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SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

You have steadfastly denied ANY role of time in this function.

I will not expect you to sove that mystery for us.

You may note that I have refrained in the past couple of days from commenting on your veritable barrage of insult at me, my profession, people who spoke in support of my position, and finally my abilities, which you are not remotely qualifed to judge.

It is never too late to develop moral courage. You will feel so much better about yourself if you will do so.

All you have to do is admit it.

The basic, underlying science behind the radio altimeter is the TIME it takes the beam to travel to the surface and bounce back. We, by some mechanism (in the broad sense of that word) measure that TIME.

[Edited 2005-01-22 06:35:33]

[Edited 2005-01-22 06:36:51]
Happiness is not seeing another trite Ste. Maarten photo all week long.

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

You have single handedly taken a simple explaination and turned it in to several days of childish debate. As I said before, I very deliberately avoided the use of the word "timed" in my discussion. The choice to do that is valid and correct.

The frequencies are compared to get a time, and from a time (and the velocity of the waves which is known) to get a distance.

How do you propose distance is measured by simply having two frequencies to compare?
Your bone's got a little machine

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

I think both slamclick and airplay have valid points. The principle of the radio altimeter is based on the speed of light (and hence radio waves) being constant, so the time taken for a signal to reflect back is proportional to altitude. I doubt airplay would contest that. If you were to sit down with a pen and paper and work out the distance to the ground you would need to know the time difference between the transmitted and received signal.

However the thread is about radio altimeters, not fundamental physical principles. A radio altimeter does not calculate altitude, so it does not need the time difference to be worked out. The indicated radio altitude is directly proportional to the frequency difference between the two signals. No need for the intermediate step of computing a time difference, although that too is proportional to the frequency difference. Timing may be involved in determining the frequency difference, but the time taken for the signal to return does not have to be computed.

So you're both right
The glass isn't half empty, or half full, it's twice as big as it needs to be.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Thanks for the olive branch, Jetlagged, I think we should take it. As my final input here, I say this: How can we both be right when Airplay had adamantly denied any role of time in the measurement of distance and I have maintained that the only way the difference in frequency can tell us the distance is because the modulation frequency is ramped up and down at a known rate and rate means increment of change over specific unit of time. True for interest rates, rate of climb, any rate you care to name. That's what it means.

So, how does the reader who still gives a damn decide what is true?

My suggestion is don't believe SlamClick because all you know about him is what you can read here, and don't believe Airplay for the same reason. Is that fair? Fine.

ASA Publications of Renton Washington publishes a series of of reference books for aviation professionals. Probably most of the people out there who have a copy of the US Federal Aviation Regulations, for example, probably have ASA's version. They are available in aviation bookstores and at FBOs and places like that. I've recommended them before.

Another thing ASA publishes is the Dictionary of Aeronautical Terms © 1988 and subsequent, ISBN 1-56027-071-3 and I recommend this book be on the shelf of any person who is, or aspires to be a professional in the aviation industry. If you care to resolve the absurd turn this thread has taken, buy or borrow a copy of this book. Among other things it defines "Radio Altimeter" on page 467 in my copy. Because it is copyrighted material we are limited in what we can quote. You may expect to find these words: "The time used in traveling from the aircraft to the ground and back . . ."

This has become the living dead - thread. As far as I am concerned the Dictionary of Aeronautical Terms is the stake I drive into its heart.

To those of you who suffered through this, and to CaptainTim who started it, my apologies for my role in this farce.

- slam
click

[Edited 2005-01-23 16:32:13]
Happiness is not seeing another trite Ste. Maarten photo all week long.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

You may expect to find these words: "The time used in traveling from the aircraft to the ground and back . . ."

Just make sure you are not reading about the type that the military typically uses which DOES measure the time for the round trip of a transmitted pulse. Or at least consider the context. You can correlate almost anything to time. I can say that the distance to my office is a function of time but I don't use my watch to measure the distance. I use the odometer...

My suggestion is don't believe SlamClick because all you know about him is what you can read here, and don't believe Airplay for the same reason. Is that fair? Fine.

Yes. Don't believe me. I've installed radio altimeters, repaired them, designed and maintained radio altimeter systems and provided certification for radio altimeter systems in all sorts of airplanes from Cessnas with the single antenna to dual antennas systems in large transports with dual systems feeding everything from TAWS to TCAS. So I guess I'm not qualifed to discuss how they work. But a pilot apparently is...

