Advanced Radar Systems

Objectives Describe how a Pulsed Doppler radar works and how it’s able to determine target velocity. Describe how a Moving Target Indicator (MTI) determines target velocity. Describe how a pulse compression can provide a high-resolution radar return at the receiver.

Velocity Determination for Pulse Radars Basic Pulse Radar system cannot determine velocity directly from a single pulse. However, radar modifications enable us to determine movement

Advanced Radar Systems Combine the ranging feature of Pulse radars with the range rate feature of CW radars Range, bearing, elevation to target Detects target’s velocity Uses radial velocity to eliminate stationary returns (clutter) or compute their velocity and future location Two types: Moving Target Indicator (MTI) and Pulse Doppler (PD) MTI uses phase measurements to ID moving targets PD uses frequency measurements to ID moving targets and compute velocity in LOS

Moving Target Indicator (MTI) Compare transmitted phase to return phase to detect motion (i.e., phase shift) uses phase comparator One full cycle of phase shift (360 degs) equals a change in range of ½ l pulse-to-pulse comparisons; resulting phase measurements are averaged to discriminate between moving targets and stationary ones cancellation circuit eliminates non-zero phase average (Clutter Rejection) Bearing, Range, and Motion Primarily used in Search Radar systems a)       MOVING TARGET INDICATOR (MTI)   i)         measures changes in the phase of the returned signal to determine target motion ii)       samples from transmitter pulse and return signal fed to a Phase Comparator iii)      returns that are in phase = largest positive value iv)     returns that are out of phase = largest negative value

Δ phase Pulse 1 time Pulse 2 Pulse 3 Pulse 4 Pulse 5 Next Pulse Avg Phase Difference Subtracted Displayed Signal Only moving Displayed

Moving Target Indicator (MTI) Delay line circuit saves previous phase evaluation for comparison. Cancellation circuit subtracts previous phase comparison from current phase comparison. Return from Stationary targets will have same phase comparison and be cancelled out. Return from Moving targets will have different phase comparison and will be retained / displayed. v)       also uses Delay line and cancellation circuit to eliminate stationary targets from display; the phase comparator output is delayed and subtracted from the next pulse, thus, stationary targets are removed (since phase difference of returning signal is the same for all pulses) vi)     commonly used in Search Radar characterized by low PRF and DC < 10%

MTI

Pulse Doppler Attributes of pulse radar / technology of CW radar. Why is a “mixer” added to Pulse Radar? Sample of transmitted and received signal compared at mixer. Mixer output is Doppler shift (velocity). Doppler sorted into velocity categories. In some applications velocities are coded by color. Standard Weather Radar. More rain / higher wind – higher Doppler.

Hurricane Isabel- Photo

Hurricane Isabel – Pulse Doppler

Pulse Doppler Radar Principle military use as Fire Control Radar High PRF Many pulses / high frequency gives large amount of range and range rate data High degree of accuracy (range, bearing, elevation, velocity) Target position now includes velocity Large receiver Bandwidth (BW) to capture larger fD    VELOCITY DETERMINATION FOR PULSE RADARS   a)       PULSE DOPPLER           common to color code signal return information displayed on PPI v)       standard weather radar; also used in Fire Control/Weapons systems vi)     only one antenna (like Pulsed) vii)    characterized by high PRF and DC > 10% viii)  Having a high PRF allows for several samples to be taken which allows for several range and range rate computations to be made which give you the high degree of accuracy.

Pulse Doppler System ft Transmitter Mixer fr Receiver Df Antenna Display Doppler Filter Mixer ft fr Df Antenna    a “mixer” is added to basic Pulse Radar System ii)       sample of transmitted signal and received signal compared at mixer iii)      output of mixer is a Doppler (frequency) shift, which is passed to a Bandpass Filter, which sorts it into categories (distinguishes targets by velocity characteristics / accurate to 3 m/s).

High Resolution Radar Pulse Compression Synthetic Aperture Radar (SAR) Inverse Synthetic Aperture Radar (ISAR)

Pulse Compression Signal Processing Technique. Same idea as FMCW Except in a pulse Combines: High energy of Long Pulse. High resolution of Short Pulse. Pulse is Frequency Modulated. Increased in frequency over duration of pulse. 1)       HIGH RESOLUTION RADAR   a)       Pulse Compression i)         Pulse Compression (revisited) ii)       combines high energy of long pulse with high resolution of short pulse iii)      pulse is frequency modulated (transmitted pulse increased in frequency over duration of pulse width) iv)     since each part of pulse has a unique frequency, multiple returns can be completely separated v)       the ability of a receiver to improve the range resolution over that of a conventional system is called the Pulse Compression Ratio (PCR)

Distance between Leading Edge Remains Same Pulse Compression Distance between Leading Edge Remains Same Return pulse passed thru Pulse Compression Circuit. Lower frequencies pass through slower. Pulse “piles up” on itself. Can distinguish multiple returns within PW Filter Output is a signal with: greater power for longer range. narrower pulse width

Pulse Compression Transmitted Pulse Received Pulse

Pulse Compression Filtering

Objectives Describe how a Pulsed Doppler radar works and how it’s able to determine target velocity. Describe how a Moving Target Indicator (MTI) determines target velocity. Describe how a pulse compression can provide a high-resolution radar return at the receiver.

Assignment Reading Assignment pp 4-16 to 4-18. Do Guided Reading Problem Set 2