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JohnPJones

quick reference radar topic

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i know i'm not the only one who doesn't know much about radar.
i understand radar horizons, and i'm familiar with looking at a radar screen, but the different types and bands etc, and what they're used for.

so can our radar nerds post about it, and i'll collect it here in the first post as a quick reference guide radar?

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The longer the frequency, the less energy it loses in traveling across the atmosphere from one point to another.   So you get longer range from it using a lower amount of energy.  However because the waveform is physically larger, you get less resolution off the target.  

The higher the frequency, the more energy it loses in traveling across the atmosphere from one point to another.   The result is shorter range, or to attain longer range, you have to pulse more energy into it.  However because the waveform is physically smaller, you get more resolution off your target.  

For these reasons, search radars use longer wavelengths, typically, from S band, to L-band, to UHF and VHF which is metric wave.  Back in the days in World War 2, it was namely VHF and UHF.  After that L-band became popular and since then S-band has became the near standard for search radars.  

For these reasons also, fire control radars use shorter wavelengths, typically X-band, but they can go down to Ku, Ka and even mmwave.  Radars used with CIWS, with fighter jets, with gunnery control, with missile seekers also fall into these regions.   Shorter wavelengths give more precise measurement of target speeds, higher angular resolution, and is better at tracking a target moving at high speeds.   It also discriminates better against clutter and noise which is why they are used with ship navigation radars.  

 

For a ship, you typically have both.  The search radar is used to paint sky for range and volume.  Once it finds something, it queues the fire control radars to get a tighter fix on it, so you can get more information out of it, such as its rate of speed, looking for a high quality track that can be used for a weapons lock on.   The fire control radar will prosecute combat with the hostile target, such as illumination of the target for a missile strike, or locking for the gunnery.  A ship can have multiple radars, especially on the fire control side.  We do not mention the use of navigation radars here since every ship has to have one.  Ships can also have weather radars and helicopter tracking radars.

Between these processes, you have the IFF which is connected to the search radar.  The IFF will interrogate cooperatively or noncooperatively, if the target is friend or foe.  IFF isn't much of a radar as it being more of a communication device.  

 

Here is a short guide along with their physical size of their wavelengths.

https://www.nasa.gov/directorates/heo/scan/communications/outreach/funfacts/txt_band_designators.html

Because the physical size of longer wavelengths are much bigger, so are the physical sizes of the radars.  With ultralong wavelengths, you can have radars that are kilometers long set up like connected telephone poles along a coast line.   And yet for mmwave size radars, they can be as small as a quarter.

Radio communication, including cellphone and wifi communication are also within these regions.  Your wifi router uses the same S-band as your naval search radars.  So does your microwave oven.  

 

I mentioned a C-band radar in my other post.  C-band is somewhere between the S-band used with search radars and the X-band used with fire control radars.   Its meant for radars with the properties of both, but not better than a dedicated one used solely for either purpose.   For this reason its not as commonly used compared to the S and X-bands.  

Edited by Eisennagel
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Here is another way that makes it more intuitively easier to remember.

 

L-Band as in Long, Long Wavelength, Long Range.

 

S-band as in Search, used for Search Radar.

 

C-band nothing in particular as between S and X-band.  Maybe C stands for Compromise.  

 

X-band as for Targeting, not literally but X is shaped like a reticle.

 

K-band as in Kurst which is German for Short.

 

There is an inverse relationship between range and resolution. 

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On March 1, 2021 at 9:11 PM, Eisennagel said:

 

Here is another way that makes it more intuitively easier to remember.

 

L-Band as in Long, Long Wavelength, Long Range.

 

S-band as in Search, used for Search Radar.

 

C-band nothing in particular as between S and X-band.  Maybe C stands for Compromise.  

 

X-band as for Targeting, not literally but X is shaped like a reticle.

 

K-band as in Kurst which is German for Short.

 

There is an inverse relationship between range and resolution. 

That is so much more helpful to us dummies lol.

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On 3/1/2021 at 10:11 PM, Eisennagel said:

 

Here is another way that makes it more intuitively easier to remember.

 

L-Band as in Long, Long Wavelength, Long Range.

 

S-band as in Search, used for Search Radar.

 

C-band nothing in particular as between S and X-band.  Maybe C stands for Compromise.  

 

X-band as for Targeting, not literally but X is shaped like a reticle.

 

K-band as in Kurst which is German for Short.

 

There is an inverse relationship between range and resolution. 

To add to this - C-band is somewhat better at dealing with closer-to-shore environments, and for this reason tends to be favored by the MMI over S-band given conditions in the Mediterranean. Ex, EMPAR, MFRA, are both C-band radars, and Kronos Dual Band will be X & C-band. 