Let's continue to feed the myth that the pilot is the ultimate authority in aviation rather than the "end user"....

vzlet
Posts: 820
Joined: Thu Mar 11, 2004 10:34 am

Well, I can say that I now know more about radio altimetry than when this thread began...
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

We all know a lot more about pedantry too.

The glass isn't half empty, or half full, it's twice as big as it needs to be.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Perhaps some of us have just learned to use a thesaurus in an attempt to appear intelligent....

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

Airplay, one simple question you haven't answered. How from a frequency difference can a distance be measured?
Your bone's got a little machine

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

distance = speed * time

In this case the time taken is for a round trip, so we are interested in half that time:

radio altitude = 0.5 * speed * time

Time is proportional to frequency difference. Let the constant of proportionality be k.

time = k * (frequency difference)

Substitute for time in the previous equation gives:

radio altitude = 0.5 * speed * k * (frequency difference)

As the speed in this case is a constant (the speed of light) radio altitude is directly proportional to frequency difference. The constants of proportionality are known, hence radio altitude can be found without intermediately computing time.

Note that time does not appear in the last equation.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

Nor is time mentioned in this sentence:

"During my flight from A to B, the big hand on my watch moved from pointing straight up to pointing straight down. My TAS was 100 knots."

And it would enable you to calculate the distance. The argument is that in this situation, timing is not a factor. Bravo Sierra...

Same thing. "That which we call a rose by any other
name would smell as sweet".
I thought I was doing good trying to avoid those airport hotels... and look at me now.

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

time = k * (frequency difference)

Given time is a base quantity, I don't think that step is suitable for this thread. If you measure frequency, you measure time. Just like if you measure a force you're actually measuring a mass, length and two times.
Your bone's got a little machine

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Airplay, one simple question you haven't answered. How from a frequency difference can a distance be measured?

Visualize a frequency comparator circuit. Basically a circuit that mixes two signals and outputs a resultant beat frequency. Forget about how the 2 signals appeared at the input of the frequency comparator circuit.

Can the resultant beat frequency independantly be used to determine the difference in time between the 2 input signals? No. All it can do is tell you the difference in frequency. It is the designer's knowledge in the rate of frequency change and the speed of radio frequency waves that allow him/her to correlate the output signal to distance. No timing takes place.

In a radio altimer, distance is indirectly measured by correlating it to a frequency.

A similar question could be, how from a voltage can a temperature be measured? A thermocouple indirectly measures temperature by supplying a voltage corresponding to some temperature. Again, like in the case of the radio altimeter, the voltage is meaningless unless someone goes to the trouble of correlating the incompatible terms.

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

It is the designer's knowledge in the rate of frequency change and the speed of radio frequency waves that allow him/her to correlate the output signal to distance. No timing takes place.

Frequency is a function of time, timing does take place.
Your bone's got a little machine

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Frequency (cycles per SECOND) is a function of time.

RATE of change is a function of time. It is this one that defines the principle on which a radio altimeter works. The time the radio signal traveled from airplane to surface to airplane is measured and reads out as height.

Any technical details regarding how that is accomplished do not change the fact that it is from the travel time alone that we determine height.

I'm afraid Airplay has dug a very deep hole for himself here.

I would cheerfully walk away from this sad affair but for one thing. I fear that he is using his credentials, trumpeted in reply #54 to reinforce what amounts to a deliberate deception of those who might not know better.

In effect he is saying that a signal is sent out and for reasons you cannot possibly comprehend, we get an altitude from it.

Here's a hint: They measure (directly or indirectly it doesn't matter) the travel time of the signal from airplane to surface and back. That travel time, compared with the speed of light gives us the travel distance. Half that is the height above the surface.

If Airplay says otherwise it is only because his personal pride will not let him admit it.

Read all his posts with that in mind.
Happiness is not seeing another trite Ste. Maarten photo all week long.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Are you familiar with the term "grasping at straws" Slamclick? The frequency in this case has no direct correlation with the time it takes for the signal to travel from the transmitter antenna to the receiver antenna.

So tell us all Slamclick. How exactly can you relate frequency to time in this case? So based on the 60Hz frequency, how long does it take for electricity to travel from the generating station to your wall socket? Remember, you said frequency is a function of time....

In effect he is saying that a signal is sent out and for reasons you cannot possibly comprehend, we get an altitude from it.

That was not my intent at all but after reading your last post, it is quite obvious that you don't understand.

If Airplay says otherwise it is only because his personal pride will not let him admit it. Read all his posts with that in mind.

