Advanced Techniques for Incoherent Scatter Radar

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					                             Advanced Techniques for Incoherent Scatter Radar
                                Frank D. Lind(1), Tom Grydeland(2), Philip J. Erickson(3),
                                            John M. Holt(4), John D. Sahr(5)
                        MIT Haystack Observatory, Route 40, Westford, MA 01886 USA
                                                          University of Tromsø, Tromsø, Norway
                                                  As (1) above, but E-mail:
                                                  As (1) above, but E-mail:
                                      Department of Electrical Engineering, University of Washington,
                                                  Box 352500, Seattle, WA 98195 USA


We briefly describe two new Incoherent Scatter Radar designs which take advantage of recent technological advances in
Software Radar technology and high power UHF transmitters. The Zenith Incoherent Profiler (ZIP) is designed to
produce vertical measurements of ionospheric properties on a 24/7 basis for use in assimilative models and real-time
Space Weather diagnostics. The system is low enough in cost to allow for deployment in large numbers as part of a
Global Radio Science Network. The second design is the Holographic Radar which utilizes a multistatic geometry, a
CW random code illuminator, and several digital beamforming phased receiving arrays to create volumetric images of
an ionospheric region, including the estimation of plasma vector quantities.


Recent advances in the development of Software Radar technology [1] have set the stage for a major revolution in the
design and operation of Incoherent Scatter Radar (ISR) systems. Fig. 1 shows a typical Software Radar architecture for
ISR applications. Incoherent Scatter radars are powerful tools for profiling the Earth's plasma environment. Current
systems are capable of measuring electron density, electron temperature, ion temperature, ion velocity, and many other

                                          Fig. 1. A generic Software Radar architecture.
derived parameters. These radars are large and powerful with peak power aperture products typically approaching a
MW-hectare. Average transmitter power of an Incoherent Scatter Radar is typically in the several hundred kilowatt
range and these systems can profile the ionosphere with integration times on the order of seconds to minutes depending
on the density of the ionospheric plasma and the altitude range to be covered by the observations.

In current configurations these instruments are large and expensive to operate. Due to this fact most Incoherent Scatter
Radars obtain data for at most several thousand hours per year. This data taking is usually for contiguous periods spaced
in a fairly aperiodic fashion that is difficult to use as real time input into geophysical and Space Weather models. The
radars are also limited by their ability to observe in only a single direction at a time. The resulting beam steering delay
creates time-space ambiguities in their observations under rapidly changing geophysical conditions. Further the pulse
mode and transmitter code selected for the radar often places strong limits on the unambiguous range and doppler
resolution possible with these systems.


A Software Radar system eliminates analog radar hardware systems in favor of digital systems that are interfaced
directly to a computer network. In a Software Radar many of the functions performed by custom digital hardware in
current radars are implemented in software on general purpose computers. This includes signal processing, transmitter
waveform generation, antenna control, and real time recording of experiments. A high speed multicasting network is
used to interconnect the hardware, computation, and control elements of the radar. Synchronization and coherence in
the radar system is created by locking frequency dependent elements of the system to the Global Positioning System or
other available UTC based site standard. When combined with digitization of the radar transmitter this approach
decouples the transmit and receive portions of a radar in a manner that can universally address both monostatic and
multistatic radar designs.

The Software Radar approach has a number of explicit advantages which derive primarily from the early transformation
of instrument information and data into the digital domain, the communication of this information using standard
computer networks, and the rapidly increasing power of computational and network platforms. The foremost result of
these advantages are in the control, flexibility, precision, and transparency of the experimental process. In particular data
can be analyzed in multiple ways to address the needs of different scientific users in an optimal manner. A simple
example of this is multi-resolution data analysis where different spatial and temporal integration scales are analyzed


The Zenith Incoherent Profiler is a low cost ISR system that is designed to be an element of a distributed network of
ionospheric instruments. Fig. 2 shows an example Zenith Incoherent Profiler design. A large zenith directed antenna is

