Andrew L. Reehorst

Glenn Research Center, Cleveland, Ohio

David J. Serke

National Center for Atmospheric Research, Boulder, Colorado

A Terminal Area Icing Remote Sensing System

NASA/TM—2014-218417

November 2014
NASA STI Program . . . in Pro¿ le

Since its founding, NASA has been dedicated to the 

advancement of aeronautics and space science. The 

NASA Scienti¿ c and Technical Information (STI) 

program plays a key part in helping NASA maintain 

this important role.

The NASA STI Program operates under the auspices 

of the Agency Chief Information Of¿ cer. It collects, 

organizes, provides for archiving, and disseminates 

NASA’s STI. The NASA STI program provides access 

to the NASA Aeronautics and Space Database and 

its public interface, the NASA Technical Reports 

Server, thus providing one of the largest collections 

of aeronautical and space science STI in the world. 

Results are published in both non-NASA channels 

and by NASA in the NASA STI Report Series, which 

includes the following report types:

• TECHNICAL PUBLICATION. Reports of 

completed research or a major signi¿ cant phase 

of research that present the results of NASA 

programs and include extensive data or theoretical 

analysis. Includes compilations of signi¿ cant 

scienti¿ c and technical data and information 

deemed to be of continuing reference value. 

NASA counterpart of peer-reviewed formal 

professional papers but has less stringent 

limitations on manuscript length and extent of 

graphic presentations.

• TECHNICAL MEMORANDUM. Scienti¿ c 

and technical ¿ ndings that are preliminary or 

of specialized interest, e.g., quick release 

reports, working papers, and bibliographies that 

contain minimal annotation. Does not contain 

extensive analysis.

• CONTRACTOR REPORT. Scienti¿ c and 

technical ¿ ndings by NASA-sponsored 

contractors and grantees.

• CONFERENCE PUBLICATION. Collected 

papers from scienti¿ c and technical 

conferences, symposia, seminars, or other 

meetings sponsored or cosponsored by NASA.

• SPECIAL PUBLICATION. Scienti¿ c, 

technical, or historical information from 

NASA programs, projects, and missions, often 

concerned with subjects having substantial 

public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scienti¿ c and 

technical material pertinent to NASA’s mission.

Specialized services also include creating custom 

thesauri, building customized databases, organizing 

and publishing research results.

For more information about the NASA STI 

program, see the following:

• Access the NASA STI program home page at 

http://www.sti.nasa.gov

• E-mail your question to help@sti.nasa.gov

• Fax your question to the NASA STI 

Information Desk at 443–757–5803

• Phone the NASA STI Information Desk at

 443–757–5802

• Write to:

 STI Information Desk

 NASA Center for AeroSpace Information

 7115 Standard Drive

 Hanover, MD 21076–1320
Andrew L. Reehorst

Glenn Research Center, Cleveland, Ohio

David J. Serke

National Center for Atmospheric Research, Boulder, Colorado

A Terminal Area Icing Remote Sensing System

NASA/TM—2014-218417

November 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identi¿ cation 

only. Their usage does not constitute an of¿ cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-218417 1 

A Terminal Area Icing Remote Sensing System 

Andrew L. Reehorst 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

David J. Serke 

National Center for Atmospheric Research 

Boulder, Colorado 80301 

Abstract 

NASA and the National Center for Atmospheric Research (NCAR) have developed an icing remote 

sensing technology that has demonstrated skill at detecting and classifying icing hazards in a vertical 

column above an instrumented ground station. This technology is now being extended to provide 

volumetric coverage surrounding an airport. With volumetric airport terminal area coverage, the resulting 

icing hazard information will be usable by aircrews, traffic control, and airline dispatch to make strategic 

and tactical decisions regarding routing when conditions are conducive to airframe icing. 

Building on the existing vertical pointing system, the new method for providing volumetric coverage 

will utilize cloud radar, microwave radiometry, and NEXRAD radar. This terminal area icing remote 

sensing system will use the data streams from these instruments to provide icing hazard classification 

along the defined approach paths into an airport. 

Strategies for comparison to in-situ instruments on aircraft and weather balloons for a planned NASA 

field test are discussed, as are possible future applications into the NextGen airspace system. 

Introduction 

Aircraft icing continues to be a major issue for the aircraft community. Despite advances in all 

aspects of icing related technologies, icing accidents still occur and aircraft winter operations are 

negatively impacted. Part of the challenge of dealing with icing conditions is the significant spatial and 

temporal variability of icing severity. It is not unusual for a series of aircraft on a landing approach to 

experience different levels of icing severity over a relatively short period of time. This variability makes 

the accurate prediction of icing conditions very difficult. While weather models and their predictions of 

icing have improved dramatically, the need for the direct detection and measurement of hazardous icing 

conditions aloft is still great. 

