Umran S. Inan
The nine-element ELF/VLF interferometer system is used as a source current
imaging array in support of the High-frequency Active Auroral Research Program (HAARP) in
Alaska. HAARP is a congressionally initiated program jointly managed by the U.S. Air Force
and U.S. Navy. The goal of the program is to provide a state-of-the-art U.S. owned ionospheric
research facility readily accessible to U.S. scientists from universities, the private sector and
One of the research topics of present interest is the generation of ELF/VLF waves through
HF heating of the auroral electrojet, the intense current system that flows in the auroral regions
between 70 and 120 km altitude. Potential DoD applications of this research are to provide
communications to submerged submarines at ELF frequencies and to use the generated waves
to locate and probe underground structures.
The purpose of the ELF/VLF current imaging array is to provide an experimental
tool that can be used to determine the HF heater-induced time varying electrojet currents that
actually radiate the ELF/VLF waves. Without this knowledge there is presently little hope of
understanding and optimizing the process by which the waves are produced.
To resolve the important unknowns concerning the spatial distribution of the modulated currents,
we propose to construct and deploy an ELF/VLF current imaging system near the HAARP
facility in Alaska. The ELF/VLF interferometer system would consist of nine ELF/VLF receivers,
each with triaxial crossed loop antennas and one vertical electric antenna, arranged in
the form of a cross with the central receiver located within~10 km of the HAARP facility. Each
leg of the cross consists of five receivers, each spaced by a distance of 70 km, with the central
receiver being common to both legs of the cross.
The array would be used to map out the modulated currents using the near field of these
currents. The known relationship between the near fields and the modulated currents provides
the link which makes this possible.
A low-power (battery operated) nine-element interferometric array is to be constructed and
assembled for ELF/VLF measurements in remote regions. An important application of this new array
include ( imaging the three dimensional structure of the HF heater modulated auroral electrojet currents
that radiate ELF/VLF waves, with specific application to the High-frequency Active Auroral Research
Program (HAARP) in Alaska. The new method is based on measurements of wideband ELF/VLF
waveforms at nine suitably distributed (Figure 1) remote sites, with a GPS-based absolute timing accuracy
between distant sites of <1 us. The GPS timing allows the waveform data from the different sites to be phase
comparable in the same sense as a traditional interferometer, except for the fact that the object being
�imaged� is in the near-field (rather than far-field as in most other interferometric applications) of the
The ELF/VLF interferometer system consists of nine low power radio receivers as
shown in Figure 1, allowing the specification of the three dimensional distribution of the source
current region with~5 km resolution in both the vertical and lateral dimensions. One of the
important applications of the new interferometer will involve its use in connection with the
powerful HAARP HF heater, for which the array design will be specifically tailored. HAARP is
a congressionally initiated program jointly managed by the U.S. Air Force and U.S. Navy with
the overall goal of providing a state-of-the-art ionospheric research facility readily accessible
to U.S. scientists from universities, the private sector and government. This facility is currently
operational at a total radiated power level of 960 kW, and has been successfully used controlled
heating of the ionosphere, for production of artificial ionospheric irregularities, and for generation
of ELF/VLF signals. When the full array of transmitting antennas are completed within the next
few years, the HAARP facility will be the most advanced and powerful ionospheric heater
in the world, capable of producing the most intense ELF/VLF signals that can be used for
remote sensing of underground structures, for probing the earth�s outer radiation belts, and for
communication with submerged vehicles.
The interferometer is a unique experimental tool that can be used to determine the HF
heater-induced time varying electrojet currents that actually radiate the ELF/VLF waves. This
knowledge will not only enhance our understanding of the fundamental processes of ELF/VLF
generation but will also allow the optimization of the process by which the waves are produced.
The data to be obtained from the proposed program will be used to analyze basic properties of
ionospheric ELF/VLF radiating systems and to assess the potential for developing ionospheric
enhancement technology for communications and surveillance purposes.
