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Rod Machado’s Private/Commercial Pilot Handbook
13-22

NEXRAD—Not Your Daddy’s Weather
The revolution in modern avionics has made it possible
for the average private pilot to have access to in-flight
cockpit weather that not long ago was only a dream. With
the appropriate receiver, a moving map GPS display of
the portable (Figures 26 and 27) or panel-installed vari-
ety and a subscription to one of several weather service
providers, it’s possible to have airborne access to all sorts
of weather data. This includes METAR and TAF informa-
tion, winds aloft, in-flight weather advisories, radar infor-
mation and much more. In this chapter we’ve covered
most of the types of in-flight weather available for uplink
to the cockpit, but there’s one weather product that
deserves its own explanation. I’m speaking of Fig. 28
NEXRAD or Next Generation Radar.
The National Weather Service supports many Doppler radar sites
As a VFR pilot, NEXRAD becomes an extremely valu- across the United States.
able source of weather information when making long
distance cross country flights. Why? Because it shows The WSR-88D radar works on the same principle as
you the location of suspended moisture, much like the other radars except that Doppler radar spends more
radar summary chart does. Only NEXRAD does it in time listening than transmitting. (Airborne radar, for
more detail and more often. Armed with cockpit instance, transmits less than 1% of the time and spends
uplinked weather, you’re now in a much better position 99%+ “listening” for the return of information).
to avoid storm systems, areas of precipitation and so Doppler radar spends approximately 7 seconds out of
on. each hour of operation in the transmitting mode. The
rest of the time (59 minutes and 49 seconds) is spent
Let’s take a closer look at how NEXRAD weather infor-
mation is generated. listening.
What does Doppler radar listen for? Like other radar,
Next Generation Radar it listens for the return of the energy pulse it sent. The
difference is that it also listens for any phase shift associ-
NEXRAD is a network of Doppler radars strategical-
ly located around the country and tied into a national ated with this returning signal. “Phase shift” is a scien-
network (Figure 28). While NEXRAD presents a nor- tific way of saying that the pitch of a sound changes as it
moves closer to or farther from the listener (human or
mal multicolor rain rate display, its Doppler circuits
radar). As a train approaches, the sound waves from its
can also depict whirling of droplets that pinpoints tur-
horn are compressed, making the pitch higher as the
bulence.
train gets closer. As the train passes, the sound waves
NEXRAD is a combination of Doppler radar (WSR-88D from the horn are stretched, which lowers the pitch
to those who know) and conventional radar.
(Figure 29). This is the “Doppler shift,” from which
Doppler radar draws its name.
The phase shift of the returning energy pulse allows
Doppler radar to detect any change in velocity of an
PANEL MOUNT GPS UNIT object, either toward or away from the radar. Particles
moving toward the antenna impart a minute shift to
THE DOPPLER EFFECT

Fig. 27

Fig. 26

Uplinked weather is available with most panel-mount GPS units Fig. 29
(Figure 26) and portable GPS units such Garmin’s 496 portable Doppler radar listens for a phase shift to determine the motion of
GPS receiver (Figure 27) and a subscription to XM Weather. atmospheric particles. You experience a similar phase shift in
sound when you listen to a passing locomotive.