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A winning proposal for the Innovative Research Program, 2009:
Passive Radio Imaging for Applications in Water Resource Management, Glaciology, and Space Weather Monitoring
Investigators: Nick Zabotin and Oleg Godin, CIRES, University of Colorado
Objective To develop a new, passive method of measurements of the water table depth, the ice thickness,
and the altitudes of ionospheric layers through observation of autocorrelation of polarization components of
the ambient radio noise with modern digital HF receiving systems.
Background and Importance An approach to remote sensing of the environment called passive imaging or
noise interferometry has become increasingly popular after its success was demonstrated experimentally a
few years ago in helioseismology and in ultrasonics. Its applications have become very popular in regional
seismology, geological prospecting, and ocean acoustics. The basic idea is very simple: instead of a dedicated
probing signal, to use correlation reception of ambient noise for interrogating the environment. It is not unlike
the way daylight is utilized by the eyes. The approach differs from conventional radiometry by coherent
processing of diffuse wave fields. It has been shown theoretically and verified experimentally that the twopoint
correlation function of a diffuse noise reproduces the shape of the Green’s function, which describes
propagation of a deterministic probing signal between the two points. Moreover, there exist applications that
require no controlled source of waves and only one receiver (the passive fathometer technique, for example).
We anticipate a possibility of expanding the passive imaging technique to the area of HF radio wave
propagation, where such applications as monitoring the water table level, measuring ice thickness or
determining an altitude of ionospheric layers, are feasible. Implementing passive techniques in this field
would have positive environmental and societal impact: the radio spectrum is densely occupied by various
vital applications, and using new active sources of radio signals is often prohibited. Passive techniques allow
using frequencies that may be unavailable for active methods. Low power consumption of passive sensors
increases the time of their autonomous operation, while their low cost allow for a large number of networked
sensors and an improved spatial resolution of tomographic inversions. Of course, frequencies and necessary
data acquisition rates are many orders of magnitude higher for radio than for seismic waves. Passive imaging
with HF radio waves would not be possible if not for the digital receivers that became available only recently.
Radio noise in the HF band is present constantly. It
is caused by multiple and globally-distributed
sources of natural (lightning) and of technogenic
(industrial noise and distant radio stations) origin.
On the other hand, now we have numerous
instruments incorporating modern fully digital HF
receivers highly appropriate for measuring and
studying the noise properties. Among them, first
and foremost, there are four VIPIR systems just
installed at Jicamarca Radio Observatory, Peru, at
NASA’s Wallops Flight Facility, Virginia, at
Tomsk State University, Russia, and in Boulder,
Colorado. The photographs on the right show
receiving antenna array and 8-channel fully digital HF radar installed at Wallops. The eight
dipole antennas form two orthogonal lines, ~100 m in length, and allow accurate measurements of
polarization, direction of arrival, and amplitude of the received signal. The primary purpose of the VIPIR
systems is active radio sounding of the ionosphere. In this project, we will be using them in an innovative
way.
Research Plan The experimental setup allows testing of at least two diagnostic schemes.
1) In many situations of practical interest, the incoming signal is a result of interference of a direct
signal and a signal reflected from underlying surface(s) where a jump of the dielectric
permittivity occurs (e.g., the water table level or the upper and lower boundaries of an ice layer;
see a sketch below).
In this situation, theory predicts a pronounced dependence of the noise auto- and crosscorrelation
on wave polarization and thickness of the layer. This scheme may be used to retrieve
the layer thickness from measurements of the correlation of polarization components of the
signal.
2) Radio noise propagating in the vertical direction is usually relatively weak. However,
its presence is still noticeable because of roughness of both the ground surface and
the ionosphere boundary. Auto-correlation function of the amplitude of the
electromagnetic noise field measured at a single location must show double peaks
corresponding to the two-way propagation time between the ground and the
ionosphere (see a sketch on the right). This property may be used to determine the
altitude of the ionosphere, a quantity that strongly depends on the space weather
conditions.
We plan a series of measurement sessions at one or two of the VIPIR locations, to be performed in the
passive mode (i.e., when only ambient noise is recorded). The data will be processed statistically to reveal the
theoretical relationships mentioned above. Results of the project will be published as papers at scientific
meetings and in the journal Geophysical Research Letters.
Why is this innovative and important? Measurements of this kind have never been undertaken in the HF
band. Note that no radiation of a dedicated signal is assumed, all proposed measurements will be done with
the use of ambient noise only. This method is energy efficient, representing real “green science." It implies no
interference for existing communication and other industrial radio systems. Measurements of this kind would
not be subject to any frequency use restrictions. Furthermore, only initial development requires cumbersome
sensor systems like VIPIR. In case of success, compact and/or mobile systems implementing this principle
can be developed. The principle itself may be patented.
Why is this interdisciplinary? The technique described involves radiophysical methods of remote sensing
and general wave physics. However, potential applications are very diverse. As it has been mentioned,
inexpensive autonomous radio sensors can be developed for permanent monitoring of the water table in
regions with problematic water supply. Similar devices could be used for monitoring of the polar ice mass, a
critical parameter for the global warming awareness. At the same time, important information about the state
of the Earth’s plasma envelope can be gained.
Expected outcome The hypothesis will be verified that critically important environmental information can
be retrieved from auto- and cross-correlation functions of HF radio noise.
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