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On 3/9/2021 at 2:45 AM, Phoenix_jz said:

To add to this - C-band is somewhat better at dealing with closer-to-shore environments, and for this reason tends to be favored by the MMI over S-band given conditions in the Mediterranean. Ex, EMPAR, MFRA, are both C-band radars, and Kronos Dual Band will be X & C-band. 

 

X-band is better for surface scanning because of its greater discrimination and anti clutter ability.  That's why ship navigation radars standardize on X-band.    But if you are going to use some longer air search ability to it, C-band is better than X-band.  S-band again is better than C-band in terms of long range volume air search, but its not as good in clutter discrimination in more littoral environments compared to C-band.   That's why C-band can mean Compromise as it can be the best compromise for some.  

 

There are other ships that use C-band.

 

Akizuki and Asahi class destroyers 

SAAB Sea Giraffe.  One user of these radars is the Independence class LCS.  

Cassidian TRS-3D.  Used with German Navy corvettes and frigates.   Also used with the Freedom class LCS and the Legend class Coast Guard cutters.

Cassidian TRS-4D.  Used with the German F125 frigate.

 

Most Chinese warships uses a secondary radar called Type 364 that is encased in a white dome and set on a mast.  It scans the sea surface for low flying objects like antiship missiles.  

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What follows is as it relates to radar in the WW 2 and immediate postwar period.  Not all of it is applicable to modern sets.

For ships radar falls broadly into one of four categories:

Air search.  These are usually meter + wavelength sets.  They are optimized for detection of airborne objects at longer ranges, generally 50 to 150 miles.  Their detection range depends largely on the output power and size of the antenna array used.  More is better.  They aren't terribly accurate in range or bearing, but this is less important than these sets making early detection of enemy aircraft.  The altitude of a target can be roughly estimated using a fade chart.

Surface search.  These typically are 10 to 50 cm sets, depending on the period of the war and technological development by country.  These sets are for detection of surface ships / objects out to about 20 to 40 miles.  Here, in addition to proper antenna shape and power, the polarization of the transmitted beam is important to minimize wave reflections and other clutter.

Fire control radars.  These started out at around 50 cm and continued to decrease during the war to 3 to 10 cm.  The shorter the wavelength, the more accurate the radar set could be.  Antenna shaping was of critical importance here.  The worst sets of this type used simple Yagi array antennas.

The last category were specialist sets.  Two late war ones were the height finding radar that would give the precise altitude of an aircraft, and the "panoramic" or "zenith" radar to detecting targets that were overhead at short range.

Displays in WW 2 came in two broad categories: the A scope that worked much like an oscilloscope, and the PPI (Plan Position Indicator).   The PPI type is the one you typically see in movies.

Most radar of this period operated as pulse sets.  That is they would send out a pulse of signal, then stop transmitting for a period to detect return signals.  Another type that became more common as the war progressed was the pulse doppler set.  It worked like a pulse radar but the receiver circuits could determine the difference between the transmitted frequency and the return frequency to determine if the target was moving and roughly how fast.  Electronics of the era made this a bit crude by today's standards, but it could be done.  One use of this type of set was to negate the effects of chaff jamming.  Here, the set used a band pass or band reject filter to eliminate any return frequencies that meant the target was not moving or moving very slow.

Many WW 2 radar sets used separate transmit and receive antenna instead of using one for both.  Some early sets were fixed in position or hand turned to aim them.  Later, most that needed to rotate got slip rings and used an electric motor for rotation.

A less common set was like the US Mk 3 - 8 fire control radars.  These were early Track While Scan (TWS) sets.  They could be set to search a fairly wide arc for targets and while doing so present ones found and show their movement on the display.  US late war nightfighter sets like the SCR 720 had this feature included in their design.

As for antenna shape, if you look at one, the beam produced will be 90 degrees out of position to the shape of the antenna.  That is, a wide, thin antenna will produce a tall narrow beam.  A rectangular one, a rectangle of the opposite shape horizontally and vertically.  A disk antenna produces a circular beam.  Yagi arrays (like those on German nightfighters) produce a less coherent beam in the same way compared to the dipoles on the array.

Allied radar starting about 1943-ish started getting IFF interrogators included on the antenna to allow them to distinguish friend from foe (hopefully).

  625x465_15079873_8868605_1513373502.jpg

The smaller rectangle on top of this SC-2 is the IFF antenna for example.  Typically only air search sets needed this feature.

 

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If a cruiser can take out a Destroyer's Torpedoe Tubes, how come a Cruiser can not have his Radar taken out? It's just an antenna, you think it would be about a zillion times easier to take out than a torpedo tube or a gun or just about anything else.

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