This may be a topic that has already been covered or is very ignorant...

"Are there any radar onboard in the cockpit of the heavys (747, A340 etc.) that shows details about other planes in the surrounding area providing (call sign, altitude, distnace and maybe even speed) instead of weather?"

Like some captions on the photos says like "there is a A340 3000ft below us heading to ORD" etc. like how do the pilots know this information? is this relayed to them on ACARS or via ATC or via a radar?

Becuase all i've seen soo far on airliners are Flight Deck Weather Radars:


View Large View Medium

Photo © Shawn Damphousse


thanks
tim

[Edited 2005-01-15 04:08:32]

That's part of the TCAS system, I believe. Has to do with collision avoidance.. While you can see the weather, that is only one setting on that particular panel.

Chris

then will other settings on the particular panel (weather panel as shwon on the picture) show a radar with information about flights around this aircraft?

i'm not sure
thanks

Its called TCAS, traffic collison avoidance system. The transponders in the planes talk to each other to let them know where the other one is, and if necessary how to avoid each other. As for other radars, there is the radar alitmeter.

Be sure to know that the TCAS system and the weather radar are two seperate systems and operate independently. It just so happens that they can both be shown on the same screen at the same time. It's nice how technology has progressed over the years.  

You arent kidding LFT 10 years ago who would have thought that a similar system (tis and nexrad) would be on 172's!!!

APFPilot1985,

It is not a RADAR Altimeter. It is a Radio Altimeter system. Though they operate on the same concept (transmit, return, measure), the radio alitmeter operates on a different frequency level than the weather RADAR system.

there is the radar alitmeter.
You mean Radio Altimeter.
A beam is send out below the Aircraft,strikes the surface & returns,The time spent is calculated to detrmine the Height.
regds
MEL

Well, it is called Radio Alitimeter. But the acronym for RADAR is radio direction and ranging. So by using radio waves for ranging, you ultimately do have primitive radar providing range on a specified azimuth... hehe

A beam is send out below the Aircraft,strikes the surface & returns,The time spent is calculated to detrmine the

in that case, where the hell does the beam returns? the plane is movieing at 500MPH?

And the beam is moving at the speed of light, approx 186,282 miles per SECOND... the beam is down & back before the plane has gone very far  

the Radio altimeter is set facing down below the a/c, the frequency of operation is around 4200-4400 MHz which makes it SHF ( super high frequency) it detecs the difference between the lowest part of the a/c and the terrain below, and i think the lag is something within micro seconds....

it is an extremely important radar system onboard, as the CAT 2 and CAT 3abc all use the Radio Altimeter for position information after descending below 200ft, possibly more important than AWR..

hope this helps


cheers

does anybody have pics of the TCAS??

tim

Hi,

This picture was taken by a friend on mine that used to fly the 744.


View Large View Medium

Photo © Julian Whitelaw


You can see on the PFD little diamonds. Each little diamond represents an aircraft picked up by TCAS (only squawking aircraft will show up). The caption details the system a little, but to add, I believe the closer they are to a potential conflict, they change colour to the corresponding risk level, and if necessary, a Traffic Advisory (TA) or Resolution Advisory (RA) is issued. At this stage, TCAS RA's are only issued in the vertical plane (ie, begin or change rate of ascent/descent). Further developments may see RA's involving the horizontal plane.

Also, TCAS can only see something like 4500' above and below the aircraft, and a range of 40nm. 40nm at cruise speed of 450kt (closure speed of 900nm/hour) equates to 2min 40 seconds till meeting (assuming head-on of course)...

Also, for the radio altimiter, it only reads when below 2500' AGL.

Speedbird128 thanks a lot.. never found that photo cuz maybe its descritpion didn't include 'TCAS' which i searched for on TCAS. thanks a lot

but if anyone can asnwer one final question before this forum dies, what happens when a plane Squawks? i read about it that ti activates some type of transponder telling other planes of their location and other navigational infomrmation?

thanks
tim

A beam is send out below the Aircraft,strikes the surface & returns,The time spent is calculated to detrmine the Height.

Wrong. It's called a "radio" altimeter and not a "radar" altimeter for a reason. It doesn't use pulse signal return timing like a radar does.

An "ramped" FM modulated continuous signal is transmitted towards the ground. The signal bounces off the ground and is received by a receiver antenna that detects the signal and determines the difference between the current transmitter modulation frequency and the frequency of the received signal modulation. That coincides wthh the distance to the ground.

