User's 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-
τ
i
). At every in-
stant, the frequency fbi of this signal is equal to: fbi = F(t) - F(t-
τ
i)
As the modulation is linear
fbi
is linked to
τ
i
and then to
Hi
by the formulae:
fbi
F
i
T
Hi
c T
ST ST
∆
= =
τ
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