Definitions Flashcards
What is primary radar?
A radar system with a reflected return
What is a secondary Radar system?
A radar system with a transponded return
What is a Monostatic radar system?
A radar system where just one antenna is transmitting and recieving
What is a Bistatic Radar system?
A radar system where multiple antennas are transmitting and recieiving
What is a Tracking Radar system?
A radar system that is following our object we are tracking.
What is a scanning Radar system?
A radar system that is not tracking a single target rather scanning a region of space.
What is an Antenna?
A mechanically steered parabolic reflector that focuses energy into a narrow beam.
What is a Duplexer?
The role of the duplexer is to allow a single antenna to be used for both transmitting and receiving signals.
What is a Transmitter
A transmitter is either a power amplifier or a waveform generator. A waveform generator produces the radar signal at low power.
What is the Receiver in a Radar system?
The receiver is almost always a superheterodyne.
What is a Mixer and local oscillator used for?
To convert the RF signal to an intermediate frequency.
Why do we use an IF amplifier and matched filter?
To detect weak signals and attenuate unwanted signals.
Why do we use a demodulator?
A demodulator assists in extracting the signal. The IF amplifier, demodulator and video amplifier act as an envelope detector.
What is threshold detection?
Threshold detection determines whether a target is present. Uses probability theory to determine the threshold value to give constant false alarm rate.
Why do we use superheterodyne reciever?
In the reciever channel, each echo has to be amplified, filtered and demodulated. All of these processes are frequency-dependent. To avoid creating banks of filters tuned to all possible carrier wave frequencies, a super heterodyne reciever is used.
The radar equation is a function of two parts, what are they?
Deterministic part which is composed of parameters under the designer’s control.
Probabilistic part in which the parameters are outwith the designers influence.
What is an isotropic antenna?
one which radiates uniformly in all directions
What is antenna gain?
Gain is a function of angle in azimuth and elevation. The direction of maximum gain is known as the antenna boresight
What can be said about the Radar Cross Section?
It is highly variable and difficult to predict without exhaustive experimentation.
What are the two loss factors?
- Atmospheric propagation losses Lp
- System losses Ls
What is a false alarm?
Occurs when noise causes the measured voltage to exceed the detection threshold but no target is present.
What is a missed detection?
Occurs when the target signature falls below the detection threshold.
What should the threshold of false alarms be set to?
It is desirable to set the threshold to minimise false alarms but maximise target detection.
What is the simplest ways to improve the SNR?
Add the target echo returns from several pulses together. This is known as pulse integration.
When can integration be performed?
pre-detection - coherent
post - detection - non coherent
What does integration improve
the probability of detection Pd, and reduces the probability of false alarm P(FA). This is because it reduces the noise varience.
What are the consequences of radar backscatter amplitude?
- Different scattering centres
- Small aspect angle changes or frequency diversity
What is swerling 1?
similair amplitudes with slow fluctuation.
What is swerling 2?
Similar amplitudes with fast fluctuations.
What is swerling 3?
One scatterer much larger than others with slow fluctuation.
What is swerling 4?
One scatterer much larger than others with fast fluctuations.
How do we eliminate ambiguous returns?
By using PRF jittering
How do we resolve ambiguities?
Using Multiple PRF’s
What is clutter?
Noise level of radar returned from the natural environment.
How do we achieve a coherent reference?
By multiplying the recieved signal with the transmitted waveform
What is sweep?
What occurs in the time between two transmitted pulses.
Why do we use a delay-line canceller?
To achieve the subtraction of the echoes from two successive sweeps.
Why do we use a time domain filter?
To reject stationary clutter at zero frequency.
What kind of spectrum does a pulse train have?
a line spectrum
What happens when the PRF increases?
The line-to-line separation increases - more usable doppler space.
What are typical filter designs
- transversal or FIR filter
- recursive IIR filters
What do we use to reduce occurrances of phase error appearing as possible targets?
Using a stalo or stable local oscillator. A stalo is mixed with a coherent reference to produce the transmitted signal.
What is Pulse-Doppler Radar?
Pulse-doppler radar uses a higher PRF to measure the radial velocity of the target umambigously as the first blind speed is significantly increased.
What is a high-prf pulse doppler radar?
is one with no blind speeds within the Doppler space.
