First passive radar observations of auroral E region irregularities
Lind, F. D.,  J.D. Sahr,  and D.M. Gidner
Geophys. Res. Lett. Vol. 26 , No. 14 , July  15, 1999;  p. 2155

On August 27, 1998 we observed auroral E-region irregularities using the Manastash Ridge Radar system. These are the first radio science results from passive radar observations of commercial FM broadcasts near 100 MHz. These are also the first reported observations of 1.5 m irregularities in 15 years. The data show scatter most consistent with type-1 and type-3 ion-acoustic turbulence as observed by 50 MHz and 140 MHz radars. The instrument is extraordinarily low in cost and provides data free of range and doppler velocity aliasing problems. The range and velocity resolution achieved by this radar is substantially superior to that of other comparable coherent scatter radars, and the time resolution is similar.

Optimal truncation criterion for application of singular value decomposition to ionospheric tomography
 Zhou, C., E.J. Fremouw, and  J.D. Sahr
Radio Science. vol.34, no.1; Jan.-Feb. 1999; p.155-66

In this paper, we present a generic method to solve the subspace-oriented estimation problem. We have optimized our approach by taking account of the a priori and a posteriori covariances of both the data and the model parameters in general linear inverse notations. In our work, singular value decomposition (SVD) was employed to provide a robust optimal solution. In computation of the generalized matrix inverse, a very simple truncation criterion on the singular value (SV) spectrum was set up which guarantees the minimal variance of the estimate. This algorithm based on SVD produces an optimal estimate independent of computing resources.  Specifically, we applied this method to studies of ionospheric tomography by inverting total electron content (TEC), which may be measured by means of satellite beacons. We processed two simulated cases. The residual variances of the a posteriori covariances of the model parameters were used as the measure to evaluate the uncertainties of the estimates. Our examples indicate that this algorithm can resolve about 60% of the a priori variance while achieving a significant decrease of the computation time by truncation of the SV spectrum.

Recursive structures and finite impulse response implementations of linear multiuser detectors for an asynchronous CDMA system
Uppala, S.V., and J.D. Sahr
IEEE Journal on Selected Areas in Communications. vol.16, no.9; Dec. 1998; p.1736-46

We present order-recursive structures for implementing the linear decorrelating and linear minimum mean square error (MMSE) detectors in a computationally efficient manner. These infinite memory length, linear time invariant, noncausal systems can be approximated by implementing them as K-input K-output finite impulse response (FIR) filters. We developed a linear multiuser receiver architecture called a recursive linear multiuser detector which has lower computational and memory requirements then an equivalent tap delay line FIR filter implementation. We establish the tradeoff between window length and bit error rate and find that relatively small window size yields performance indistinguishable from the ideal decorrelating detector and the ideal MMSE detector.

Computing probability of errors of linear multiuser detectors in white Gaussian noise and Rayleigh fading channels
Uppala, S.V., and J.D. Sahr
ICC '98. 1998 IEEE International Conference on Communications. Conference Record. Affiliated with SUPERCOMM'98 (Cat. No.98CH36220). IEEE, New York, NY, USA; 1998; 3 vol. xxxvii+1838 pp. p.360-4 vol.1

We present a tool for rapidly computing the bit error rate of linear multiuser detectors. Computing bit error rates of linear multiuser detectors involves averaging the probability of error over the bits of all the users which is an exponentially complex operation. The technique presented in this paper is based on computing the Laplace inversion integral of the moment generating function of the variate which determines the probability of error of a user. We apply this technique for computing the probability of error of an arbitrary linear multiuser detector operating in an additive white Gaussian noise and in a Rayleigh fading channel.

Passive radio remote sensing of the atmosphere using transmitters of opportunity
Sahr, J.D.,  and F. D. Lind
Radio Science Bulletin. no.284; March 1998; p.4-7

Commercial FM broadcasts near 100 MHz illuminate the natural environment with continuous, high power illumination. By fortunate coincidence these broadcasts are superb for radar applications. With proper signal processing and a bistatic, synchronous receiver system, target range may be resolved to about one kilometer, and Doppler spectra resolved to a few meters per second, with integration times of the order 30 seconds. Such a system suffers no large ambiguities in range or Doppler shift, an extremely attractive feature in studies of meter scale irregularities in the ionosphere.

