Research

Possible Application of "Passive Radar" for the Detection and Tracking of Tornadoes

Prof. Robert K. Vincent
Bowling Green State University

(Submitted For Presentation at the Multinational Conference on "Passive and Covert Radar 2002",
Jun 18-20, 2002, Roke-Manor Research LTD, United Kingdom)

Abstract

Tornado detection with Doppler radar and seismic stations has already been proved possible. The type of signals expected from cellular telephone microwaves as they pass through some parts and reflect off other parts (containing hail or airborne objects from the ground) of tornadic clouds may permit the detection of funnel clouds and the tracking of their direction and propagation speed along the ground. This would be far less expensive than Doppler radar because the microwave signal from "Passive Radar" is freely broadcast for other purposes, and the only cost would be for passive receivers (multistatic condition, with sources and receivers on many different platforms) that were connected wirelessly to a central processing center. Other sensors, such as low-cost seismic stations and acoustical detectors, would also aid in detection of tornadoes that touch the ground. The same data collection sites would be useful for mapping small aircraft in rural areas (passive radar), detecting and locating terrorist "practice bombs" (seismic sensors), and possibly the unloading of large trucks (acoustic and seismic) in rural areas. A strong recommendation is that well-distributed, multi-sensor data collection sites with dual use for catastrophic weather and terrorist action detection be funded by the anti-terrorism community as part of homeland defense networks.

Background

Unlike typhoons and hurricanes, which form over an ocean surface, tornadoes are intensive windstorms that form over land. Typically, wind speeds of tornadoes exceed wind speeds of typhoons and hurricanes by 50% or more. Most tornadoes occur in the central part of North America, which has two North-South-trending mountain ranges (Appalachians and Rockies) that direct cold, high-latitude air in the direction of warm, low-latitude air from the Gulf of Mexico. Figure 1 shows a tornado just before it hit Newcastle, Nebraska on August 6, 1986.

Because of the quick on-set and highly destructive behavior of a tornado, detection methods for the purpose of warning populated areas in its path are continually being sought. Doppler radar is the principal method currently used for tornado detection, the most sensitive of which is Terminal Doppler Weather Radar, or TDWR (Vasiloff, 2001). Doppler Radar has the source and receiver located on the same platform (monostatic condition) or on two different platforms (bistatic condition) and measures the speed of the wind rotating in the funnel of a tornado, as well as the speed of its propagation. However, TDWR stations are expensive to install and maintain, which severely limits the number of them around the USA. Such stations cannot distinguish between funnel clouds in the air from ones that have touched the ground and they are limited by curvature of the Earth from mapping tornadoes at great distances.

More recently, seismic detection has been considered (Tatom and Villers, 2001; Vincent et al, 2002) as an additional tool for tornadoes that have made contact with the ground. Two tornado touchdowns in Ohio were recently recorded (Vincent et al, 2002) on multiple seismic stations of the OhioSeis network, which consists of 20 seismic receiving stations around the state of Ohio, funded by the Ohio Department of Natural Resources Geological Survey Division and the Federal Emergency Management Administration (FEMA). The OhioSeis network began recording vertical-displacement seismic waves and reporting their data over the internet in early 1999 (Ruff, 1999). Each station has an EAI S-102 seismometer (manufactured by Engineering Acoustics, Inc.) and an Apple Macintosh G3 computer, with a 16-bit, National Instruments A-to-D board converter. This is one of only a very few such networks of digital seismic stations in the USA, most of which are on the West Coast. When tornadoes touch down, a high-intensity, low-frequency (0.05-0.128 Hz) seismic signal occurs and most of the higher frequency signals (from 0.128 Hz, up to the higher frequency limit of the EAI S-102 seismometer, which is 1.5 Hz) either disappear or display very low intensities (Vincent, et al, 2002). That same paper recommends research into "tornado fences", which are arrays of seismometers that can communicate with one another, such that the detection and velocity vector of the tornado can become the basis for a warning about an impending tornado that has touched the ground.

Possible Role of Passive Radar for Tornado Mapping

The term "passive radar" is an apparent oxymoron, because in the past, radar was taken to mean "active microwave sensing." With the addition of many active sources of microwave radiation for non-military purposes (including cell phone and TV microwaves) in recent years, it is possible to detect these microwaves with numerous, inexpensive, passive detectors fixed on the ground, such as cell phones, and to use complex computer software of all of their signals to determine the velocity parameters of objects flying through this microwave "soup." Another term for this would be multistatic radar, where the source is on one or more platforms and there are multitudes of receivers located at sites other than the source platforms. The most publicly proclaimed use of passive radar has been as a threat for stealth aircraft of the U. S. Air Force and its allies (McWethy, 2001). Aircraft of any kind can reflect and occult beams of microwave radiation, causing time delays in the signal reaching a particular sensor. When these time delays are analyzed by intelligent software from a multitude of passive receivers, each at a different location, it is possible to recreate the flight path of an aircraft passing through the microwaves.

The same passive radar array could likely be used to map the approach and passage of a tornado, although signal intensity may be more important than the signal time of reception as data from which a tornado detection algorithm would be created. Because of their longer wavelengths, cell phone signals, which range from 33-35 cm (0.9-0.86 GHz), are less likely to be reflected off a funnel cloud than are microwaves from WSR-88D weather surveillance radars, which operate at wavelengths from 8-15 cm (8-4 GHz). Research needs to be performed to determine the extent to which a tornado would attenuate a microwave signal from cellular telephone towers, but the facts that cell phone reception is often changed by inclement weather and that Doppler radar signals are reflected from tornadic clouds indicate that both attenuation and reflection of the signal from tornadoes occur with microwave radiation. There are many particulates and sometimes larger objects whirling around in a funnel cloud that might reflect microwaves in the 33-35 cm wavelength range, including hail ranging in size from golf balls to baseballs.

