Installation Manual

THALES Communications
OIM
AHV1600-SYSTEM
Page 2
July 16/09
The AHV1600 Transceiver performs FM/CW modulation transmissions that are beat against the received
reflection. The variable slope modulation allows the beat frequency to be maintained in a given bandwidth
(window of 60 kHz to 110 kHz around centre frequency of 80 kHz). The window is then analysed through the
equivalent of a 1 kHz bandwidth filter sweeping from 15 kHz to 110 kHz. The evaluation of the aircraft altitude
is based on the measurement of the saw tooth duration and the position of the echo frequency in the window.
The detection of the beat frequency spectrum is performed by a digital signal processing function.
The transmitted wave is linearly modulated in frequency by a saw tooth.
A beat signal is then obtained by mixing the transmitted waves F(t) and received waves F(t-

i). At every in-
stant, the frequency fbi of this signal is equal to: fbi = F(t) - F(t-
 i)
As the modulation is linear fbi is linked to
 i and then to Hi by the formulae:
fbi
F
i
T
Hi
cT
ST ST



2.
.
The fbi frequencies form the beat signal spectrum.
This spectrum is constituted of all the frequencies from
the ground and the thermal noise as well.
To enable a measurement of fbo with a probability of
noise detection compatible with the integrity require-
ments of the Radio Altimeter, a detection level is de-
fined.
Only frequencies, which appear in the beat signal with
energy above this level are taken into account.
As the frequencies fbi and the heights Hi are propor-
tional, the minimum distance to the ground Ho is linked
to the minimum frequency fbo of the spectrum.
The Radio Altimeter then measures this frequency fbo,
the leading edge of the spectrum.
In the case of the Radio Altimeter, the frequency excursion
F

is fixed and T
ST
is made proportional to Ho by
a feedback loop that keeps fbo in a constant frequency range. The accurate measurement of the minimum fb