Ideally all aircraft would have onboard systems to indicate icing conditions in their flight path, 

analogous to thunderstorm detection with airborne radar. However, even if these systems were developed, 

and became practical and affordable, the likelihood of all aircraft being adequately equipped is low. 

Therefore a better approach is to provide coverage to indicate icing conditions in areas of greatest traffic 

volume and flight risk: the terminal area. 

Development of a ground-based hazard detection algorithm test bed, the NASA icing Remote Sensing 

System (NIRSS), began in 1997 (Ref. 1). NIRSS consists of three vertically pointed instruments: a Metek 

Ka-band cloud radar, a Radiometrics Corporation multi-frequency microwave radiometer, and a Vaisala 

laser ceilometer. The radar provides cloud base and tops and particle density distribution information, the 

radiometer provides temperature profiles and integrated liquid (water path) measurements, and the 

ceilometer provides refined cloud base measurements. The NIRSS algorithm combines these 

measurements to determine the presence of icing conditions aloft and assigns a severity based on the 

calculated local liquid water content (LWC) intensity. NIRSS is shown notionally in Figure 1 and its 

components are shown in Figure 2. 
NASA/TM—2014-218417 2 

Figure 1.—NIRSS concept. 

Figure 2.—NIRSS hardware. METEK Ka-band radar in foreground, Vaisala ceilometer in middle left, and 

Radiometrics MP3000 in back left. 
NASA/TM—2014-218417 3 

A statistical analysis examining the agreement of NIRSS and the Current Icing Product (CIP) with 

pilot Reports (PIREPs) found that NIRSS performed well with an 80 percent positive PIREP agreement 

and a 70 percent negative PIREP agreement (Ref. 2). 

An acknowledged shortcoming of the NIRSS technology has been that it only provides a vertical 

profile of icing condition severity. To help fully protect a terminal area and provide information that 

accounts for the temporal and spatial variability of icing conditions, a volumetric remote measurement 

capability is required. 

To provide the most utility to the widest range of aircraft, an icing remote sensing system with airport 

terminal volume coverage is envisioned and is shown notionally in Figure 3. 

A terminal icing remote sensing system needs to provide reliable and timely information to operators 

and traffic managers. The volume of coverage needs to be of a reasonable range and altitude, allowing for 

routing flexibility to avoid hazardous conditions for both arriving and departing traffic. 

This paper describes such a terminal area remote sensing system that can practically determine icing 

severity aloft with coverage over an entire airport terminal area. This capability is being developed by 

NASA and NCAR and has reached a level of maturity that warrants prototype testing. An initial prototype 

testing of this system at NASA Glenn Research Center is discussed along with a strategy to compare 

remotely sensed data with in-situ flight data within a terminal area. 

Figure 3.—A terminal area icing remote sensing concept. 
NASA/TM—2014-218417 4 

Hardware Systems 

An early concept for a terminal area icing remote sensing capability made use of the current vertical 

pointing NIRSS algorithms with the addition of a scanning radar and a scanning radiometer to provide the 

required 3-D measurements. However, there are physical constraints with both radar and radiometer 

technologies that prevent this extension. 

Meteorological radar systems primarily span a frequency range from S-band (2 to 4 GHz or 15 to 

7.5 cm) to W-band (75 to 110 GHz or 4 to 2.7 mm). S-band radars such as NEXRAD (around 10 cm 

wavelength) have large antennas and powerful transmitters to allow them to observe precipitation and 

severe weather at long ranges (hundreds of kilometers). However they are generally unable to accurately 

quantify the macro- and micro-physical details of non-precipitating cloud particles because of their 

insensitivity to small hydrometeors, relatively coarse spatial resolution, and ground clutter issues. Ka- and 

W-band meteorological radars (3 to 9 mm wavelengths) are designed specifically to monitor clouds and 

have excellent sensitivity and spatial resolution, but suffer from significant attenuation in the presence of 

liquid water that limits their range to 30 km or less (Ref. 3). Thus, both the NEXRAD and Ka-band have 

potential drawbacks, but as will be discussed shortly, the availability of both provides complimentary 

parts of the answer toward the problem of in-flight icing detection within a terminal-area sized volume. 

