Aerospace and Electronic Systems Magazine November 2016 Tutorial X - 95

Fasano et al.
In order to estimate the maximum distance where an object can
be detected Rdet, the value of S can be estimated by means of the
radar range equation [17] that is reported in expression (28):

(28)

where
Ptx is the power emitted by the transmitter;
G is the gain of the antenna, i.e., for a parabolic antenna, i.e.,

Fig. 13.

for a parabolic antenna, where

Attenuation in dB/km for Ka and X band radars.

d is the diameter of the dish;
ηA is a term called "aperture efficiency" that depends on the quality
of antenna manufacturing. Typical values of ηA range from 0.55
to 0.70;
σ is a term called object radar cross section (RCS). It expresses
the reflectivity of the object and for man-made targets, as is the
case for SAA, it depends on several parameters such as the type of
the object, the materials of its surface, its relative pose with own
aircraft, and the radar carrier wavelength λc. The statistical model
of σ as a function of the type of aircraft is discussed in [23]. The
typical limit for detection applied for SAA is σ= 1 m2, i.e., a small
double seat aircraft. It is worth noting that last generation aircraft
can be challenging to be detected, since they are manufactured by
exploiting several nonmetallic components;
R is the range where the object is placed;
L is the transmission loss, i.e., the loss determined by atmospheric
scattering and absorption. This term has high impact on high frequency systems rather than low frequency ones. Fig. 13 reports
typical values of L for the Ka and X band frequencies and different
meteorological conditions.
At the same time, the value of noise N can be determined as
reported in (29):
N=kTB (29)
where k=1.38 10-23 [J/K] is the Boltzmann constant, T [K] is the
temperature that is typically assumed T=290 K, and B [Hz] is the
radar bandwidth. By combining (28) and (29) the signal-to-noise
ratio (SNR) can be computed as reported in (30):
(30)
Assuming σ equal to the minimum RCS that must be detected σmin,
since Rdet is determined when S/N = εth, it can be computed by substituting this term in (30). The resulting value is given in (31):
(31)

NOVEMBER 2016, Part II of II

This reveals that the best performance is provided when a large
wavelength and a large pulse width is assigned. With current transmitters and receivers, an airborne pulsed radar can be developed
to detect objects with 1 m2 RCS from a distance Rdet = 10 NM.
Nevertheless, a large wavelength also determines the need for a
large antenna.
As reported in the previous section, some false alarms, i.e., objects that have been detected but are not of interest for the user,
can be rejected even if just a single scan is realized. In particular,
single scan false alarm rejection can be performed by exploiting
the following issues.
1. Altitude threshold-All detected objects that are at an altitude
lower than a stated threshold are rejected as false alarms. Nearground objects are called ground "clutter" and they are usually
not of interest for detection unless the radar is used to detect fixed
or moving obstacles in the proximity of an airport. Own aircraft
altitude and attitude data must be provided to the radar processing
unit to perform the clutter removal by altitude thresholding.
2. Velocity threshold-In this case the Doppler signature of objects
can be used to remove stationary objects that are likely to be on
the ground. This type of processing is more hazardous than altitude thresholding, since it can determine the rejection of stationary objects in flight such as balloons and hovering helicopters.
In summary, radars are the most adequate noncooperative sensing
source for SAA applications. However, they are also demanding
in terms of onboard resources. For this reason, they can be easily
installed onboard large aircraft but alternative solutions are needed
for small ones.
Airborne LIDARs: Airborne LIDARs are systems equivalent
to radars that exploit the light emitted by a laser rather than the
microwave energy emitted by the radar antenna. The laser generates the light by applying a high voltage to a cavity filled with
a mixture of noble gases, such as He-Ne or Nd-Yag. This voltage determines the Townsend effect, i.e., the production of a large
quantity of photons in the wavelength of visible and IR due to the
high-energy ionization of gases. These photons are obliged to pass
through a collimating lens that is located at the exit of the cavity.
As a consequence, the laser can be modelled as a light source with
the following attributes.

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