That is quite pathetic Slamclick. Right from the start of this little "debate" you have used tactics to try to discredit me rather than discuss the issue. And now you imply that all my posts should be ignored. Well...I won't let you get away with that crap unanswered.

I have stated nothing wrong in this thread. I made one analogy (to the triangle wave shape of the function that modulates the signal) that was quite awkward and I addressed it. But nothing I have said is technically wrong.

Like I said before, feel free to dumb it down for your pilot buddies when you explain it to them...along with the fairy dust that creates lift. I really don't care. As long as they respect the contents of the AFM.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

I just realized something....Slamclick (aka 411A) has argued technical issues to the death before. We have crossed paths on DC generation and on AC generation and 28 volt systems. (To name just a few)

You didn't know what you were talking about then either Slamclick....

goboeing
Posts: 2581
Joined: Mon Jun 05, 2000 5:31 am

Airplay.

Why in the world are you posting a link to a thread in this forum that 411A and skipper and you and others were discussing? It has absolutely to relevance to this thread; skipper is not here anymore and 411A is not Slamclick so stop living in the past.

The correct ones and incorrect ones stand out to me in this radar argument. Somebody should close this thread to new posts at this time.

Nick

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Airplay I mentioned that history way back in reply #23 and you denied it in reply #24. Weak attempt there, and way too late.

"just realized" my ass!

Back to the subject:
The basic, underlying science behind the radio altimeter is the TIME it takes the beam to travel to the surface and bounce back. We, by some mechanism (in the broad sense of that word) measure that TIME.

Happiness is not seeing another trite Ste. Maarten photo all week long.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Moderators, perhaps it is time to lock this thread.

But please, first, allow Airplay the chance to retort.
Would not want to miss something.
Happiness is not seeing another trite Ste. Maarten photo all week long.

SATL382G
Posts: 2679
Joined: Sat Aug 14, 2004 12:02 am

If you guys would exhibit some self control a Mod lock would not be necessary.

Sheesh...
"There’s nothing quite as exhilarating as being shot at and missed" --Winston Churchill

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Why didn't you answer my question about the 60 Hz in your outlet Slamclick? Frequency is a function of time. But purely in a sense of the period of the wave. Any single frequency wave has a fixed period. So how far away is the generating station? Is it a shorter distance away in Europe where they use 50 Hz? Or maybe its farther away because the wavelength of a 50 Hz signal is longer?

Why in the world are you posting a link to a thread in this forum that 411A and skipper and you and others were discussing? It has absolutely to relevance to this thread; skipper is not here anymore and 411A is not Slamclick so stop living in the past.

The relevance here, is that I am being accused of not knowing what I'm talking about, and being argumentative for the sake of argument. Opinions are opinions and everyone has one. Backhanded insults are a different matter and they won't go unanswered.

Those threads illustrate that Slamclick/411A (and I'm pretty sure based on the style of writing etc that they are the same person) has argued to death his point(s) in the past using the same smarmy style and flawed understanding of the subject matter. Even when presented with proof.

We really don't need posts like #21 that started us on this path. A little civility would have helped.

Perhaps this is just a question of what "paraphrasing" is. Slamclick claimed he was "paraphrasing" but I (the writer of the statement he was "paraphrasing") stated that his statement did not retain the fundamental meaning of my statement where I very deliberately avoided using any reference to "timing".

Does this mean that we can all offer a twisted interpretation a each other's statements under the guise of "paraphrasing" and then attack when the writer of the original statement rejects the interpretation?

Go ahead and post your twisted "dumbed down" view of technology Slamclick/411A, just don't tie it to me in any way and we'll get along just fine.

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

Are you familiar with the term "grasping at straws" Slamclick? The frequency in this case has no direct correlation with the time it takes for the signal to travel from the transmitter antenna to the receiver antenna.

Wrong, frequency is ALWAYS the inverse of a time. If you measure a frequency, you have a measure a time - just like if you measure an acceleration, you have to measure a length and two times.

We really don't need posts like #21 that started us on this path. A little civility would have helped.

But then...

Go ahead and post your twisted "dumbed down" view of technology Slamclick/411A, just don't tie it to me in any way and we'll get along just fine.

You have time and again been shown to be wrong. Just cut your losses and leave this thread alone.
Your bone's got a little machine

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Thank you Airplay for locating those two threads. To the two or three people left in the western hemisphere who still care, they shed some light on this absurd debate.

Second thread first: In it you had a debate with 411A and I never even posted. For some reason you decided that he was me and vice-versa. Why? Because there could not be two people in the whole world who disagree with you? Get a clue. There are many, and many more who cannot be bothered to say so.

