# Using the L35 WX Radar

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Just Flight has the Advantage WX Radar on sale for less than \$18 so I thought it would be a good time to learn a little about using weather radar.  I installed it in the L35 and have been thinking about how to use it to estimate the height of storm tops in relation to cruise altitude.

If we assume the radar puts out a nice thin coherent beam (which it does not) we can use some simple trig as below:

If we let the Slant Range (SR) be 1NM we get the following H heights:

Note that for a Tilt Angle of n, the height is on the order of n hundred ft – easy to remember and probably close enough given the inherent approximations.

And the height increases in a linear manner with the SR – double the SR, the height doubles, etc.

So, for example, with a Tilt Angle of 5 degs and a target at 10 miles, we get a height of about 5000+ft.

It would seem you could increase the Tilt Angle in 1 deg steps until the return just disappears to find the 'angle' of the storm top.  Now, however, since the beam does have a width, you would probably have to reduce the tilt angle by half the beam width for calculation purposes (this assumes the tilt angle is measured to the beam's center). So if the beam width was 8 degrees, you would reduce the tilt angle by 4 degrees before using the above table of values.

“Weather” this has much validity in practice I don’t know.  Perhaps someone with real world wx radar experience could comment.

Al

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Hi Ai,

be aware that you also have a certain beamwidth which you have to take into account for your calculations.

Typically for a narrow beam that is about 2,5° and up to 4° for a wider beam, depending on your range setting.

So when you set your tilt be aware that you actually just set the middle of the beam to the angle and actually scan a few degrees more to either side.

If you want to know if something's at your altitude a setting of approx 2° up will put the bottom of the beam to your altitude while actually scanning upwards and vice versa.

Also keep in mind that the WXR antenna is connected to the airplanes attitude reference system, so that the antenna tilt will always be set in reference to the horizon, *not* in reference to your airplanes pitch attitude.

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Hi Ai,

be aware that you also have a certain beamwidth which you have to take into account for your calculations.

Typically for a narrow beam that is about 2,5° and up to 4° for a wider beam, depending on your range setting.

So when you set your tilt be aware that you actually just set the middle of the beam to the angle and actually scan a few degrees more to either side.

If you want to know if something's at your altitude a setting of approx 2° up will put the bottom of the beam to your altitude while actually scanning upwards and vice versa.

Also keep in mind that the WXR antenna is connected to the airplanes attitude reference system, so that the antenna tilt will always be set in reference to the horizon, *not* in reference to your airplanes pitch attitude.

Hi Emi,

Yes, as I mentioned in my post, you do have to take the radar beam width into account. What I don't know for this particular radar is what the beam width is -- what does the simulation use?

Very good point regarding  the reference to the horizon. I did not realize the radar accounted for the planes attitude so the tilt angle is always in reference to the horizon.

Thx much,

Al

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what does the simulation use?

That's the question I find interesting. Since we are not using "real" radio waves travelling at the speed of light to scan for "rain" and its density, how does the WX Advantage work?

Since it "detects" my simulations weather, which is collected from real world data by the program FSGRW, or Flight Sim Global Real World Weather and "drawn" as pixels on my monitor by REX Soft Clouds and Texture Direct. I think the way it works is very interesting.

Some weather radars, in the flight sim, require ASN before they work. But not so the REX WX Advantage.

Clever guys those WX coders!

Cheers for an interesting Sunday evenings discussion.

Regards

David

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Code7700.com is an amazing (and huge) website by a very professional pilot (James Albright, who calls himself Eddie on the site), who has flown everything from jet fighters to B747s. He currently flies a Gulfstream G450. On this site there is extensive information about the use of wx radar. While looking at some of this, at http://code7700.com/radar_techniques.html,  near the bottom of the page where the use of wx radar during a descent is discussed, I came across the  statement, "Contrary to popular belief, there is no "auto" function on most radars, no automatic tilt compensation for aircraft pitch attitude".

Whether this statement applies to the Advantage WX Radar I don't know. Has anyone seem a definitive statement regarding a/c pitch attitude and the Advantage Radar tilt angle?

Al

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"Contrary to popular belief, there is no "auto" function on most radars, no automatic tilt compensation for aircraft pitch attitude".

This statement I find interesting, I got to admit, I'm still in my ATPL training, not having flown any aircraft equipped with WXR yet, however in the theory exams our authority definately wants to hear all WXR are always compensating for aircrafts pitch.

They even ask the details how this system works...

I'll check up on this some more.

If I'm on it anyway I'll also ask the REX/MilViz devs about the beam width of their radar.

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I'll check up on this some more.

If I'm on it anyway I'll also ask the REX/MilViz devs about the beam width of their radar.

Emi,

I would guess some aircraft do take pitch into account and some don't, but I don't have any experience to say what is most common.

When you ask REX about the beam width, you might also ask whether they compensate for the a/c pitch as far as the tilt angle is concerned. The a/c pitch info is certainly available in the sim so their wx radar code could make use of that info if they wanted to model a pitch compensated system.

BTW, I saw one post in another forum that said the REX radar models either a 30" or 24" dish which would make the beam fairly narrow -- 5 degs or less.

Thanks for looking into all this.

Regards,

Al

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The Milviz-Rex info says their Advantage WX Radar was modeled after the Bendix (now Honeywell ?) RDR-4B WX radar. At http://www.faam.ac.uk/index.php/faam-documents/science-instruments/915-rdr-4b-doppler-radar-manual/file is the manual for the real RDR-4B radar. In a quick scan through the document I noticed:

- There is a larger antenna array and a smaller size antenna for smaller a/c. The larger antenna has a beam width of 3 degs. I could not find what the beam width for the smaller antenna is, but it will be greater than 3 degs (maybe 5 degs?).

- There is a whole section on antenna stabilization, and stabilization can be turned on and off. When off, the tilt angle is referenced to the a/c, and when on the tilt angle is referenced to the horizon.

All this begs the question of what exactly did Rex-Milviz actually model?  Regardless, the manual looks like interesting reading.

Al

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It seems to me the REX-Milviz Advantage Radar tilt value might be referenced to the horizon.

I tried the following experiment:

- While in ALT HLD at 300KTs IAS, I adjusted the radar to just barely display some ground clutter. I then pulled the power all the way back and watched the radar display as the a/c slowed and pitched up in an attempt to hold altitude. I didn't see much change in the display even though the a/c pitch attitude changed by over 5 degs.

Thx,

Al

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Good one you found with the manual Al!

I unfortunately still haven't gotten a reply from REX/MilViz.

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The beam width is 3 deg and the tilt is horizon stabilized. The API allows for developers to control the beam width if they create their own display.

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