There are some lower cost single antenna systems that work on the same basic principles, except the antenna must switch between transmit and receive. Because of the delay induced by the switching these single antenna radio altimeters can only read down to about 40 feet. Dual antenna systems can read down to the ground.

Captain Tim,

The term "squawk" relates the the transponder replay. Mode A/C transponders that have been in use for decades can "squawk" the selected code, the aircraft's altitude and "ident" which is just a mode that allows positive identification of a radar target when the pilot presses the transponder "ident" button.

The new Mode S transponders are capable of transmitting much more and as ATC evolves more is being added to the squawk stream from the transponder.

One last thing...

NEXRAD is a service available for aircraft with compatible receivers and displays. It allows for the display of weather and other information uploaded from ground stations. It is not a form of on-board radar but is capable of displaying "radar" information uploaded from other sources.

[Edited 2005-01-17 04:17:44]

Here's a pic from my profile:



The one that I am trained on has lightening detector, weather(NexRad), terrain advoidance system, and traffic advoidance.

XJR

in that case, where the hell does the beam returns? the plane is movieing at 500MPH?

Quick calculation by an English major (read: probably incorrect) says that a plane flying at 2,500ft AGL at 140kts will have moved .014 inch by the time a radio beam makes the round-trip.

But according to Airplay, this really doesn't matter since a radio altimiter uses a continuous beam.

But it does matter since (paraphrasing Airplay) the signal is timed by the change in frequency modulation. It still requires a beam to go to the ground, return, and the time that took to be quantified in some manner.

As Lemmy says, the distance covered waiting for the signal to come back is inconsequential. Like, you don't need to adjust a logo light so that it will still point at the tail when you are doing M.82 Aiming it while standing still shall be adequate.

But it does matter since (paraphrasing Airplay) the signal is timed by the change in frequency modulation

I never used the term "timed". The frequency of the demodulated received signal is compared to the reference frequency. There is no "timing" done.

Airplay I have long felt that the engineering curricula in colleges was a little too focused. Once again you have done nothing to dispel that idea.

Some high school level English:

Par"a*phrase, n. [L. paraphrasis, Gr. ?, from ? to
say the same thing in other words; ? beside + ? to speak: cf.
F. paraphrase. See {Para-}, and {Phrase}.]
A restatement of a text, passage, or work, expressing the
meaning of the original in another form, generally for the
sake of its clearer and fuller exposition; a setting forth
the signification of a text in other and ampler terms; a free
translation or rendering; -- opposed to metaphrase.

In putting the word "timed" in italics I was setting it apart for an example. You are correct; you did not use the word "timed." If your English skills were up to your educational level you would have understood that the your description of how a Radio Altimeter works was grammatically, and factually flawed. You said:

An "ramped" FM modulated continuous signal is transmitted towards the ground. The signal bounces off the ground and is received by a receiver antenna that detects the signal and determines the difference between the current transmitter modulation frequency and the frequency of the received signal modulation. That coincides wthh the distance to the ground.

Now a moment's thought about this would tell you that the difference between the "current transmitter modulation frequency" and the "frequency of the received signal modulation" is going to be in Hertz or cycles per second or something like that. The distance to the ground is going to be in feet.

So how do Hertz and feet "coincide.?" By knowing the rate of change over TIME of the "current transmitter modulation frequency."

In other words this will not work without some reference to measured time. In other words it is timed. Just because it uses a time-indexed FM change instead of a simple timer does not mean it is not being timed.

I would never presume to quarrel with your engineering knowledge. If you are going to debate what is essentially a point of language you might recognize the limitations of your education.

The signal is timed.
It is "timed" by the rate of change of the frequency.
If the machine did not know what the rate of change over time was it could not work as it does.
However it works, the yardstick by which it is made to work is the knowledge of how long it will take a radio beam to travel to the surface and reflect back. The rest is just mechanics, detail.

edit: spelling


[Edited 2005-01-18 20:26:50]

[Edited 2005-01-18 20:28:54]

Slamclick,

In your attempt to make me look like an idiot, you just expose your own ignorance.

When one "paraphrases" one shouldn't lose the fundamental meaning of the original statement. That is exactly what you do when you use the word “timed”. It is not “timed”. That is the point. When you use the word “timed” you take from the fundamental theory of operation of the system. As a layman perhaps you don’t understand the significance of the use of the word “timed” here.

Before you chose to get pissy again, I suggest you review the definition of “paraphrase” you provided. You obviously misused it in this case.

