Aerospace and Electronic Systems Magazine January 2018 - 28

Malanowski et al.

Figure 8.

Elevation pattern for 16-element antenna with half-wavelength spacing.

Figure 9.

Elevation angles vs. target altitude for different transmitters.

is placed close to the launch site. The rocket is moving almost
straight up and its current altitude is h. Since the launch site is
close to the receiver, the elevation angle of the target from the
receiver position will change rapidly as the target ascends. This
is not a big problem, as the elevation pattern of the receiving
antenna is relatively wide (the antenna gain is ca. 5 dBi), and the
antenna can be positioned in such a way that the target is illuminated during the whole time.
The illumination of the target by the transmitter antenna elevation pattern is a more important issue. The transmit antennas
are designed to focus emitted power at a narrow elevation angle,
as the aim is to illuminate the receivers placed on the ground,
and not the airspace. In the case of all transmitters used during
the experiment, 16-element (in the elevation direction) antennas with half-wavelength element spacing are used. A theoretical elevation pattern for such antenna is shown in Figure 8. The
3dB beamwidth for this pattern is 6.8°. As the target climbs, the
elevation angle seen from the transmitter increases, which may
cause losses, due to influence of the elevation pattern. In addition, antennas are frequently tilted down to illuminate the terrain
beneath the mast, which can further reduce the illumination of
high flying targets.
In Figure 9 the target elevation angles seen from different
transmitters are plotted versus target altitude (compare Figure 7
with the target at altitude h, transmitter-receiver distance equal
to Rb and the target elevation angle θ). The difference in the
elevation angles for different transmitters results mainly from
the different distances between the target position and each of
the transmitters. For example, the transmitter Tx2 is closest to
the target position (44 km), therefore, the elevation angle is the
greatest.
In Figure 10 relative losses for different transmitters caused by
the elevation pattern are plotted. In calculations, it was assumed
that the antenna elevation tilt angle is 0°, as the real value is unknown. It can be seen that the most significant losses occur for
transmitter Tx2, which is the closest to the target, therefore the
elevation pattern influence is most significant.
In the considered experiment, the maximum rocket altitude
was ca. 1,500 m, therefore the losses introduced by elevation patterns are negligible. If the rocket altitude was larger, elevation pattern losses could be a limiting factor of target detection.

ROCKET DESCRIPTION

Figure 10.

Elevation angles vs. target altitude for different transmitters.

28

The rocket that was detected by the passive radar was named Strega and it had been designed by rocket enthusiast Damian Mayer.
The rocket is 2.3 m in length and 80-84 mm in diameter. It has
three stabilizers. Everything except the rocket nose/head is made
from aluminum 6060 alloy. The nose was made from fiberglass
composite. The engine used on board Strega was powered by 5.2
kg of KNO3 and Sorbitol propellant. The mass of the rocket without the propellant is 8.5 kg. The rocket during the launch is shown
in Figure 11.
The rocket is equipped with a simple flight parameters recorder named Altimax. Altimax features an accelerometer and a
pressure sensor which operates at the frequency of 200 Hz for
IEEE A&E SYSTEMS MAGAZINE

JANUARY 2018



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