flying the ADF to convince me that any given swing of an ADF needle is due to any one cause.
............
The hardest thing about flying is knowing when to say no.
Several years ago I was required to write the U.S. Commercial rotorcraft exams for a U.S. licence and to my surprise there were seven ADF time and distance / bearing change questions on the exam. Can anyone tell me what advantage knowing the answers to such inaccurate methods would give me?
Of course it is nice to be able to chat about the errors that are inherent in the ADF but at the end of the conversation we still are left with the same result...the ADF is an aid for cloud breaking procedures when flying IFR and is without doubt a true non precision aid. Knowing this very simple fact we also must be aware that it will soon go the way of the radio range method of navigation.
As a closing comment I defy anyone that is flying the ADF to convince me that any given swing of an ADF needle is due to any one cause.
............
The hardest thing about flying is knowing when to say no.
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Glad to see we've started a bit of discussion on ADF dip.
With my combination of NDB & aircraft equipment, the ADF is a waste of time, because I'm not prepared to accept an error of +/- 180 deg!
On one hand, we should allow for dip as it would seem to be the only quantifiable (at least in apporximate terms) of all potential ADF errors.
On the other, from reading fireflybob's remarks, whilst we should be aware of it, it has little practical effect in light aircraft.
Does anybody agree/disagree - and is there anubody who flies "grown up" aircraft that can explain how they cope with?
With my combination of NDB & aircraft equipment, the ADF is a waste of time, because I'm not prepared to accept an error of +/- 180 deg!
On one hand, we should allow for dip as it would seem to be the only quantifiable (at least in apporximate terms) of all potential ADF errors.
On the other, from reading fireflybob's remarks, whilst we should be aware of it, it has little practical effect in light aircraft.
Does anybody agree/disagree - and is there anubody who flies "grown up" aircraft that can explain how they cope with?
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Jessfix:
What do you mean by "Grown up airplane"?
No matter what you put an ADF in it will still be an inaccurate navigation aid.
For instance tracking outbound from a radio transmitter using ADF the further you go from the transmitter the less accurate the track information.
Why are we even discussing antiquated nav aids when we should be concentrating on modern aids such as GPS?
Unfortunately for flying training we seem to cling to old tecnology rather than learn new and far better methods. One aid that comes to mind here in Canada is the use of the E6B flight computer in flight schools when we have far better computers to do our calculations with. Never could figure that one out.
.................
The two most dangerous words in aviation.. "WATCH THIS"
What do you mean by "Grown up airplane"?
No matter what you put an ADF in it will still be an inaccurate navigation aid.
For instance tracking outbound from a radio transmitter using ADF the further you go from the transmitter the less accurate the track information.
Why are we even discussing antiquated nav aids when we should be concentrating on modern aids such as GPS?
Unfortunately for flying training we seem to cling to old tecnology rather than learn new and far better methods. One aid that comes to mind here in Canada is the use of the E6B flight computer in flight schools when we have far better computers to do our calculations with. Never could figure that one out.
.................
The two most dangerous words in aviation.. "WATCH THIS"
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Cat, we still have to study the ADF here in pretty little England cos its scattered with them. Unfortunately even in a western country, I still have to do NDB approaches on a regular basis. Always ends up being a lottery as to the actual final approach track.
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Is not the simple fact that NDB's are cheap to install, and in many locations they are already there. Better than nothing, and nobody is about to replace them with something more expensive.
Some people like crosswords, I suppose some people like NDB's.
Some people like crosswords, I suppose some people like NDB's.
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NDB's are featured all over the place. I did an approach into a West African airfield in a 767 a few months back using an NDB procedure. Similar problems encountered to light aircraft, plus the ever present tropical thunderstorms thrown in just to liven it up.
The other problem with the 757 and 767 with regard to NDB's is the design philosophy. At the time the aircraft was designed, it was thought that the ADF was an unnecessary peice of ballast, as a result you can't display the needle on the HSI, and have to use the RMI needle, which is small and usefully placed out of view behind the control yolk (in my seat position anyway!). On the plus side we do have a track line, so once established on the inbound course, you just steer the trackline to match it and (in theory) you should stay well on track!
