John D. Sahr

The Manastash Ridge radar: A passive bistatic radar for upper atmospheric radio science

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.

Observations of auroral E-region plasma waves and electron heating with EISCAT and a VHF radar interferometer

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 × 104 cm−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. From the observations and our analysis, we conclude that the electron temperature increases were caused by plasma wave heating and not by either Joule heating or particle precipitation.

Passive Direction Finding Using Airborne Vector Sensors in the Presence of Manifold Perturbations

This paper studies direction finding of electromagnetic energy sources using passive vector sensor arrays whose manifold is only nominally known. The problem of direction finding is studied in the context of an airborne array, thus facilitating the observation of a ground-based source from multiple look-angles. A calibration algorithm explicitly accounting for the polarization diverse nature of the vector sensor is developed. It is also shown that the direction finding performance provided by a conventional array calibration algorithm is greatly enhanced with usage of the proposed polarization diverse calibration algorithm

The E-region rocket/radar instability study (ERRRIS): scientific objectives and campaign overview

Abstract 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 176km. The lower apogee rockets were flown into the unstable auroral electrojet and encountered intense two-stream waves driven by d.c. electric fields that ranged from 35 to 115 mV/m. The higher apogee rocket returned fields and particle data from an active auroral arc, yet observed a remarkably quiescent electrojet region as the weak d.c. electric fields (~ 10–15 mV/m) there were below the threshold required to excite two-stream waves. The rocket instrumentation included electric field instruments (d.c. and wave), plasma density fluctuation ( δn / 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 50MHz CUPRI system, which was the primary tool used to determine the launch conditions. Two of the rockets were flown in conjunction with plasma drift, density, and temperature measurements taken by the EISCAT incoherent scattar radar. The STARE radar also made measurements during this campaign. This paper describes the scientific objectives of these rocket/radar experiments, provides a summary of the geophysical conditions during each launch, and gives an overview of the principal rocket and radar observations.

The altitude of type 3 auroral irregularities: Radar interferometer observations and implications

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 (O{sup +}) or, less frequently, the molecular (O{sub 2}{sup +} and NO{sup +}) gyro frequencies. These type 3 echoes have been attributed to an electrostatic ion-cyclotron (EIC) 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 CUPRI (Cornell University portable radar interferometer) shows that type 3 events occur at typical electrojet altitudes (100-120 km), however. At these altitudes the ion collision frequency is greater than the ion gyrofrequency and there can be no cyclotron motion. The cause of the observed type 3 echoes hence remains a mystery.

First passive radar observations of auroral E‐region irregularities

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.

On the design of quadratic filters with application to image processing

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.

Spectrum estimation of moderately overspread radar targets using aperiodic transmitter coding

We present 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 unraveled, 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.

Optimal truncation criterion for application of singular value decomposition to ionospheric tomography

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.

Self-calibration of an airborne array

Unlike external array calibration, array self-calibration does not assume that source angles of arrival are known. This paper studies novel self-calibration algorithms and tests them on experimental data. Results indicate that self-calibration algorithms based on an appropriate cost function may achieve source-localization performance comparable to that of external array calibration and better than without calibration.