The Manastash Ridge radar: a passive bistatic radar for upper atmospheric radio science
Sahr, J.D.,  and F. D. Lind
Radio Science. vol.32, no.6; Nov.-Dec. 1997; p.2345-58

We describe a novel method for radar remote sensing of the upper atmosphere which relies upon commercial FM broadcasts near 100 MHz. These broadcasts have high average power and excellent radar ambiguity function. With proper processing we can study the spatial and temporal distribution and Doppler spectrum with excellent and completely unambiguous resolution. Since this passive system has no transmitter, there are enormous benefits in safety, expense, shielding, antenna and receiver design, and licensing issues. Some new problems are introduced, but these are solved with relatively little expense. After presenting the technical basis for such a radar, we describe an instrument that we are building at the University of Washington to study high-latitude plasma irregularities in the E region.

Performance analysis of the decorrelating detector in the presence of synchronization errors
Uppala, S.V., and J.D. Sahr
1997 IEEE Pacific Rim Conference on Communications, Computers and Signal Processing, PACRIM. 10 Years Networking the Pacific Rim, 1987-1997 (Cat. No.97CH36060). IEEE, New York, NY, USA; 1997; 2 vol. xxiii+1021 pp. p.18-21 vol.1

We present a semi-analytic method for computing the bit error rate of the decorrelating detector in the presence of phase and timing errors (synchronization errors). These errors in effect introduce multiple access interference. We first compute the moment generating function of the variate which determines the probability of error of a user. It can be shown that the probability of error is obtained by computing the Laplace inverse integral of a modified version of the moment generating function. Helstrom (1986) used this approach to analyze a BPSK system corrupted with intersymbol interference. In this paper are show that this method can be used for computing the probability of errors of users in the presence of multiple access interference. Due to the analytic approach used systems with larger packet sizes and larger user populations can be tackled.

On the design of quadratic filters with application to image processing
Uppala, S.V., and J.D. Sahr
IEEE Transactions on Image Processing. vol.6, no.4; April 1997; p.608-14

We present a systematic design method for quadratic filters, and show an example for an edge detector. Our design maximizes the output signal-to-noise ratio (SNR) constrained to detect desired signals while ignoring specified error signals such as shot noise. This filter compares favorably with Sobel and morphological detectors of similar complexity.

Auroral electrojet plasma irregularity theory and experiment: a critical review of present understanding and future directions
Sahr, J.D., Fejer, B.G.
Journal of Geophysical Research. vol.101, no.A12; 1 Dec. 1996; p.26893-909

We review the experimental observations of meter scale plasma irregularities in the auroral E region and the status of their theoretical understanding. Most of the experimental data is derived from VHF radar scatter experiments, but sounding rockets also provide crucial information not obtainable from radars. Linear theories correctly predict the altitude of occurrence, strong magnetic aspect sensitivity, marginal instability, and typical phase velocities. Subsequent nonlinear theories have been developed to account for other observed features but with less satisfying application. Further understanding of auroral electrojet irregularities is impeded by precision limitations of existing instruments, by radar data which may seem incompatible, by the usually poor knowledge of the ambient conditions during these experiments, and by some confusion in the nomenclature (e.g. "type 2") used to describe the irregularities. We hope to clarify some of these experimental and theoretical issues. We discuss observational "facts" that need to be refined and point out weaknesses of existing theories or their common interpretations. Finally, we outline some avenues for future experimental and theoretical pursuit.