In the first thread you linked is the apparent origin of this. In reply #4, third paragraph I made a simplistic statement (dumbed down if you insist on being rude) about DC power not transmitting over distance as well as AC. It was a correct statement. Attach a wire from a 24v NiCd battery and it will start a PT-6A just fine. Make the wire ten miles long and I'll bet against it making the start. Your response in reply #6 "huh? that makes no sense?" Well, perhaps it was not very technical but enough of us have had that experience to know that it is true. You then used HIGH VOLTAGE DC as if that was a refutation of the statement. All of this is relevant only in that it marks our first disagreement here.

More pertinent is this: In between those two posts (#5) EMBQA wrote: "The reason airliners use AC power is very simple.........weight. With AC power you can use much smaller wires, and with that you save hundreds of pounds verse the much heavier DC." ( a point I had also raised in #4.

Your response is the oldest example I can find of how utterly dishonest you are. They go on, but here it is, you wrote in #6: "Weight? Kind of ironic isn't it EMBQA? AC does NOT weigh less than DC in itself. Thats like saying running weighs less than jogging..."

That kind of pretense of non-comprehension does not belong here. His statement was true. His statement was easily understood. For personal reasons you chose to try to cloud the issue. Shame on you! If you really think that he was saying that alternating CURRENT weighs less than direct CURRENT you are simply too stupid to matter. If you are intelligent enough to have read his statement as the rest of us did and still write that, you are too dishonest to bother with.

To the one person left in the western hemisphere who still cares at this point I have two parting suggestions.

In reply #53 I made reference to ASA Publicatons Dictionary of Aeronautical Terms and its description of the radio altimeter. Read that. I did not write it. Airplay did not write it.

Thus I end my participation in this farce. I am not proud of the part I have played in keeping it alive. I am not ashamed, however, of my efforts to make Airplay give a straight answer to a simple true/false question and to stop trying to cloud the issues.

Our posts are there to be seen. You can all judge for yourselves.
Happiness is not seeing another trite Ste. Maarten photo all week long.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Wrong, frequency is ALWAYS the inverse of a time. If you measure a frequency, you have a measure a time - just like if you measure an acceleration, you have to measure a length and two times.

So...tell me then. When the radio altimeter "measures" the frequency it gets from the mixer, how does it relate to time and how does that correlate to distance? Can you tell how far a radio station is by the frequency it transmits on? Or (as I know is correct) can you only determine the period and therefore the wavelength of the signal and not the distance it has travelled?

You have time and again been shown to be wrong. Just cut your losses and leave this thread alone.

I disagree. What exactly is the proof that I have "proven to be wrong"?

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

So...tell me then. When the radio altimeter "measures" the frequency it gets from the mixer, how does it relate to time and how does that correlate to distance? Can you tell how far a radio station is by the frequency it transmits on? Or (as I know is correct) can you only determine the period and therefore the wavelength of the signal and not the distance it has travelled?

Are you contradicting yourself here? Are you now saying that from a frequency shift you CAN'T find the distance?

Check out Jetlagged's (!) equations, which are correct. The distance is found using a frequency change, which is a change in a function of TIME.

What exactly is the proof that I have "proven to be wrong"?

There is no "timing" done.

There is timing done - frequency is a function of time.
Your bone's got a little machine

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

All I can do is grin and nod. Like I do whenever I hear a pilot tell me how things work...

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

All I can do is grin and nod. Like I do whenever I hear a pilot tell me how things work...

Is frequency a function of time?
Your bone's got a little machine

Staffan
Posts: 3879
Joined: Tue Mar 18, 2008 9:21 am

Don't you guys understand eachother or don't you want to?

Staffan

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

And engineers, apparently. Must be a joy working with you.
I thought I was doing good trying to avoid those airport hotels... and look at me now.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

I thought of an interesting analogy to this problem. It’s much less clunky than the “measuring tape” analogy I offered earlier.

I'm fairly sure that most of us have operated older analog (tuning knob) ADFs with a BFO selection. BFO generates an audio tone to allow you to identify NDBs that use interrupted-carrier keying. You’ll have a hard time finding such a beacon these days…but that’s another story.

Before digitally tuned ADFs were the norm, many pilots used BFO to help tune in weak stations. With BFO on, a tone is generated that is relevant to the tuning error. A sample can be heard here:

http://members.shaw.ca/airplay1/electronics018.wav

In other words, if you are trying to tune in 1 Mhz with the BFO on, you will hear a relatively high pitched tone that reduces in frequency as your dial approaches 1MHz. When the dial is exactly on 1MHz, the tone will cease. (or for lack of a better term, reach 0 Hz) As you pass 1Mhz, the tone will reappear and again increase in frequency as you tune away.

