WEBVTT NOTE This file was exported by MacCaption version 7.0.06 to comply with the WebVTT specification dated March 27, 2017. 00:00:00.000 --> 00:00:04.922 align:center line:-1 position:50% size:46% Hello, and welcome to Part 3 of our Radar Back-to-Basics class. 00:00:04.922 --> 00:00:11.887 align:center line:-1 position:50% size:50% In this section, we'll be talking about pulse compression and FMCW radar. 00:00:11.887 --> 00:00:18.852 align:center line:-1 position:50% size:64% Primary radars suffer significant signal losses from the transmitted pulse to the received echo. 00:00:18.852 --> 00:00:22.940 align:center line:-1 position:50% size:61% Transmitted signal must bounce off and travel back from the target to the receiver 00:00:22.940 --> 00:00:26.109 align:center line:-1 position:50% size:34% without any amplification. 00:00:26.109 --> 00:00:30.906 align:center line:-1 position:50% size:63% One way to overcome these large signal losses is to transmit longer pulses 00:00:30.906 --> 00:00:35.494 align:center line:-1 position:50% size:49% and integrate that larger total energy in the received echo. 00:00:35.494 --> 00:00:39.414 align:center line:-1 position:50% size:55% A longer pulse width thus provides longer operating range for a given antenna 00:00:39.414 --> 00:00:42.709 align:center line:-1 position:50% size:39% and transmit power amplifier. 00:00:42.709 --> 00:00:47.422 align:center line:-1 position:50% size:58% Radar range resolution is also an important characteristic related to pulse width. 00:00:47.422 --> 00:00:49.967 align:center line:-1 position:50% size:46% The ability to resolve small objects allows a radar 00:00:49.967 --> 00:00:53.428 align:center line:-1 position:50% size:63% to provide a more detailed picture of the target. 00:00:53.428 --> 00:00:56.807 align:center line:-1 position:50% size:42% A radar that can resolve details say down to 1 m 00:00:56.807 --> 00:01:00.936 align:center line:-1 position:50% size:45% will provide a lot more information about the approaching targets. 00:01:00.936 --> 00:01:04.356 align:center line:-1 position:50% size:46% A resolution of 100 m might render one large target 00:01:04.356 --> 00:01:08.944 align:center line:-1 position:50% size:58% indistinguishable from several smaller ones in a close formation. 00:01:08.944 --> 00:01:10.904 align:center line:-1 position:50% size:41% If a radar's pulse width is long, 00:01:10.904 --> 00:01:16.576 align:center line:-1 position:50% size:63% echoes from adjacent targets can bounce back together, overlapping in time. 00:01:16.576 --> 00:01:22.249 align:center line:-1 position:50% size:60% To the radar, this appears as one large target instead of adjacent smaller targets. 00:01:22.249 --> 00:01:28.005 align:center line:-1 position:50% size:57% Therefore, to get the best radar resolution, a narrower pulse width is desirable. 00:01:28.005 --> 00:01:32.718 align:center line:-1 position:50% size:54% You can see that optimal range and resolution involve conflicting criteria. 00:01:32.718 --> 00:01:38.348 align:center line:-1 position:50% size:61% Best range implies a long pulse, whereas best resolution implies a short pulse. 00:01:38.348 --> 00:01:40.726 align:center line:-1 position:50% size:45% How do we address this tradeoff? 00:01:40.726 --> 00:01:44.021 align:center line:-1 position:50% size:60% We use something called pulse compression which we'll talk about next. 00:01:47.399 --> 00:01:51.486 align:center line:-1 position:50% size:53% To solve the range versus resolution optimization problem, 00:01:51.486 --> 00:01:55.032 align:center line:-1 position:50% size:49% many radars use pulse compression or modulation. 00:01:55.032 --> 00:01:57.743 align:center line:-1 position:50% size:55% Very often, this is a linear frequency chirp, 00:01:57.743 --> 00:02:03.040 align:center line:-1 position:50% size:52% and in concept it's a simple modulation to create and decompress. 00:02:03.040 --> 00:02:06.543 align:center line:-1 position:50% size:51% Frequency modulating the radar pulse with a linear voltage ramp 00:02:06.543 --> 00:02:09.463 align:center line:-1 position:50% size:43% creates a frequency chirp pulse. 00:02:09.463 --> 00:02:14.092 align:center line:-1 position:50% size:62% The chirp pulse is then transmitted as an uncompressed pulse would normally be. 00:02:14.092 --> 00:02:16.053 align:center line:-1 position:50% size:46% This gives the best of both worlds. 00:02:16.053 --> 00:02:19.181 align:center line:-1 position:50% size:49% Long pulse widths for easy detection by the receiver, 00:02:19.181 --> 00:02:21.350 align:center line:-1 position:50% size:45% due to the longer integration time, 00:02:21.350 --> 00:02:25.437 align:center line:-1 position:50% size:59% and higher total energy as well as the ability to resolve close-together targets 00:02:25.437 --> 00:02:29.149 align:center line:-1 position:50% size:42% and get better range resolution. 00:02:29.149 --> 00:02:30.942 align:center line:-1 position:50% size:35% Another important feature of many radars 00:02:30.942 --> 00:02:35.447 align:center line:-1 position:50% size:50% is the ability to measure Doppler shift from moving targets. 