                                       Fig. 2. The Zenith Incoherent Profiler (ZIP)
         Fig. 3. Zenith Incoherent Profiler Performance for Solar Maximum using an MSIS model ionosphere.

combined with a medium power Digital Television Transmitter based on Constant Efficiency Amplifier (CEA)
technology [2]. An estimate of the potential performance of this system is shown in Fig. 3 based on a 100 meter antenna
and using an MSIS model ionosphere. The ZIP design has low construction and operation costs, excellent performance,
and is well suited to automated 24/7 operations. Elements which contribute to the low cost of this instrument include a
modest average power level (20-30 kW), high transmitter efficiency (60-70%), a simple offset feed spring suspended
mesh antenna design, the use of high power coax instead of waveguide, and full remote operations using Software Radar
technology. Additional cost optimization can be obtained by dynamically varying the output level of the CEA in
response to changing geophysical conditions. A mid-class incoherent scatter radar system, such as the Zenith Incoherent
Profiler, can provide access to physical parameters that are unavailable using other experimental techniques. An
excellent example of this is the electron to ion temperature ratio. Modestly more costly and complicated design
variations can also produce measurement of ionospheric electric fields.

A low cost Incoherent Scatter Radar design is useful because it can be widely deployed in reasonable numbers. Such
systems can also be constructed and operated by organizations that lack the resources for a full facility class ISR. This is
in sharp contrast to current ISR designs which are all unique and expensive instruments. A network of such low cost ISR
systems would also be much more powerful collectively than any individual instrument. This coordinated operation is
easily enabled through the use of a uniform design based on Software Radar technology. The Zenith Incoherent Profiler
will be a key element of a planetary network for monitoring the Earth's Ionosphere.


The Holographic Radar (Fig. 4) is a an Incoherent Scatter Radar system that is designed to provide optimal imaging of
ionospheric structures within the limits imposed by the correlation time of the ionospheric medium. This radar design is
multistatic in nature and operates in a manner that is most similar to laser holography. The system uses a CW CEA
transmitter to illuminate a wide ionospheric volume at high average power with a coherent broadband random phase
code. The transmit antenna is optimized for medium gain but with low sidelobes that are carefully directed away from
the receiving arrays. Digital beamforming receiver arrays are used at multiple locations to image the illuminated region.
All possible beams intersecting the ionospheric plasma are simultaneously evaluated by this architecture using
                                            Fig. 4. The Holographic Radar

high performance computing and optical correlation techniques. This allows the system to make volumetric
measurements of ionospheric parameters, including plasma vector quantities, without the typical time space ambiguities
that are produced by scanning radar systems. The gain mismatch between the transmit and receive antennas is offset by
high average transmitter power, software beam integration, adaptive integration time, and rapid code speed. The system
shares many of the advantages of the low cost ZIP systems, but is somewhat more expensive to construct and operate.
This is primarily due to the computational intensity of the imaging problem and the cost associated with the high power
transmitter. The additional cost is justified scientifically by providing near optimal high resolution imaging for
ionospheric regions with rapidly varying structures.


Recent technological advances in digital receivers, UHF transmitters, and software radar technology have enabled a new
generation of Incoherent Scatter Radar systems that can be significantly optimized for different performance and
operational characteristics. These advanced architectures offer new possibilities for addressing the observational needs
of communities interested in real time Space Weather data assimilation. In particular global “now casting” and
predictive modeling will not be possible without a major network of radio science instrumentation that can constrain
model parameters with appropriately measured physical values. Many of these parameters are only available using the
Incoherent Scatter Radar technique. The development of a Global Radio Science Network is a major challenge for the
21st century and new Incoherent Scatter Radar designs are a fundamental part of addressing this goal.


[1] J.M Holt, P.J. Erickson, A.M. Gorczyca, T. Grydeland, “MIDAS-W: A workstation based incoherent scatter radar
data acquisition system,” Annales Geophysicae, vol 18., no. 9, pp. 1231-1241, 2000.

[2] R.S. Symons, “The constant efficiency amplifier,” NAB Broadcast Engr. Conf. Proc., pp. 523-530, 1997.

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