|Dec 22, 2012, 12:09 AM|
NEXRAD radar interference info
Several weeks ago one of our club members mentioned that in late November several members who were fun flying out in the general direction of the "radar ball" experienced signal loss for several seconds on the same day. Since I had done some research about the NEXRAD radar site as a possible interference source a while ago I thought it would be a good time to share what I know.
Out of respect for those who don't have the time or desire to read this whole article, here's the bottom line on whether our NEXRAD radar could potentially interfere with your plane's radio link. Answer: yes it could but it's a low probability event. Worse case is on clear days when your plane is flying low (<500 feet above ground) in the NE quadrant of our field (i.e. out toward the radar ball) and headed either straight out or straight back from your TX position. The combination of those conditions is a worse-case recipe. Like I said, not very likely but if you do get such a radio hit it should only last for a few seconds. Now for the fun details.
Back in Dec 2010 while flying my "new" Xplorer 3.8 I experienced total control failure when I was flying in the NE corner of our field about 2300 ft west of the radar ball. The weather was clear and only a day after a storm had blown through. Luckily the ground was soft and I had set up my Airtronics 10-ch receiver's failsafe mode with 1/2 flaps and 20% right rudder. So when the radio signal failed at about 1200ft altitude the plane immediately snapped into a slow spin down to the soft ground. It never reacquired the radio link in spite of my powering the TX off then on and moving the TX antenna around. The fuselage had snapped clean just behind the front of the canopy opening but the rest of plane was unharmed. I was lucky it wasn't busted up worse. Extra thanks to Scott Meader who repaired it stronger than new.
So began the task of determining the cause. Being an engineer by trade I decided to take my time finding the signal failure's cause before I tried to fly the plane again. Since the Airtronics SD-10G radio/receiver link worked perfectly when I did a thorough range test back at the field that same afternoon, I began to suspect interference from the radar ball which was ~1/2 mile away from the plane at 1200 feet when it was "hit. "
The NEXRAD Radar
I was used to watching the Davis NEXRAD radar on Weather Underground and knew that the radar's "center +" on the map was essentially smack dab on our flying field . After some judicious Googling I found some great references (provided below) that explained how these radar sites work. Here it is in a nutshell:
The radar "ball" protects a 28 foot radar dish that rotates on the horizontal plane for a 360 degree scan of weather conditions. The site itself continuously consumes 50kW of electricity because, well, the radar is a high power long range 250 mile radar. To call the NEXRAD radar system high power is like calling the Great Salt Lake "just a touch salty." NEXRAD pushes out 750 kilowatt peak power pulses. The power is needed to provide strong enough signal reflections so at max distance they can be detected and interpreted as rain, snow, sleet, Santa Claus, etc.
Thankfully the radar signal is sent out in very short pulses followed by much longer quiet times while the radar antenna listens for reflections. So the pulses are either 1.6 microseconds or 4.7 microseconds at between 318 and 1300 pulses/second, which means the average power is much less: only a "mere" continuous average 1300 watts of 2.7-3.0gHz microwave energy that sprays over our flying field. Contrast that with our RC transmitters that push out a whopping 0.1W at 2.4gHz.
So the next question I had to answer was "How often does the NEXRAD radar beam cross any part of our flying field?"
The NEXRAD's antenna focuses the microwave beam into a very narrow one, 0.96 degrees wide to be exact. This makes the beam's half power width only 50 feet at a 1/2 mile distance (FYI, the radar ball is 4000 feet from our grassy landing area). The NEXRAD operates in either "Clear Air Mode" or "Precipitation Mode" which effects both the dish rotational speed AND the number of angular "cuts" it makes through the sky. In Precip mode it rotates faster at once every 33 seconds and at 9 separate angular cuts from 0.5 degrees to 19.5 degrees up. In Clear mode it rotates once every 86 seconds and only beams at 5 angles from 0.5 degrees to 4.3 degrees elevation.
So armed with this info I created a beam analysis spreadsheet and annotated field map (PDFs attached below) that are used to show where the beam's center is for a given NEXRAD mode and angular beam scan. Looking at the spreadsheet, at the "SVSS launch area" field position when the radar is in Clear mode (i.e. mostly clear skies, no rain imminent) the center of the radar beam is between 135 - 400 feet above ground. That is, all angular scans over our field are below launch height.