Look Airplay we both know that the reason you even bothered to take issue with my Reply #19 relates to our sparring in other threads in the past. What I said in #19 does no offense to your words, and in no way deceives the people who come to this forum to learn how things work. It matches the "free translation or rendering" part of the definition of paraphrase just fine.

It would match the definition entirely if one condition was met: That condition is if a radio altimeter functions because of the constant - the length of time it takes a radio wave to travel to the ground and return from a given height above the ground. In other words the speed the wave travels. Speed = distance traveled OVER TIME.

So your entire argument with me is based on denial of that premise. In having any dispute with me whatever you are stating that a radio altimeter does not use the length of time it takes a radio signal to travel to the surface and be reflected back based on the airplane's height above that surface.

Is that your statement?
Is that not the constant that makes the radio altimeter work?
Now, personally I believe that that you are so stubborn that you are willing to deceive the readers here rather than admit it.

If a radio altimeter functions in some manner that does not index it to the passage of time, please post it. I'll have engineer friends explain the parts I don't understand.

It would not be truthful or accurate to narrow the definition of "timed" down to where it relies on reference to my Seiko watch. Timing is done any number of ways where a rate of change is known, and it doesn't matter if it is ice melting, counting gear teeth or changing a frequency. So long as it references a "rate" it is still timing.

So be an adult about this. Either tell the readers here that a radio altimeter DOES NOT USE the length of time it takes the beam to travel to the ground and return as a benchmark or admit that it does.

One or the other.

Slamclick,

First of all let me say that I have better things to do than to post for no other reason than to "spar" with you. People come to the Tech/Ops forum for accurate technical facts. I provided accurate facts and you provided confusion. I deliberately avoided reference to timing because radio altimeters DO NOT “time” anything. They use an FM continuous wave and measure along the wavelength of the modulated signal.

Relating the entire process to time is misleading. I’m not saying that it can’t be related to time but only in the same context that any radio wave can be. We don’t however speak of a VOR “timing” anything.

I’ve tried to come up with an analogy of a radio altimeter system to help you to understand.

Take a tape measure and remove the tape from the holder. Form a loop and feed it through two pulleys. One at ground level and the other directly above it as high as it can go (4 feet for an 8 foot tape measure)

I think we can agree that if you hold your finger horizontally anywhere along the length of the tape measure and read the numbers on the two segments sitting side by side, we can derive the distance between our finger and the ground. Now start spinning the tape measure loop around the two pulleys. Again, we can derive the distance between our finger and the ground by comparing the two numbers that instantaneously appear on the tape segments next to our finger. This is fairly representative of what is going on in a Radio Altimeter system.

Is any “timing” taking place here? Nope. If we tried really hard, can we describe the function of the tape and pulleys with respect to time? Probably. But to what end? Just to add confusion? Just to try to explain something in irrelevant terms? Just to be the cause of why the majority of pilots still think that radio altimeters work like radars?

If you wish to inaccurately “paraphrase” my comments in the future and get called on it, try to avoid the dramatics and the accusations of some imaginary will to “spar”. Suck it up.

The signal bounces off the ground and is received by a receiver antenna that detects the signal and determines the difference between the current transmitter modulation frequency and the frequency of the received signal modulation. That coincides wthh the distance to the ground.

What you're trying to explain, I think, is a continuous wave radar, where transmitted signals are frequency modulated and constantly changing. Once the signal is recieved back from the ground, the time between the two frequencies is measured! The system knows at what time the recieved frequency was sent out, and so can measure the time it took for the signal to bounce back from the Earth.

SlamClick is right. The current and recieved frequencies are compared, and the result is a time.

Nothing to do with radar.

IIRW it goes like this: the signal (which is continuously transmitted, and freq. modulated) changes continuously in (carrier) frequency, we may call it a sweep.
(between + and - 25MHz from f0 ?)

At the receiving end, a (permanent) comparison of frequencies takes place.
The difference at any given time indicates the terrain altitude.
So, there is no time measurement nor even a time reference, but of course time is the underlying fixed factor.

The scuffle is about semantics.... for me, they are both right somehow.

Look guys, my handicap is years of experience training aircraft mechanics, pilots and dispatchers. I have to be able to tell them things in a way they can understand and not just memorize. I think excessive reliance on memory is contrary to safety of flight. This is one of those places where the FAA and I agree.

