About radars and 'the spinning thing'

This post is a translation of a post previously published on my Swedish-language blog in September 2018.

Something I have noticed during the last few years is that if someone asks what I work with and I answer 'radar development', there is a very common follow-up question. Namely, 'does it have one of those spinning things?'

Usually, 'spinning thing' here means the typical radar display that you see in ships and airplanes, especially in movies and tv-series, with a greenish beam that sweeps around a circular display showing bright dots where the radar sees a target. Of course, sometimes 'spinning thing' also refers to the antenna in itself, since ship radar antennas are usually mounted fairly high and keep spinning around. It's a fairly natural question, since these types of radars can be found in popular culture and are in any case very visible.

As it happens, though, the types of radars that I work with do not have any sort of spinning thing. If I say so, the next question is usually something along the lines of 'then how does it work?' which is complicated and takes time to explain. It's also a very good question, so I thought I would take the time to explain it here.

If you think a bit more about that classical green radar display you quickly realize that it is a very good way to show two things: Distance to the thing the radar is detecting, and the direction the antenna was pointing in when the object was detected. Distance is one thing that is very straightforward to measure with radars: You send out a signal or radio wave pulse and wait to see if it is reflected by anything and therefore comes back, and then you calculate the distance from the time elapsed between emission of the pulse and reception of the reflected pulse. Direction is often more difficult, but for the spinning ship radar there is a simple solution - the thing the radar sees must be in the same direction as the antenna is facing when the reflected signal was received. 'The spinning thing' is thus something that helps you determine in which direction the radar target is located.

If your radar does not have a spinning thing, you need to find another way to determine which direction a signal or pulse of radio waves comes from. One way to understand how this can be achieved is to start with a type of waves that are easier to visualize, say water waves. If you are angling at a lake, your float (the red thing in the figure below is supposed to be the float, by the way) will bob up and down with the waves, sometimes being at the top of a wave and sometimes in the trough between waves.

Now imagine that you instead, for some reason, have four floats in a row in the water. If the waves are coming straight towards you, all your floats will be on the peak of a wave or in a trough at the same time. If the waves are coming from some other direction, some of your floats could be in a trough while others are not. Even if all four are affected by the same wave motion, the phase of the wave can be different for each float. If you know the wavelength and speed of the waves and the distance between the floats, you can calculate the (approximate) direction that the wave comes from the vertical position of the floats.

You can also imagine that you, on a day when the lake is calm, equip your floats with something that can create waves instead. The waves created by the different floats will interfere with each other, so if two peaks from two different floats end up in the same place at the same time they will reinforce each other, but if a peak and a trough end up in the same place at the same time the waves will cancel each other out. The end result of this will be that the resulting waves are steered in a specific direction depending on the relative phase at the different floats.

The connection between this thought experiment and radars is that the antenna elements in a radar that does not have 'the spinning thing' (say a front-looking car radar) act a lot like the floats. If the antenna that your radar emits radiation from has multiple antenna elements, you can decide that they should transmit the radio waves with small differences in phase. The emitted waves will cancel each other in some directions but not in others, with the effect of steering the resulting wave in a specific direction. By changing the relative phase between the elements it would be possible to change the direction and thereby scan the surroundings.

Another common alternative is if the receive antenna has multiple antenna elements. In this case you could have a somewhat simpler transmit antenna that illuminates a larger area with radio waves. When the waves are reflected by objects in this area they are received by the receive antenna, and the relative phase of the signal at the different receive antenna elements gives the direction of the object.

Of course, there are things that make this more complicated in reality - for example, you may need quite a lot of antenna elements if you want to look at a large area with high resolution. It can also be difficult when there are multiple objects that reflect the waves. A thing that makes the last problem easier is that reflections from objects at different distances will reach the radar at different times, so they can be separated with respect to distance before the direction is determined. In automotive radars the radial velocity of the targets relative to the radar is usually also determined, which provides another way of separating targets. An approaching car and a lamppost at the same distance from the radar can be separated based on their velocity, and then you can calculate the direction of each object separately.
Even without 'the spinning thing'.