Probing electric fields near sprites and jets using multiparameter radar and chaff
Seliga, T.A., J.D. Sahr, and R.H. Holzworth
IGARSS '96. 1996 International Geoscience and Remote Sensing Symposium. Remote Sensing for a Sustainable Future (Cat. No.96CH35875). IEEE, New York, NY, USA; 1996; 4 vol. lxxi+2383 pp. p.569-73 vol.1

The discovery of electric discharges above thunderstorms has generated intense scientific interest. Studies of these `sprites' and `jets' have focused on their characterization by optical and radio techniques, with radar measurements of the causative storms providing insight into related weather. The authors describe a method for investigating the electric field structure above thunderstorms using ground-based radar to observe chaff dispersed by rockets. Slender conducting or dielectric chaff will generally align itself with the ambient electric field. This alignment is readily detected by appropriate configurations of polarimetric radar(s) such as are now used in meteorology to observe the nature and motion of hydrometeors. This is especially convenient as it permits the thunderstorms associated with sprites and jets to be characterized with the same experimental facility. This paper renders a preliminary examination of factors such experiments would entail and features that a multiparameter radar might utilize to probe chaff dispersed by small rockets. Monostatic and bistatic radar measurements of scatter from chaff provide a powerful tool to study electric fields associated with sprites and jets as well as other atmospheric electric fields.

Aperiodic transmitter waveforms for spectrum estimation of moderately overspread targets: new codes and a design rule
Uppala, S.V., and J.D. Sahr
IEEE Transactions on Geoscience and Remote Sensing. vol.34, no.5; Sept. 1996; p.1285-7

Presents several new classes of aperiodic waveforms suitable for FFT-based spectral analysis of moderately overspread radar targets. In previous work, the authors explained the principle of operation with a particular code. They now provide codes with higher performance as well as a design rule for the user to apply these codes to real targets.

Application of closure phase and self-calibration to radar interferometric imaging of atmospheric and ionospheric irregularities
Sahr, J.D.
Journal of Atmospheric and Terrestrial Physics. vol.58, no.8-9; June-July 1996; p.959-64

Describe extensions to radar interferometric techniques used to study irregularities in the upper atmosphere, based on techniques developed by radio astronomers. In particular, the techniques of 'self-calibration' and 'closure phase' offer great promise to radars that measure properties of stochastic geophysical targets. The author shows how useful extensions arrive and comments upon implications of such techniques for the analysis of existing data and the design of new instruments.

Correlative range - Doppler detectors and estimators in bistatic radar using commercial FM broadcasts
Hall, P.W.
MS thesis, Univ. of Wash., Seattle, 1995

Commercial FM radio broadcasts entertain us with music and keep us informed of current events.  With the help of digital signal processing and fortunate geography we learn more from FM radio than today's news - we can use commercial FM broadcasts as a radar waveform to probe the auroral electrojet.  As the earth moves through the Solar Wind, charged particles set up a current in the ionosphere at about 70 degrees latitude and a height of 100 to 120 km.  This current carries a million amperes and is responsible for the Northern Lights, communications impairments and sometimes affects power distribution networks.  Using two receivers on either side of the Cascade Mountains in Washington state, we correlate the FM broadcast scatter off the auroral electrojet to learn more about the physics of this important part of our ionosphere.  the ambiguity function of FM broadcasts is measured and agrees w9ith our thoretical model but differences are found between stations.  The "dead-air" between spoken words greatly affects the range and doppler resolution available.  We also propose an unbiased and consistent estimator for the four product correlation which is computationally efficient.

Three-wave coupling in the auroral E-region
Sahr, J.D., and D.T. Farley
Annales Geophysicae. vol.13, no.1; 1 Jan. 1995; p.38-44

Analysis of a simple model for two-stream weak fluid turbulence suggests a straightforward mechanism for the excitation of plasma wave modes whose phase velocities are roughly half the ion-acoustic speed. This slow wave is linearly stable but is excited through three-wave interaction with two other waves, at least one of which is an unstable two-stream wave. All three waves satisfy the linear dispersion relation. Careful consideration of the coupling coefficients and resonance conditions shows that the driven (linearly stable) modes have k-vectors that are perpendicular to the local magnetic field and are at some large angle to the Hall drift direction. These waves have phase velocities in the neighborhood of half the ion-acoustic speed and they could be responsible for type-3 radar echoes observed at 50 MHz. This mechanism cannot, however, generate fast waves moving at type-4 velocities.