It is also possible that the attenuation of passive radar signals may detect the approach of dust storms. Reflection off dust storm clouds is not as likely to reflect these longer wavelength microwave signals.

Integration of Passive Radar With Other Sensors
At Data Collection Sites

The most likely successful method for detecting tornadoes with passive radar would include other types of sensors along with the passive radar detectors at each data collection site, using cell phones with wireless internet capability to transmit the data back to a central processing system. However, passive radar may not be able to tell the difference between funnel clouds in the air from those that have touched the ground, a constraint shared by Doppler radar. As discussed above, seismic sensors at each site (costing less than $5,000 each) would aid in detecting the track of tornadoes that have touched the ground up to at least 75 km away. Three seismometers spaced out over a few tens of kilometers would yield sufficient information to track the velocity vector of the funnel cloud along the ground. Acoustical sensors might also be valuable tornado detection devices, once the tornado approaches a data collection site.

Passive radar, seismic, and acoustical sensors offer dual use for data collection sites. Besides the tornado tracking application discussed above, these data add valuable information to the war on terrorism for homeland defense. Passive radars can be used to detect aircraft in regions where there is no large airport. A seismic detector array can be used to detect small explosions, such as were set off in the "Thumb" of Michigan by the terrorists that attacked the Oklahoma City Murrah Office Building. This would likely be restricted to "practice bomb" blasts within a radius of approximately 80 km of the seismometers, depending on the size of the blasts. The FBI asked a seismologist at the U. of Michigan several months after the Oklahoma City attack for seismic evidence of such practice bombs in the Thumb area, but the answer was negative because there was only one seismometer within 150 km of the blast site. A signal must be recorded on 3 seismometers in different locations to determine the precise location of a seismic disturbance. OhioSeis seismologists regularly record quarry blasts on our network, and the sites of the quarries are well known, such that a practice blast outside of those quarries would likely be uniquely identified if it occurs within about 80 km (depending on blast size) of three OhioSeis seismometers. However, most of Ohio still does not have sufficiently dense coverage by seismometers to detect and locate practice bomb blasts over any but a small percentage of the state. As a prerequisite to such a capability, funds would have to be made available to perform research and write software for the automatic detection and location of small man-made explosions with seismic data inputs. Acoustical detectors would also help in recording loud noises, such as explosions or heavy unloadings from trucks, but the range of detection would be very much less than seismic detectors, and probably on the order of a few km.

The dual use of data collection systems with several types of sensors, which is a proposed addition to a new passive radar program (Guice et al, 2002) at Louisiana Tech University in Ruston, Louisiana, is very attractive for anti-terrorist funding because such systems will be useful even when no terrorist activity is taking place. For this reason, Bowling Green State University in Bowling Green, Ohio, is proposing to assist Louisiana Tech with passive radar and seismic signal processing and software algorithm research to make automatic tornado detection and small blast detection a reality with data collection facilities that were originally designed for anti-terrorist tasks. North Louisiana is an area that has moderately- frequent destructive tornadoes.

Conclusions and Recommendations

Tornado detection with Doppler radar and seismic stations has been proved possible. The addition of passive radar data collection sites to rural areas prone to tornado activity would likely improve the warning time of impending tornado paths. They would also help in the detection of aircraft flying outside the detection limits of large airports, such as terrorist-piloted crop dusters. If seismic detectors were added to these data collection sites, detection of tornadoes that have made some ground contact would become possible, as well as the detection of "practice" blasts for terrorists that wish to test explosive devices before their actual use, as occurred in the Oklahoma City bombing. Acoustical detectors may also have a dual use between tornado detection and anti-terrorism tasks.

My strong recommendation is that well-distributed, multi-sensor (including passive radar) data collection sites with dual use for tornado detection and other civilian monitoring tasks be funded by the anti-terrorism community as part of homeland defense networks.

References

Guice, L.K., L. Roemer, D.R. Copsey, and C.G. McWright, 2002, Passive Coherent Radar (PCR) Test and Evaluation for Enhanced Operational Safety and Security at Small Airports, Multinational Conference on "Passive and Covert Radar 2002", 18-20 June 2002, Roke-Manor Research LTD, UK.

Harris, Gary, 2000, former pastoral intern at St. Mark's Lutheran Church, Bowling Green, Ohio; personal communication of hand-held camera photo, with permission to publish.

McWethy, John, 2001, Emerging Threat for U.S. Air Force: New "Passive" Radar, Under Development by Russia and China, Could Threaten Stealth Aircraft, ABC News Article, http://abcnews.go.com/sections/world/ DailyNews/radar_stealth_010614.html, June 14, 2001. Note: The two spaces in this URL are actually underscores.

Ruff, L. J., M. Hall-Wallace, 1999, Instructional Software for Seismology, Seismological Research Letters, Vol. 70, no. 1., p. 85-86.

Tatom, F.B. and S.J. Vitton, 2001, The Transfer of Energy from a Tornado into the Ground, Seismological Research Letters, Vol. 72., No. 1, pp. 12-21.

Vasiloff, S. V., 2001, Improving tornado warnings with the Federal Aviation Administration's Terminal Doppler Weather Radar, Bulletin of the American Meteorological Society, V. 82, No. 5, pp. 861-874.

Vincent, R.K., Z. Zheng, S. Ping, and Z. Shaofen, 2002, Wavelet Packet Transformation Analysis of Seismic Signals Recorded from a Tornado in Ohio, accepted for publication in the Bulletin of the Seismological Society of America.