One shortcoming of current microwave radiometer technology is the requirement of a dwell time for 

each measurement. Scanning a volume of space with even a limited number of microwave frequencies 

would require approximately an hour. The icing environment may vary appreciably during this time, 

making a volume scanning radiometer, with the current required dwell times, unacceptable for use in a 

terminal area icing remote sensing system. 

At first glance these constraints of radar and radiometer technologies appear to indicate that a remote 

sensing system that can provide volumetric coverage for a terminal area is impractical. However, utilizing 

the strengths of these sensing technologies along with an additional processing step allowing the 

movement of features with time, a useful remote sensing capability is possible. The icing remote sensing 

system described here will make use of a vertical pointing Ka-band radar, a multi-frequency microwave 

radiometer with limited azimuth and slant elevation positioning, and NEXRAD feature extraction and 

tracking. The instruments that are specifically planned for use in the terminal area icing remote sensing 

system are as follows: (1) the cloud radar is a Metek MIRA-36 vertical pointing Ka-band Doppler radar 

(Ref. 4), (2) the microwave radiometer is a Radiometrics Corporation MP-3000A, a multi-frequency, 

microwave radiometer (Ref. 5), and (3) the NEXRAD radar is referenced as KCLE and is located 

adjacent to NASA Glenn Research Center and Cleveland Hopkins Airport and operated by the National 

Oceanic and Atmospheric Administration (NOAA), National Weather Service (NWS). 

Terminal Area Algorithms 

The algorithms to determine icing hazard within an airport terminal area have their foundation in the 

algorithms used in NIRSS. The primary data used by the NIRSS are the temperature profile and 

integrated liquid water (ILW) from the microwave radiometer, and the cloud layering information from 

the cloud radar. The radiometer derived temperature profile is used to identify altitudes in the atmosphere 

where liquid water could be present. The ILW from the radiometer is then distributed within the potential 

liquid cloud heights using the cloud layering information from the cloud radar to determine a liquid water 

content (LWC) profile. Liquid water that has been identified at sub-freezing altitudes is then identified as 

an icing layer. Finally, the icing layer LWC values are mapped into qualitative in-flight icing hazards 

values. 

The development of a terminal area icing remote sensing capability requires the coverage of critical 

terminal airspace while living within the hardware technology constraints discussed in the previous 

section. Thus, a scanning cloud radar is not sought. Rather, the vertical cloud profile is assumed to apply 

to the entire terminal volume. This is a reasonable assumption since most weather events do not vary 
NASA/TM—2014-218417 5 

significantly within 50 horizontal kilometers, when dealing with 1 to 5 min time resolution. One notable 

exception to this assumption is strong cold-frontal passages. 

Since the radiometer can be azimuthally positioned to allow measurements at multiple discrete 

headings within 5 min, slant-angle radiometer ILW and temperature profile can be practically collected 

along all of the airport approach azimuths. Having a radiometer that can scan along each respective 

approach runway at multiple elevations can provide individualized runway approach vector hazard 

warning values. Previous research also shows that slant angle products from the radiometer have 

improved vertical resolution in the lowest few kilometers and thus higher overall accuracy compared to 

vertically profiled values (Ref. 6). 

The domain of concern for the software is the total volume of airspace within 25 horizontal 

kilometers of an airport. The software needs to ingest slant elevation ILW fields at various azimuths along 

with all of the vertically pointing NIRSS fields (temperature, ILW, LWC, droplet size, cloud base, cloud 

top, number of layers, etc.), package these profiled fields into data 'packets', and use one of two methods 

to advect the data packets into the 3-D airport terminal environment. This needs to be done every 5 min, 

so that several packets of various ages are located within the terminal domain at any given time—each 

containing all of the relevant meteorological fields for in-flight icing diagnosis. 

The first advection method will ingest NEXRAD reflectivity data and run it through previously 

existing pattern recognition and tracking software (Ref. 7). This code is setup to output East-West and 

North-South vector components of the horizontal advection as it follows the 0 dBZ reflectivity contour in 

time at the 1 km AGL height level. The second advection method relies on surface wind speed and 

direction values from the NWS Automated Surface Observing System ground station for use in cases 

when NEXRAD data is not available. In this manner, patterns in NEXRAD reflectivity or surface winds 

speed/direction are used to advect packets of profiled input and derived output fields within the airport 

domain. 

