Zapped by Radar
Join Date: Feb 2001
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Inflight BBQ's,
When maintenance has the wx radar turned on on ground, it's most probabaly in test mode. I fly an old piece of junk with an AVQ10 wx radar, with that in test mode, it basically only tests the screen, not the TX. I wouldn't worry about it too much, but would avoid walking in front of the nose dome....I don't know what the test mode does on newer generation airplanes/wx radars.
I don't agree with having the wx radar off on the ground. Reason for that (call me nuts) is that on our old system it actually scares off the birds on the runway. If we depart from a runway with birds around it, we usually leave the thing on. Believe it or not, but they all disapear off the runway.
When maintenance has the wx radar turned on on ground, it's most probabaly in test mode. I fly an old piece of junk with an AVQ10 wx radar, with that in test mode, it basically only tests the screen, not the TX. I wouldn't worry about it too much, but would avoid walking in front of the nose dome....I don't know what the test mode does on newer generation airplanes/wx radars.
I don't agree with having the wx radar off on the ground. Reason for that (call me nuts) is that on our old system it actually scares off the birds on the runway. If we depart from a runway with birds around it, we usually leave the thing on. Believe it or not, but they all disapear off the runway.
Join Date: May 2001
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I think superpilut makes the best point about inhibiting the wx radar on the ground. Linking the radar to the ground proximity sensors and the flaps is the only way that this could be acceptable. Once, back in the day while flying the J32 I was number 7 in line for takeoff and the airport was on a ground stop because of storms within the boundary. Tower calls us up and says if you guys want to depart first you can be the "pathfinder". From our perspective, the wx did not look that bad so we said sure. Once on the runway with the radar turned on and tilted up, we quickly realised why we were given priority over the heavies. (19 pax vs. 200+). I'll think twice about being a pathfinder in the future.
Join Date: Jun 2003
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Ground inhibit for the radar? That's just making a simple problem complicated. Why not just put it on the checklist (in BOLD letters) so that it won't be forgotten? Then make it company policy for whoever forgets to be subjected to 10 minutes exposure infront so he/she will not do it again! 
On more than a few occasions I have noticed the WX radar sweeping along merrilly and displaying a nice solid bank of buildings in front of us.
Join Date: Apr 2002
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Confusing your Mw and Kw
Someone wrote "I'm not sure what the effective radiated power is for a weather radar - I assumed 50,000 watts" and continued "When I did the radhaz course, there was an exercise to find leaks in the waveguide of a 2 MW radar transmitter (the radiated power was much higher)"
Small point. Radar sets of this sort send out a pulse of energy then go quiet. That's because the radar set waits to see what comes back - which is of course a reflected pulse of radio energy from the storm cell (or hangar...).
If a radar set was emitting 50Kw of effective energy, think about the efficency of the radar transmitter. Say 50%? Then you'd need 100Kw of input energy with the other 50% appearing as heat. Is that a reasonable amount of power to obtain from the aircraft's generators? On that basis of efficency, a radar set of 2Mw (that's megawatts) would probably require a small ground based power station to run it...
Main point being a radar set can be thought of as a sophisticated microwave oven hence don't get very close to the radome when the radar is running. The effect of the weather radar on people is the same as that of (if you could) putting your body inside a working microwave oven. As a side effect, overcoming the safety switch on a microwave oven so that it works with the door open means that you have a nifty radar jammer which has be known to fool guided missiles...but don't stand beside the oven when doing this in a war zone.
Small point. Radar sets of this sort send out a pulse of energy then go quiet. That's because the radar set waits to see what comes back - which is of course a reflected pulse of radio energy from the storm cell (or hangar...).
If a radar set was emitting 50Kw of effective energy, think about the efficency of the radar transmitter. Say 50%? Then you'd need 100Kw of input energy with the other 50% appearing as heat. Is that a reasonable amount of power to obtain from the aircraft's generators? On that basis of efficency, a radar set of 2Mw (that's megawatts) would probably require a small ground based power station to run it...
Main point being a radar set can be thought of as a sophisticated microwave oven hence don't get very close to the radome when the radar is running. The effect of the weather radar on people is the same as that of (if you could) putting your body inside a working microwave oven. As a side effect, overcoming the safety switch on a microwave oven so that it works with the door open means that you have a nifty radar jammer which has be known to fool guided missiles...but don't stand beside the oven when doing this in a war zone.
ex jump pilot,
When talking about the power of a radar, it's customary to be talking of the peak power during the pulse. The average power is much less - by the duty cycle.
Assume you want to see out to 40km. That means the time between pulses is a minimum of 266microseconds. If you want resolution to 300 metres, the maximum pulse length is 1 microsecond. This gives a duty cycle of 1/266 or .00375. So a radar giving 50kW peak at the transmitter would actually be only 187.5 watts average.
I also said 'effective radiated power'. This is the power of the transmitter multiplied by the gain of the antenna. A 2 foot dish at 5Ghz will give around 20dB, so my example of the 50kW ERP is actually 500 watts (peak) of RF, and with a duty cycle of .00375, is only a few watts. In all probability, a magnetron based thing will run at around 20 or so watts average, and quite likely around 20kW peak. So the power requirements are well within the aircraft generator capabilities - unlike the start of WW2, where things like Whitleys and Blenheims reportedly had only about 500 watts for everything!
