REQUISITOS DE ACCESO Y CRITERIOS DE ADMISIÓN

3.7.1 Frequency Modulation Ranging

The use of pulsed radar techniques has hitherto been described to measure target range.

However, frequency modulation may also be used to determine range as depicted in Figure 3.24.

Beam 1

Beam 3

Beam 2

Vx

Vy Vz

f 2 x V∝ V

∆ λ

V

f + f∆

f + f∆ f - f∆

Figure 3.23 Doppler radar.

124 BASIC RADAR SYSTEMS

The transmitted signal consists of a triangular wave modulation, as shown, that sweeps across the frequency spectrum, completing one cycle in 0.01 s in the example given. The received frequency will lag the transmitted frequency by an amount
f owing to the time taken to complete the out and return journey. The example shows a measurement taken when the reflected received frequency, f₁, is compared with the current frequency at the transmitter, f₂, with the difference in frequency being
f . The associated time difference signal,
t, is proportional to the range of the target.

The figures shown on the diagram relate to the use of this technique in a radar altimeter, where the radar returns are used to calculate the instantaneous altitude of the aircraft above the terrain over which the aircraft is flying. In this example, the transmitter is sweeping in a linear manner over a frequency range of 4250–4350 MHz in 0.01 s. The use of radar altimeters is described in Chapter 7.

3.7.2 Terrain-following Radar

Whereas the radar altimeter is useful in informing pilots where they are in relation to the terrain underneath the aircraft, it does not tell them where the terrain is in front of the aircraft. To do this, the pilot needs to use a terrain avoidance (TA) mode or, better still, a dedicated terrain-following radar (TFR). The TA function can be crudely achieved by using a normal pulsed radar in a single-bar scan mode with a fixed depression angle. This will tell the pilot where he is in relation to the terrain ahead of the aircraft, but it is not a sophisticated mode and does not readily lend itself to coupling into the autopilot (Figure 3.25).

The TFR is a dedicated radar coupling into a dedicated functional system and autopilot that allows the pilot much greater performance and flexibility when penetrating at low level at night. The TFR scans the terrain ahead of the aircraft and receives ground returns that are

Frequency (MHz)

Time (sec) 4350

4250 f2

f1

t1 t2

∆t

0.01 Transmit Signal

Receive Signal

∆f

Figure 3.24 Frequency modulation ranging.

used for guidance. Normally, a simple box scan is used where the active sweeps are those in the vertical direction (sections 1 and 3). In some circumstances a figure-of-eight scan is used which provides broader lateral coverage than the simple box scan. The TFR therefore builds up a range/elevation picture of the terrain ahead of the aircraft and calculates an imaginary

‘ski-toe’ profile that reaches out ahead of the aircraft. This profile is calculated taking into account such factors as aircraft speed, manoeuvrability, etc., and provides an envelope within which the aircraft will not be able to avoid the terrain ahead. The system is configured so that, whenever the terrain ahead broaches the ski-toe envelope, the aircraft pitches up to rectify the situation. Similarly, if the terrain drops away in front of the aircraft, the aircraft pitches down until just operating outside the profile. The system operates just like the toe of a ski, moving up or down to follow the terrain ahead of the aircraft but always ensuring the aircraft can safely manoeuvre.

The measurements from the radar altimeter are also fed into the terrain-following system which calculates the ‘most nose-up command’ provided by either TFR or radar altimeter.

This has the advantage of providing the pilot with an additional altitude safety buffer directly beneath the aircraft as the TFR is looking several miles ahead.

The TFR/radar altimeter commands may be coupled into the autopilot to provide an auto-TF mode while the aircraft is approaching the target area, thereby enabling the aircraft to fly at low level automatically while the pilot performs other mission-related tasks. The TFR may be an embedded system forming part of the aircraft primary radar, alternatively it may be provided in a pod that is loaded on to the aircraft. The AN/AAQ-13 LANTIRN navigation pod fitted to F-15 and F-16 aircraft performs a TFR function for these aircraft.

3.7.3 Continuous Wave Illumination

On some weapons systems a continuous wave (CW) illumination mode is provided. This mode is used when aircraft are fitted with semi-active air-to-air missiles; that is, missiles that can receive incoming RF energy and once fired can track and engage the target. As the

Terrain

Figure 3.25 Terrain-following radar operation.

126 BASIC RADAR SYSTEMS

missiles are unable to transmit, the aircraft radar has to provide the target illumination and it does this by using a CW illuminator co-boresighted with the aircraft radar antenna.

Therefore, when the aircraft radar is locked on to the aircraft it can simultaneously illuminate the target (Figure 3.26). The disadvantage of this technique is that the aircraft radar has to remain locked on to the target and transmitting CW illumination until the engagement is complete. In high-density air-to-air combat this may not always be possible.

3.7.4 Multimode Operation

Modern radars such as those on the F-15E and F-22 have the capability of operating simultaneously in a number of modes, an example of which is shown in Figure 3.27. In this hypothetical example, three simultaneous modes are depicted:

Target Data Set : Range

Azimuth Angular Rates Identification

Range Rate Elevation

Figure 3.26 CW illumination.

Ground Mapping

Track While Scan (TWS) Synthetic

Aperture Radar (SAR)

Target 1

Target 3

Target 2

Figure 3.27 Simultaneous multimode operation.

 Sector ground mapping;

 Synthetic aperture (SA) spot mode;

 Track-while-scan (TWS) mode engaging three separate targets.

The radar achieves this capability by interleafing the radar modulation required for each mode on a pulse-by-pulse basis and effectively operating as several radars in one. This offers immense flexibility to the aircraft as a weapons platform.