Active Radar Homing Air-to-Air Missile Systems
What is an active radar homing or radar-guided air-to-air missile system?
An active radar homing system consists of a ground radar and a large antenna, an active radar launcher carrying multiple radar-guided missile bodies, and the active-radar missile itself. The ground radar can be fixed or mobile. A fixed radar installed on site is typically called an air-surveillance radar. Radar-guided missiles include an onboard radar transceiver, unlike semi-active radar homing missiles which only include a radar receiver. A transceiver provides both transmitter and receiver functions.
The diagram above shows the components of an active radar homing system. It consists of three parts: a fixed or mobile radar, a missile launcher, and radar-guided missiles. The working principle is as follows:
- The ground radar continuously illuminates the airspace with electromagnetic energy and receives reflections when a target enters its range.
- Once the ground radar detects a distant target, it provides a signal to the missile launcher to release an air-to-air missile toward the target.
- When the radar-guided missile approaches the target, it uses its onboard radar transceiver to fine-tune range and guidance to intercept the target.
- To be effective, the missile detonates near the target, destroying nearby objects by blast and fragmentation, or it directly impacts the target.
Advantages of Active Radar Homing
- More accurate than ground-radar-guided surface-to-air missiles due to onboard sensing.
- Onboard radar transceiver enables closer engagement and higher kill probability.
- After launch, the missile is responsible for terminal guidance, freeing the launch platform to track other targets.
Disadvantages of Active Radar Homing
- More expensive than semi-active radar homing systems because of the onboard transceiver.
- Limited effective radiated power and coverage due to onboard power constraints.
- Without support from ground or airborne radar networks, long-range hits may be limited.
Differences Between Semi-active, Passive, and Active Radar Homing Missiles
Radar homing guidance is the most common guidance form for air-defense missiles. Based on operation, there are three radar-homing types: semi-active, active, and passive.
Semi-active Radar Homing (SARH)
The diagram above illustrates normal operation of semi-active radar homing. Key characteristics:
- Uses only a receiver; the target is illuminated by radar or another external source.
- Reflected energy from the target is received by a receiver mounted on the missile.
- A computer connected to the receiver determines the target relative trajectory; the missile uses this information to intercept accurately.
- Used for long-range air-to-air and surface-to-air missile systems.
- Used in all-weather air-defense guidance systems.
Example: first-generation surface-to-air missiles used by RSAF.
Active Radar Homing (ARH)
The diagram above shows the normal operation of active radar homing. Key features:
- The missile transmits energy and receives reflections. Unlike semi-active systems, this is performed by the missile itself since it houses both transmitter and receiver components.
- The homing system does not require an external illumination source.
Examples: AMRAAM air-to-air missile, Harpoon anti-ship missile.
The AIM-120 AMRAAM combines active and semi-active modes and supports roughly 50 km medium-range engagements.
Passive Homing
The diagram above shows passive homing operation. Key features:
- Uses heat radiation or other emissions from the target; the missile uses that energy to determine target parameters.
- Operates independently of external guidance systems.
- Only receives signals and does not transmit, similar to semi-active systems in their receive-only aspect.
Examples: Mistral deployed by RSAF is an infrared passive homing system; AIM-9L/M uses a passive infrared seeker head. Passive seekers are harder to detect and easier to break lock compared with semi-active and active seekers.
AESA vs PESA Radars
A comparison of AESA and PESA radars. AESA stands for active electronically scanned array, and PESA stands for passive electronically scanned array.
PESA Radar
PESA radars use a common shared RF source where the signal is modified by digitally controlled phase shifter modules.
- Uses a single transmitter/receiver module that feeds multiple antenna elements.
- Generates electronically steerable beams in different directions.
- Antenna elements interface with the single transmitter/receiver; unlike AESA, PESA does not have separate TRx modules at each element.
- Because a single frequency is used, PESA systems are more susceptible to enemy RF jamming.
- Scan speed is slower and typically can track or process fewer targets simultaneously.
AESA Radar
AESA uses many individual transmit/receive (TRx) modules for electronic beam steering without mechanically moving the antenna. It is generally regarded as an advanced version of PESA.
- Uses many transmit/receive modules, one per antenna element.
- Multiple TRx modules interface with multiple antenna elements to form an array.
- AESA can generate multiple beams at different RF frequencies simultaneously.
- Because it can operate across a wide frequency range, AESA is less likely to be disrupted by enemy RF jamming.
- Fast scan rates allow tracking of multiple targets and multitasking.
Both modern PESA and AESA share similarities:
- Both are typically pulsed radars.
- Both support frequency agility and frequency hopping over different times and frequencies.
- Both can operate in narrowband or wideband modes.
- Both can be used for ECM, passive scanning, beamforming, etc.
Monostatic vs Bistatic Radars
Monostatic Radar
The monostatic radar block diagram uses the same antenna for transmit and receive. Because a single antenna is used for both directions, a duplexer is required to separate transmit and receive chains.