Strangely enough, this is quite close to what a radio altimeter does. But what distance is being measured? The distance to the radio station? No. The distance the dial has moved. If I take the time to measure (or calculate like the design engineer does) I could correlate the resultant beat frequency to the distance the dial travels.

Notice that no timing is done in this process. Yes, frequencies are compared but these frequencies are not directly associated with the measurements being taken. As a matter of fact, if BFO was actually used to measure the distance the dial moves, higher resolution can be achieved simply by changing the mechanism that moves the dial.

The dictionary tells us that “frequency” is:

fre·quen·cy (frç'kwən-sç)
n., pl. -cies.
1. The property or condition of occurring at frequent intervals.
2. Mathematics & Physics. The number of times a specified periodic phenomenon occurs within a specified interval, as:
a. The number of repetitions of a complete sequence of values of a periodic function per unit variation of an independent variable.
b. The number of complete cycles of a periodic process occurring per unit time.
c. The number of repetitions per unit time of a complete waveform, as of an electric current.

Radio altimeters operate at around 4Ghz. The beat frequency they detect ranges from a few Hz to a few hundred kHz. Can anyone here take 1Hz to 300kHz and directly relate it to a distance? Remember, the speed of light is constant no matter what the frequency. (although the transmit medium is a factor…but again that’s a different subject) We can calculate the period and wavelength rather easily. But how do we relate it to distance?

In this case, saying frequency is a function of time, is like saying voltage is function of time. The actual voltage/frequency is irrelevant. The process can happen with various frequencies as long as the design engineer knows about it.

The answer is that the design engineer already did the math for the radio altimeter and all that is left to do is translate the beat frequency to a voltage that drives the indicator. A very simple process, and an ingenious invention that, in my opinion, loses the fundamental theory and elegance when “timing” is discussed. The timing was resolved on the drawing board, not in the radio altimeter. But are we discussing what the engineer does or what the radio altimeter does?

So…again. Don’t agree with me. Tell pilots whatever you want. But remember, you are in the Tech/Ops forum that is defined by A.Net as:

The more advanced forum for those of you who want to dig deeper into technical issues as well as airline management and operations. A forum for both professionals and knowledgeable amateurs.

When you “dig deeper into technical issues” you typically don’t “dumb things down”.

And engineers, apparently. Must be a joy working with you.

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

OK, as this has gone beyond the ridiculous, let’s make entertainment out of it! Now, let us all sing together!

Melody: Europe – Superstitious

Keep on digging that hole
I won’t follow
Keep going the wrong way
I’ll stay here

But that’s easy to take
Honestly, I don’t really care

I’m not superstitious
I have no doubt
That there’s timing
While heights are changing
From ground to plane
There’s still timing
To keep me safe

[...]

Let me guess? People grin and nod a lot? No wait, wouldn’t be while you’re looking, would it?

fre·quen·cy (frç'kwən-sç)
n., pl. -cies.
1. The property or condition of occurring at frequent intervals.
2. Mathematics & Physics. The number of times a specified periodic phenomenon occurs within a specified interval, as:
a. The number of repetitions of a complete sequence of values of a periodic function per unit variation of an independent variable.
b. The number of complete cycles of a periodic process occurring per unit time.
c. The number of repetitions per unit time of a complete waveform, as of an electric current.

And yes, a voltage proportional to a beat frequency proportional to a frequency difference proportional to a time delay will be proportional to the time delay. Any engineer worth his salt will see this and one of us, as you said (paraphrasing?) “did the math for the radio altimeter” to get this proportionality represented up in the pointy end.

As Airplay here seems to think there’s a contest between the different professions within aviation on this matter, you can take my posts as representative of engineering. SlamClick, you volunteer to speak for the pilots I take it? Any other professions who care to be represented?

Pilots may be a bunch of overpaid gits and glorified bus drivers, but I love’em anyway... well, except when they break my aircraft. Group hug, everyone?

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

FredT,

There are plenty of examples of equipment that use a timer to resolve distance. A DME is a great example.

So what is the difference between a DME and a radio altimeter?

A DME must make use of a plethora of circuits that the radio altimeter doesn't. Not only does the DME need to tansmit interogation pulses to the ground station, but it must randomize the timing with a squittter circuit.