ELF/VLF waves are important not only as communication tools, but also as remote diagnostic
probes to determine the characteristics of the Earth, the ionosphere, and the magnetosphere.
Since the controlled production of these waves on the ground generally requires large, elaborate
and expensive antennas and associated facilities [C. H. Richard, Sub vs. Sub, Orion Books, New
York, 1988], it is important to explore alternate means of generating these waves.
One promising alternate method for producing ELF/VLF waves is to use a powerful ground
based HF radar to modulate the intense auroral electrojet currents that flow in the auroral D
and E regions, causing these natural currents to radiate ELF/VLF waves from an altitude range
of 70-110 km, depending upon the HF frequency. This technique works in the following way.
The HF waves heat the ambient electrons in the D and E-regions where the electrojet current
flows, causing the electrons to collide more frequently with the ambient neutral particles. The
increased electron collision rate changes the local conductivity of the medium, and this in turn
changes the electrojet current flow. If the HF heater is modulated at a frequency in the ELF/VLF
range, the electrojet current will vary at this same frequency and will then radiate waves at this
Controlled experiments using this technique have been successfully conducted in Norway
[Stubbe et al., 1982; Barr and Stubbe, 1984; Reitveld et al., 1986; Barr and Stubbe, 1991], Russia
[Kapustin et al.,1972], and Alaska [Ferraro et al.,1982; McCarrick et al., 1990; Villasenor et
al., 1996]. Furthermore, VLF waves generated in the D-region by ground based HF heaters
have been detected by a number of spacecraft, including ISIS 1 [James et al., 1984], Aureol 3
[Lefeuvre et al., 1985], DE 1 [Inan and Helliwell, 1985], and Akebono [Kimura et al., 1991,
Although it has been amply demonstrated that HF heaters can modulate the auroral electrojet
currents to produce ELF/VLF waves in the D-region, there is very little known about the actual
details of the mechanism through which the waves are generated. For example, the horizontal
spatial distribution of the currents that radiate the waves is not known. Thus the waves could be
produced through quadrupole radiation rather than through dipole radiation. Furthermore the
altitude of the modulated currents is not known, other than the fact that they must be located above
70 km where the conductivity is high enough to support intense electrojet currents [Rowland et
Since very little is known about the spatial distribution of the modulated currents that produce
the ELF/VLF waves, there is no experimentally verified model which could be used to optimize
the wave generation process. Instead, trial and error methods have been employed which are
very difficult to interpret because of the well known variability of the auroral ionosphere.
To resolve the important unknowns concerning the spatial distribution of the modulated currents,
we propose to construct and deploy the ELF/VLF inteferometer array near the HAARP
facility in Alaska. As shown in Figure 1, the system would consist of nine ELF/VLF receivers,
each with triaxial crossed loop antennas and a vertical electric antenna, arranged in the form of
a cross with the central receiver located within ~10 km of the HAARP facility. Each leg of the
cross consists of five receivers, each spaced by a distance of ~70 km.
Fig. 1. Schematic representation of the nine element ENIACarray proposed for theHAARP facility. The basic configuration is that of a cross with perpendicular arms. At each site the 4 channel VLF/ELF receiving system measures the signals on 3 orthogonal loop antennas and one vertical electric antenna. For clarity in the figure, only two loop antennas are shown at each site. The spacing between each site is ~70 km. The source region where the HF modulated auroral electrojet currents are located is in the 70-120 km altitude range.
