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CRJ 200 Radar - Insights?

Old 08-04-2008, 10:40 AM
  #1  
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Default CRJ 200 Radar - Insights?

Hello,

Does anyone know if the beam width on the CRJ 200 Radar is the same as with other radars where you can apply the 1degree/1NM rule?

If so, why is it that at +1.0 or +0.7 at 80NM range you can get ground clutter? Isn't that supposed to be 8000 feet up or down at 1.0 TILT?

Would anybody happen to have the TWR-850 user manaul?

Also, do you guys know how the GCS work? Does it look for radar shadows only and if the return does not have a shadow it'll treat it as ground reflectors?

Thanks!

-schone
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Old 08-04-2008, 10:54 AM
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Just use your TCAS to follow someone with a better radar.
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Old 08-04-2008, 10:58 AM
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I have realized something about the -200's... each radar setting is different in each airplane at altitude. Some work great at 250-300 at 0deg on the 80 mile ring, while some show ground clutter with the same settings. I prefer the 20 mile ring when in the terminal area and low with about +4 on the tilt. Takeoff +5.7 works well. Cruise depends on altitude and many other variables but I am not a fan of auto-tilt but I am a fan of the GCS. Obviously the situation impacts how useful and what settings you will use. Nexrad and a storm scope with sat uplink would be very nice, but then again I got used to the G1000 setup in the TRAINERS before coming to an airliner with a radar that leaves much to be desired.
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Old 08-04-2008, 11:06 AM
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Math is Math. 1 degree is always 100 feet per 1NM.

what size plate does your book say it has. Keep in mind that even the "modern" plate will have side lobes, they just are not as pronounced as the old dish style.

if you have a 12 degree beam width, and tilt is at +1; then you are sending 5 degrees down, and 7 degrees up. That would be 500 feet per nautical mile down, and 700 feet per nautical mile up, for a total beam width of 1200 feet per nautical mile.

Getting ground clutter (ground returns) is from having the tilt too low at lower altitudes. In your post, you do not say from what altitude you are getting ground returns from, since you can get them from any altitude.

If you have a 12 inch antenna, the mathimatical formula translates into about an 8 degree total beam width, with side lobes extending up to 12 degrees total. The side lobes are not as pronounced as on older dish antenna and shouldn't cause much problem unless down low. Aircraft structure, and installation factors can make the side lobes more pronounced on different aircraft. For instance, the EMB-145/140/135 has a 12 inch plate which should be a 8 degree beam. Using the 1/1/1 formula, I have yet to see one that didn't put out 12 degrees... in spite of the company manuals saying it was 8 degrees.

Another thing to do, is to visit the website of the radar manufacturer and download the user manual.... or check the cockpit library during a longer flight. Determine what the base DB level is for the MFD display. Then find out what each tick mark on your gain control translates into in terms of DB's.

With some skillful use of the gain, you should be able to actually determine storm levels accurately. You can use the National Weather Service scales as a reference. For example. The Honeywell Primus in the EMB has a base DB of 20 decibells. For ANYTHING to show up at all, it has to return at least 20 DB's. Each tick mark on the gain control is 3 DB's. Adjust the gain control from full, down until your target just disapears. Count the number of tick marks on the gain control, multiply times 3 and add the baseline of 20 DB's. That will give you the actual DB return of the cell/area in question. Use the number of DB's as compared to the NWS scale and you now know what level storm you are looking at. Re-adjust the gain knob to display the appropriate colors.

The latest and greatest radar, not in airline service yet that I am aware of, also includes a GPS derived terrain database... the radar compares returns, does the math for you, and removes anything that is a ground return. It also does vertical sweeping as well as horizontal. The display gives TWO pictures. The traditional plan view we are all familiar with, and then underneath, a PROFILE view showing tops and bases. Really cool stuff.

Hope this helps.
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Old 08-04-2008, 11:08 AM
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Originally Posted by higney85 View Post
I have realized something about the -200's... each radar setting is different in each airplane at altitude. Some work great at 250-300 at 0deg on the 80 mile ring, while some show ground clutter with the same settings. I prefer the 20 mile ring when in the terminal area and low with about +4 on the tilt. Takeoff +5.7 works well. Cruise depends on altitude and many other variables but I am not a fan of auto-tilt but I am a fan of the GCS. Obviously the situation impacts how useful and what settings you will use. Nexrad and a storm scope with sat uplink would be very nice, but then again I got used to the G1000 setup in the TRAINERS before coming to an airliner with a radar that leaves much to be desired.