Aircraft are most vulnerable to in-flight icing during approach, flying at lower speeds at near-constant 

altitudes for a prolonged period of time. Each airport has predetermined approach routes for aircraft, with 

rules that define the aircraft altitude at given distances for each different runway heading. The software 

needs to model these approach vectors by inflating the 2-D centerline, as defined by FAA approach 

plates, into a 3-D volume to represent the air volume that an aircraft could be affected by during 

approach. This volume is defined by centering a box of 0.25 km (~1000 ft) vertical by 2 km (~6500 ft) 

horizontal on the vector (the lateral size of the boxes are doubled at the outer portions of the approach 

path to accommodate defined holding areas). The resulting approach volumes are then sectioned 

horizontally into 9 zones based on runway heading and radial distance from the tarmac: Zone (1) contains 

all of the approach volume within a 5u5 km box centered over the airport is defined in one volume zone; 

Zones (2) through (5) contain 4 volume zones along the runway headings outside the center box and 

inside a 25u25 km box and; Zones (6) through (9) contain four more volume zones extending from the 

middle box out to the outer 50u50 km box (Figs. 4 and 5). Figure 4 is a top-view, 2-D representation of 

all the approach zones for the prototype system at Cleveland Hopkins Airport and Figure 5 shows an 

isometric view with more details of zones 1 through 9 for one approach path. The most recent hazard 

output profile that is determined by the software to be located within a given volume zone is applied to 

that zone at 5 min intervals. The highest hazard level designation (on a scale from 'none', 'trace', 'light', 

'moderate' to 'severe') that exists within the defined vertical bounds of the given volume zone is assigned 

as that zone's in-flight hazard value at a given time. 

Takeoffs are typically at higher energy levels and gain airspeed and altitude rapidly thus spending 

minimal time in icing conditions that tend to be stratified. While considered a lower priority than the 

approach coverage, a separate warning output for departure quadrants will need to be added in the future 

to achieve full terminal area coverage. 
NASA/TM—2014-218417 6 

50 x 50 km boundary

25 x 25 km boundary

CLEVELAND

PS-1332-3

Figure 4.—Terminal area approach zones for Cleveland Hopkins Airport. 

Outer box

50 x 50 km (27 x 27 NM)

Middle box

25 x 25 km (13.5 x 13.5 NM)

Inner box

5 x 5 km (2.7 x 2.7 NM) x 25,000’

1000’

(typ)

13,000’

6.7 NM

1.7 NM

6.7 NM

5.4 NM

1.3 NM

6,500’

13.5 NM

1.3 NM

PS-1332-4

Zone 1

2

3

4

5

6

7

8

9

Figure 5.—Terminal area approach zones detail. 
NASA/TM—2014-218417 7 

Comparison Strategy 

To test the accuracy and feasibility of the system described, a field test including ground remote 

sensing operations and a series of in-situ measurements aloft are planned. Since the terminal area icing 

remote sensing system must first calculate temperature, LWC and cloud droplet size before determining 

icing severity, it is most desirable to have comparable in-situ measurements of these quantitative fields 

and not the potentially subjective icing severity. This is best accomplished with a research aircraft 

equipped to measure and record air temperature, LWC, cloud particle size distribution, aircraft position 

and altitude, and time. The planned field test will include flights with the NASA Twin Otter aircraft 

(Fig. 6). The in-situ measurements are primarily desired along the airport approach paths, but also 

elsewhere in the terminal area volume. A combination of approaches, constant heading and altitude 

transects, and climbing or descending spirals are desired to maximize the breadth of the comparison 

database. A record of the evolving meteorological synoptic conditions during test periods should also be 

obtained. If discrepancies are identified during the post-test analysis, a good understanding of the 

surrounding weather conditions will be enormously valuable. 

Additionally, it is desirable to also use weather balloons (Fig. 7) to supplement the data provided by 

research aircraft. Weather balloons are capable of operations at times and conditions that would preclude 

aircraft operations. While the data gathered from instrumented balloons is typically more limited than 

aircraft data (spatially and temporally and by instrument type) balloons are relatively inexpensive. 

Weather balloons can be used for a “quick look” at atmospheric conditions before committing the 

resources for an aircraft launch and can fill data gaps between aircraft flights. Through the Small Business 

Innovation Research (SBIR) Program, NASA sponsored Anasphere, Inc. for the development of a 

supercooled liquid water content sensor for use with weather balloons (Ref. 8). This sensor, along with 

the traditional weather balloon sensors, will be a valuable part of the icing in-situ validation activity by 

providing valuable data for direct comparison to the remote sensing measurements and for weather 

assessment before a research flight. 

The post analysis of all the comparison data will likely be a significant effort. Due to the size of the 

sample dataset of both in-situ and remote measurements, some form of statistical assessment will be 

necessary. It is very likely that the terminal area algorithms will evolve during the process, and a 

statistical score method will be necessary to categorize the strengths, and compare between different 

versions. 