The interferometer array would be used to map out the modulated currents using the near field
of these currents. The known relationship between the near fields and the modulated currents
provides the link which makes this possible. The spatial resolution of the imaging will depend
upon the signal-to-noise ratio of the near field measurements. However since this ratio can
generally be improved by averaging, we believe a 20 dB ratio is achievable. In this case the
imaging should be able to achieve a spatial resolution of~5 km or better in both the horizontal
and vertical directions. Achieving this goal will require operations in which the HF beam is
directed away from the vertical, but always well within normal operating configurations. A
resolution of~5 km appears sufficient for most applications, since simulations suggest that
ELF/VLF radiation is enhanced for a fixed HF power level when the heated region is made as
large as possible [Taranenko et al., 1992]. The modulated auroral electrojet currents produce
a magnetic field at ground level which is described by Maxwell�s equation
whereB is the magnetic field, E is the electric field, c is the velocity of light, t is the time, .is the
divergence operator, andJ is the sum of the modulated source current density and eddy currents
in the ground induced by the time varying magnetic field. TheE field in (1) is predominently
produced by space charge. in the source region which is associated with the current flow. The
quantitiesJ and . are related through the equation of continuity.
Within the source region the local charge density. is significant only in regions where there are
significant gradients in the local conductivity, such as near the boundaries of the heated volume.
Since it is the time derivative of. which enters into (2), it is clear that the effects of . on J
decrease as the driving frequency is decreased. In the limit of decreasing driving frequency, (1)
and (2) can be approximated by the relations:
In cases when ground eddy currents are negligible the relationship between the B field at the
ground and the source currents can be described through the generalized Biot-Savart Law [Inan
& Inan, 1999]:
whereB is the field produced by the currents, J is the current density in the source region, R is a
unit vector along the line connecting the source and observation points,R is the distance between
the source and observation points, and the integral includes the volume of space containing the
modulated currents, anddV is a volume element in the source region.
To solve (1) forJ, given B at the ground, we divide the source regions into identical small
volume elements .V and approximate the integral by the finite sum:
Assuming that the summation hasi elements and that we have measured B(r) at i separate
locations, we can then write (5) as the matrix relationship
whereB is a column vector containing the B measurements and J is a column vector containing
the 3i unknown values of J in the discretized source region. Given that M is not singular, we
can invert (3) to obtain the current density from the measured values ofB:
In cases where ground eddy currents are significant, (4) must be modified through the inclusion
of the image currents. However, the solution proceeds in a similar manner.
In general the horizontal scale of the modulated currents is just the full width of the HF beam
at~80 km altitude, which is generally less than 50 km. The vertical scale is unknown, but
probably of the order of 25 km. For a spatial resolution of 5 km we would then havei 500 in
the summation of (5). To obtain 500 separate measurements ofB at our 9 sites, it is necessary
to use the HF beam pointing capability to move the heated region with respect to the fixed sites.
This can be done quite simply for example, by tilting the HF beam a fewdegrees from the vertical
and then samplingB at the 9 sites as the azimuth is changed in steps. Then tilting the beam a
few degrees more from the vertical and repeating the azimuth sean.
Given that the maximum inclination angle from the vertical is 30. for the HAARP HF beam
and that the source currents are located somewhere between 70 to 120 km altitude, the mimimum
effective area that can be sampled with the current imaging array is shown in Figure 2. In this
figure the center circle includes all the points with respect to the center of the heated region that
can be sampled by the central site of the array as the HF beam moves from 0 to 30. with respect to
the vertical, and at each zenith angle moves through 360. of azimuth. The other circular regions
represent the effective area sampled at each of the remaining 8 sites. The dashed circle in the
center of the central circular area represents the projection on the ground of the area illuminated
by the HF beam at 80 km altitude when the HF frequency is 2.8 MHz and the beam is vertical.
It is clear that the central site of the array can effectively sample all points on the ground directly
beneath the heated region and that measurements can be made at any point up to 175 km from
the center of the heated region along each leg of the cross. In the central region the sample
points can be arranged in a rectangular grid with axes parallel to the cross axes and with 5 km
separation between sample points. This scheme produces~150 measurements of B. At the 4
sites closest to the central site the sample points can be more widely separated because of the
increased distance from the source region. With a rectangular sampling grid with a separation of
8.5 km between sampling points we get~220 measurements of B in these 4 sites. Finally in the
outermost sites with a separation of 11 km between sampling points we get~130 measurement
ofB for a grand total of 500 measurements of B, as required.
