| Abstract: |
Icing conditions pose a threat to aircraft, including unmanned aerial vehicles. For aircraft without deicing equipment, a standoff system that could detect hazardous conditions before they are encountered is strongly desired. Three-dimensional radiative transfer modeling, including the effects of multiple scattering, suggest microwave and millimeter wave radiometers have sensitivity to the key environmental variables required to identify hazardous conditions even at horizontal (flight path) geometries. Recent work by Adams and Bobak is built upon and related more specifically to this problem. The requirements for such a system are examined, as well as an examination of the intersection of frequencies with high information content with existing (or near future) systems that can be used for field experiments.
For aviation, icing severity can be broken down into four categories: Trace, Light, Moderate, and Heavy. The severity categories were developed to provide consistent definitions for Pilot Reports (PIREPS), which is the source for much of the current aviation icing conditions information. In order to make a connection with meteorological conditions, rather than a basis on ice growth on an airframe, several authors connected the categories with a specific level of liquid water content (LWC). The other key meteorological parameters for determining the threat level are static air temperature and droplet size.
Rotary wing aircraft (helicopters) are particularly sensitive to small amounts of ice build up. Severity for a certain set of conditions will be dependent upon the specific airframe. A 1974 study in which freezing spray was directed onto helicopters in flight showed that as little as 4 minutes of flight in moderate icing conditions or 10-11 minutes in light icing conditions was enough for an AH1 attack helicopter to suffer significant performance degradation. Flight speed for these tests was 90 knots, or about 46 m/s.
The flight speed and time to significant degradation can be combined to determine a resolution volume which must be detectable. Ten minutes (600 seconds) at 46 m/s results in a path length of about 27.6 km. A cloud with flight path dimensions of 27.6 km or more, and liquid water content commensurate with light icing should be detectable in order to have a good standoff system for this particular airframe. Similarly, at LWC levels that result in moderate icing conditions, a cloud that extends about 11 km should be detected.
The publicly-available ARTS (Atmospheric Radiative Transfer Simulator) code is used to simulate the brightness temperature of various scenes (no cloud, subarctic mixed phase cloud, midlatitude winter cloud) at various view geometries, including horizontal. This code allows for multiple scattering. The subarctic cloud is largely based on data from Shupe, et al., [2008] while the midlatitude cloud is a general representation of typical conditions. Brightness temperatures were simulated from 30-200 GHz, while varying liquid water content, temperature, ice water content, distance from the cloud and (new) thickness of the cloud. Adams and Bobak [2017], hereinafter referred to as AB17, has additional information about the simulations and information content as a function of frequency. |