I believe that the way a radio altimeter works is based on an underlying constant - the speed of light. So a beam goes out. A beam bounces back. Some mechanism determines how long that took. That yields the distance to the surface. One mile out and one mile back equals one "radar mile" or 10.75 microseconds.

That is how it works.

Now as an instructor and as a technical writer I have to make it comprehensible without falsifying it.

So, Iakobos you wrote "So, there is no time measurement nor even a time reference, but of course time is the underlying fixed factor" [underlining mine]

Now how can I explain that to students? That time is the underlying factor but we IN NO WAY measure it? So you are saying that the frequency modulation changes could be utterly random, that they could vary at random rates, or not at all and we could still tell how far the signal traveled?

Look guys, just because no one looked at their Rolex doesn't mean timing did not occur.

Airplay I have a different problem with your analogy. Picture me putting that up in front of students, who are not fools. A hand goes up.

"Captain Slam I assume the pulley at the bottom is the surface, but is the pulley at the top the airplane?"
"No, the airplane is my finger somewhere in the middle."
"Okay, then, WHAT IS the pulley at the top? Do they have transmitters hanging in the sky?"

I submit to you that the pulley at the top is the constant - the speed of light. There is just no way you can make this work without measuring time. You may measure it by knowing electronic or mechanical rates of change, but those are just details. The underlying truth that makes it work is the speed of light.

In recent years there has been a trend in training US airline pilots, away from so-called "nuts and bolts" type of courses. I have mixed feelings about this because of my own mechanical background but they sign my paycheck I teach the way they want it. If a student comes to me during the break and wants to know more I'll try to accommodate.

So how does a radio altimeter work?

25 words or less.

Whatever the electro-mechanical minutiae in the details of how it is done, the simple answer (and not incorrect) is "We know how long it takes for a radio signal to travel a distance and bounce back."

If I'm teaching someone how to calibrate a radio altimeter - well, I would not be doing that because I am not qualified, never going to be qualified. But this student should get the full "nuts and bolts" of how it works.

For pilots and lay people, it is the speed of light, the radar mile. That depends on our ability to quantify the passage of time. Some people call that timing, no matter how it is accomplished.

Lucky me, I know two guys who are more Catholic than the Pope...

"We know how long it takes for a radio signal to travel a distance and bounce back.
You can even make it 3 words shorter, the "and bounce back" is unnecessary.

Still you would not have teached them the principle of operation, which is the measurement of a frequency shift, the denominator being time.

Maybe I truly do not understand how it works.

Iakobos are you saying that it is not necessary for the signal to bounce back in order to measure the distance to the planet?

If that is the case, there should be no receiver component to a radio altimeter.

Is that correct?

Ah....

SlamClick, I didn't realize we were talking about how to dumb things down for pilots. Yah...tell them whatever you want. It really doesn't matter. Just don't present it in the tech-ops forum as gospel.

 

I have always wondered why pilots feel they need to know every technical detail of the systems they use. Are they expected to pull a box out and repair it in flight?

Pilots need to know that the radio altimeter reads the altitude above ground. Period.

Something I deliberately avoided in my discussion about Radio Altimeters was the fact that actual "radar" altimeters exist as well. You won't find them in the average airliner though. The military seems to prefer them because they have a less detectable spectrum signature. These radar altimeters work like a radar and time their pulse signals departure and arrival.

So...as far as pilot training goes, use the radar analogy. It is correct for airplanes like F-18s and such and for the purposes of flying an airplane, its "good enough".

 

Airplay most pilots I know feel they need to know only what they need to know. What they need to know is dictated by the FAA or other nation's equivalent agency. They are going to sit in class. They are going to take written exams. They are going to have an oral exam. They are going to take simulator training that will assume they know some things about the aircraft systems. They don't know what questions will be on the exams. They don't know what questions will be asked on an oral exam. They don't know exactly what events are going to be encountered in the simulator. They are curious.

If they get past these things, they will go out and fly the line. They know they will experience equipment failures. They just don't know what equipment will fail during their careers, or in what way.
They are curious.

Knowledge, to an airline pilot is not at all theoretical. Most pilots at the professional level believe that one day before they retire, an airplane is going to hand them a wild card. Their own life and the lives of possibly hundreds of people will depend on how they play that card.

They do not have the option of sitting at their desk saying: "I wonder why that happened."

"Are they expected to pull a box out and repair it in flight?" No, they are expected to get the plane on the ground safely no matter which box fails or how that failure manifests. It would be the height of conceit and arrogance to say that pilots need to know "X" but not "Y" Especially if one were not, himself, a pilot.

