Radalt Question

[Mr Irons]

Why is it that the frequency modulated continuous wave radio altimeter can achieve better accuracy than the pulse type radio altimeter?

[Captain Smithy]

A CW Radar Altimeter can achieve much better accuracy than if a pulsed signal was used. Not exactly sure why but I think it's due to doppler effect of the CW signal.

That said I wasn't aware of such a thing as a pulsed Radalt. I thought CW was standard?

Smithy

[Wodrick]

Going back about 40 years so might have recalled incorrectly but it's down to the duration of the pulse. While transmitting it cannot receive so no matter how brief the pulse is there will be a minimum range. Could be crucial at fifty feet. Seem to remember seeing a pulse Radar Altimeter for high altitude applications long ago, perhaps military.

And as to accuracy very small changes due to Doppler effect are detectable.

[Checkboard]

FM altimeters are industry standard for aviation, and measure the time delay by comparing the phase shift in the reflected signal - which makes them very accurate. Pulse altimeters must use a deramping chirp to mix with the reflected chirp to measure the shift, and the measured frequency shift is thus only an estimation, giving lower accuracy (3.125 nanosecond chirp gives a resolution of about half a meter).

Frequency modulated radar altimeters have a height limit of about 5000 feet. Pulse altimeters are used for higher altitude applications (mostly satellites).

[NSEU]

Frequency modulated radar altimeters have a height limit of about 5000 feet.

I thought so, too, and I was quite surprised to hear that the 747-400ER uses the Rad Alt signal to trigger a change in the Aux Tank configuration at <8190' (Descent Pressurisation Mode).

Rgds.
NSEU

[cdeavionics]

Pulse radio altmeters

These were fitted to the BOAC DC7C fleet circa 1961. They were high level rad alts, hence the current FM rad alts were originally referred to throughout the industry as LRRA (Low Range Radio Altimeters).

[Mr Irons]

Thank you all for your assistance, that has cleared things up for me.

[Bullethead]

When I was in the RAAF flying Hercs the Navs used the hi range rad alt to determine actual altitude to be used in their calculations relating to pressure pattern flying.

Regards,
BH.

[Checkboard]

I was going to mention that, actually. Pressure pattern flying involves flying an accurate physical altitude (using a rad alt over the ocean) and using the pressure altimeter to determine the pressure change over a period of time. The pressure change (due to flying through a pressure pattern) can be shown mathematically to relate to the crosswind (not headwind), and thus drift, aircraft has experienced. It's a technique which was used by aircraft for long range over-water navigation, before area navigation systems such as INS/IRS/GPS were developed.

When I was studying it, I was always curious at how they obtained rad alts at high level - or if they simply flew below 5000'. Now I know.