Calibrating Airborne Weather Radar

darling pretty

Well-known member
Joined
Feb 1, 2002
Posts
57
Total Time
6000+
Does anybody have any good tips, know of any web sites/publications on how to "calibrate" an aircrafts weather radar?

I've seen video clips w/ incomplete formulas/techniques that you can use to determine the height, the relative intensity at your current altitude and other useful info, but I never had the opportunity to copy it down.

Any info/techniques would be greatly appreciated.


DP
 

chperplt

Registered User
Joined
Nov 25, 2001
Posts
4,123
Total Time
.
There was a thread not too long ago that discussed this.

A good rule of thumb is to double your altitude (10,000 ft = 20 miles), paint the ground at that distance, and then raise the tilt 5 degrees.
 

darling pretty

Well-known member
Joined
Feb 1, 2002
Posts
57
Total Time
6000+
I'm assuming that gives me the relative intensity at my current altitude, any suggestions on how to figure out the height of the cell to determine if it is overflyable?

Thanks

DP
 

Checks

Well-known member
Joined
Dec 23, 2001
Posts
447
Total Time
7,000
I watched the Archi video a couple of years ago and use the following:


Flying along at 9,000ft AGL I would paint the ground with the bottom of the radar out 9nm.

Raise tilt 10degrees from that point. Now the bottom of the radar beam will be parallel to the ground(assuming even terrain). I believe he called this the calibrated 0 degree position(or something like that)

Paint the "tops" of the cells and note how many degrees up you moved the radar up from the previous position. Then mulitply that number by the distance of the cell in NM and multiply by 100ft.
5degrees x 25nm x 100ft = 12,500ft

The painted tops are 12,500ft above your current altitude=
12,500 plus 9,000ft=21,500ft "tops"

Then Archie said to add 20% to the 21,500ft because 21.500ft is the altitude of the "tops" that is showing up on your radar. There is convective activity above what you can pick up on the radar.

About 25,000ft.
 

capt_zman

Well-known member
Joined
Nov 28, 2001
Posts
462
Total Time
>7000
There's an excellent book on weather radar called Airborne Weather Radar, A User's Guide by James C. Barr. Book is everything you would ever want to know about weather radar. I've read it probably 10 times and keep cheat sheets in the plane.

It all starts here (figure out what size dish you have, most light GA stuff has either a 10 or 12 inch antenna):

30 inch antenna = 3-degree beam
18 inch antenna = 5-degree beam
15 inch antenna = 7 degree beam
12 inch antenna = 8 degree beam
10 inch antenna = 10 degree beam

Why is this important? Well, we need to figure out what our beam width is at a given distance. Here's a formula for determining this:

Distance (NM) x 100 = beam width in feet per degree of beam angle.

For example, 40 NM x 100 = a beam width 4,000 feet wide per degree.

If you have a 10 degree beam, then the beam width is (10 degrees x 4000 feet) = 40,000 feet at 40 NM.

Now you need to figure out what is actually "filling" the beam. For instance, if you are at 40 NM and you have a 30,000 ft thunderstorm in front of you, it doesn't fill the entire beam width and can be devastatingly understated on the screen.

Now is where tilt management comes in. We must be able to paint the storm from the ground on up. For example, if your pointing your antenna at the upper 1/10th of the storm, you are overscanning or if you have your tilt set too low, you are underscanning, which is really bad. Therefore, we must be able to establish a ground base (or zero tilt). Here's the formula to determine zero tilt:

Take 2 zeros off your altitude, and divide that number by half of your beam angle.

Using our previous example, we are at 10,000 ft and we have a 10 degree beam. The true zero tilt position should begin to paint ground returns at approximately 20 NM.

(10000/100) / (.5 x 10) = 20

Sometimes the zero tilt mark on your radar is not correct, so use that formula to calibrate the zero tilt.

Now, we already know that if we are using the 40 NM scale on the radar screen, each degree of tilt is equal to 4000 feet. So if we establish our zero tilt and we see ground returns at 20 NM, we can scan what's in front of us by tilting up one degree (4000 ft) and we can basically tell how tall a thunderstorm is or where the precip begins and ends relative to altitude. This all assumes that what you are painting is reflective (rain and wet snow).

There's a ton of other stuff to learn about weather radar (attenuation, gradients, ground tilt management, shapes, etc), so I would highly advise getting the aforementioned book.

I just put a new RDR2000 in my baron and have made 4 trips back and forth to FL recently. On every trip, I've had to apply these principles and they have worked like a charm everytime.
 

Kaman

Well-known member
Joined
Nov 26, 2001
Posts
947
Total Time
8.2
Don't forget contour folks...

Hi guys and gals,
I may be short on piloting experience, but I'm long on airborne RADAR ops, because I did it for a living in the military. Tops are an important factor when considering a line of cells and it's height compared to your cruising altitude and the capability of climbing over it. Not to mention that tops are an indication of the intensity of vertical movement in the atmosphere.
Contour is what is really important to understand, and a lot of pilots don't know how to properly interpret Wx RADAR. In the "old" days before color weather radar the only real way to tell the intensity of a cell was the characteristics of the echo. A sharp defined edge with a narrow "dog-bone" shape was a really BAD cell and the narrow area is NOT where you want to go....The more precip, the better the reflectivity it has. So, look for the fuzzy stuff on the scope thats where the precip is least intense and better chance for avoiding the "bad" stuff. Also, watch out for the classic "fishhook" Could be teh beginning of a tornado!
I hope that this doesn't insult the intelligence of the more learned weather pilots out there, but this is my "thing" and I'd be more than happy to share more info with anyone interested, so drop me a PM....Fly safe out there....And remember...."INCREASE GAIN and DECREASE INTENSITY" on the scope for the best presentation and have your avionics folks make sure that the carrier freq. is locked in where it's supposed to be...I learned on a set you had to tune that in yourself!

Regards,

ex-Navy Rotorhead
 
Top