notes about pulsed Doppler radar FM altimeter

Well, the main problem was its radar. You see, the F-111's altimeter was a radar altimeter. With the ash on the ground in Vietnam interfering with the radar, it was very rarely accurate.

Generally 2500 ft is the standard

They are one in the same. They use radio waves for RAdio Detection and RAnging. Radar is an acrynom, why they flip flopped the R and the A in ranging is beyond me.
They're both the same thing

Radar altimeter gets altitude information by bouncing a radio wave off the surface of the Earth and determines the aircrafts altitude by measuring the length of time it takes for the signal to return.

Barometric altimeters get altitude information by measuring the barometric air pressure outside the aircraft. Barometric pressure decreases as altitude increases.

Radar altimeters will give altitude above ground level while barometric altimeters give altitude above sea level.