FLY2HMO From , joined Dec 1969, posts, RR: Posted (7 years 8 months 3 weeks 3 days 22 hours ago) and read 4777 times:
So I got my IFR stage check tomorrow, and I get to explain how the ADF and VOR works.
I know how the old school ADF's worked, you got the loop antenna and the sense antenna, and you turn the loop antenna to where it receives the signal the strongest.
As far as modern day ADFs are concerned, from what I remember it's all solid state, and pretty much what happens is that you have the equivalent of two loop antennas perpendicular to each other, which eliminates the need for the sense antenna. Is this correct?
Also, I've heard a zillion different ways to explain how a VOR generates radials. I know the usual light house analogy. But my instructor introduced me to a new explanation which I hadn't of heard before.
The way he put it was that every radial has its own unique phase difference. So what the onboard receiver does is compare the phase difference selected on the OBS for a given radial, and compares it to the phase difference of the signal the receiver "feels" from the VOR. Which technically means there's only one signal coming out of the VOR. It sounded simple enough to me.
KELPkid From United States of America, joined Nov 2005, 6232 posts, RR: 4
Reply 1, posted (7 years 8 months 3 weeks 3 days 22 hours ago) and read 4769 times:
IIRC, a VOR station sends a timing pulse over an omni-directional antenna, followed by a "sweep" (which is a signal sent to a series of highly directional antennas). The receiver in the aircraft times the difference, and that is how it knows which radial it's on...a good, old-fashioned OBS head is a three-phased motor driving the needle, and the receiver translates the radial data into a phase difference which biases the needle towards the proper direction, depending on the OBS setting...it's been almost two years since my IFR checkride, so the grey matter is getting a little fluffy in this area. I'm expecting prompt correction from the experts here...
ADF: when it works, it points to the station, be it a LOM, genuine still-commissioned NDB site, or your favorite AM broadcast station (probably not real helpful here!) It also passes the pre-flight test (dependent on the specific make and model, some use a small transmitter in the aircraft's wing that it points to, some require that you tune to a known good station, and while in test mode, the needle does a slow 360, when you take it out of test mode it should point back towards the station immediately).
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
FLY2HMO From , joined Dec 1969, posts, RR:
Reply 2, posted (7 years 8 months 3 weeks 3 days 17 hours ago) and read 4730 times:
Quoting KELPkid (Reply 1): IIRC, a VOR station sends a timing pulse over an omni-directional antenna, followed by a "sweep" (which is a signal sent to a series of highly directional antennas).
Ah yes, the lighthouse theory
I dunno though. My instructor said that there really only is one signal coming out of the VOR, and each degree has its own phase difference. Apparently our flight department approves that explanation. So I guess their grey matter is trustworthy (cough cough). Although, when I tune in my scanner to a VOR, I'm sure I hear two tones. One constant tone in the background (sweep signal perhaps?) and a slightly different pitched highly-intermittent tone (reference signal?).
I think the only people that really know the answer are those trolls living inside the antenna shack
BTW, my checkride got cancelled for some mysterious reason. So you guys have more time to answer lol
KELPkid From United States of America, joined Nov 2005, 6232 posts, RR: 4
Reply 3, posted (7 years 8 months 3 weeks 3 days 4 hours ago) and read 4704 times:
Quoting FLY2HMO (Reply 2): Although, when I tune in my scanner to a VOR, I'm sure I hear two tones. One constant tone in the background (sweep signal perhaps?) and a slightly different pitched highly-intermittent tone (reference signal?).
What modulation is your scanner using? If it's using AM, I thought all you were supposed to hear is the morse code ID, HIWAS broadcast, and/or the flight service station if it's in a pretty remote place...(ever talked to flight service over a VOR? That's a real trip!).
Its FM, I can pick up HIWAS, the morse ID and everything else, just like in a plane. But to really be able to distinguish between the two tones, I have to be at least within a mile of a VOR and in unobstrocted line of sight, otherwise it's too faint.
As a random fact, I've been within 200 ft of a localizer antenna (by the fence by 3R in PRC) and it seems to be silent. All you hear is the morse code.
Quoting Mir (Reply 4): I think you've got it reversed.
Well my reasoning behind my explanation (and its only my theory) is that the reference signal only transmits when the sweep signal goes north, and the sweep signal is constantly on, therefore, the sweep signal is the constant tone, the reference signal is the intermittent one. I doubt my scanner is good enough to "feel" when the sweep tone is going through it and just "feels" it like if its constantly on.
If only the FAA would de-classify the true VOR operating fundamentals
Tg 747-300 From Norway, joined Nov 1999, 1318 posts, RR: 0
Reply 6, posted (7 years 8 months 3 weeks 2 days 19 hours ago) and read 4657 times:
According to my JAA Radio Nav book, a VOR sends out two signals. A reference signal and a variable signal. In the VOR receiver the phase of the REF and VAR signal is compared. The radial (angular position with reference to the VOR station) is proportional to the phase difference.
