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
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.