Eli5: Radar are built to spin around, send out, and capture a signal to create a 360 degree image of the surrounding area that regularly updates.
One would think that you could build a stationary antenna that electronically pulses and limits the area it is searching to do the same thing, removing the complication of the moving parts.
Why isn’t this the norm? And is it even possible?
In: Engineering
Modern radars can [point the beam electronically](https://en.m.wikipedia.org/wiki/Active_electronically_scanned_array) but there are some limits. It only covers about 120°, so either it has to spin or you need 3 of them for full coverage.
Also, older tech can be just cheaper and simpler.
Because a spinning antenna is a simple device with an array of simple horns driven by a single oscillator that doesn’t require much power. It’s also compact and lightweight.
A phased array antenna has a huge number of radiating surface elements that must be driven by more complicated electronics that phase shift the oscillators individually for each radiating element, causing an interference pattern between the elements to cause the main lobe to go out at an angle depending on the phase shifts. This type of antenna isn’t compact at all, it’s big, it’s heavy, and it requires a large area to mount it on.
On top of that, a phased array antenna doesn’t have the same amount of gain, so it needs a lot more power.
[https://commons.wikimedia.org/wiki/File:DDG-125_acceptance_trials.jpg](https://commons.wikimedia.org/wiki/File:DDG-125_acceptance_trials.jpg) – that ship has phased array radar antennas on the angled faces of the forward deckhouse, but it also has regular rotating antennas inside those spherical domes.
It’s a lot cheaper to rotate the antenna. active scanned antenna arrays exist. Look at a picture of a modern US navy ship and you’ll see four big white panels pointed forward, aft, port, and starboard. Each of those contains an array of hundreds (thousands?) of emitters and receivers. Carefully timing the pulses to each of them lets you send the radar beam in a specific direction. But they’re hideously expensive — thousands? of channels plus all the pulse timing stuff and pulse receiving stuff. And there’s four panels because obviously the front panel can’t transmit backwards.
So they’re really only cost effective if you have to be able to jump the radar between 6 different directions rapidly (tracking 6 incoming missiles) and also take a fraction of a second to re-sweep the rest of the sky 10 times a second searching for more missiles.
the electronics to sink this and the electricity need it to run properly can be between 2-3 time more than investing in a chicken rotiserie attachement
Short answer, technically they don’t.
Longer answer is it depends on the kind of RADAR. Most of the RADAR systems we are acostom to seeing in movies or TV or whatever are the spinning kind. The easiest way to have a system see 360 degrees is to have a single transmitting point and have an antenna spin around in a circle. This allows for the “simplest”, RADAR systems aren’t really simple in any capacity, setup to get the largest field of view.
There are other setups, the two I’m most familiar with are phase arays and doughnut arays, the second may have another name, but that’s what we called it. A phase aray RADAR is what you see on the front of the super structure of a Destroyer. The rectangular panels are actually RADAR arays. A doughnut aray us kinda similar but it’s a series of phase aray panels set up in a circle… like a doughnut.
There is also one phase aray RADAR I’m familiar with that spins.
A lot of why we use what we do comes down to how much space there is for the equipment, what we actually need that equipment to do, and how practical it is. Most of the phase aray RADARs I’m familiar with are 3D, meaning they show position from the antenna on a 2D grid, so how far away it is, as well as it’s position left or right. 3D radars how the same plus altitude. The only phase aray I know of that doesn’t show that isn’t really a RADAR but we kinda treat it as one.
Source for all of this, RADAR tech in the Navy for way to many years.
Radar works somewhat like sound. A conventional radar uses a single transmitter and receiver (think single speaker and single microphone), due to a combination of cost, size, and signal processing capability (the last 2 were only limitations historically). On its own, neither of these can determine the direction they are transmitting or receiving in, they can only transmit a certain frequency at a certain power for a certain period of time, same with receiving.
The way this was conventionally solved is by using radar dishes, which can reflect radio waves, both incoming and outgoing in a certain direction. Your (mostly) Omnidirectional transmitter can now be focused into a beam only a couple degrees or even milliradians wide, and your (mostly) omnidirectional receiver would now (mostly) only receive signals from that specific direction (receiving from a single direction is less important, because you can encode your transmit signal with some information to let your receiver know that it’s the radar signal it’s receiving, and it’s not just some random guy on his phone). This let you know where something was, rather than that there was just something in that general vicinity. Now, to cover the entire sky, you could build a radar array that had dozens or hundreds of transmit and receive nodes pointing in all directions, likely costing billions, requiring gigawatts of power, causing interference issues… or you could just spin the dish around. You can see why this second method was preferred.