I agree that it's not a good idea to stand in front of one while it's working, though. Incidentally, although it's never as far as I know been formally studied, it is apparently well known amongst engineers working on high power microwave transmitters that when they father children, it's almost always daughters........
When talking about the power of a radar, it's customary to be talking of the peak power during the pulse. The average power is much less - by the duty cycle.
Assume you want to see out to 40km. That means the time between pulses is a minimum of 266microseconds. If you want resolution to 300 metres, the maximum pulse length is 1 microsecond. This gives a duty cycle of 1/266 or .00375. So a radar giving 50kW peak at the transmitter would actually be only 187.5 watts average.
I also said 'effective radiated power'. This is the power of the transmitter multiplied by the gain of the antenna. A 2 foot dish at 5Ghz will give around 20dB, so my example of the 50kW ERP is actually 500 watts (peak) of RF, and with a duty cycle of .00375, is only a few watts. In all probability, a magnetron based thing will run at around 20 or so watts average, and quite likely around 20kW peak. So the power requirements are well within the aircraft generator capabilities - unlike the start of WW2, where things like Whitleys and Blenheims reportedly had only about 500 watts for everything!
I agree that it's not a good idea to stand in front of one while it's working, though. Incidentally, although it's never as far as I know been formally studied, it is apparently well known amongst engineers working on high power microwave transmitters that when they father children, it's almost always daughters........
Join Date: Nov 2003
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Ok, interest piqued so had to look up some figs for this.
Radar safe distance is Max Permitted Exposure Level (MPEL). MPEL is a function of antenna/dish diameter, rated peak power output and duty cycle (time from start of one pulse to start of next pulse transmitted) - radeng explains a lot of this.
The American National Standards Institute (ANSI) gives an MPEL of 0.005 Watts per square centimetre though other national standards of 10mW/cm squared are sometimes used.
Old weather radar sets had to transmit high power because the receiving processor electronics were noisey and needed a higher power return to be able to show a picture of the weather. This isn't the case anymore. Some of the power outputs quoted above would do an Air Intercept radar on an F15 proud.
With newer, more sensitive processing electronics, a good return for showing weather can be of a much lower strength so the transmitter power can be a lot lower.
Transmitted power outputs on modern weather radars range from 1,300 watts (1.3kW) to 10kW giving Radar Safe Distances of 2.7ft for the Bendix RDS-86 fitted to the BAe ATP to 13ft for the Honeywell RDR-4B on large Boeings and Airbus.
So to answer the original post's question - the plane in front was quite safe.
Rule is though, don't switch the thing to transmit until you are taxiing out. Some aircraft have system disables installed but the pilot is the best controller of all.
Sorry you asked now?
Radar safe distance is Max Permitted Exposure Level (MPEL). MPEL is a function of antenna/dish diameter, rated peak power output and duty cycle (time from start of one pulse to start of next pulse transmitted) - radeng explains a lot of this.
The American National Standards Institute (ANSI) gives an MPEL of 0.005 Watts per square centimetre though other national standards of 10mW/cm squared are sometimes used.
Old weather radar sets had to transmit high power because the receiving processor electronics were noisey and needed a higher power return to be able to show a picture of the weather. This isn't the case anymore. Some of the power outputs quoted above would do an Air Intercept radar on an F15 proud.
With newer, more sensitive processing electronics, a good return for showing weather can be of a much lower strength so the transmitter power can be a lot lower.
Transmitted power outputs on modern weather radars range from 1,300 watts (1.3kW) to 10kW giving Radar Safe Distances of 2.7ft for the Bendix RDS-86 fitted to the BAe ATP to 13ft for the Honeywell RDR-4B on large Boeings and Airbus.
So to answer the original post's question - the plane in front was quite safe.
Rule is though, don't switch the thing to transmit until you are taxiing out. Some aircraft have system disables installed but the pilot is the best controller of all.
Sorry you asked now?
Join Date: Apr 2002
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Sorry you asked now?
Not at all. Thank you everyone who added to the debate. I was a bit unclear how the 'power' went from "50kW ERP is actually 500 watts (peak) of RF, and with a duty cycle of .00375, is only a few watts". Now I know.
Doesn't take much power to reach the upper advised limit (10mW/cm squared).
Thanks all who contributed - I'll make sure I keep "13ft for the Honeywell RDR-4B on large Boeings and Airbus" away when its wx radar is on....
Not at all. Thank you everyone who added to the debate. I was a bit unclear how the 'power' went from "50kW ERP is actually 500 watts (peak) of RF, and with a duty cycle of .00375, is only a few watts". Now I know.
Doesn't take much power to reach the upper advised limit (10mW/cm squared).
Thanks all who contributed - I'll make sure I keep "13ft for the Honeywell RDR-4B on large Boeings and Airbus" away when its wx radar is on....