Then timing is done as it measures the time difference between the transmitted pulses and the return pulses from the ground station. BUT...the ground station is not only replying to your DME, its replying to others. And its spraying the airways with even more pulses. So the DME must figure out which pulses are are "its own". It uses a "hit and miss" ciruit that tallies the probability of having the right pulses. Once a good "lock" is made on it's data stream it validates the timing and processes the information into a useable format...namely a signal capable of driving the distance indicator.

Radio Altimeters by-pass all this complexity with a simple and elegant solution using a mixer. A circuit found in pretty every radio receiver.

Isn't science marvelous?

And FredT, after reading your little tirade where you bold words associated with "timing" I am quite suspicious of your understanding of the process....

Frequency is a measure of repitition. But unless you are discussing wavelength of a fixed velocity wave, there is really nothing to derive any sort of "distance" from. So maybe you can tell me how far away the generating station is if the frequency of the voltage coming out of your plug is 60Hz....

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

Wow, I didn't think this thread would still be generating such heat since I lasted looked.

Firstly to defend myself:

777236ER, there's nothing wrong with the equations I wrote above in reply 59. I didn't say the constant k was dimensionless (in fact it has units of seconds squared). It's a mathematical way of expressing the statement "time (taken) is proportional to the frequency difference". Are you going to argue with that, without taking it out of context?

If you measure force with a spring balance you are actually measuring a distance (which happens to be proportional to force). Same problem with units for you? There's no rule that says you have to measure everything in terms of fundamental units.

I stand by that post. It clearly shows how you can measure radio altitude without timing the radio signal as long as certain things are known and constant.

Finally, to the point:

The timing enthusiasts (who "know" time must be measured) have now resorted to the assertion that you need to measure time to measure a frequency, so timing is involved. QED. Even if true, this is not the time taken for the radio signal to be reflected, which was the time period the argument began over.

All this stems from a simplistic understanding of how radio altimeters work. That the time taken for the radio wave to return is proportional to the altitude. Armed with this knowledge it has been asserted that obviously time must be measured.

Even though it has been pointed out that radio altimeters are continuous wave devices, so do not directly measure the time taken for radio waves to travel a distance a few die-hards remain unconvinced.

For the last time: the time taken for the radio wave to be reflected is important, in theory. But not to a radio altimeter, which concerns itself with comparing two frequencies and outputting a signal proportional to the difference between them.

Look, this isn't Airplay's pet theory on how these things work. It is actually how these things work. And it's such an elegant device it's worth defending the argument even to this degree.

I notice Slamclick has repeatedly called for this thread to be closed. He obviously knows the game is up.

The glass isn't half empty, or half full, it's twice as big as it needs to be.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Oh yeah Jetlagged you really got me there!
I retreated with my tail between my legs.

P.S. "k" in your little formula (reply #59) is the rate of change of the frequency in Hz per microsecond (or some such value the numbers matter not so long as they are indexed in some way to the speed of light)

That known rate is used to

time

. . . how long the radio signal has been traveling. It does that because that is the one and only way you can get the distance.

Sticking time in earlier in your equation and then pointedly not using the word "time" in the latter part is deceptive. It is dishonest.

Airplay has refused to admit that, much less to comment on it throughout this thread. He has used every cheap trick in the book from insulting whole groups of people to grabbing one non-essential word in the post and running off on some silly-ass tangent with it, to pretending he did not understand what was being said. He does that in other forums as well. It is almost the entirety of what he does here.

Airplay is a very dishonest person. I will no longer debate this with him because of the points in the paragraph above. He has a long history of deflecting the very straightforward reasoning of others.

So lest you think I am retreating from this argument out of some uncertainty on the subject at hand:

A radio altimeter gets the height above the surface directly from knowing how much time a radio beam has taken to travel from the airplane to the surface and back.

No one ever said there was a dedicated time-only measuring device or circuit. That has been reiterated many times. That does not change the fact that the time is being measured by the frequency shift which occurs at a known rate.

Finally, the reason I called for the thread to be closed is because it has been hijacked. The original question was whether or not there were other radars in the cockpit. No one here, to my knowledge has ever argued that a "radio altimeter" was a radar. Incredibly, about half the posts on here are because Airplay refuses to acknowledge the role of time in measuring distance by radio wave, and several of us have not wanted to let that piece of deception pass for truth. We have readers who come here for truth.

Happiness is not seeing another trite Ste. Maarten photo all week long.

777236ER
Posts: 12213
Joined: Sat Aug 18, 2001 7:10 am

I stand by that post. It clearly shows how you can measure radio altitude without timing the radio signal as long as certain things are known and constant.