Now as to your statement: "Pilots need to know that the radio altimeter reads the altitude above ground. Period. " This statement is untrue and irresponsible.

Nov. 8, 1965: Fifty-eight people die, four survive when an American Airlines Boeing 727 crashes during landing at KCVG.

Nov. 20, 1967: A TWA Convair 880 crashes just short of the same runway. Seventy of the 85 people on board die in the worst air disaster in Greater Cincinnati history.

Do you know what these two events had in common? Answer: The pilots knowledge that the radio altimeter "reads altitude above the ground" as you put it.

These two planes were on night visual approaches and the RA was telling them distance to the ground - straight down. Problem is, they were over the Ohio river and its valley. The airport is on higher land to the south. A tenth of a second before impact they might still have been reading 300' or so. Both struck rapidly rising terrain just south of the Ohio River.

Don't you think you might add a little bit of knowledge to the "height above the ground" part. Like maybe "it does not look forward" or "it does not provide protection from rapidly-rising terrain" something like that?

Better yet, perhaps you could let others decide what pilots need to know.

Now, back to the question.

I say that a radio altimeter functions because we know how long it takes the radio signal to travel to the surface and bounce back.

Is that right?

Is that wrong?

Now as to your statement: "Pilots need to know that the radio altimeter reads the altitude above ground. Period. " This statement is untrue and irresponsible.

Irresponsible? Not in the least. Untrue? Not at all.

The majority of aircraft accidents are caused by so-called “pilot error”. 70% of these “pilot error” accidents are caused by the pilot deviating from the flight manual procedures or the published miniums.

How many accidents are caused by pilots who second guess the published procedure because he/she thinks they know better?

Those examples you quoted are irrelevant. Those pilots failed to maintain adequate clearance from the ground because they descended below the published safe altitude. The accident report of the Convair you quoted states:

Probable Cause

The Board determines that the probable cause of this accident was
an attempt by the crew to conduct a night, visual, no-glide-slope approach during deteriorating weather conditions without adequate altimeter cross- reference. The approach was conducted using visual reference to partially lighted irregular terrain which may have been conducive to producing an illusionary sense of adequate terrain clearance.

Sounds like poor airmanship to me. No mention of the radio altimeter. Or were you “paraphrasing”?

http://www.avsaf.org/reports/US_reports/1960/1967.11.20_TransWorldAirlines_Convair880.pdf

The 727 accident was caused by similar circumstances. That reports says:

The board determines that the probable cause of this accident was the failure of the crew to properly monitor the altimeters during a visual approach into deteriorating visibility conditions.

http://www.avsaf.org/reports/US_reports/1960/1965.11.08_AmericanAirlines_Boeing-727.pdf

It’s never a good idea to blow your altitude minimums. Accidents like these were precursors to legislation requiring GPWS. And all the GPWS AFMs that I’ve seen or wrote will tell you that they give little or no warning of flight into precipitous terrain.

This reminds me of a story about one particularly stubborn pilot. The airplane he flew locked the AC generator that provided power to the engine/prop de-ice loads from being reset if it detected a ground (earth) fault. He learned enough about the system to know that you could bypass the lockout by cycling the control breaker and he used the technique for days rather than risk reporting the ground fault to maintenance and losing flying time.

This resulted in thousands of dollars of damage to props and intakes before we figured out what was happening. Instead of minor localized damage, his actions cause much more serious damage and the potential to FOD the engine.

So in this case, the pilot only needed to know that the indication means the system is locked out. Nothing else.

Better yet, perhaps you could let others decide what pilots need to know.

Actually...I'm one of those pesky people who writes flight manuals and FCOMS/AOMs. So I DO decide what pilots need to know.

Unfortunately I don’t have control over what you guys do in the classroom to tear down the information that the authorities take so much care in building....

Airplay that was not an answer. It was a change of subject.

I say that a radio altimeter functions because we know how long it takes the radio signal to travel to the surface and bounce back.

Is that right?

Is that wrong?

I say that a radio altimeter functions because we know how long it takes the radio signal to travel to the surface and bounce back.

Is that right?

SlamClick is right. The time between the recieved frequency and current frequency is known, and from THAT the height is found.

Well, that's what I think.
I just wish the "engineer" who "designs aircraft electrical systems" who has the really ironic mis-quote from A Few Good Men would cowboy up and admit it.

I'm beginning to feel like a bully. It is almost embarassing at this point, but I hate to let someone with perceived authority get away with that stuff in this forum.