The NEXRAD datalink will not give you bases or tops... the data image is also anywhere from 5 to 15 minutes old... USELESS for picking your way through a line. Great for long range planning, but pretty useless for splitting cells at 480 kts.
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Old 08-04-2008, 11:19 AM
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You are right its not useful at 480, but it does show tops... and the longest ive seen on the G-1000 is 5mins. its usually at 1-3.
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Old 08-04-2008, 11:34 AM
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The GCS is only a basic feature of the processor, basically differentiates between the "type" of echo that solid objects reflect from those that water do. I use the GCS a lot. I use it in my first few scans while I figure out the tilt, and while on the ground when taking off with cells around the airport. Also, about the tilt, auto-tilt sucks and about the manual tilt angle varying from aircraft to aircraft is just installation error. When the radar is inspected, manitenance usually checks the mounting of the antenna. It could "de-adjust" with a few hundred hours flying in turbulance. Also, some antennas are leaning to one side, that's the reason you get more ground return from one side. I think it's a pretty good radar, I used to fly lights with the old Bendix R160 black and white, and the radar in the CRJ is awesome compared to that. Also, I wish we had stormscope combined with radar, it's more useful than most people think.

Also, rembember, interview question:

Q-What's the best weather avoidance device on any airplane?

A- TCAS !!!
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Old 08-04-2008, 11:39 AM
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You people use math when utilizing the airborne radar???

Above FL300 0 degrees tilt is a fine place to start in the CRJ-200. Tilt down until you see ground clutter behind the weather to see the lower moisture/energy, then tilt it up until the return starts to dissipate to see the moisture/energy in its upper half. Find the middle ground that works best and leave it there. If there's a shadow behind a cell, DON'T FLY THERE.

I also used to Gain up +1 in the CRJ to help avoid the worst weather when trying to pick through a line...NEVER gained down though.

I'm certainly no Dave Gwinn or Archie Trammel, but the guy that taught me hands-on to use weather radar has a Ph.D in synoptic meteorology. He didn't bother with any mental gymnastics when flying around thunderstorms...just adjusted the tilt until he saw what he wanted to see.

And for full disclosure, we have a Garmin 496 in our Citation for the sole purpose of datalink NEXRAD. Between it, the factory airborne radar and the good ole' Mk. I eyeball, we've yet to have a problem working around the worst of southern and midwestern thunderstorm lines.
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Old 08-04-2008, 11:58 AM
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Originally Posted by Mason32 View Post
With some skillful use of the gain, you should be able to actually determine storm levels accurately. You can use the National Weather Service scales as a reference. For example. The Honeywell Primus in the EMB has a base DB of 20 decibells. For ANYTHING to show up at all, it has to return at least 20 DB's. Each tick mark on the gain control is 3 DB's. Adjust the gain control from full, down until your target just disapears. Count the number of tick marks on the gain control, multiply times 3 and add the baseline of 20 DB's. That will give you the actual DB return of the cell/area in question. Use the number of DB's as compared to the NWS scale and you now know what level storm you are looking at. Re-adjust the gain knob to display the appropriate colors.
Hope this helps.
I have never flow any of the RJ's but in every type of radar i've used the gain knob was used primarily for ground mapping. In fact in many of the radars i've used the gain knob was only functional in the ground mapping mode. In other radars you had to select a varible gain swith to activate the gain knob. This is so that in normal weather mode the colors will correctly depict the level of returns.

My understanding, and correct me if I am wrong, but a radar is callibrated to show level 1 as green, level 2 yellow, 3-4 red, 5-6 magenta.

With all this in mind why would you have to play with the gain knob to determine the level of the weather?
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Old 08-04-2008, 12:45 PM
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Originally Posted by rustypigeon View Post
My understanding, and correct me if I am wrong, but a radar is callibrated to show level 1 as green, level 2 yellow, 3-4 red, 5-6 magenta.

With all this in mind why would you have to play with the gain knob to determine the level of the weather?
The variable gain is basically resetting the value at which the baseline threshold for the display to show anything starts at. In the example I spoke of the baseline was 20 DB's to get a return. By being able to turn the gain down to make the "magenta" areas go away, you get to actually count the total DB's being returned.

Not all radar's are four color, most of the newer ones are, but not all.
Also, not all radar's are set to match the national weather service scales for level 1-6. You are correct, on most models, level 3 & 4 will get shown as the same color; and level 5-6 will get their own color for both levels. It can be very beneficial to be able to diferentiate between the levels... even though they are being displayed as the same colors.

being able to find the "weaker" area in a line, or the lower tops (tilt), are all tools in the arsenal for doing battle with the beast.

I know you haven't had much time with the units in the RJ's, but trust me, being able to do this stuff can make a huge difference. Our units are running with such wide beam widths they they are pretty useless at long ranges. They are very good up close and out to around 25 miles.... but for picking off stuff at long range you need a much larger plate, with a much smaller beam width.

The typical mainline aircraft will have a 2 or 3 degree beamwidth, compared to an RJ's 8 to 12 degree beamwidth... it is much like going out in your backyard on a dark night with a spotlight, and a floodlight. One works great for long distance, and the other works great to light up the small patio. To use the floodlight effectively, you need to make full use of it's limited abilities.
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