Fig. 2. Effective area sampled by the proposed ELF/VLF interferometer array. The center solid circle includes all points with respect to the center of the heated region that can be sampled by the central site of the array as the HF beam moves from 0 to 30 degrees from the vertical and at each zenith angle moves through 360 degrees of azimuth. The other circular regions represent the effective area sampled at the other 8 sites as the beam swings through the same angular range. The dashed circle at the center of the central solid circle is the projection of the source region onto the ground when the HF beam is vertical and the frequency is 2.8 MHz.
The dwell time of the HF beam at each location would be~1 second. Thus the entire
measurement sequence requires less than 10 minutes to complete. When the HF frequency is
higher than 2.8 MHz, the HF beam width will decrease, as will the horizontal scale of the heated
region. In these cases we divide the heated volume into 500 sections as before so that the spatial
resolution is inversely proportional to the frequency and always a fixed percentage of the diameter
of the beam at~ 80 km.
Since it is possible that space charge effects may be important at all ELF frequencies, we
monitor these effects by measuring the vertical component ofE at each site. The relation
between the verticalE and . is given by the relation
wherez is the vertical axis, z is the altitude of the charge, and . is a quantity that depends upon
the local ground conductivity. For a highly conducting ground,. ~= 1. Since we know the values
of Ez at the ground, (8) represents an integral equation for .. We solve it by the same procedure
we used to arrive at (7). That is, we approximate the integral by a summation over 500 volume
elements to create a matrix relationship:
whereEz is a column vector containing the 500 measured values of Ez and . is a column vector
containing the 500 unknown values of.. Inversion of (9) gives the relationship for .:
In general, J can be expressed as the sum of two parts:
where. J1 = 0,.J2 = 0, and J2 is associated with the space charge .. The quantity J1 can
always be found from (7), whileJ2 is found from (2):
Since by definition.J2 = 0, we can express J2 as the gradient of a scalar F, i.e., : J2 = .Φ.
Now (12) becomes similar to Poisson�s equation:
Since the right hand side of (13) is known through (10), (13) can be solved numerically to find
F, and then to findJ2. Thus the complete current J = J1 + J2 can be found in the source region
for any driving frequency in the ELF range.
At each sitethe ELF/VLF signals from the three orthogonal loop antennas and the vertical whip
antenna are fed into a four channel preamplifier box. The 4 outputs from the preamplifier are sent
to a Stanford-built 4 channel presampling filter and clipper circuit where the signals are broadband
filtered to remove high frequency noise and clipped to prevent saturation from impulsive signals
(due to lightning discharges). The 4 signals are then fed into a programmable i386-Engine-M
processor, which extracts the average variation of each field component over the course of a
single cycle of the HAARP heater. The HAARP transmission and modulation schedules are
known a priori and the processor is programmed to process the data only during these times and
at the specified frequencies, thus minimizing on-time for the processor (to save power) and data
volume. The processed data is stored in flash memory cards. The stored data is downloaded
(by connecting a portable NoteBook computer) from the memory cards during regular visits
to the site. Any changes in recording schedules and frequencies are also downloaded to the
programmable processor during these regular site visits.
The preamplifier is a wideband instrument (300 Hz to 90 kHz) with high linear dynamic range
(~100 dB) and is similar in design to Stanford-built systems used worldwide for highly sensitive
ELF/VLF measurements. For the interferometric measurement proposed here, the sampling rate
of the i386 processor will typically be set at 10 kHz, providing the ability to measure signals in
the frequency range of 300 Hz to 4 kHz. The processor will carry out �on-board� analysis of
the data as followsby partitioning data into sections of 1 second duration, and in each section
determining the average variation of the field component over the course of a single cycle of the
HAARP heater through a superposed epoch analysis. The information concerning this average
field variation will then be stored in flash memory. This procedure markedly improves the
signal-to-noise ratio by which the field components can be determined and also greatly reduces
the amount of data that must be subsequently stored.