You're perfectly right, and time is DIRECTLY measured. That being said, I don't think Airplay will apologise.

SlamClick is right. The time between the recieved frequency and current frequency is known, and from THAT the height is found.

Oh good....another armchair "expert" in the tech/ops forum....

You're perfectly right, and time is DIRECTLY measured. That being said, I don't think Airplay will apologise.

There is nothing to apologize for. It is incorrect to say that the time between the received and current frequency is known. It can be derived, but is not measured by the radio altimeter.

There is no “timer” circuit in the unit. How much more clearly can I put it? In a device like a DME there certainly is a measurement of time. A pulse is transmitted, and received, and the time between the event is monitored by a timer circuit.

There is no such circuit in a radio altimeter. Can the process be correlated to time? Certainly. But again…to what end? To confuse the process?

In a VOR system two modulated signals are compared. An FM modulated 30 Hz reference and an AM modulated 30 Hz variable. The phase difference between the two are compared and the resultant bearing to the station is derived.

Is there "timing" done by the VOR receiver? Nope. Can the process be correlated to time? Again, certainly. We can discuss the time difference between the 2 signals, but again to what end?

Is it accurate to say that VOR systems “time” the difference between the variable and reference signals? No. That is misleading. And it is my guess that it isn’t typically taught that way except maybe by non-technical pilots trying who need to dumb the process down so that other pilots can ignore the AFM.

Is it accurate to say a DME system “times” the difference between the transmitted and received frequency? Yes.

Argue amongst yourselves all you want. But you’ll get no apology from me. I have stated the facts. Slamclick tried to “paraphrase” my statements and was incorrect. Perhaps the right thing to do here is to realize your error and retract your “paraphrase” Slamclick and refrain from posting to technical topics you know nothing about….

Okay, thenAirplay how about a couple more definitions?

Obfuscate
verb {T} to make something less clear and harder to understand, especially intentionally:

Dissemble
verb {I} to hide your real intentions and feelings or the facts:

Let's cut through the crap.

I say that a radio altimeter functions because we know how much TIME it takes the radio signal to travel to the surface and bounce back.

TRUE?

FALSE?

Over to you Airplay

Slamclick, "WE" all know how fast radio waves move. Does the radio altimeter know? No. Does it measure it? No.

Your question is out of context. It's like arguing whether during landing, an airplane comes down or the ground comes up. Although an argument can be made for either, one theory is quite impractical and misleading. Can you guess which? It’s actually even more irrelevant than that but the old “apples and oranges” term is overused in my opinion.

By the way, I'm not guilty of the definition of either of your obscure words. I certainly hope you don't make a habit of using big words in an attempt to impress your pilot students. It just makes them even more confused. Students learning about aircraft systems shouldn’t have to bring a dictionary to class.

I'll leave you to your thesaurus. I'm sure a recalcitrant dilettante like yourself has many other facts to oppugn.....

All right. I've been reluctant to step into this, but here goes nothing.


I wouldn't say a VOR receiver measures time. In that case, the phase shift is proportional to the radial. Yes, there's a time constant associated with any phase shift, but in this case the phase shift is all important. The time involved is entirely dependant upon the base frequency, which could be any old frequency.


What a radio altimeter most certainly does is measure the time it takes for the radio waves to reach the ground and bounce back.

As for there not being a timing circuit in a radio altimeter, that's just plain wrong. There's no circuit for counting fractions of seconds, but timing can be done in many ways. At the heart of most modern timers are oscillators. There's an oscillator in a radalt as well. There's also timing to ramp the frequency of said oscillator up and down at a given rate. Yes, rate. To have a given rate assumes that you have timing.

The time is measured, as a beat frequency between the transmitted and received signal. You can argue that this is measuring a beat frequency, not measuring time. You can also argue that you are measuring angles rather than time when looking at the hands of a watch.



Now, can we put an end to this. Please? I took the armrests off my chair at work. I could get it photographed and sent off to you both to prove that I'm not qualified to be an armchair expert... unless the engineering diploma above it requalifies me, of course.

Try to shake hands or make up, or just agree on a gentlemans disagreement. If you can't, take it by email? Or meet in a pub. You'd probably have a good time over a cold one or three. The net has a way of bringing out the conflicts in what would be fun discussions when seated around a table.

Cheers,
Fred

In a VOR system two modulated signals are compared. An FM modulated 30 Hz reference and an AM modulated 30 Hz variable. The phase difference between the two are compared and the resultant bearing to the station is derived.

