Continuous Wave Radar system Principles and applications

Continuous Wave Radar Employs continual RADAR transmission Separate transmit and receive antennas Relies on the “DOPPLER SHIFT”

EEE381B Aerospace Systems & Avionics FM-CW radar An unmodulated CW radar is incapable of detecting range, as there is no reference point in the transmitted or returned signal for measuring elapsed time. By frequency modulating the CW signal, differences between the transmitted and received frequencies can be used to estimate range. The further the target, the larger the frequency difference. Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics FM-CW radar theory [4] The modulation parameters are frequency deviation, f, and modulation period, Tm . Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics FM-CW radar theory [4] Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics FM-CW radar theory [4] closing target Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics FM-CW radar theory [4] Given an FM-CW radar with triangular frequency modulation of fm and frequency deviation f, the range of a stationary target can be derived as follows: fb = tr dft/dt, where the round-trip transit time, tr = 2R/c, and the changing transmit frequency, dft/dt = 4fmf. Therefore fb =(8Rfmf/c), or R = cfb/(8fmf) The round-trip transit time, tr = 2R/c, see figure on slide 14. The changing transmit frequency, dft/dt = 4fmf, from the slope of the transmitting waveform = f/(¼Tm) Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics FM-CW radar theory [4] Recall that the range resolution of a radar is a measure of its ability to distinguish closely spaced targets. The range resolution of a FM-CW radar is a function of its modulating bandwidth, and is c/(4f). The range ambiguity is the range beyond which the radar yields ambiguous range results. The range ambiguity of a FM-CW radar is a function of its modulating frequency, and is cTm. This is usually well beyond the signal range. For a FM-CW radar with a modulating frequency of 1 kHz, and a maximum frequency deviation of +/- 0.6 MHz The range resolution is c/(4f) = 3x108 / (4(0.6x106) = 125 m The range ambiguity is cTm = (3x108)(1/1x103) = 300,000 m = 300 km (well beyond the range of the radar!!) Radar - Continuous Wave Radars

FM-CW radar architecture [4] EEE381B Aerospace Systems & Avionics FM-CW radar architecture [4] Radar - Continuous Wave Radars

CW Radar applications [1] EEE381B Aerospace Systems & Avionics CW Radar applications [1] Radar altimeter Terrain-following radar CW illumination Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics Radar altimeter Triangular FM-CW radar is commonly used in aircraft to determine the instantaneous altitude above the terrain it is flying. Radar - Continuous Wave Radars

Terrain-following radar [1] EEE381B Aerospace Systems & Avionics Terrain-following radar [1] “The TFR scans the terrain ahead of the aircraft and receives ground returns that are used for guidance. Normally, a simple box scan is used where the active sweeps are those in the vertical direction (sections 1 and 3). In some circumstances a figure-of-eight scan is used which provides broader lateral coverage than the simple box scan. The TFR therefore builds up a range/elevation picture of the terrain ahead of the aircraft and calculates an imaginary ‘ski-toe’ profile that reaches out ahead of the aircraft. This profile is calculated taking into account such factors as aircraft speed, manoeuvrability, etc., and provides an envelope within which the aircraft will not be able to avoid the terrain ahead. The system is configured so that, whenever the terrain ahead broaches the ski-toe envelope, the aircraft pitches up to rectify the situation. Similarly, if the terrain drops away in front of the aircraft, the aircraft pitches down until just operating outside the profile. The system operates just like the toe of a ski, moving up or down to follow the terrain ahead of the aircraft but always ensuring the aircraft can safely manoeuvre.”[1] Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics CW illumination [1] Used in conjunction with semi-active missiles. The aircraft radar “illuminates” the target, while the missile uses the received return signal to track the target. What are the advantages and disadvantages? The main advantage of this system is that the missile need not be equipped with a transmitter (volume, weight & $). The main disadvantage is that the aircraft radar must remain “locked-on” to the target. Radar - Continuous Wave Radars

Pulse Vs. Continuous Wave radar Pulse Echo Single Antenna Gives Range, usually Alt. as well Susceptible To Jamming Physical Range Determined By PW and PRF. Continuous Wave Requires 2 Antennae Range or Alt. Info High SNR More Difficult to Jam But Easily Deceived Amp can be tuned to look for expected frequencies

EEE381B Aerospace Systems & Avionics exercise Recalling the radar range equation, why is it possible for a CW radar to achieve much greater ranges than a pulsed radar? Can you think of an application in sports where a simple Doppler radar may be employed? The average power of a CW radar is much higher than that of a pulsed radar because of its “continuous” duty cycle. In order to offset, recall that the pulsed radar requires very high peak power levels. Doppler radars are used in sports such as baseball to measure the velocity of pitches. Radar - Continuous Wave Radars

EEE381B Aerospace Systems & Avionics Doppler calculation Just after take-off you realize that you are following a military CC-138 (Twin Otter) in a Cessna 152. Your air speed is 190 km/hour. You estimate that the Twin Otter is at an approximate 15 angle above you. You have a “home-made” 10.6 GHz Doppler radar installed on the Cessna oriented straight ahead. If the beat frequency on your Doppler radar is 1517 Hz, what is speed of the Twin Otter? What range resolution can you get with this crude radar? a. Closing speed is vclosing = c(1- ft / fr ) / (2 cos( )) = 3x108 (1- 10.6/10.599998483 ) / (2 cos(15)) = -22.224 m/s = (-22.224m/s)(3600s/h) = -80,000 m/h = -80 km/h. vCessna = vTwinOtter + vclosing Twin Otter speed, vTwinOtter = vCessna - vclosing = 190km/h + 80 km/h = 270 km/h. b. Trick question – a Doppler radar cannot determine range without some form of modulation scheme. Radar - Continuous Wave Radars

Radar altimeter calculation EEE381B Aerospace Systems & Avionics Radar altimeter calculation An aircraft is equipped with an FM-CW radar altimeter with a modulation frequency of 1.0 kHz and a frequency deviation of 0.60 MHz. Compute the beat frequency as a function of range. If the system has a measured beat frequency of 60 kHz, what is the aircraft altitude? What is the range resolution of the altimeter? What frequency variation in MHz is required to give a range resolution of 10m? a. fb = 8Rfmf/c = 8R(1000 Hz)(600,000 Hz)/(300,000,000 m/s) = 16R Hz /m ; meaning that for every 16 Hz of beat frequency, altitude increases 1 m. b. If fb = 60 kHz, R = (60,000 Hz)/(16 Hz/m) = 3,750 m = 3.75 km. c. Range resolution = c/(4f) = (300,000,000 m/s)/((4)(600,000 Hz)) = 125m. d. f = 300,000,000 m/s / ((4)(10 m)) = 7,500,000 Hz = 7.5MHz. Radar - Continuous Wave Radars