In the late Cold War, miniaturization technology got to the point where you could cram a couple hundred to a couple thousand antenna elements into a reasonable area. The transmit modules would be connected to phase shifters, which slightly delay the transmitted signal, and due to some EM black magic, this causes interference that makes the outgoing radar wave particularly strong in a specific direction and weak in all others. This effectively replicates turning the radar, but without any moving parts. Note that these transmit antennas are still all driven by a singular signal generator and the receivers are also going to a single signal analyzer, so you can still really only look at one segment of the sky. This is called passive electronically scanned array, or PESA. The primary benefit of PESA include no moving parts (unless you want to scan outside the electronic slew angle of the array, typically a couple dozen degrees off axis) and being able to scan the sky VERY quickly, as in a couple thousand times a second, because there’s no moving parts.
In the modern day, miniaturization tech has made even the signal generators and analyzers small enough to cram a couple thousand in a reasonable space, so modern active electronically scanned arrays or AESAs are basically a couple thousand radars in a trench coat. The really cool thing about AESAs is that because these are all independent transceivers, they can all be doing different things. The radar can effectively split into a dozen smaller, weaker radars that all use phase shifting interference to look in different directions, and they can all be using different frequencies to reduce the chance that an enemy realizes that he’s being painted by a radar. You also get true track while scan, where you can keep a radar beam on a target while scanning the area, which was previously done with some high speed trickery that reduced scan area and track fidelity. You can even task some of the transceivers to do fancy stuff like jam the enemy radar or send communications, as they’re all just radio transmitters and don’t have to be acting like radars. This is all still preserving the psychotic scan rate of PESAs.
The main reason both of these pieces of tech aren’t really widespread outside of military (although PESA may be in civilian hands already, idk) is for a couple of reasons.
1. It’s largely unnecessary. Airports rely on aircraft transponders that broadcast their position, so super accurate and fast ground based tracks aren’t really needed. And navigation radars don’t really need the speed or accuracy either.
2. It’s really hard. The amount of processing power and miniaturization, especially for AESAs, is still cutting edge stuff. Hell, Russia doesn’t even have widespread AESA adoption for their military (although then again what do they have for their military).
3. It’s really expensive. It should go without saying that a thousand really small radars is going to be a lot more expensive than a single big radar.
4. There are some drawbacks. They can’t look beyond about 90 degrees off axis due to obvious physics, so you still need to turn them or get more of them if you want that effect. The interference method also does cut back on the raw power of the beam.
If you want to see some completely stationary radars though, look up any ships or ground stations using the US AEGIS air defense system. They usually have 4 panels of AESA or PESA elements all offset 90 degrees from each other to see in 360 degrees. The new E7 Wedgetail AWACS also uses 3 AESA arrays, which is why it looks like a stick on a plane. Funnily enough, the naval AWACS, the E2D Hawkeye, used to have a rotating dish, but I think the new radar is fully electronically scanned but still looks like it has the old spinning dish’s housing.
Edit: actually, PESAs are definitely in civilian hands. Texas Instruments makes phased array millimeter wave radars for car self driving, which is pretty neat. I somehow forgot that radars are used on that kind of small scale too.
Three things roughly determine how far a radio signal goes – the power output of the transmitter, the frequency you transmit at, and the shape of the antenna. The frequency and power are usually dictated by other requirements, so we’re left with the shape of the antenna. We can either use a very focused antenna that will send a signal in a tight beam over a long distance, or we can use an omnidirectional antenna that sends out signals in all directions over a much shorter distance. However, if we spin a highly directional antenna in circles, we can get most of the benefits of both.
They don’t. It’s possible to make a stationary radar system that behaves as though there is a spinning dish.
https://en.wikipedia.org/wiki/RAF_Fylingdales#/media/File:Radar_RAF_Fylingdales.jpg
This is one of the UK’s ballistic missile early warning system radars.
Imagine focusing a certain amount of energy in a straight line, it’s going to go a certain distance. Now take that same amount of energy and send it in all directions. It’s not going to go as far.
basically the same reason we let lawn sprinklers rotate with maybe one or two outlets instead of a static one with i.e. 12 in total.
you also would have to consider inevitable black spots in a static configuration. which you could counter through overlap thus making it even more inefficient economically.
Latest Answers