1. Introduction a Flashcards
List the 9 Radar Performance Characteristics
Peak Power / Average Power
Frequency
PRF / PRI
Pulse Width / Bandwidth
Dwell Time
Beam Width
Sensitivity
Range
Electronic Protective Measures
What are the 3 basic functions of the majority of radars?
Search / Detect
Track
Image
Describe Search / Detect function of a radar
Radar searches a volume in a scan pattern
Describe the Track function of a radar
Targets are tracked, measurements are combined and smoothed to estimate the current position.
Describe the Imaging function of a radar
Obtaining detailed information on targets for surveillance and terrain mapping.
Monostatic vs. Bistatic
Bistatic: Separate antennas for transmission and reception.
Monostatic: One antena for both.
Describe Continuous Wave radar
Transmitter continually transmits a signal. Range is determined through frequency modulation which puts a timing mark on the EM wave.
Described Pulsed Waveform radar.
Pulse Width
Pulse Repitition Interval (PRI)
Short pulses sent out.
What is Duty Cycle?
The fraction of time the transmitter is transmitting during one PRI.
What is Average Power?
Non-coherent vs. Coherent
Non-coherent: Detect only amplitude.
Coherent: Detect both amplitude and phase. This allows target motion characteristics and can differentiate the target from clutter.
Often an oscillator is used to compare the phase of the received signal to a known reference.
Unambiguous Range
If you send out two signals before the first signal has time to return, you now have two signals out at once. When you get the next return, you cannot be certain which pulse it was.
When the travel time (delta T) is greater than the IPP (interpulse period), there is an ambiguity in the range.
Therefore, for a given range, there is a maximum PRF which allows umambiguous range determination.
Unambiguous Doppler Measurement
The Doppler Shift is measured by sampling at the pulse repetition frequency.
By Nyquist Theorem, the sampling rate must be 2x the target frequency.
So the maximum Doppler shift which can be unambiguously measured is half the PRF.
As PRF increases, so does the maximum unambiguous Doppler shift.
What are the 3 PRF regimes and why are they used?
High PRF gives low umabiguous range but high unambiguous Doppler shift. You cannot have both, you must balance.
What are the advantages and disadvantages of a Phased Array
Reliable, high bandwidth, no moving parts
Expensive
What is the Array Factor and Gain Loss
Array Factor is the sum of the voltage response at all elements.
When signals are received at an angle to the array, they will have different phases, decreasing the Array Factor when they combine.
This is just to say that the directivity of the array increases when it is normal to the incident wave. It performs best when it is pointing directly at an incoming wave.
Higher scan angles result in Gain Loss. This is practically limited to 60 degrees.
How do Phase Shifters work?
Phase Shifters are placed behind each radiating element, allowing the wavefront to be steered by constructive and destructive interference.
What are Grating Lobes? Why are they important?
Undesirable maxima in the Array Factor occuring at angles other than 0 degrees, due to constructive interference occuring when the wave travels exactly 1 wavelength between elements.
This leads to angular ambiguities, where the radar may report that a target is in the main lobe when it is really in a grating lobe or vice versa.
We prevent Grating Lobes by spacing elements closer than 0.5 wavelength together so that there are fewer possible frequencies with wavelengths which will be able to fit between elements.
