5. Doppler Phenomenology Flashcards
Why do we prefer to use higher frequencies to measure Doppler shift?
Dopper shift is larger at higher frequencies. Higher Doppler shift is easier to measure with more precision.
How does the direction of the target wrt the radar affect Doppler shift?
The larger the velocity component which is in the LOS of the radar, the larger the Doppler shift.
Why does the Continuous Wave Signal result in the Power Spectral graph shown?
With a continuous wave over an infinite amount of time, all the power is focused into a single frequency and its reciprocal when.
Explain the Single Rectangular Pulse Spectrum
A square wave can be represented by a spectrum of frequencies, not just a single frequency. This shows the overarching shape of the frequency distribution of the square wave. Power is distributed within this distribution.
Explain the Infinite Pulse Train Spectrum
Over an infinite amount of square waves (pulses), the power in the square waves are ideal and confined to some discrete set of frequencies which are distributed as such.
Explain the Finite Pulse Train Spectrum
With a finite set of pulses (dwell), the power distribution accross the frequencies which make up the square waves, is not ideal, resulting in a main lobe and smaller ajacent lobes at each component frequency.
Explain the Modulated Finite Pulse Train Spectrum
Modulating each pulse lets you change the power distribution.???
Explain the following graph. How does this tie into Doppler Resolution?
We can compare the spectral analysis of stationary clutter to the moving target by measuring the shift between the main lobes in frequency spectrum.
The Doppler resolution is how well we can distinguish the shift in the lobes, which is related to PRF. The higher the PRF, the thinner the lobes (closer to ideal straight lines) and easier they are to distinguish.
Coherent vs. Noncoherent (more advanced definition)
Non-coherent: Detect amplitude and time delay
Coherent: Can also measure phase changes using two channels I and Q, which allow amplitude and phase to be independently measured.
Expand on the following image to explain how Pulse data is stored.
The information for a single CPI at a single bearing is stored in the vector above.
Multiple CPIs are sent at a single bearing and stored as well, adding a dimension.
When there are multiple receivers collecting the returns, that information is stored in another dimension, making it a cube.
There is a Data Matrix for every bearing. The data matrix is compared with previous matrices to detect changes.
Draw the corresponding frequency domain response
Explain I/Q channels
I order to achieve coherent detection, the received signal must be split into two channels (I/Q) to allow amplitude and phase to be independently measured.
They must have a reference sinusoid so pulses are sent out at the same phase so they can be compared to incoming phases.
Those are range reslution bins.
Multiple pulses are sent for every bearing. The entire row is the CPI.
What is MTI
No FFT is performed.
For every row of range bins, accross the CPIs, it measures frequency shifts to determine IF anything passes the threshold. To do so it must have a filter which removes the zero-doppler shifts.
Describe Blind Speed (Better)
the frequency response of the MTI filters are periodic in multiples of the PRF. Anything at multiples of the PRF will show up as a zero-doppler return and be filtered out.
The reason is because of aliasing.
How do you compensate for blind speeds?
Staggered PRF.
What is Pulse Doppler?
Uses a FFT, which allows you to sample the frequency return, which allows you to separate the frequency measurements into Doppler bins. Now you can determine WHICH doppler responses that pass threshold are which.
So now you can also track the clutter because you aren’t just removing all the zero-doppler returns, you are still tracking them. You can also track the blind speeds.
The downside is you need a high PRF or a long dwell time.
What is MTD
Moving Target Detector.
If you are using Pulse-Doppler, you can now keep track of the zero-doppler returns and blind speeds so you can create a clutter map and determine if it is moving or rather, if there is something within the clutter which is moving over time.
