I have seen the location of the radar altimeter is different between Airbus and Boeing. Airbus has it almost at the tail, Boeing has it forwards. I would like to know which one gives the height more accurately?
In the aircraft I fly, the radar altimeter (aka radio altimeter) is calibrated for the installation, and the calibration compensates for the flight attitude of the aircraft at touchdown. The radar altimeter is considered radio navigation avionics, and in many cases is required equipment.
So in theory, a flight attitude other than a normal approach attitude may cause some error. However, since the primary use is during approaches, the consequence of this error is negligible, assuming flaps and gear extended.
In practice the avionics tech verifies that the altimeter reads a certain reading on the ground, and that reading is within spec for the installation. Practically, this means that the radar altimeter does not always read "0" when the aircraft is on the ground in a taxi or parking mode, however it is normally close.
My assumption in answering this is civilian aircraft, using normal low level short range radio altimeters, as opposed to delay Doppler or synthetic aperture radar which are used for other types of navigation applications.
First, let's get the equipment straight - a RADAR altimeter uses a "pulse" transmission characteristic, and a RADIO altimeter is FM CW - both transmitters operate near or at 4.3GHZ.
The operating characteristics are different, the "pulse" system is fairly simple - you transmit a pulse, receive the pulse, and calculate the delay between the two for your altitude.
The FM CW transmits a Frequency Modulated Continuous Wave signal where the transmit signal varies in frequency from low to high within the 4.3GHZ center frequency (usually 400KHZ centered on 4.3GHZ) to give a 'sweep from 4.3GHZ minus 200KHZ to 4.3GHZ plus 200KHZ. The FM CW receiver is phase locked to the transmit signal and compares the frequency shift (time delay of the FM CW sweep) of the receive signal to this signal, and derives a 'frequency change' which is translated to an altitude signal. The FM CW technique typically is less dependent on ground reflection anomalies and is more stable during approach.
The pulse type RADAR altimeter is widely used in military applications due to it's lower RF 'footprint' and is also used for terrain following autopilots. Both systems respond to ground conditions, (moisture content) lakes, oceans - in different manners, and any object on the aircraft within a multiple wavelength of either system can result in a 'multi-path' signal reception and contribute to altitude error. These errors from objects (other antennas, gear, flaps, even propeller blades at a particular RPM) may cause errors that are only observed during flight - this is the main reason for a flight performance evaluation.
The attitude of the aircraft during approach, say at the outer marker when flaps or speed settings are changed, will have slight variations, usually no more than +/- 10 feet, and as you approach closer to the ground this variation will become smaller - to the point of flare where the variation at 3 feet is less than 3 inches (with a 1.73 degree pitch angle approach). At altitudes above a normal approach, the atmosphere (atmospheric rarefication, temperature, density, humidity and moisture content) can influence a radar pulse more than a continuous wave signal, and lightning can also have adverse affects on both radio and radar altimeters.