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#1486604 - 24/01/2019 15:04 Why are Radar ranges 64, 128, 256 & 512km?
galumay Offline
Cloud Gazer

Registered: 23/01/2019
Posts: 9
Hi all, my first post on your forum, I stumbled across it searching for an answer to my question which has been the subject of fierce debate at the dinner table and consequently on social media!

My question is why the rain radars that BoM produce are available in ranges of 64, 128, 256 & 512km? The numbers are odd in that they are all 2 to the power of n, which is rather odd based on my limited understanding of radar from my marine qualifications.

My suspicion is that its an arbitary distance dating back to some earlier system, its not even really an accurate description because the radar images are square so the range on the vertical and horizontal axes is 128km, but its quite a bit more on the diagonal axes.

Hopefully one of the expert weather watchers on the forum can resolve the debate for me!

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#1486606 - 24/01/2019 15:22 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
Steve777 Offline
Weatherzone Addict

Registered: 20/10/2011
Posts: 4908
Loc: Artarmon (Sydney North Shore)
Australia converted to the metric system over the decade early 70s to early 80s. Before then, for distances, miles were used.

Here is a quick conversion table: MIles & Kilometres

You'll see that 64 km is 39.77 miles, just under 40. 128 km ≈ 79.54 miles.

Now the distance that the radar can see is not a precise number. It would be affected by conditions and topography in different directions, so for practical purposes, 64 km is 40 miles. The conversion would have been done the other way, so 40 miles ≈ 64.37 km --> 64 km.

And I think that's your answer:

64 km ≈ 40 miles
128 km ≈ 80 miles
256 km ≈ 160 miles
512 km ≈ 320 miles (in fact, 320 miles is 514.99 km, but it's near enough)

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#1486715 - 24/01/2019 21:15 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
galumay Offline
Cloud Gazer

Registered: 23/01/2019
Posts: 9
Thanks Steve, that looks a possibility, I did the same conversion with nautical miles because I suspected they would have been the old range measurement.

If you are correct that suggests that the fact that the ranges are all numbers that are 2 to the power of n, is entirely coincidental.

My only question with that theory would be, did the BoM use radar in the 60's? I am old enough to remember the change of currency in '66 and the subsequent change to km's from '74, but I dont remember seeing radar for weather until quite recently.

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#1486800 - 25/01/2019 09:20 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
Tom Offline
Weather Freak

Registered: 26/06/2004
Posts: 75
Loc: Elwood, Victoria
Another possibility: in imaging applications, including remote sensing, image dimensions are often in powers of two. This has some advantages during various types of computation. For example, something called a Fourier transform, which is widely used in image processing, is much much quicker when the image size is a power of two.

I donít know if this is the reason for weather radar, but it could be.

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#1486805 - 25/01/2019 09:36 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
galumay Offline
Cloud Gazer

Registered: 23/01/2019
Posts: 9
Also sounds reasonable Tom.

Someone also mentioned elsewhere that its a hang over from the use of "8 bit radars" - whatever they are!

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#1486809 - 25/01/2019 09:53 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
Steve777 Offline
Weatherzone Addict

Registered: 20/10/2011
Posts: 4908
Loc: Artarmon (Sydney North Shore)
Originally Posted By: galumay
...My only question with that theory would be, did the BoM use radar in the 60's? I am old enough to remember the change of currency in '66 and the subsequent change to km's from '74, but I dont remember seeing radar for weather until quite recently.


Radar has only been publically available on the internet since the 90's, but radar was used before then, possibly back in the 60's. I recall from the 70s and 80s that the Weather Report on the news would show the radar. Also sometimes at sporting events like the Cricket.

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#1486857 - 25/01/2019 12:27 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
Raindammit Offline
Weatherzone Moderator

Registered: 14/02/2002
Posts: 13341
Loc: Townsville & Bilyana NQ
The now defunct Mt Stuart radar at Townsville was turned on in late 1971, just in time to capture images of Severe Tropical Cyclone Althea.

http://www.bom.gov.au/cyclone/history/pdf/althea.pdf
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#1486906 - 25/01/2019 14:24 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
galumay Offline
Cloud Gazer

Registered: 23/01/2019
Posts: 9
Interesting, so most radars and all public radar images are post metric conversion. The radar images of Cyclone Althea have range rings of either 40m, 20m or 10m - but this is different to the range of the radar that I am asking about. eg the range rings in our 128km radar are 50km apart, 256km has 100km range rings and 512km has 200km range rings.

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#1487281 - 27/01/2019 09:00 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
retired weather man Offline
Meteorological Motor Mouth

Registered: 01/07/2007
Posts: 5055
Loc: Wynnum
The mainstay of radars for BoM from the mid 60's for about 30+ years was a British design called the WF44, based on an older warship type called the 277F. Being British, distances and range rings were in nautical miles, namely 30,60,120,240 nautical miles, with the radar having a 3 degree beam. The chosen distances particularly the closer ones roughly aligned with the then airport Air Traffic Control area responsibility sectors.

The more modern radars in km's are roughly based on the older nautical miles equivalents, with narrower beam widths for greater accuracy, hence slightly longer ranges, still bearing in mind the Earth's curvature restrictions.
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#1487287 - 27/01/2019 09:24 Re: Why are Radar ranges 64, 128, 256 & 512km? [Re: galumay]
Ruckle Offline
Meteorological Motor Mouth

Registered: 03/08/2005
Posts: 6438
Loc: Woodend VIC 579m ASL - where w...
Interesting topic, thanks for raising.
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