retrieval algorithms, noise, and Mie scattering present
requirements of the system, the environment to be
major radar technological challenges. Although radar can
sensed, and the technology available to accomplish the
be used in a variety of orientations, scanning vertically
task. This report identifies the state of knowledge,
from the ground or horizontally from the air, its size,
strengths, weaknesses, major issues, barriers, and oppor-
weight, and power demands make ground-based radar a
tunities, and it identifies the research and investment
more viable near-term technology.
needed to create a prototype icing remote-sensing sys-
Microwave radiometer development is less mature
tem.
than radar technology. However, the recent introduction
2.2 Scope
of a radiometer that scans and profiles temperature, water
This report provides a framework for the develop-
vapor, and cloud liquid water, and experimentation with
ment of a plan for creating in-flight icing-avoidance
techniques to use radiometers in a horizontal sensing
remote-sensing systems for use in the national airspace.
mode, in addition to the more traditional vertical or near-
There are three strategic elements:
vertical modes, are promising. Microwave radiometers
are passive, an advantage to the military, but they lack
Identifying the operational needs of pilots, opera-
absolute ranging capability--a disadvantage. Identifica-
tors, manufacturers, and regulators as to functional
tion of cloud glaciation and drop size with polarization
requirements, system utilization, aircraft integra-
is an additional useful radar and microwave radiometer
tion, and human factors
capability.
Identifying sensing requirements
Lidar is not considered a viable technology for remote
Identifying technologies, and their state of devel-
sensing of icing conditions because it cannot sense deeply
opment, for an integrated sensing system.
into optically thick clouds. Remote detection of temper-
ature is possible with microwave radiometers and RASS
2.3 Goal
from the ground, but perhaps only by using microwave
This report is intended to provide background infor-
radiometers from aircraft. Considerable development is
mation to facilitate the creation of a development plan
needed in this area.
to improve aircraft operational capabilities and safety
The most critical needs in operational research are to
in icing environments. This will be accomplished by
assess cockpit and aircraft system integration, develop
developing remote-sensing systems that provide pilots
avoid-and-exit protocol, and assess the human factors
with information about the location and intensity of in-
in using remotely sensed icing information. In addition,
flight icing hazards, giving them the ability to avoid
remotely sensed icing information must be integrated
and exit icing expediently. This will
into weather and air traffic control infrastructures, and air-
Increase safety
craft flight envelopes and the hazard of icing to aircraft
Reduce delays
performance in these envelopes need better definition.
Increase aircraft utilization
Improved absolute and spatial characterization of cloud
Increase military readiness.
and precipitation liquid-water content, drop-size spectra,
and temperature are needed to develop remote-sensing
One method of either avoiding or escaping icing is
system specifications. An icing metric must also be devel-
to sense remotely, either from the ground or from air-
oped that will allow the sensed microphysical conditions
craft, atmospheric icing potential (Ryerson 1996, 1997,
to be converted into a measure of icing potential for air-
1998). This requires scanning the airspace ahead of an
craft. Technology development requires refinement of
aircraft for supercooled water and presenting that infor-
inversion techniques for unambiguously retrieving
mation to the pilot in a manner consistent with effi-
liquid-water content, drop size, and temperature from
cient cockpit risk assessment.
clouds and precipitation. These goals can be accom-
Currently, no dedicated system exists for remotely
plished with strong leadership and collaboration among
sensing the icing potential in a projected flight path for
federal agencies, including NASA, the FAA, the National
an individual aircraft. A remote-sensing system that
Center for Atmospheric Research, NOAA, and DoD.
advises pilots of the icing risk ahead of an aircraft will
Partnership between government and industry will bring
be an information management system that senses the
viable technologies to prototype and to market.
environment, processes the sensed information, and
presents it in a useful form. This requires that the proper
environmental parameters be sensed with an accuracy
2.0 INTRODUCTION
suitable for providing useful information, that the infor-
2.1 Purpose
mation be processed with sufficient speed to assist pi-
Development of a remote-sensing icing-avoidance
lots, and that information be presented in a manner that
avionics system requires assessment of the operational
aids pilots in making icing risk-management decisions.
2
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