How can range ambiguities be alleviated?
By using multiple PRFs
What are the main differences between MTI and Pulse Doppler Radar?
MTI has no range ambiguities but multiple doppler ambiguities. High-PRF Pulse Doppler has no doppler ambiguities but multiple range ambiguities.
What is a matched filter?
A matched filter yields the highest signal-to-noise ratio as its output.
Why is it called a matched filter?
Because its impulse response is a ‘flipped-in-place’ version of the original signal.
How can we achieve a detectable echo return for long range operation?
- Either increase the peak power or
- Increase the pulse duration
however peak power is limited by the voltage breakdown characteristics of the radar electronics.
=> use pulse compression
How can we improve range resolution?
By increasing the bandwidth of the pulse
What us linear frequency modulation (LFM) chirp signal?
A linear increase of frequency with time.
How is LFM chirp modulation commonly decoded by?
A technique called Stretch processing or deramping.
What is the Swath?
The footprint made by the radar beam intersecting with the ground plane.
How can we discern the changes in frequency in the stretch processing of LFM chirp?
although their pulse echos almost completely overlap, the slight stagger in their arrival times results in clearly discernible changes in frequency.
What criteria must be met for the matched filter to resolve two closely-spaced echoes?
the instantaneous difference in their delay-shifted frequencies must meet or exceed 1/𝜏.
What constitutes an electro-optic system?
A remote sensing equipment operating in the infra-red, visible or UV frequency bands that have the following four main components,
- Sensor
- Optics
- Sightline pointing and stabilisation
- Electronics.
What are the pros and cons of an electro-optic system?
Pros
- Passive technology - much stealthier
- much shorter wavelength significantly improves the diffraction-limited resolution
- angular resolution
cons
- limited range
- not all weather
A definition?
Derivation of performance of a device usually taking into account external factors and handling of output
Why do we use radiometric concepts
For a quantitative understanding of flux through an electro optical system.
What is throughput?
Through put or flux gathering capability is the AΩ product
What is a lambertian radiator?
One where the radiance L is independent of view angle
How does the flux fall off in a lambertian radiator?
The flux falls off proportional to cos θs as the projected source area reduces with increased viewing angle.
What are the three mechanisms that atmosphere attenuates optical radiation?
- Absorption
- Scattering
- Refraction
What is absorption?
This is the process whereby energy contained in the photons is devoured by gas molecules and aerosols.
What are transmission windows
Spectral bands where attenuation is minimal
What is scattering?
The prices whereby photons are restricted along a different progression path following collision with gas molecules and aerosols.
What is refraction?
Radiation is refracted, in the same way as light through a lens, due to the refractive index of the atmosphere.
What is the simplest type of target model?
A black body source.
What does Kirchoff’s law relate?
The absorption and emission properties for a body in thermal equilibrium.
What is emissivity?
Hemispherical spectral emissivity is defined as the ratio of the radiant exitance of a real body too that of an ideal body.
What is a black body?
Any object that emits the theoretically maximum amount of infrared energy at any given temperature value is called a black body.
What is the sensor cut off wavelength?
The longest wavelength to which the sensor transducer is sensitive.
What is the optical train
The functions of the optical system are to gather radiative flux from a scene
What is the instantaneous field of view?
The instantaneous field of view of an optical system is the angular coverage
What is the point spread function?
The spreading of the flux
What is the modulation transfer function?
Is a measure of the effectiveness of the optical system for specific spatial frequencies.
What happens when the spatial frequency increases?
Objects become blurred.
What does the MTF tell us?
how the image is degraded.
How is the MTF related to the PSF?
the MTF is the Fourier transform of the PSF
Optical sensing is based on what two principles?
- Thermal effects of radiation
- Quantum effects of radiation
What is the photo electric effect?
Photons interact with the atoms, generating a measurable release of electrons via the photo electric effect.
What process is defined by the spectral responsivity?
A detector is essentially a transducer that converts electromagnetic flux into measurable electrical signals.
What is the cut off wavelength?
The wavelength that contains just enough energy to cause an electron to cross the band gap.
What is the noise equivalent bandwidth?
The width of a flat band pass filter that will pass the same amount of white noise power as the original transfer function.