Fig. 3. System diagram of one of the nine identical ELF/VLF receivers constituting the proposed inteferometer array.At each site, the ELF/VLF signals from the 3 orthogonal loop antennas and the vertical whip antenna are fed into a Stanford-built 4 channel preamplifier box. The 4 outputs from the preamplifier are sent to a Stanford-built 4 channel line receiver where the signals are broadband filtered to remove high frequency noise and clipped to prevent saturation from impulsive signals (due to lightning discharges). The 4 signals are then fed into a programmable i386-Engine-M processor, which extracts the average variation of each field component over the course of a single cycle of the HAARP heater. The HAARP transmission and modulation schedules are known a priori and the processor is programmed to process the data only during these times and at the specified frequencies, thus minimizing on-time for the processor (to save power) and data volume. The processed data is stored in flash memory cards. The stored data is downloaded (by connecting a portable NoteBook computer) from the memory cards during regular visits to the site. Any changes in recording schedules and frequencies are also downloaded to the programmable processor during these regular site visits.
The logistics of the field deployment and use of the proposed ELF/VLF interferometer is envisioned
to involve the following steps:
1) Identify nine sites that are electromagnetically quiet (away from power lines) and accessible
by road (with car or snowmobile).
2) Visit each site and deploy the ELF/VLF antennas (see Figure 3). This deployment could be
semi-permanent, and can be done in advance to save time.
3) Visit each site (this could also be done durign the previous visit) and deploy the system
shown in Figure 3, except for batteries.
4) At this point, the ELF/VLF interferometer is in place and ready for measurement of HAARP
signals. One day before the start of a HAARP experimental campaign, visit each site and
connect the batteries.
5) During the experimental campaign, visit each site once every two days (40-hours; see subsection
on Data Storage below) to download data from Flash Memory cards, by connecting
them to the poratble NoteBook computer. Downloads of new recording schedules (based
on new planned HAARP transmission schedules) into the i386 processor EEPROMare also
done in these visits.
6) Visit each site once every five days (see subsection on Batteries) to replace batteries.
Items (5) and (6) above will be continued until the end of the experimental campaign.
The Stanford-built preamplifier and line receivers are very similar in design to those previously
constructed by Stanford University and currently in use at many sites worldwide. These systems
have operated successfully in each case and planned goals were uniformly achieved. Thus, the
construction of nine such units for the proposed ELF/VLF interferometer poses no schedule, cost,
or performance risk. In past applications, the analog outputs from these units would be fed into
an A/D board connected to a Pentium computer, with the digitized broadband signals recorded
on hard disk and ultimately on CD-ROMs or digital tapes. The primary difference between the
new systems proposed here and the previous systems is the fact that a low-power procesor (i386)
is used instead of the Pentium computer and data is stored on Flash Memory cards instead of
CD-ROMs, both features being necessary to facilitate battery-powered operation.