A new radar technique for remote sensing of atmospheric irrecgularities by passive  observation of the scattering of commercial FM broadcasts
Hansen, J.M.
MS thesis, Univ. of Wash., Seattle, 1994

This paper investigates the properties of public broadcast FM radio and how it can be used as a technique for remote sensing.  In particular we study how three different receiving systems can make use of FM broadcasts to achieve high range and Doppler velocity resolution of scatter from atmospheric irregularities.  Calculations of system performance are provided based upon other observations of auroral electrojet irregularities.  The ambiguity function of FM radio signals is measured using simulations and from this further simulations are performed to model the three methods of detecting targets.  The results presented here are promising, indicating that further study and actual experimental implemention is warranted.

Spectrum estimation of moderately overspread radar targets using aperiodic transmitter coding
Uppala, S.V., and J.D. Sahr
Radio Science. vol.29, no.3; May-June 1994; p.611-23

Presents a new method of spectrum estimation of moderately overspread targets using a pulse-to-pulse method, with nonuniform pulse spacing. This method is especially applicable to VHF observations of the auroral electrojet irregularities, for which the ratio of the target range extent and the autocorrelation time exceeds the speed of light by a modest amount. For a suitable choice of pulse spacings, the range and frequency aliasing can be uniquely unravelled, and the estimation algorithm can be expressed in terms of the fast Fourier transform, for excellent computational efficiency. This technique should find application to other moderately overspread targets, such as Doppler weather radar and equatorial spread F.

Simulated annealing searches for long binary phase codes with application to radar remote sensing
Sahr, J.D., E.R. Grannan
Radio Science. vol.28, no.6; Nov.-Dec. 1993; p.1053-5

The authors apply the technique of simulated annealing to search for long discrete codes with special autocorrelation and ambiguity function properties, suitable for radar applications. This technique provides access to codes which are far too long for exhaustive search, and to codes whose correlation properties do not lend themselves to algebraic investigation. The authors illustrate the process with two new kinds of binary phase codes.

Observations of 3-m auroral irregularities during the ERRRIS campaigns
Sahr, J.D., D.T. Farley, W.E. Swartz, J.F.  Providakes, and R.F. Pfaff
Journal of Atmospheric and Terrestrial Physics. vol.54, no.6; June 1992; p.809-18

In the late winter of 1988 and 1989, three NASA sounding rockets were flown through the auroral electrojet from ESRANGE (Sweden) as part of the E-region Rocket-Radar Instability Study (ERRRIS). Many ground-based instruments supported these flights, including the EISCAT, STARE, and CUPRI radars, as well as all-sky cameras, riometers, and magnetometers. The authors summarize the observations of the Cornell University Portable Radar Interferometer (CUPRI), which detected coherent backscatter from 3-m irregularities in the auroral E-region. Twenty hours of power spectra and interferometry data are available, and, during the 1989 campaign, three weeks of nearly continuous Range-Time-Intensity (RTI) and first moment data were recorded.

The E-region Rocket/Radar Instability Study (ERRRIS): scientific objectives and campaign overview
Pfaff,R.F., J.Sahr, J.F.Providakes, W.E.Swartz, D.T.Farley, P.M.Kintner, I.Haggstrom,  A. Hedberg, H.Opgenoorth, G.Holmgren, A. McNamara, D.Wallis, B. Whalen, A.Yau,  S. Watanabe, F.Creutzberg, P. Williams, E.Nielsen, K Schlegel, and T.R. Robinson
Journal of Atmospheric and Terrestrial Physics. vol.54, no.6; June 1992; p.779-808

The E-region Rocket/Radar Instability Study (Project ERRRIS) investigated in detail the plasma instabilities in the low altitude (E-region) auroral ionosphere and the sources of free energy that drive these waves. Three independent sets of experiments were launched on NASA sounding rockets from Esrange, Sweden, in 1988 and 1989, attaining apogees of 124, 129 and 176 km. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation ( delta n/n) receivers, d.c. fluxgate magnetometers, energetic particle detectors (ions and electrons) ion drift meters, and swept Langmuir probes to determine absolute plasma density and temperature. The wave experiments included spatially separated sensors to provide wave vector and phase velocity information. All three rockets were flown in conjunction with radar backscatter measurements taken by the 50 MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with measurements taken by the EISCAT incoherent scatter radar.