00:02:35.447 --> 00:02:40.202 align:center line:-1 position:50% size:59% Measuring the change in frequency of the RF carrier or the phase shift with time 00:02:40.202 --> 00:02:44.831 align:center line:-1 position:50% size:58% allows some radars to accurately determine the target's speed. 00:02:44.831 --> 00:02:49.086 align:center line:-1 position:50% size:47% MTIs (moving target indicators) use Doppler shift in the return echo 00:02:49.086 --> 00:02:50.921 align:center line:-1 position:50% size:33% to sense the movement. 00:02:53.715 --> 00:02:59.846 align:center line:-1 position:50% size:53% Here's a frequency domain visualization of what chirp pulses would look like. 00:02:59.846 --> 00:03:03.892 align:center line:-1 position:50% size:59% This is shown on a real-time signal analyzer. 00:03:03.892 --> 00:03:07.229 align:center line:-1 position:50% size:56% You can see that as the radar frequencies change within the pulses, 00:03:07.229 --> 00:03:12.192 align:center line:-1 position:50% size:67% you can see they sweep across the horizontal axis of the display by varying amounts 00:03:12.192 --> 00:03:15.320 align:center line:-1 position:50% size:45% depending on the amount of chirp applied to the pulse. 00:03:15.320 --> 00:03:20.617 align:center line:-1 position:50% size:55% You can see, for example, here on the left-hand side is a chirp pulse 00:03:20.617 --> 00:03:25.831 align:center line:-1 position:50% size:59% and two others very similar to it in the center and on the right side of the display. 00:03:25.831 --> 00:03:33.130 align:center line:-1 position:50% size:61% There's one very wide chirp pulse that occupies the majority of the display here, 00:03:33.130 --> 00:03:39.386 align:center line:-1 position:50% size:40% indicating a much wider chirp. 00:03:39.386 --> 00:03:43.432 align:center line:-1 position:50% size:47% Another form of pulse compression is called digital modulation, 00:03:43.432 --> 00:03:48.228 align:center line:-1 position:50% size:43% and it offers other advantages in terms of unambiguous range. 00:03:48.228 --> 00:03:51.731 align:center line:-1 position:50% size:53% Adding digital modulation to each pulse allows the adjacent pulses 00:03:51.731 --> 00:03:54.109 align:center line:-1 position:50% size:32% to be uniquely encoded. 00:03:54.109 --> 00:03:56.695 align:center line:-1 position:50% size:57% Then using digital modulation techniques-- 00:03:56.695 --> 00:04:01.283 align:center line:-1 position:50% size:47% for example, we could use binary phase-shift keying (BPSK)-- 00:04:01.283 --> 00:04:06.580 align:center line:-1 position:50% size:59% it encodes pulses so that the roundtrip delay of each pulse is easily measured. 00:04:06.580 --> 00:04:10.292 align:center line:-1 position:50% size:45% We can do this unambiguously using each pulse's unique coding 00:04:10.292 --> 00:04:18.800 align:center line:-1 position:50% size:28% as a separating tool. 00:04:18.800 --> 00:04:23.555 align:center line:-1 position:50% size:55% Another technique that's sometimes used is using FMCW radar 00:04:23.555 --> 00:04:29.853 align:center line:-1 position:50% size:55% where the signal is not even pulsed at all and instead, over time, 00:04:29.853 --> 00:04:33.106 align:center line:-1 position:50% size:52% we shift the frequency--it's like a chirp-- 00:04:33.106 --> 00:04:38.028 align:center line:-1 position:50% size:57% very often in say a triangular shape pattern 00:04:38.028 --> 00:04:42.616 align:center line:-1 position:50% size:59% where we, over time, increase the frequency and then decrease it. 00:04:42.616 --> 00:04:47.871 align:center line:-1 position:50% size:47% The main advantage of CW radar is because the energy's not pulsed, 00:04:47.871 --> 00:04:50.999 align:center line:-1 position:50% size:38% these are a lot simpler to manufacture and operate. 00:04:50.999 --> 00:04:53.543 align:center line:-1 position:50% size:58% They have no minimum or maximum range, 00:04:53.543 --> 00:04:56.421 align:center line:-1 position:50% size:63% although the broadcast power level can impose 00:04:56.421 --> 00:05:00.258 align:center line:-1 position:50% size:34% a practical limit on the range of the radar. 00:05:00.258 --> 00:05:04.721 align:center line:-1 position:50% size:47% Continuous-wave radar maximizes the total power on a target 00:05:04.721 --> 00:05:09.100 align:center line:-1 position:50% size:39% because a transmitter is broadcasting continuously. 00:05:09.100 --> 00:05:12.062 align:center line:-1 position:50% size:54% The military uses continuous-wave radar 00:05:12.062 --> 00:05:18.318 align:center line:-1 position:50% size:46% to guide semi-active radar homing air-to-air missiles, for example. 00:05:18.318 --> 00:05:24.407 align:center line:-1 position:50% size:65% FMCW radars are also widely used in automotive collision avoidance radar systems. 00:05:26.660 --> 00:05:36.962 align:center line:-1 position:50% size:61% Those are just a few of the techniques used to deal with range resolution and range. 00:05:36.962 --> 00:05:45.762 align:center line:-1 position:50% size:64% If you'll stay for the next section, we'll talk about radar measurements. Thank you.