BWilliams From United States of America, joined Sep 2007, 212 posts, RR: 0
Reply 7, posted (5 years 10 months 1 week 5 hours ago) and read 4317 times:
Quoting FLY2HMO (Reply 5): As a random fact, I've been within 200 ft of a localizer antenna (by the fence by 3R in PRC) and it seems to be silent. All you hear is the morse code.
I believe that this would occur if you were "behind" the localizer (for lack of a better word), which you would be if I'm picturing this right, so you aren't getting a signal from the directional/glideslope beams like you would from a VOR that transmits along 360 degrees of arc.
FredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (5 years 10 months 6 days 21 hours ago) and read 4288 times:
Quoting FLY2HMO (Thread starter): I know how the old school ADF's worked, you got the loop antenna and the sense antenna, and you turn the loop antenna to where it receives the signal the strongest.
The common way of doing it is actually by determining where you find the weakest signal. It is easier to distinguish due to the directional properties of the antenna.
As for the "just one signal theory of VOR operation", that'd be a good example of dumbing something down to the degree that it can be spoon-fed to the daftest of pilots without frying their brains and giving them whole new ways of cocking it up through advanced misinterpretation. All you need to know to push the right button at the right time, but nothing more. In my opinion, you need to understand more and do better than to simply push the right buttons to be a pilot. Not all airline training departments/certificating authorities agree.
On the carrier of a VOR you have an AM ident/voice, a 30Hz AM signal and an AM subcarrier. The subcarrier is a 9960 Hz AM modulation, FM modulated with a 30Hz signal at a modulation depth of 480Hz.
At a given location, you'll see both 30Hz modulations with a given phase relationship giving you your radial. If it is the AM or FM 30Hz modulation which will change phase as you move around depends on if it is a conventional VOR or a doppler VOR, but is by and large insignificant, as the phase difference is what matters for finding a radial.
Counting the signals, I get more than one.
[Edited 2008-05-11 01:30:06]
[Edited 2008-05-11 01:31:12]
I thought I was doing good trying to avoid those airport hotels... and look at me now.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 69
Reply 10, posted (5 years 10 months 6 days 14 hours ago) and read 4247 times:
So here is the old retired aviator sitting at his computer and smiling his satisfaction at what he is reading. Thank God the FAA is still requiring the teaching of things that:
1. No one can prove.
2. Cannot be changed by the pilot.
3. Make not a particle of difference in how the pilot operates his aircraft in the regulated airspace over our heads.
4. Might very likely be out of date before the ink is dry.
The lighthouse model is workable, requires no further modification unless the real explanation is simpler.
...may very well be true. In fact I'd bet a very small amount of money that it is. HOWEVER we cannot compare a thing with itself and learn anything. So if there IS a phase difference (whatever that means) by radial, well then there must be at least two components to that one signal - and it follows that one would be, in effect, omnidirectional and the other unique to the radial. Hey! That sounds just like the lighthouse model!
As for ADF, well, I used to fly the old stuff: Sense antenna from tail to top of canopy and loop antenna somewhere out in the slipstream. The most primitive I flew had a hand-cranked loop with a small, crude compass rose on the control head to show you loop orientation reference the aircraft nose. It was easier to turn the airplane than to crank the loop. It was very easy to head the wrong direction down the line bearing you got. Better to turn 90° to it, fly a while then take another cut. They were bound to come together somewhere.
Last I heard, some years back was that a solid-state directional antenna (of the day) had a ring of paired dipole antennae and it compared signal strength between the sets of pairs to find the line bearing (TO or FROM) then used some feature in these dipoles that made them receive better in one direction than the other to resolve which was TO the station.
At that point the needle pointed to the station and you either fly the head or the tail of the needle depending on which way you were going. It was important only to remember that you cannot move the head of the needle, you can only use aircraft heading to drag the needle tail left or right through that little fulcrum in the middle of the needle. Easier to demonstrate in the airplane than to explain in text.
Some day they are going to use accelerometers and ring-laser gyros and small computers that can sense heading and speed and track over the ground to find winds aloft and they will use air data computers to correct airspeed and altitude and... Wait a minute. That happened thirty years ago!
Well, some day they are going to launch a bunch of satellites that will tell ground stations where they are and these are going to be small enough to put in airplanes and ... Wait a minute! That happened twenty years ago.
Some day they are going to have VLF and Microwave nav systems and... No. Really, they're not going to. Those were just empire building programs by the same folks who require that a pilot who is not going to be allowed to visit a VOR understand how it works - as opposed to understanding how to work it.
Well, don't feel bad. Last I heard it is still necessary to teach aircraft dispatchers Adcock Range (Morse "A" and "N") but not necessary to teach them anything about computer-based flight planning.
Happiness is not seeing another trite Ste. Maarten photo all week long.