Is there "timing" done by the VOR receiver? Nope. Can the process be correlated to time? Again, certainly. We can discuss the time difference between the 2 signals, but again to what end?

What has this got to do with anything? There is NO time difference between the two signals, only a phase difference.

There IS a time difference between the current transmitted and recieved signals in a radio altimeter, and it's because of the time difference that the altitude is known.

Five second Google search. http://www.britannica.com/eb/article?tocId=61201

Using your logic, a speedometer doesn't measure speed, but angular speed, or a quartz watch doesn't measure time but rate of change of electrical pulses.

What has this got to do with anything? There is NO time difference between the two signals, only a phase difference.

Thanks for that. A radio altimeter measures the phase difference between the transmitted signal and received signal too. The measurement must be made within the same wavelength of a single ramp. Just like a VOR.

There IS a time difference between the current transmitted and received signals in a radio altimeter, and it's because of the time difference that the altitude is known.

Oh really? When you “time” the return of something, you send out a signal and then wait until you detect that IDENTICAL signal back. But in the case of a radio altimeter, you don’t get the identical signal back. Ever. You receive a carrier with the modulation phase shifted from the current transmitter frequency, representative of the distance to the ground.

Your example suggests that we can time the return of an apple by comparing it to an orange…

Using your logic, a speedometer doesn't measure speed, but angular speed, or a quartz watch doesn't measure time but rate of change of electrical pulses.

A speedometer INDICATES speed of a vehicle and it does it by measuring rotational speed. A “quartz” watch INDICATES time and it does it by measuring the vibrations of a crystal. A radio altimeter INDICATES altitude above the ground and it does it by comparing the phases of two signals. What’s your point? The entire issue here is HOW these devices do what they do.

Are you suggesting that the only way to measure distance is by the use of some time based measure? Better synchronize your tape measure to your watch. Are you suggesting that every indicator actually directly measures the magnitude of the indicated property?

By the way 777236ER, your five second Google search takes me to an incomplete article that ends just as it is about to describe a “A more common form of radio altimeter”. Any guess to what this “more common form” is?

At the heart of most modern timers are oscillators. There's an oscillator in a radalt as well. There's also timing to ramp the frequency of said oscillator up and down at a given rate. Yes, rate. To have a given rate assumes that you have timing.

I’m a little disappointed FredT. Are you suggesting that every device that uses an oscillator employs a practical “timing” process?

You can also argue that you are measuring angles rather than time when looking at the hands of a watch.

Remember that we are discussing HOW the system works. So in that context it is quite relevant, with respect to a watch, to state that the position of the hands and the placement of the numbers on a watch dial correspond to the resultant angle of the hands from a specified datum. So…what’s your point?

The modulation frequency and the ramp rate in a radio altimeter really isn’t that critical. As long as your indicator drive is adjusted for it, you could use a fairly large range of frequencies. Why? Because the display of altitude relies on the difference of phase between the two signals not the timing difference between the transmitted signal and the received signal. Frequency and ultimately voltage measurement (or rather “comparison”) is the critical function here.

The clock/timer circuits in GPS and DME systems is quite critical. Because it relies on precise timing. In GPS and DME, a second is a second and the timer circuit must be done in terms of actual time.

Thanks for that. A radio altimeter measures the phase difference between the transmitted signal and received signal too. The measurement must be made within the same wavelength of a single ramp. Just like a VOR

Wrong.

The phase shift of a VOR varies with each radial. The FM signal will have a phase shift of 0/2ð compared to the base signal at the 0 degrees radial, a phase shift of ð at the 180 degrees radial. The measurement of phase shifts is caused by a completely different process, and is measured to a completely different ends. There is no timing done at all when recieving a VOR signal, either directly or indirectly. The phase shift merely indicates the direction of the VOR.

A radio altimeter INDICATES altitude above the ground and it does it by comparing the phases of two signals. What’s your point? The entire issue here is HOW these devices do what they do.

Wrong. A radio altimeter indicates height by analysing the time it takes for the signal to travel from and to the the aircraft by comparing the frenquencies of the signals. There is a big fundamental difference with a VOR - a phase shift with a VOR directly indicates direction, a frequency shift with a rad altimeter indirectly indicates time, which can be used to find distance.

[Edited 2005-01-21 23:16:23]

I’m a little disappointed FredT. Are you suggesting that every device that uses an oscillator employs a practical “timing” process?