That's a bit like saying you can measure time by measuring angles. If you measure frequency, you measure time. End of.
Your bone's got a little machine

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Check your pesonal messages Slamclick. This mudslinging and flamebaiting doesn't belong here.

[Edited 2005-02-06 20:02:31]

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

To reiterate* the view of the no timing camp:

“I do not measure the time between event A and event B. I only compare the position of the big hand on the clock when event B occurs with the position of the big hand when event A occurred.”

That’s comedy at a high level!

Cheers,
Fred

*) Instead of paraphrasing, although the latter would have been more fun.
I thought I was doing good trying to avoid those airport hotels... and look at me now.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Airplay no one likes being called dishonest. Most people avoid it by being honest.

Now I have already copied into Notepad half a dozen examples, (complete with links and reply# references) of the things I accused you of. If you'd like I can paste them here for all to see.

I would not expect you to acknowledge that you had been dishonest. I would expect you to do what you always do - twist and turn and dodge and sling back a veritable barrage of red herrings.

Want to continue that? Say the word and I'll paste them here.

Now I've posed this simple, direct question to you many times. One more time:

A radio altimeter gets the height above the surface directly from measuring how much time a radio beam has taken to travel from the airplane to the surface and back.

Airplay your reputation is on the line with this. Will you answer?

True?
False?

P.S. Please cease and desist sending me personal messages. You and I have nothing to say to one another that cannot be said in a public forum.
Happiness is not seeing another trite Ste. Maarten photo all week long.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Slamclick...I have decided to maintain the high road. Do whatever you want. I don't think petty insults belong here and I won't participate in your game.

Cheers.

[Edited 2005-02-08 05:45:36]

bcbhokie
Posts: 196
Joined: Fri Sep 24, 2004 7:30 am

I'm coming into this thread as an indepenent electrical engineer to contribute my two cents to the "timing" debate without addressing a lot of the pedantic stuff...

Since a frequency is just the inverse of time, isn't a frequency measurement a time measurement? I mean, as an electrical engineer, when I say I'm putting out a 1000Hz square wave, what I am saying is that the wave repeats every 1/1000 of a second. Frequency and a time period are the same thing; the only real difference is that frequency implies repetition. If you get up every day at 8am and go to bed every night at 8pm, that's a 43200 second period, or a .0000231 Hz frequency

Frequency = 1 / Time (general relationship)
Hertz = 1 / Seconds (most commonly used units)

I would define the act of timing as the act of comparing two time intervals. If a frequency comparison is involved, that would also seem to be timing to me. (Maybe we could coin a new term, "one over timing", to be more specific...?   ) So my view here is that it was legitimate for SlamClick to use the term timing initially and it is also legitimate to clarify that the measurement being discussed is based on comparing two repeated signals, not measuring delay on a single pulse like some distance measurement schemes.

Not trying to stoke the flames here - mostly posting to clarify my own thoughts since I managed to temporarily confuse myself reading all the back and forth replies

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

Yes we are measuring a frequency. Frequency is a function of time. We can relate frequency and just about everything in the universe to time at some level. We can relate time to a brick sitting on a desk because time is a tangible dimension. But with respect to the operation of a radio altimeter, nothing is being “timed”.

The frequency we are detecting is not “directly” related to the distance from the airplane to the ground. Radio altimeters can arguably be designed to operate at various carrier frequencies and modulation rates while still indicating the same distance. So, as I’ve mentioned before, BCBHokie, there is no direct correlation between a frequency and a distance unless you speak of wavelength, and that is not what is being measured here.

Just like my BFO analogy described, the sound that represents the distance the dial travels works for any frequency in the band. The scale of an AM radio of course is logarithmic so don’t expect a linear relationship along the entire band, but as long as you consider that, the distance the dial travels can be computed based on the beat frequency. No “timing” is being done when you tune a radio. There is no “timer” circuit in a tuner.

Now…don’t try to tell me that a tuner measures the period (time) of the received signal therefore a time measurement is made. Because again…even if this were true (which it is not) the “time” measured is not related to the distance the wave has travelled.

Cheers

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

Yes we are measuring a frequency.

No, we are measuring how much a frequency has changed between the time when we transmitted the beam and the time at which we received the reflected beam.

But with respect to the operation of a radio altimeter, nothing is being “timed”.

Stay tuned, kids!

The frequency we are detecting is not “directly” related to the distance from the airplane to the ground.

The received frequency isn’t, the beat frequency is. You have to be more specific! As any engineer knows, the devil is in the details.