The monostatic radar equation is:
PR = (pt * G^2 * λ^2 * σM) / ((4 * π)^3 * d^4 * Lt * Lr * Lm)
Where:
- PR = total power received by the receiving antenna
- G = antenna gain
- λ = wavelength = c / frequency, where c = 3 x 10^8 m/s
- pt = peak transmit power
- d = distance between radar and target
- Lt = transmitter loss
- Lr = receiver loss
- Lm = medium loss
- σM = radar cross section of the target
Bistatic Radar
The bistatic radar block diagram uses two separate antennas as transmitter and receiver located at different positions.
Example: A CW radar can operate as bistatic or monostatic when the distance between antennas is very small.
Where:
- PR = total power received by the receiving antenna
- Gt = transmit antenna gain
- Gr = receive antenna gain
- λ = wavelength = c / frequency, where c = 3 x 10^8 m/s
- pt = peak transmit power
- dt = distance between the object and the radar transmit antenna
- dr = distance between the object and the radar receive antenna
- Lt = transmitter loss
- Lr = receiver loss
- Lm = medium loss
- σB = (4 * π * Ae^2) / λ^2, where Ae is the object projection area
Doppler Radar
Doppler radar uses continuous wave transmission and is therefore also called CW radar. The principle is that the frequency of the returned signal from a fixed target is the same as the transmitted wave, while the returned signal from a moving target is shifted in frequency according to the Doppler frequency. By measuring the difference between transmit and received frequencies, the radar extracts the target relative velocity.
Because CW transmission lacks precise transmit-receive time stamps, basic Doppler radar does not provide range measurements. This concept is used in traffic police radar to measure vehicle speed and in variometers and aircraft speed sensors.
As shown above, for a stationary target the number of transmitted waves equals the number of received reflected waves. For a moving target the number changes depending on whether the target is moving toward or away from the radar.
Assume a target is moving toward the radar. The distance decreases at a rate determined by the target speed, producing a frequency shift in the returned signal called the Doppler shift. When transmit frequency is in the GHz band, the Doppler shift is typically on the order of 1 kHz. Based on this frequency change, the radar determines the target speed and movement direction. The Doppler radar equation for frequency shift is:
fd = 2 * f0 / c * dR/dt = 2 * (dR/dt) / λ0 = 2 * V * cosθ / λ0
Where v is the relative velocity of the target along the position vector R and θ is the angle between velocity vector and the radar line-of-sight.
When the target moves directly toward the radar, θ is 0 and fd is positive; when moving away, θ is 180 degrees and fd is negative; when movement is perpendicular, fd is zero.
In Doppler radar, target radial velocity is determined by measuring Doppler frequency shift and its sign.
Traffic Police Doppler Radar
The block diagram above shows modules used in traffic police radar to detect and measure vehicle speed. A microwave signal generator serves as the transmitter. A Gunn-diode-based oscillator generates the microwave signal. The microwave signal is transmitted through an RF circulator and horn antenna. A portion of the transmitted signal is leaked and used as a reference for comparison with the reflected signal. An RF mixer produces sum and difference frequencies of these two inputs. The sum component is ignored and the difference component, caused by Doppler shift, is used. The processed Doppler frequency is displayed on an oscilloscope or display unit, which provides the vehicle speed.
The following formula can be used to estimate vehicle speed for police radar:
Speed (mph) = 0.26 * (Doppler shift in Hz) / (microwave frequency in GHz)
FMCW Radar Systems
FMCW radar is a frequency-modulated continuous-wave radar. The carrier frequency f0 is modulated by a waveform fm(t), so the transmitted frequency can be expressed as:
f_t = f0 + fm(t)
Since Doppler CW radar does not provide time stamps for transmit and receive, it cannot measure range directly. FMCW overcomes this by using frequency variation over time to derive range. The FMCW waveform and beat frequency concept allow range measurement.
Range for an FMCW radar can be given by:
Range = c * fd / (2 * a)
where a = fd / Te
FMCW radars typically operate at lower power than pulsed radars and are used for very short-range targets.
Ground-Penetrating Radar (GPR)
Ground-penetrating radar (GPR) is a radar developed to analyze subsurface structure. It uses radio waves in the frequency range from about 1 to 1000 MHz to map subsurface features.
The typical GPR system includes a transmitter and receiver section. The transmitter generates a source signal, performs modulation and RF upconversion, and feeds the signal to an antenna for transmission into the ground. The receiver performs signal sampling, digitization, data storage, signal processing, and display.
Working principle:
- The transmitter emits RF signals into the solid ground.
- Echoes are detected and recorded at different time instances to construct an image of the subsurface.
- The display provides image information based on time delay and signal strength.
Applications
- Measuring ice and snow thickness in polar regions
- Locating buried utilities
- Assessing mine workings
- Forensic investigations
- Archaeological excavations
- Searching for buried landmines
- Avalanche prediction