Please put in a bit more effort to read and reply in context. A few post above that, someone erroneously stated:
there is no “timer” circuit in the unit. How much more clearly can I put it? In a device like a DME there certainly is a measurement of time. A pulse is transmitted, and received, and the time between the event is monitored by a timer circuit.

There is no such circuit in a radio altimeter.

Remember that we are discussing HOW the system works. So in that context it is quite relevant, with respect to a watch, to state that the position of the hands and the placement of the numbers on a watch dial correspond to the resultant angle of the hands from a specified datum.

Yes, but while we are talking cogs, hands and pendulums, we must also keep in mind the purpose of this intricate mechanism for changing the angles of the hands at a precise rate: To keep track of time.

Because the display of altitude relies on the difference of phase between the two signals not the timing difference between the transmitted signal and the received signal.

Great! I think we have found the heart of the misunderstanding here. As the saying goes: We all think and act far below our capability for fifteen minutes or so every day. We can only hope that it's not the wrong 15 minutes! The minutes when we are reaching back into our memory banks to dig up the functions of an avionics box to post on a.net would constitute the wrong minutes.  

A radalt does not rely on phase differences. In fact, it is completely oblivious to any phase difference. What it does measure is a frequency difference.

By now, you're probably already going "oh darn!", but for the benefit of those not in the know who are probably terminally confused by now: The basic carrier frequency of the radalt (4300 MHz) is frequency modulated at the transmitter. It ramps from 50MHz below to 50 MHz above carrier and back at a fixed rate. Note that it is, for once, not a sin(w*t) function.

At the receiver, the received wave is mixed with the transmitted wave. As the received wave is what was transmitted a short while ago, what returns will not be at the same frequency as what is currently transmitted. The received signal has been delayed by a time directly proportional to the distance to the ground, and in the meantime the transmitted frequency changed - at a known and constant rate. When you mix these two frequencies, you get a beat frequency, as always when you mix two different frequencies. This beat frequency is the difference between the two frequencies, which is directly proportional to the time by which the received signal was delayed, and thus directly proportional to the distance to the ground.

You feed the beat frequency to a beat frequency counter, which converts the beat frequency to a voltage which is then fed to your radalt indicator.

Cheers,
Fred

A radalt does not rely on phase differences. In fact, it is completely oblivious to any phase difference. What it does measure is a frequency difference.

FredT, the modulation frequency is ramped up and down continuously. The rate can be represented (as it often is in the maintenance manual) by a triangle wave.

In a (albeit convoluted) way the radio altimeter measures the phase difference between the ramp that represents the received signal and the reference signal. So yes...its not the best way to explain the system. It shows how confusing things get when we stray from the simple facts. And the simple fact is.....no timing is done.

777236ER,

First you say:

There IS a time difference between the current transmitted and recieved signals in a radio altimeter, and it's because of the time difference that the altitude is known.

Now you (correctly) say:

A radio altimeter indicates height by analysing the time it takes for the signal to travel from and to the the aircraft by comparing the frenquencies of the signals.

Which is it mate?

For crying out loud. There IS a time difference and it's from the TIME difference that the height is found. The TIME is found by comparing the frequencies. Just admit that the time matters, and that SlamClick was right all along.

Read what FredT wrote, and stop being bloody minded.

TIME is found by comparing the frequencies.

Or is it the distance that is found by comparing the frequencies.....

What are we measuring here? Time or distance. I say distance.

Of course we are measuring distance. Most adults would be embarassed to pretend that they did not understand that after having debated it for several days.

HOW are we measuring it?

By referencing something, anything that can be made to happen at a known RATE, hey - frequency modulation will do, against the known amount of TIME it takes a beam to travel to the surface and bounce back.

A couple more definitions. After this I'm going to have to charge you for English lessons.

Obtuse
[adj] slow to learn or understand; lacking intellectual acuity
[adj] lacking in insight or discernment

Disingenuous
[adj] not straightforward or candid; giving a false appearance of frankness



[Edited 2005-01-22 05:41:35]

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 rest of this bickering is just decoration designed show us all just how far a pilot will distort the correct interpretation (dumb down if you will) in an attempt to learn about systems at a level they really don't need to.

Pilots like you are very dangerous Slamclick.

You haven't crawled up from your level of discussion either. You chose to directly insult rather than discuss the issue. Don't ever change Slamclick. I wouldn't recognize you if you started to act mature and actually knew what you were talking about....