Let's take another example: Say we're in Precip mode with a weather front fast approaching and you're flying over near the SE corner of the field (point C on the field map). The NEXRAD's beam center will be anywhere from 131 feet to 1288 feet above ground depending on the angular scan.
Estimating Worse Case Interference Conditions
First thing to understand is that the NEXRAD's transmitting frequencies are not directly overlapping the RC 2.4 gHz band. Under normal conditions one might be able to eliminate NEXRAD as out of band and not a concern but it's not that simple. If an overwhelmingly strong out-of-band signal is present at the front end of a receiver who's passband is close, the receiver can still be swamped out (overwhelmed) by the nearby RF source. Without having detailed specs on all the different 2.4 receivers it's impossible to know if any of them have poor enough selectivity that they would fail to reject a signal that is many orders of magnitude greater than the RC signals of interest. So NEXRAD can still be an interfering source.
Next task was to step back and consider when the NEXRAD interference might be worse. If you look at the spreadsheet and the two different weather modes that NEXRAD can run in, it's obvious that in Precipitation Mode the interference can be much worse by virtue of the fact that it rotates slower and makes more angular slices in the sky above our flying field. So if you're flying when it's cloudy with a storm fast approaching, it's almost certain NEXRAD will be in Precip mode. Thus it's a potential interference source for flying altitudes over our field between ~50 feet and 2100 feet above ground (assuming for the moment that only the radar beam's half power width is the danger zone).
The Russian Roulette part of all this is that you don't know when the radar beam is going to hit your plane, if ever. During Precip mode the NEXRAD makes 11 circular sweeps (lower 2 repeated) at different elevation angles taking a total of 6 minutes for a full cycle. During Clear mode the full cycle of 7 sweeps (lower 2 also repeated) takes 10 minutes. So there is definitely a random element to when your plane's path might intersect with the radar beam.
Weather conditions, Plane Orientation, and NEXRAD Interference
After doing many ground-based RC range tests where I rotate planes to determine where the reception nulls are, the received RC signal strength is almost universally worst when the plane is either flying low and directly at you or directly away from you. i.e. when the fuselage is pointed straight toward or away from you. So coupling this with the NEXRAD beam center height calculations, the absolute worst potential for a signal interference hit is in clear weather (beam rotating slowly) when you are flying in the NE corner of the field (closer to NEXRAD) and you are low (under 500 feet and in the path of all possible beam rotation cycles) and heading straight back to you. I would call that a worse-case combination.
Keep in mind that if you do get hit that the beam is always moving and should only result in a second of actual signal loss*, then a short delay of a second or two to reacquire receiver lock. So keeping a cool head for a few seconds after any radio hit is important. And DO program your plane's failsafe mode if your RX/TX combination supports it.
* unless you happen to be flying in the radar's scan direction .
Recent change was made to NEXRAD
It bears mentioning that on June 27 of 2012 our NEXRAD site was upgraded to support dual polarization. This means that the radar pulses are no longer just transmitted with horizontal polarization, but rather they now also use vertical polarization. This gives the NEXRADs a much better ability to delineate between rain, snow, sleet, and hail. This also means we are in kind of a new coexistance trial with the radar since there is an additional "signal plane" of interference that could exacerbate problems with RC links. Time will tell.
So what about my plane?
Well, it turns out that two weeks later I had another signal loss event but this time I was on the downwind leg of a landing approach. Since the plane was only a few hundred feet away and very low that strongly reduced the possibility of a NEXRAD problem. Turns out, to make a longer story short, the used SD-10G transmitter I had bought was defective with a not-very-often intermittent signal failure. When the TX was replaced the signal loss events stopped. Q.E.D.
NEXRAD Theory of Operation: PDF attached (excellent explanation of how it all works)
NOAA NEXRAD site: http://www.roc.noaa.gov/WSR88D/ (loaded with good NEXRAD info)
NEXRAD dual polarization mod installed 6-27-12 (http://www.roc.noaa.gov/WSR88D/Publi...l/DPstatus.pdf)
Previous RCG Thread discussing NEXRAD: http://www.rcgroups.com/forums/showthread.php?t=971683
Feedback and comments welcome.
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