Radio altimeters can arguably be designed to operate at various carrier frequencies and modulation rates while still indicating the same distance.

This is true, but irrelevant. Muddying the waters? Wrap the pill in ham and the cat will swallow it?

So, as I’ve mentioned before, BCBHokie, there is no direct correlation between a frequency and a distance unless you speak of wavelength, and that is not what is being measured here.

Absolutely correct. We’re not measuring a frequency, we’re measuring the time it took the beam to get from here to there and back again.

Now, here’s yet another analogy:

Take a circular plate. Add a moving hand, pivoted in the middle of the plate. Let the hand move around the pivot at a fixed rate. We’ve created a crude clock.

The position of the hand in this analogy corresponds to the emitted frequency.

Now add a conveyor belt. Let this conveyor belt go past the circular plate, out to a point an unknown distance away and back at a known speed. Put pieces of paper on the conveyor belt, a foot or so apart.

Have a small imp sit by the clock and conveyor belt. Have the imp draw the current position of the hand on the clock on each piece of paper as it goes by him.

The conveyor belt is the radio altimeter beam, the imp is the transmitter and the distance to the point where the conveyor belt turns back is the distance to the ground.

Put another, slightly brighter imp, by the returning side of the conveyor belt. Tell this imp to compare the position of the hand on the drawn pictures on the pieces of paper with the current position of the hand on the real clock. This is your receiver, mixer and beat frequency converter.

Then have the receiver imp yell the current difference between the angle of the drawn hand on the pieces of paper and the current angle of the hand on the clock to the nearby pilot. This is your radio altimeter display. The angle will be directly proportional to the time the note spent going out and back on the conveyor belt. As the belt moves at a fixed speed, it is also directly proportional to the distance to the turning point.

If the difference between the position of the hands is small and decreasing, have the imp hit the pilot on the toes with a peen hammer and yell “pull up, you daft git!” Attach the hammer to the landing gear, so the imp won’t be able to use it when the landing gear is extended. This is your GPWS.

In a real radio altimeter, the hand goes from twelve o’clock to twelve o’clock in one direction, then turns around and goes back in the other direction. This gives a small area of uncertainty around the turning point where you will not be able to easily tell the distance by the difference between the drawn and actual angle.

If the angles grow large or the notes return all aged and yellow (weak return signal), we will not know if the hand has moved a full turn. When this happens, we train the imp to shut up – the radio altimeter is not reliable above a given altitude.

This is exactly the way it works. A transmitter and a receiver, both hooked up to the same triangle wave generator (clock with only one hand). By comparing how much the hand moved (how much the triangle wave changed) between sending the note off (transmitting the beam) and receiving the note (receiving the reflected beam) we know the distance to the turning point of the conveyor belt (to the point where the beam was reflected) as we know the speed of the conveyor belt (the speed of the radio wave).

You have a clock (triangle wave frequency modulated signal generator). You compare the position of the hand (the frequency generated) when transmitting with the position of the hand (the frequency generated) when receiving what you transmitted a while ago. You then know the time it took for the signal to get there and back. That is timing.

If it is to be described any simpler, it will take colourful pictures of cute kids. “John has a signal generator”. “See John’s signal generator”. “See the signal generated”. “See the signal bounce”.

Frankly, I think we are better off sticking with Helfrick as is.
I thought I was doing good trying to avoid those airport hotels... and look at me now.

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

I really think I am having more fun with this than I should have. Oh well, I've always liked the Monthy Python kind of humour where the fun is in the absurdity of the statements made.
I thought I was doing good trying to avoid those airport hotels... and look at me now.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Damn, FredT that was eloquent!

If I ever go back to teaching ground school that is going to be my presentation on the radio altimeter.
I'll credit you for the first three classes, after that it is mine.

For whatever it is worth, you now represent all of Scandinavia on my RR list.

Slam
is this a ceasefire?
Click
Happiness is not seeing another trite Ste. Maarten photo all week long.

airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

I think I've said as much as I need to say without bringing "imps" into it.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

Yeah, let's go back to the tape measure and skyhook in reply #24

Happiness is not seeing another trite Ste. Maarten photo all week long.

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

Thanks, SlamClick. That's an RR entry I appreciate.

I think it has all been said now, most of it about a dozen times. At this point, I'm prepared to make a gentlemans agreement to disagree and let readers of this thread decide who to believe.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

I'm onboard for that.

All we need is one more gentleman.
Happiness is not seeing another trite Ste. Maarten photo all week long.
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