5.2 Introduction
However, capabilities of some technologies are either
A remote-sensing system designed to detect icing
enhanced or severely limited by the direction they are
potential ahead of aircraft would consist of three com-
sensing, either horizontally or vertically.
ponents:
Remote sensing of aircraft icing conditions is devel-
oping along two parallel paths, depending upon the tech-
A suite of detectors to measure conditions that cause
nology. One requirement is to create an ability to
icing
remotely detect icing conditions ahead of aircraft from
A processing system to integrate information, as it is
sensors mounted on the aircraft. However, since more
received from the sensors, to assess the icing hazard
aircraft are exposed to icing in the departure and
An information display system to provide pilots with
approach phases of flight, another requirement is to
timely, useful maps of the icing hazard.
develop ground-based systems that are capable of scan-
The sensing system, which acquires the information
ning the airspace around airports. Airport-based sens-
to provide a measure of icing potential with distance
ing systems are likely to be developed before airborne
ahead of the aircraft, must accomplish several tasks.
systems because greater technological development has
First, it must detect, either directly or through the use
occurred with ground-based systems, and because they
of surrogates, cloud and precipitation liquid-water con-
present fewer weight, power, and size restrictions.
tent, temperature of the droplets (or the existence of
It is currently unlikely that one technology will be
supercooling), and some measure of the breadth of the
able to sense liquid water, temperature, and elements
drop-size spectrum. It must map the magnitude of these
of the drop-size spectrum, so a remote-sensing system,
conditions ahead of the aircraft for avoidance purposes,
whether ground-based or airborne, will probably con-
and it must locate where icing conditions do not exist
sist of multiple technologies to obtain all the necessary
ahead of the aircraft for escape purposes if the aircraft
information. Technologies under consideration include
is immersed in icing. In addition, the location of each
radar, lidar, passive microwave radiometers, and radio
condition must be measured in three-dimensional space
acoustic sounding systems (RASS).
often enough to provide a continuously updated image
5.3 Radar
of conditions to the pilot. The system must also scan a
sufficiently large volume of atmosphere, to a great
Radar is the most mature technology under consid-
enough distance and with sufficient detail, to provide
eration. Initially developed in the 1930s by the British,
pilots with avoidance and escape options. Since remote
it was first used for detecting weather phenomena imme-
sensing is a stand-off technique, sensing systems typi-
diately after World War II (Battan 1973, Toomay 1982).
cally detect and measure phenomena without being
Weather radar operates in the shorter wavelengths of
immersed them. To effectively assist escape from icing,
the microwave spectrum and into the centimeter wave-
however, they must also be able to sense that icing con-
length spectrum, with longer wavelength (lower fre-
ditions do not exist in a volume of air ahead of the air-
quency) radars used to detect larger drops such as rain
craft while sensing from within icing conditions.
and shorter wavelengths used to detect cloud droplets.
A variety of technologies are available for remotely
Radar wavelengths for atmospheric sensing can be spec-
sensing atmospheric properties (Westwater and Krop-
ified by band, frequency, or wavelength (Table 1).
fli 1989). Current technologies are designed to operate
As wavelength shortens, the ability to detect smaller
from ground positions to zenith or from satellites to
drop sizes improves, but range decreases and attenua-
nadir. Some sensors can scan the atmosphere at inter-
tion by precipitation and other atmospheric constitu-
mediate angles, and some can scan in the horizontal.
ents such as water vapor increases as well (Houze 1993).
Table 1. Relationships between radar band, frequency,
wavelength, and weather parameter sensed.
Nominal
frequency
Nominal
Meteorological
Band
(GHz)
wavelength
condition sensed
S
3 GHz
10 cm
Precipitation (NEXRAD)
C
6 GHz
5 cm
Precipitation (ships)
X
10 GHz
3 cm
Precipitationclouds (aircraft)
Ku
15 GHz
2 cm
Precipitationclouds
K
30 GHz
1 cm
Precipitationclouds
Ka
35 GHz
8.7 mm
Cloud droplets
W
94 GHz
3.2 mm
Cloud droplets
31
To Contents
Previous Page
