a radar with the dish antenna can tell us the direction to a contact by just being pointed at it if you cycle the emitter on and off in pulses then you can measure the time delay of The Echoes to find that Target's range you can even figure out a target's height by its angle above the ground so this lets us figure out where it's at in three dimensions but is that all let's find out how the Doppler effect can tell us more there's an additional factor that we can track besides Azimuth range and height we can also acquire a Target speed by measuring how much the frequency of our radar changed from the time it left the radar to when it returned that's because the frequency will shift depending on how fast the target is moving anytime radar waves are reflected from a stationary object they'll return with the same wavelength they started with but once an object starts moving then the waves will compress in front of it this shortens the distance between waves and we know that a shorter wavelength means a higher frequency conversely it means the waves behind it get longer here's an example an s-band radar can send out waves at a frequency of 2800 megahertz if those waves come back one kilohertz higher then we know the target is closing with the radar at a speed of 40 meters per second which is about 80 knots and if it was two kilohertz higher then it would be closing at 160 knots the lower frequency would mean the target is moving away so if you can measure the difference in frequency you can determine the target speed however there's a caveat only the waves in front and behind the moving object have their wavelengths changed these Reflections that are 90 degrees to either side come back at exactly the same wavelength this means we can only measure the speed towards or away from the emitter the name for this is Radio velocity that's because it's the movement on these lines that radiate away from our emitter on its own a radar with the capability to measure the Doppler effect can take simple speed measurements in fact that's exactly how law enforcement radar guns work that Doppler capability also allows weather researchers to see the speed of various parts of a weather pattern in real time that lets them identify severe weather like the tornado forming in this image whenever a Doppler pattern indicates intense high speed rotation like this hook we see here it's a strong sign that a tornado might form there on its own this Doppler capability doesn't add much value to Airborne radars but that changed with the Advent of digital computers that were small enough to fit inside of a fighter this happened in the 1970s ushering in a new era in aerial Warfare the most noteworthy effect is the ability to filter out returns that are moving at the same radio velocity as the Earth's surface here's how that would look from the perspective of a mobile radar like what you would find on an aircraft this big spike is from the main lobe of a radar reflecting off of ground clutter it's at a higher frequency than what's being transmitted this is because the aircraft's forward movement is introducing closure with the terrain so this Spike will always be at the aircraft's velocity relative to the terrain in other words it will move outward as you speed up and come closer as you slow down all this back here is ground clutter returns from the side lobes this includes some rearward-facing side lobes which is why we have negative frequency returns if an Airborne Target is inbound to our radar we'll see an additional Spike here and if it's outbound we'll see a spike here the difference is due to our relative speeds the outbound Target is closer to our own so naturally its frequency will be closer and show a smaller Doppler shift since we're not interested in this ground clutter Spike we want to filter it out of our results so we can place boundaries around it like this and anything inside those boundaries won't be displayed this type of filter is informally known as the notch as you've probably guessed this filtering isn't perfect any Target moving at or near the radio speed is our ground clutter is going to be hidden in the returns that are being filtered out so when we see a Target moving at a lower altitude than our radar like this fighter moving below the horizon it won't show up because it's within the Doppler shift range that we're filtering out but if you notice this second target at a higher altitude is showing up we can still see it even though it's at a longer range that's because the signal processor in our radar is smart enough to only do Doppler filtering when there's ground clutter present just remember that this is a feature that needs to be added by the Builder so not every radar will have it that's not all there is to Doppler radars just like how pulse signals have an ambiguity problem so does Doppler measurement instead of being a range problem with Doppler measurement it's a frequency problem imagine we send out a radar wave at one wavelength like this now let's say we're sending out pulses at an interval represented by these lines we can then measure the reflected waves at these points as well if the wavelength is slightly shorter then we can use that to determine the target is moving towards us but if it gets to an even shorter wavelength that matches our samples like this then we have a problem Our receiver can't tell the difference between the shorter wavelength and the wavelength we send out in other words the shorter radar Echo looks exactly the same to Our receiver as a stationary Target this is an oversimplification but it illustrates the concept of aliasing aliasing happens at a certain combination of wavelength and prf but you'll also find aliasing at multiples of this prf the radial speeds where this happens are known as blind speeds aliasing can also happen if the target is moving the same speed as our emitter so this is an additional level of masking Beyond being hidden in ground clutter like with this fighter that's flying ahead of us at the same speed because it's flying at one of the blind speeds it's getting filtered out of our radar display but watch what happens once we accelerate now that we're significantly faster than the target it shows up on our radar display there is a solution to these ambiguity challenges when you increase the pulse repetition frequency then you take more samples and then these blind speeds move farther apart this gives you a broader range to work with increasing prf can help if you have a target with a high radio velocity but it has a big downside too remember that unambiguous range works in the opposite direction a lower prf increases that unambiguous range as you increase prf to overcome velocity ambiguity you decrease the maximum unambiguous range so when it comes to selecting a prf you have a trade-off between accurate speed and accurate range this is called the Doppler dilemma and it's the reason why there isn't one perfect prf for all situations what happens with ambiguous results varies from one system to the next on one system the range or velocity shown May jump back and forth between possible results on another the contact may just be filtered out entirely it depends entirely on the builder of the system in early Radars this could be a significant problem those early Radars operated at a single frequency with the single prf with little or no digital processing capability but as technology improves radar systems were introduced that could change these parameters in real time and then do a lot more with the signal Echoes they pick up we'll go over some of these more advanced Radars like phased arrays and stealth's interaction with radar in a future video but this should give you enough to understand the basics of how Radars work I hope you found this video useful and thank you for watching