In general, it is important to note that line drivers and line receivers are required to drive
(i.e., power is sent to the preamplifier from the line receiver) and receive (preamplified signal
transmitted back to the line receiver) signals off on long lines. These units provide for correct
cable termnation, common mode rejection, power-from-signal isolation, clipping to prevent
saturation (due to impulsive signals produced by lightning discharges) and other important
signal conditioning. When a wire of any length is used for signals of large dynamic range and
significant bandwidth (in this connection ELF/VLF is extrememly wide bandwidth compared to
Fig. 4. Stanford ELF/VLF receivers at unmanned automatic geophysical observatory sites.This system, currently operational at five different remote high latitude Antarctic sites is currently deployed at the AGO sites P1, P2, P3,P4, and P5. It consists of a magnetic loop antenna (either a 1.7x1.7m or 4.9x4.9m square � both shown in the photo above) connected to a preamplifier (grey box with sealed lid shown in lower left) buried immediately underneath, which is powered by and sends data over a ~500 ft coaxial cable (shown rolled up) to the station (shown in upper left), in which resides the main receiver unit (the box with the blue front panel) consisting of a line receiver, various fixed band filter channels, and a broadband snapshot system, as depicted in the block diagram. This �low power� ELF/VLF system specially designed for unmanned sites with limited power nevertheless consumes ~6 to 7 W, depending on operational modes. The broadband snapshot system in these units (the upper left corner of blue box) has recently been redesigned to utilize the programmable i386-Engine-M processor, which also forms the basis for the individual elements of the proposed ELF/VLF interferometer system (see Figure 3).
most things, covering, usually, three decades (30 Hz to 30 kHz) of frequency) it is necessary to
have special terminations at both the send and receive ends.
The use of the programmable i386 processor also does not pose any risk since Stanford University
instruments currently located at unmanned automatic geophysical observatories in Antarctica
(see Figure 3) have recently been redesigned to use this processor for capturing wideband
snapshots of ELF/VLF activity. As a result, expertise in the use and programming of this
particular card for processing analog outputs from preamplifier systems already resides in our
research group. The graduate student funded under the proposed program will take advantage
of this expertise and will be responsible for programming the software interfaces for flexible use
of the i386 processor for extraction of ELF/VLF waveforms at specific times and frequencies
during which modulated heater transmissions will be carried out with the HAARP facility.
In terms of our application the i386 processor has the following key features:
Low power consumption: 1.9 Watts in normal operation, and 0.15 Watts in sleep mode.
512-KB 16-bit EEPROM, programmable in C/C++, with quick debugging/testing.
512-KB 16-bit SRAM, with memory expansion board available.
100 kHz 16-bit A/D, with 8-channel multiplexer.
These features allow for minimal power consumption, especially in view of the fact that the
processor will need to be ON only during specified (a priori) periods of HAARP transmission/
modulation, and can be in the SLEEP mode at other times. On average, we expect the duty
cycle of HAARP transmissions in any given targeted hour to be no more than 30-seconds every
5-minutes, which means that the processor can operate at anaverage power draw level of ~0.3
Watts. At other times, when no HAARP transmissions are made, the system will SLEEP while
consuming only~0.15 Watts.
The proposed VLF receiver incorporates a compact GPS receiver card as the source of a stable
time and frequency reference. The power and size of GPS receivers has been shrinking rapidly,
and the Motorola UT Plus Oncore card we propose to use operates at a power level of~0.9
Watts. At first, this might seem to be a significant power draw, compared to the preamp/line
receiver and the i386 systems. However, it should be noted that the~1 usec time resolution
needed in an ELF/VLF receiver (note that the maximum bandwidth proposed is 100 kHz so that
1usec resolution is well enough) can be realized by using the GPS receiver only occasionally,
for example once or twice per day, to synchronize a thermally isolated crystal resonator which
is then the source for a timing generator and frequency reference. A thermally isolated, good
quality crystal resonator has an Allen Deviation of typically better than 10-10, for 1000 to
10,000 sec, which is about~1 usec/day. The drift due to aging is larger, about ~10 usec/day.
By synchronizing to GPS once or twice per day, the effects of drift and variation could easily be
compensated to the precision needed by the ELF/VLF receiver. GPS satellite lock is achieved
(with the Motorola UT Encore card) generally in at most~4 minutes, so that the duty cycle of
the GPS receiver would be 4 minutes every 12 hours, so that a~0.9 W GPS card would only
consume a negligible average power of only~5 mW.