Figure 6.—NASA Twin Otter aircraft. 
NASA/TM—2014-218417 8 

Figure 7.—Weather balloon just after launch. 

Concluding Remarks 

NASA has been developing icing remote sensing technologies since 1997. This research and 

development activity is now on the verge of a major milestone, prototype testing, of providing a remote 

sensing capability to determine icing severity with coverage over an entire airport terminal area. 

Since this capability will be completely new, for maximum operational value it will very likely 

require further development after the initial prototype testing. The field test will, to some extent, require 

looking into the future and predicting potential areas of system improvement. For example: will the 

proposed approach-path oriented scheme provide sufficient icing information for departing aircraft? 

During the field test, it may be necessary to provide more remotely sensed and in-situ sampling than just 

the approach paths to allow future development. Also the optimal methods for relaying the remotely 
NASA/TM—2014-218417 9 

sensed information to aircrews, air traffic control, and airline dispatch will need to be thoroughly 

examined so that it blends well with the rest of the data being utilized for flight under the FAA’s 

Next Generation Air Transportation System (NextGen). 

The development and eventual fielding of this kind of new capability should have significant impact 

to the flying community in the future. With this kind of icing severity information feeding into the 

NextGen airspace system, flight crews, air traffic control, and airline dispatch will have solid information 

of the current icing situation in an airport terminal area, removing the need for a flight crew to first “dip 

their wings” into potentially hazardous conditions. This technology also has the potential of feeding into 

decision support tools (DSTs) (Ref. 9) that would enable the airspace system to assess a specific aircraft’s 

unique, current safety margins based upon its capability, phase of flight, past exposure, and (now) known 

future icing condition exposure. 

References 

1. Reehorst, Andrew; Brinker, David; Politovich, Marcia; Serke, David; Ryerson, Charles; Pazmany, 

Andrew; Solheim, Frederick, “Progress Towards the Remote Sensing of Aircraft Icing Hazards,” 

NASA/TM—2009-215828, November 2009. 

2. Johnston, Christopher J.; Serke, David J.; Adriaansen, Daniel R.; Reehorst, Andrew L.; Politovich, 

Marcia K.; Wolff, Cory A.; and McDonough, Frank, “Comparison of In-Situ, Model and Ground Based 

In-Flight Icing Severity,” NASA/TM—2011-217141, December, 2011. 

3. Kenneth P. Moran, Brooks E. Martner, M.J. Post, Robert A Kropfli, David C. Welsh, and Kevin B. 

Widener, “An Unattended Cloud-Profiling Radar for Use in Climate Research,” Bulletin of the 

American Meteorological Society, Vol. 79, No 3, pgs. 443-455, March 1998. 

4. “Meteorologische Messtechnik GmbH, Doppler Radar 35 GHz,” METEK GmbH, Elmshorn, Germany, 

viewed March 26, 2014 <http://www.metek.de/product-variants/doppler-radar-35-ghz.html>. 

5. “MP-Series Profilers,” Radiometrics Corporation, Boulder, CO, viewed March 26, 2014 

<http://radiometrics.com/products-services/product/mp-series/>. 

6. Guirong Xu, Randolph Ware, Wengang Zhang, Guangliu Feng, Kewen Liao, and Yibing Liu, “Effect of 

off-zenith observations on reducing the impact of precipitation on ground-based microwave radiometer 

measurement accuracy,” Atmospheric Research, Volumes 140-141, pgs 85-94, April-May 2014. 

7. Tuttle, John D., and Foote, G. Brant, “Determination of the Boundary Layer Airflow from a Single 

Doppler Radar,” Journal of Atmospheric and Oceanic Technology, Volume 7, pgs 218-232, 1990. 

8. David Serke, Emrys Hall, John Bognar, Allen Jordan, Spencer Abdo, Kirstin Baker, Tom Seitel, Marta 

Nelson, Andrew Reehorst, Randolph Ware, Frank McDonough, and Marcia Politovich, “Supercooled 

liquid water content profiles with a new vibrating wire sonde compared to ground-based microwave 

radiometry,” American Meteorological Society, 16th Conference on Aviation, Range, and Aerospace 

Meteorology, pres. 5.3, January, 2013. 

9. “Weather – Air Traffic Management Integration,” Massachusetts Institute of Technology, Lincoln 

Laboratory, viewed April 18, 2014 <http://www.ll.mit.edu/mission/aviation/wxatmintegration/ 

usedecisionsupport.html>