The Design of Passive Radar Systems for Studying Ionospheric Coherent Scatter
Frank D. Lind (1), John D. Sahr (2), Melissa G. Meyer (2)
(1) MIT Haystack Observatory, Route 40, Westford MA 01886 USA. (2) Department of Electrical Engineering, University of Washington, Box 352500, Seattle WA 98195 USA.
Abstract
In this paper we discuss the system architecture of the Manastash Ridge Radar in sufficient detail to allow for the construction of similar passive radar systems for observing E-region ionospheric irregularities. Passive radar systems are well suited for observing E-region irregularities due to the wide availability of high power signals with meter scale wavelengths (FM radio). The design space for constructing ionospheric passive radar systems is not well explored however several lessons have been learned from the development and operation of the Manastash Ridge Radar system which may aid others interested in implementing such a radar. The most important of these lesssons include the necessity of high quality digitization and coherence in the function of the radar system as well as the importance of software as an element of the radar.
Introduction
Passive Radar is the use of signals already present in the environment to make radar observations (Sahr and Lind 1997). It is distinguished from other passive observation techniques (refs) by its ability to resolve range, doppler shift, and bearing in a manner similar to traditional active radar systems and with similar or better resolution. Passive Radar has been used to make observations of coherent scatter from E-region irregularities for some time now (Lind et al 1999). Other communities have also applied passive radar techniques to the observation of aerospace targets. Recently the technique has improved to the point where interferometery has been demonstrated (meyer et al 2003) and the Manastash Ridge Radar system has proven to be a productive monitor for the statistical characteriation of E-region irregularities (Meyer et al 2004).
A basic passive radar system for the observation of coherent scatter from E-region irregularities is suprisingly easy to construct. The design space of possible system architectures and modes of operation has not however been widely explored.
Requirements for a Passive Radar System
To function a passive radar system must intercept a signal in the environment which will then propagate and scatter from a target of interest. The scatter from this target must then be received using a system which creates sufficient dynamic range to allow the weak scattered signal to be detected in the presence of the strong transmitter signal. The intercepted transmit signal (X) and the received signal from the scatterer (Y) must be obtained with sufficient relative coherence to allow the relative delay and phase of the signals to be resolved.
Sensitivity Analysis for Coherent Scatter Observations using Passive Radar
System Architecture of the Manastash Ridge Radar
The Manastash Ridge Radar has evolved through two major designs to arrive its current form. During this evolution several characteristics of the architecture have remained consistent. These include the essential system geometry, the operating frequency range, and the method of maintaining system coherence. The radar uses a bistatic receiver geometry where the signals from FM radio station transmitters are intercepted at the University of Washington in Seattle. The antenna used to intercept these high SNR signals is extremely modest consisting of an off the shelf folded dipole antenna for the FM band. A remote receiver site located at Manastash Ridge Observatory where several different antennas have been used to observe scatter from the ionospheric irregularities.
=== Hardware Design ===
The first generation of Manastash Ridge Radar hardware used a direct conversion receiver architecture to convert the FM radio signals from a single channel to baseband Inphase and Quadrature (IQ) components which were then digitized at a sampling rate of 250 kHz using a 12 bit A/D converter.
The second generation hardware uses a direct RF digitization approach where FM radio signals are subsampled by the digital receiver. In this approach the entire FM radio band is amplified and filtered to a level appropriate for digitization by a 14 bit A/D converter operating at 72 MHz. The current digital receivers used are from the Echotek corporation and are based on the Gray Chip digital downconverter.
=== Operations Software Design ===
=== Data Flow and Network Requirements ===
Passive Radar Signal Processing
=== Basic Range-Doppler Signal Processing ===
=== Two Element Interferometry ===
=== Ambiguity characteristics of FM radio signals ===
=== Computing Requirements for Typical Operations ===
Future Directions
There are many areas of passive radar system design for observing ionospheric coherent scatter which remain to be explored. These areas include the development of passive radar networks, extension of the systems to exploit a wider range of signals, the development of interferometric imaging, and experimentation with different deployment locations and geometries.