The altitude of type 3 auroral irregularities: radar interferometer observations and implications
Sahr, J.D.,  D.T. Farley, W.E.  Swartz, and  J.F.  Providakes
Journal of Geophysical Research. vol.96, no.A10; 1 Oct. 1991; p.17805-11

VHF coherent scatter radars at auroral latitudes have observed scatterers with narrow power spectra at sub-ion acoustic mean Doppler shifts. These spectra have been designated 'type 3'. The mean Doppler shift of these waves is often near the atomic or, less frequently, the molecular gyro frequencies. These type 3 echoes have been attributed to an electrostatic ion-cyclotron instability in the upper E region (h>140 km), where the ion collision frequency becomes low enough to permit ion gyromotion.  Interferometric analysis of recent coherent radar observations with the Cornell University portable radar interferometer shows that type 3 events occur at typical electrojet altitudes (100-120 km), however. The cause of the observed type 3 echoes hence remains a mystery.

Ion cyclotron harmonics in auroral radar echoes: real effect or analysis artifact?
Farley, D.T.,  J.D. Sahr, and J.F. Providakes
Geophysical Research Letters. vol.18, no.4; April 1991; p.709-12

Certain analyses of Canadian radar studies of auroral electrojet echoes have apparently shown spectra with multiple ion cyclotron harmonic peaks. On the basis of these, it has been argued that ion cyclotron effects must be important for all echo types seen by VHF radars, not just type 3. The authors reject this conclusion for two reasons: (1) at the altitude of auroral backscatter  (approximately 100-120 km) the ion-neutral collision frequency is much higher than the gyro frequency-ions are essentially unmagnetized; (2) the authors show that the 'high resolution' signal processing algorithm employed does not distinguish between echoes with and without harmonic structure. With a simulation, they show that the algorithm itself can impose apparent quasi harmonic structure onto the spectrum of random noise.

Removal of aliasing in pulse-to-pulse Doppler radar measurements
Sahr, J.D., D.T. Farley,  and W.E.  Swartz
Radio-Science. vol.24, no.6; Nov.-Dec. 1989; p.697-704

Frequency aliasing is a problem often encountered in pulsed radar studies of some deep targets, such as the radar aurora. It is possible to resolve the aliasing ambiguities by transmitting closely spaced pulse pairs, rather than single pulses. Instead of the usual power spectrum, the authors compute a cross spectrum of the received signals. The amplitude of this cross spectrum is just the normal, possibly aliased, power spectrum, and the phase determines the aliasing, if any, at each frequency bin. The technique works as long as the spectrum is not seriously wrapped around upon itself, that is, as long as each frequency bin is dominated by signals with a single Doppler shift, no matter what that Doppler shift may be. The price paid for this extra information is the addition of radar clutter, uncorrelated signals from unwanted ranges, but this price may be acceptable.

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer
Providakes, J., D.T. Farley, B.G. Fejer, J. Sahr,  W.E. Swartz,  I. Haggstrom, A. Hedberg, and J.A. Nordling
Journal of Atmospheric and Terrestrial Physics. vol.50, no.4-5; April-May 1988; p.339-56

Two radars were used simultaneously to study naturally occurring electron heating events in the auroral E-region ionosphere.  During a joint campaign in March 1986 the Cornell University Portable Radar Interferometer (CUPRI) was positioned to look perpendicular to the magnetic field to observe unstable plasma waves over Tromso, Norway, while EISCAT measured the ambient conditions in the unstable region. On two nights EISCAT detected intense but short lived (<1 min) electron heating events during which the temperature suddenly increased by a factor of 2-4 at altitudes near 108 km and the electron densities were less than 7*10/sup 4/ cm/sup -3/. On the second of these nights CUPRI was operating and detected strong plasma waves with very large phase velocities at precisely the altitudes and times at which the heating was observed. The altitudes, as well as one component of the irregularity drift velocity, were determined by interferometric techniques. The authors conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.