DCS F-18 - Electronic Warfare Theory

[VBF-12 Gosling]

Gents

We have been WWII pilots for a long time. Electronic Warfare (EW) has not really been much of an issue for us. Moving into the modern age with the F-18 and similar aircraft changes all that. Also, if our intent is to be a VAQ squadron then EW should be our “bread and butterfly”.

So, I’m going to write a few posts on the theory of Electronic Warfare.

I’ll add to this post as I cover various subjects. The intent is a short description and a Youtube Video that will support that explanation.

There will also be another thread that covers specifics about EW on the F-18.

EW01 - Theory - Electromagnetic Spectrum
EW02 - Theory - Inverse Square Law
EW03 - Theory - Reflected Radar Energy
EW04 - Theory - Doppler
EW05 - Theroy - Radar Shape

[VBF-12 Gosling]

EW01 - THEORY - ELECTROMAGNETIC SPECTRUM

The Full Spectrum
The Electromagnetic Spectrum covers everything from low frequency radio through visible light to the likes of X-ray and Gamma-rays...



Atmospheric Propogation
One of the key points is that various frequencies propagate through the atmosphere at different amounts. Some frequencies are attentuated all together:



Radar Frequency Band Use
Looking at the frequencies used for radar, the lower frequencies propagate further but that lower frequency means that the accuracy and update rates are lower. So different frequencies have different purposes like Search Radars or Fire Control Radars. Also, lower frequencies require larger antenna so are impractical for an aircraft radar for instance. The physical size of a radar determines its angular accuracy and resolution.



A and B Band (HF & VHF Radar)

• Early Warning Radars and Over The Horizon (OTH) Radars as the lower frequencies are refracted more by the atmosphere
• Easier to obtain high-power transmitters
• Less atmospheric attenuation
• Limited accuracy because a lower frequency requires antennas with very large physical size
• Stealth technologies have less effect at extremely low frequencies

C Band (UHF Radar)

• Good frequency for the detection and tracking of satellites and ballistic missiles over a long range
• Early warning and target acquisition radars like the surveillance radar for the Medium Extended Air Defense System (MEADS)

D- Band (L-Band Radar)

• Long-range air-surveillance radars out to 250 NM (≈400 km)
• high power pulses over a broad bandwidth and possibly an intrapulse modulation
• Curvature of the earth reduces detection of low flying targets at maximum range

E/F-Band (S-Band Radar)

• Due to atmospheric attenuation this band requires higher transmitting power to achieve a good maximum range
• Also this frequency range suffers from weather conditions more than D-band
• Can detect and display the position of aircraft with a medium range up to 50…60 NM (≈100 km)

G- Band (C-Band Radar)

• Mobile military battlefield surveillance, missile-control and ground surveillance
• Short or medium range
• Antenna size are relatively smal
• Excellent accuracy and resolution allowing very manoeuvrable anntenna
• The effect of bad weather conditions is very high. Therefore air-surveillance radars use an antenna feed with circular polarization often

I/J- Band (X- and Ku- Band Radars)

• Relationship between used wave length and size of the antenna is better than in lower frequency-bands
• Popular band for military applications like airborne radars for performing the roles of interceptor, fighter, and attack of enemy fighters and of ground targets
• Very small antenna size provides a good performance
• Missile guidance systems of convenient size for applications where mobility and light weight are important and very long range is not a major requirement
• This frequency band is widely used for maritime civil and military navigation radars
• Very small and cheap antennas with a high rotation speed are adequate for a fair maximum range and a good accuracy
• Usually slotted waveguide and small patch antennas are used as radar antenna, under a protective radome
• Also for airborne imaging radars based on Synthetic Aperture Radar (SAR) for military electronic intelligence

K- Band (K- and Ka- Band Radars)

• The higher the frequency, the higher is the atmospheric absorption and attenuation of the waves
• Achievable accuracy and the range resolution rise
• Short range, very high resolution and high data renewing rate

[VBF-12 Gosling]

EW002 - THEORY - INVERSE SQUARE LAW

As a radar signal propagates it spreads out and so as the range increase less power will arrange at the target. This is affected by the inverse square law. Depending on the [purpose of the radar, the shape of the antenna will be designed to focus the signal and thus transmit as much of the generated power towards the target as possible.

The reflected energy from the target also is affected by the inverse square law and so the target may well be designed to absorb and/or spread the power that hits it as much as possible using stealth technologies.



The time taken for the signal to travel out and return is directly related to the range of the target. As such the receiver must be particularly sensitive to very small signals.

[VBF-12 Gosling]

EW003 - THEORY - REFLECTED RADAR ENERGY

Detection of the reflected energy is crucial for the radar system output. With two lots of inverse Square Law working against it the receiver must be really sensitive. It’s likely the target has stealth design to spread the energy that hits it,

Atmospheric conditions will also attentuate the signal depending upon the frequency of the radar.

Finally, it is not just the target that reflects the radar energy; rain, sea surface and defensive weapons like chaff can all confuse the picture.

• Rain - THe droplets reflect the signal causing areas of return in which a real trarget will be hidden
• Sea Surface - Wave also cause radar reflections called clutter. This can be reduced using clutter rejection processing. Also, a low flying aircraft may return a valid signal, but the signal may also reflect on the sea’s surfaces giving a mirror image of the target, also called multipath. This will generate a range ambiguity
• Chaff - By dropping chaff the target can increase the number of targets from which the radar has to identify the real one. Clutter rejection software within the radar system is designed to reject chaff

[VBF-12 Gosling]

EW004 - Theory - Doppler

As the target moves towards (or away) from the receiver, the reflected frequency is compressed (or stretched) proportionally. Measuring this allow the radar system to calculate the closure rate of the target. This principle is used to remove clutter by rejecting low closing or relatively stationary targets.



This can be exploited by turning abeam to the radar receiver and so reduce the closing rate as to be rejected as clutter.

[VBF-12 Gosling]

Radars come in all sorts of shapes.

Curved dishes used to be common but are steadily being replaced by flat pate arrays


Circular - Pencil Beam - Fire Control or Target tracking radar

SHAPE	BEAM SHAPE	GENERAL USE
Bead Stead	Narrow Fan Beam	Search Vertical Fan for accurate azimuth resolution - Rotates - Old
Circular Dish	Pencil Beam	Target Tracking - Usually a Continuous Wave
Horizontal Apple Skin Slice	Narrow Fan Beam - Vertical	Search Radar in Azimuth - Usually rotates
Vertical Apple Skin Slice	Narrow Fan Beam - Horizontal	Altitude Finding - Wobbles up and down
Flat Plate	Electronic Beam Forming	Modern, Can angle the beam electronically, Can be stationary (multiple aerials for coverage) or rotate. Can allocate parts of the beam to track while rest searches (Track While Scan) . Airborne, shipborne (AEGIS) or land based