The other problem with the 757 and 767 with regard to NDB's is the design philosophy. At the time the aircraft was designed, it was thought that the ADF was an unnecessary peice of ballast, as a result you can't display the needle on the HSI, and have to use the RMI needle, which is small and usefully placed out of view behind the control yolk (in my seat position anyway!). On the plus side we do have a track line, so once established on the inbound course, you just steer the trackline to match it and (in theory) you should stay well on track!
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Compared to most other aids its prety poor, but if you have nothing else it can be pretty useful, especially when you are overhead.
MJR
The only phase sensitivity in an ADF is the sense circuit. Otherewise, it just responds to the modulation envelope comprising of the alebraic sum of all the different signals in the pass band.
MJR
The only phase sensitivity in an ADF is the sense circuit. Otherewise, it just responds to the modulation envelope comprising of the alebraic sum of all the different signals in the pass band.
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A lot of very good theory above none of this i doubt but from practical observation of the two most common ADF,s in use in GA the king KR87 has considerably less tendancy to "dip" than the Collins ADF650 this i find surprizing because they both use the same type of system to find the sorce of the radio signal to which the unit is tunned and both units have a good reputation with the avionics guys that i know.
The King KR87 recever has a tighter band width than the ADF650 but in theory this should not effect the dip when near the station ,i am left to speculate that it is something to do with the "damping" of the signal within the unit but this all goes far beond my electrical know how.
Comments from an avionics engineer please.
The King KR87 recever has a tighter band width than the ADF650 but in theory this should not effect the dip when near the station ,i am left to speculate that it is something to do with the "damping" of the signal within the unit but this all goes far beond my electrical know how.
Comments from an avionics engineer please.
A better way of describing dip is to imagine an aircraft with wings level with a beacon on a relative bearing of 045 degrees on an intercept heading, the needle will of course 045 degrees off the nose. Now bank the aircraft, i.e turn onto the inbound track. You now have a horizontal and vertical component of the bearing. The vertical component increases, the horizontal reduces. If you have problems imagining this, consider if the aircraft was to use 90 degrees angle of bank. The horizontal component will be zero as the needle attempts to point directly at the beacon indicating a relative bearing of zero. This is the dip, and it only occurs with bank causing the relative bearing to reduce.
To counter it, anticipate by lagging your roll out a bit . I'm sure someone more clever than I could work out how much lag would be necessary depending on the cosine of the angle of bank, but I was taught to use th TLAR method. (That Looks About Right). It is a function of the angle of bank, regardless of aircraft type.
To counter it, anticipate by lagging your roll out a bit . I'm sure someone more clever than I could work out how much lag would be necessary depending on the cosine of the angle of bank, but I was taught to use th TLAR method. (That Looks About Right). It is a function of the angle of bank, regardless of aircraft type.
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Another way to describe the actions of an ADF needle would be to compare it to the actions of the magnetic compass.
Both are subject to multible errors and in practice you strive to adverage their errors.
From a practical viewpoint I am making these obversations based on starting my airline flying in the DC 3, as I recall we became quite proficient in out thinking the damn thing and actually did some rather impressive approaches.
Also the old Lear ADFs with a bfo select were great in the Arctic for long range relative bearings.
Then again I am half senile now so don't pay to much attention to my ramblings.( I was told when I was young I would go blind doing what I did , but all it did was ruin my memory. )
.......................
The hardest thing about flying is knowing when to say no.
Both are subject to multible errors and in practice you strive to adverage their errors.
From a practical viewpoint I am making these obversations based on starting my airline flying in the DC 3, as I recall we became quite proficient in out thinking the damn thing and actually did some rather impressive approaches.
Also the old Lear ADFs with a bfo select were great in the Arctic for long range relative bearings.
Then again I am half senile now so don't pay to much attention to my ramblings.( I was told when I was young I would go blind doing what I did , but all it did was ruin my memory. )
.......................
The hardest thing about flying is knowing when to say no.
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You must be doing an IR to be interested in this topic.
I've never seen anything in publications about it. All I ever heard was from the instructors.
Unfortunately you need to learn it a lot for the IRT. RANT does a very good job of simulating it.
I've never seen anything in publications about it. All I ever heard was from the instructors.
Unfortunately you need to learn it a lot for the IRT. RANT does a very good job of simulating it.