punkalouver
8th Jan 2010, 20:00
An interesting incident report here on how newer wx radars operate differently than older units. Has anyone noticed significant differences in their own experience.
http://www.cad.gov.hk/reports/main3.pdf
It is recognized that all airborne weather radars do
operate in a similar fashion and on broadly the same
principles. However, there are significant differences
between modern, ‘flat-plate’ antenna radars, such as the
RDR-4B, and older, parabolic antenna radars. These
older radars, on which many senior pilots gained
experience, have up to 15 times the power of modern
radars, are able to detect close-in weather at lower
altitudes relative to the aircraft due to large side lobes
and generally require less ‘effort’ to interpret a radar
picture. In comparison, the RDR-4B radar focuses
radar energy in a narrow ‘pencil’ beam, the power of
which is greatest at the centre and reduces as the
angular distance from the centre of the beam increases,
with little energy emitted as extraneous side lobes. The
narrow beam and loss of side lobes make tilt and range
control more critical. The beam width of the RDR-4B
radar is nominally 3 degrees. The radar energy is
focused at the middle of the 3-degree beam, reducing to
half power at the edge of the beam. Thus while radar
targets having good reflectivity may produce returns
from the 3 degree periphery, targets with less
reflectivity may not ‘paint’ at all.
The Honeywell RDR-4B Forward Looking Windshear/Weather
Avoidance Radar System User’s Manual gives a very detailed
description of the operating procedures for the weather radar, with
particular emphasis on range selection and antenna tilt management.
Adopting the procedures in this document, or alternatively, those
described in FCOM 3.04.34, would have adequately established a
protection zone ahead of the aircraft. Both documents stress that it
is important for the antenna tilt angle to be lowered progressively
as the aircraft approaches weather to maintain a clear radar picture
of the weather ahead. This will help to ensure that a deviation will
clear any weather hazard by a safe margin. However, there was no
evidence that these procedures were adopted prior to the
occurrence. Although the Honeywell RDR-4B radar has a feature
that provide an automatic increase in gain above 25,000 feet to
compensate for the lower reflectivity of the ice crystals in the upper
levels of a storm cell, the radar remains limited by the reflectivity
characteristics of the target being scanned and the available gain. It
is for this reason that the RDR-4B Users Manual stated repeatedly
that effective tilt management is the single, most important key to
more informative weather radar displays.
http://www.cad.gov.hk/reports/main3.pdf
It is recognized that all airborne weather radars do
operate in a similar fashion and on broadly the same
principles. However, there are significant differences
between modern, ‘flat-plate’ antenna radars, such as the
RDR-4B, and older, parabolic antenna radars. These
older radars, on which many senior pilots gained
experience, have up to 15 times the power of modern
radars, are able to detect close-in weather at lower
altitudes relative to the aircraft due to large side lobes
and generally require less ‘effort’ to interpret a radar
picture. In comparison, the RDR-4B radar focuses
radar energy in a narrow ‘pencil’ beam, the power of
which is greatest at the centre and reduces as the
angular distance from the centre of the beam increases,
with little energy emitted as extraneous side lobes. The
narrow beam and loss of side lobes make tilt and range
control more critical. The beam width of the RDR-4B
radar is nominally 3 degrees. The radar energy is
focused at the middle of the 3-degree beam, reducing to
half power at the edge of the beam. Thus while radar
targets having good reflectivity may produce returns
from the 3 degree periphery, targets with less
reflectivity may not ‘paint’ at all.
The Honeywell RDR-4B Forward Looking Windshear/Weather
Avoidance Radar System User’s Manual gives a very detailed
description of the operating procedures for the weather radar, with
particular emphasis on range selection and antenna tilt management.
Adopting the procedures in this document, or alternatively, those
described in FCOM 3.04.34, would have adequately established a
protection zone ahead of the aircraft. Both documents stress that it
is important for the antenna tilt angle to be lowered progressively
as the aircraft approaches weather to maintain a clear radar picture
of the weather ahead. This will help to ensure that a deviation will
clear any weather hazard by a safe margin. However, there was no
evidence that these procedures were adopted prior to the
occurrence. Although the Honeywell RDR-4B radar has a feature
that provide an automatic increase in gain above 25,000 feet to
compensate for the lower reflectivity of the ice crystals in the upper
levels of a storm cell, the radar remains limited by the reflectivity
characteristics of the target being scanned and the available gain. It
is for this reason that the RDR-4B Users Manual stated repeatedly
that effective tilt management is the single, most important key to
more informative weather radar displays.