The technology in FLASH memory has been leapfrogging its capacities with no sign of slowing
down. Present commercially available memories have up to 190 Mbyte capacities, so that a few
of these chips would have the capacity to store 1 Gbyte of data. The great advantage of FLASH
memory is retention at zero power. The power used by these memories in writing to them is
dependent on data rates, so that for the small few kHz bandwidths of ELF/VLF applications only
~1 mW is needed. For our application, we propose to use two Sandisk 160 MB flash memory
cards at each site, for a total storage capacity of 320 MB. Within a few months, and certainly
by program start (1 April 2000) we expect at least twice as much capacity to be available at the
The data storage requirements are obviously determined by the bandwidth and duty cycle of
operation. As a benchmark, 10 kHz sampling rate at 16-bits allows for~4 kHz bandwith and
produces~20 kBytes/s. At that rate, 320 MB of storage allows for continuous recordings of a
single waveform for~5.3 hours.
In practice, however, our expected duty cycle for HAARP transmissions in any given targeted
hour will be no more than 30-seconds every 5-minutes (i.e.,~10%), so that 320 MB storage
allows recordings of a single HAARP-induced waveform for~53 hours, or more than two
24-hour days. With four channels (Bx, By, Bz, and Ez), and noting that typically HAARP
operations will be conducted for periods no longer than~8 hours/day, we can record 4-channels
at 4 kHz bandwidth during HAARP transmissions (30-seconds every 5-minute, for 8-hours/day)
for a period of~40 hours. In view of the overall logistical approach described above, and with
the distance between sites of~70 km (see Figure 2), this provides ample time for visiting the
sites, to download flash memories and replace batteries (if needed; see below).
The above data volume estimates are in fact quite conservative, and assume that the raw 16-bit
waveforms of signalswould be retained with full 4 kHz bandwidth. The actual data requirements
are likely be substantially lower due to the facts that:
The processor can carry out �on-board� analysis of the data by partitioning data into sections
of 1 second duration, and in each section determining the average variation of the field
component over the course of a single cycle of the HAARP heater through a superposed
epoch analysis. Only the information concerning this average field variation (rather than the
full waveform) will have to be stored in flash memory. This procedure markedly improves
the signal-to-noise ratio by which the field components can be determined and also greatly
reduces the amount of data that must be subsequently stored. In cases where data volume
is not a limitation, full waveforms can be retained and the superposed epoch procedure can
be carried out after the data is extracted from Flash Memory.
In general, HAARP transmission schedule and modulatioin frequencies will be known in
advance so that the waveform needs to be captured only in a narrow band (e.g., 400 Hz)
centered around the ELF/VLF frequency used (e.g., 3 kHz), rather than the full 4 KHz
bandwidth. This approach would relax data storage requirements by a factor of~10.
Based on the above discussion, the total average power consumption of each of the nine ELF/VLF
interferometer elements is as follows:
Component Duty Cycle Average Power Consumption
Stanford-built Preamplifier 100% 0.3 Watts
Stanford-built Line Receiver 100% 0.3 Watts
i386-Engine-M Processor 30-s per 5-mins 0.3 Watts
Motorola Oncore GPS Card 4-min per 12-hrs 0.005 Watts
TOTAL 0.95 Watts
With a total continuous power consumption of only~1Watt, the proposed ELF/VLF receiver
system can be ideally powered by batteries. For this purpose, we propose to use batteries
produced by Hawker, Inc., which are currently used in remote regions such as Antarctica. Our
preliminary investigation indicates that their Cyclon series of the Hawker batteries works down
to-65. C. To supply ~1 Watt for 24-hours at 4 Volts, we need ~6 amp-hours. The Hawker
J-cell specifications, in the Cyclon series, is 8 amp-hours, at 2 V, in an approximately 5 cm
diameter, 14 cm high, rigid plastic container that weighs 840 gm. For our purposes, we can
connect two of these in series and then five such pairs in parallel to provide 30 amp-hours at 4
Volts, or 5-days of continuous power. The batteries would then weigh~8.4 kg, and would still be
relatively easy to handle.
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