What are some of the noise sources?
- Photon noise
- Johnson noise
- Shot noise
- Dark current
- Fixed pattern noise
What is the noise equivalent power?
The amount of flux that would produce an output equal to the RMS value of the noise, assuming linear responsivity.
What is the specific detectivity?
A figure of merit used to specify the performance of a detector.
How is specific detectivity related to NEP?
it is proportional to the NEP, so the bigger the specific detectivity corresponds to better sensitivity.
What is Johnson’s criteria?
- detection
- recognition
- identification
What are the two fundamental functions for LOS stabilisation/tracking systems?
- Dynamic disturbance isolation
- Pointing
What are the two performance criteria
- Stabilisation performance (or jitter)
- Tracking performance
What are the requirements of automatic tracker?
- Sensor sensitivity
- Resolution
- Frame-to-frame jitter
What is passive isolation?
A spring and damper system used in conjugation with active systems.
What is electronic stabilisation?
An electronic shift of display to keep stable images. However it has several draw backs including limited angle of travel and limitation on vibration frequencies.
What are the different methods of sightline stabilisation?
- Strap down stabilisation
- Direct gyro stabilisation
- Gyro-on-gimbal
What is the target tracking problem?
It’s essentially one of estimation
Why do we need to estimate?
Because of noise
What is the operation of a modern automatic detection and tracking system?
- Quantise in range and angle to create discrete cells
- Apply dynamic thresholding to obtain a constant false alarm rate
- Integrate the pulse returns from each cell
- Declare detection it any m out of n pulses exceed the threshold
What are the two main approaches to detecting a target within an image?
Correlation and centroiding
When do we use correlation vs centroiding?
For extended target tracking use a correlation approach. Point targets are usually characterised as hot spots so can use centroiding.
What is the track initiation phase?
The one that effectively bridges the gap between hardware and software.
What is the function of the initiation segment?
Apply some logical heuristic to the measurements delivered by the sensor and if it is true it will initiate a new reach entity.
What is the objective of track association?
To match a new observation to an existing track. This process uses track gates extensively.
What is track filtering
The observations for each track must be filtered because radar resolutions and noise levels are finite.
What are the issues with track filtering
Long revisit times
Inaccurate position estimation
Sensor inaccuracies
What is the alpha - beta filter?
A sub-optimal filter closely related to the Kalyan filter
How do we select the value of alpha?
alpha -> 1 = rapid response to manoeuvring targets = wide bandwidth
alpha -> 0 = good smoothing of errors = barrier bandwidth
What is a more accurate soothing/prediction model?
Kalman filter
Why is the Kalman filter better?
Because it incorporates models of both the target dynamics and the errors in the model and measurements.
When does the Kalman filter reduce to an alpha - beta filter?
For Gaussian white noise and a constant velocity trajectory.
What is automatic detection and track?
An ADT radar is used almost exclusively in civilian air traffic control systems. It can track many targets simultaneously. Hence much slower revisit time.
What is track while scan?
Can track multiple targets but can rapidly scan a relatively small sector of space.
What is single target tracker?
It is designed to maintain the radar boresight on a single target at all runes using a relativity high data rate.
How do we know which direction the antenna should be moved to maintain track?
Use two overlapping antenna patterns each offset from the bore sight by an equal known amount.
What is monopulse tracking radar?
It sends offset pulses simultaneously and operates by having the feed horns offset from the antenna focal axis.
What are some demand characteristics for system agility?
- Step position demand
- Rate demand
- Acceleration demand
What are the subsystems of an electro optic system
Sensor - visible light transducer - IR camera
Optics - refractive or reflective optical train
Sight line pointing & stabilisation - gimballed servomechanism
Electronics - sight line control, image processing, tracking.
What is the function of each electro optic system component
Sensor - convert photons into an electrical signal
Optics - relay the light from the objective lens onto the sensor
Sight line control - stabilises the sight line against own ship motion and tracks the target
Electronics - performs all calculations needed by the EO system
What are the 4 types of electro optic systems used in aerospace equipment?
- Forward looking infra red (FLIR)
- Targeting and laser designation
- IR countermeasures
- Laser/missile warner
What are the four main functions of a target tracking system?
- Detection
- Initiation
- Association
- Filtering
