Predictive windshear avoidance radar

[barberpoll]

Hello,
just trying to get some info on Predictive windshear avoidance radar. Does anyone know what type CX uses? Is it actually a function of the airborne WX radar using doppler shift on the precipitation or is it more along the lines of using EGPWS and Mode 7? And which type uses which, or is common equipment used on all types?

In addition to this, does anyone know where the radar beam itself gets its power from?

Thanks.

[swh]

BP,

AFAIK they use doppler Wx radar will measure wind speed and direction. The radar is only capable of measuring horizontal head or tailwinds, cannot measure vertical or cross wind variations.

The software makes assumtions that for the corresponding horizontal variation there is a corresponding vertical component, leading to a "prediction" of windshear, when a threashold is exceeded a warning is generated.

My simplistic understanding.

:ok:

[Jerboa]

From my experience EGPWS Mode 7 is not Predictive Windshear.
If you get a EGPWS Mode 7 Windshear you are already in it.
Predictive is a function of the weather Radar and the alerts and cautions will be such that you can take avoiding action.
ie : 'Monitor radar display' or 'go-around windshear ahead'

[IBTheseus]

Predictive windshear will use the radar.

Reactive windshear is from the EGPWS

[barberpoll]

Thanks everyone for taking the time to provide answers and to share your knowledge.

Still not getting how excactly it (the radar) works though, sounds like I was correct in saying that it uses the radar to determine doppler shift on precipitation returns? But how does it sense wind direction and velocity ahead of the airplane? (horizontal head/tail) What is it painting? As it is true your not always going to have rain with windshear.

As for my question regarding the beam itself, I understand it is x wave and is transmited as a frequency signal is in watts. But how is the power generated?

Thanks again.

[betaboy]

Predictive windshear requires precipitation to work. Thus, it cannot detect CAT.

"Windshear prediction is radar based and is available below 1500’ AGL. It looks out to 5 nm ahead of aircraft. A warning message reading WINDSHEAR AHEAD will appear on PFD and ND. Color of the warning will be red or amber depending on level of warning. Levels include Advisory (display only) and the Warning and Caution messages have an aural warning alert as well. Predictive warnings are inhibited during takeoff after 100 kts. until 50’ AGL and then again inhibited on landing once below 50’ AGL. Windshear prediction uses the normal weather radar and there is only one radar installed. If the normal radar is turned off the windshear prediction will still operate normally if set to Auto. Prediction means that a possible windshear is ahead of you. Predictive windshear will not warn for CAT (Clear Air Turbulence), system must have precipitation to work."

From:
http://www.chipsplace.com/helpful/Airbus/Instrument.htm


Traditional windshear detection (GPWS Mode 7) uses the ADC to compute presence of windshear. It depends on sink rate at the time vs rate of speed loss in knots/sec.

[swh]

BP,

Remember reading somewhere that the radar can now pick up the movement of nitrogen molecules, well apparently they always could, just they were designed to filter them out in older radar types, newer types have better digital processing to allow this to be picked up.

Doppler only works on the axis of transmission, to get crosswind and vertical wind movements my thoughts would be multiple doppler units, say one on each wing tip and one on the tail, the sum of the momvement would get the movement of the air, as a vector, direction and speed.

However present day types are basically along only one axis, so the signal processing measures the head or tailwind change, and assumes this corresponds to a similar vertical movement.

Having been in 20+ of windshear on finals, or in CAT, without warning from these types of radars, they should be thought of as tools thats will not always show everything, but aid in your situational awareness

:ok:

[palgia]

Remember reading somewhere that the radar can now pick up the movement of nitrogen molecules, well apparently they always could, just they were designed to filter them out in older radar types, newer types have better digital processing to allow this to be picked up.

Can you give us a source for this information?

I personally find it very hard for the radar to pick up molecles... maybe it can pick up aerosol particles... but not nitrogen molecues...imho

I know a lot of research has been done on LIDARS whch are able to pick up the doppler effect using aerosols (therefore working even without the presence of precip).
However I never heard of this technique being implemented YET. It was research that is still ongoing. The cost to retrofit existing aircraft is the main obstacle... as always.

As far as airborne wx radars being able to detect aerosols, I would appreciate any information. I am familiar with ground-based wx radars but I never heard of an airborne radar being able to do that.

Thanks,

palgia

[saline]

Still not getting how excactly it (the radar) works though, sounds like I was correct in saying that it uses the radar to determine doppler shift on precipitation returns?

Yes it is doppler shift. If you don't understand doppler do a little research, it is a basic concept.

But how does it sense wind direction and velocity ahead of the airplane? (horizontal head/tail) What is it painting? As it is true your not always going to have rain with windshear.

Predictive radar systems are targeted at downbursts coming from thunderstorms. Even so-called "dry" microbursts contain some precipitation so the radar system should be effective to some extent in that environment.

Predictive windshear using radar requires the presence of precipitation. This is not really a limitation. There is no historical evidence of windshear bringing down commercial aircraft in the absence of significant precipitation.

As for my question regarding the beam itself, I understand it is x wave and is transmited as a frequency signal is in watts. But how is the power generated?

The microwave power is generated like any radio signal. There is both a transmitter and a receiver as components of the radar. The key to radar is, the transmission is focused into a narrow beam which is deliberately focused on the area ahead. Traditionally that is what the saucer shaped antenna is for. (Much like the light of a flashlight is focused into a narrow beam.) The reason it detects rain so well is that rain also has a concave surface to focus the return back at the antenna. This is why snow and ice particles don't paint well, if at all. Instead the signal gets dispersed in all directions whereas rain drops focus the return signal back at the transmission point. The X frequency was selected because it is the best compromise of frequencies as far as ability to penetrate into weather and give a more accurate display. But it is still very prone to attenuation of signal by heavy rain.

I have flown the system for a few years in thunderstorm environments. I've found it to be reliable and not prone to false alarms in sharp contrast to reactive systems.

[barberpoll]

Thank you again everyone for your replies, they are a great help.
Much appreciated.

Regards, BP

[ICT_SLB]

Barberpoll,
FYI there is another form of radar on smaller aircraft which can provide similar WS alerting over & above the reactive from GPWS. This is Turbulence Detection - it is a normal WXR with a very stable transmitter and a more accurate frequency gate provided by an airspeed input into the radar. This gives a theoretical center frequency - turbulence will be shown up by doppler-shifted signals both above or below the center frequency as the precip or dust moves towards or away from the aircraft.

As far as I can remember this does not have a height limit - the crew can just select TURB mode at any altitude.