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Automatic processing of ionograms to extract ionospheric propagation characteristics and electron density profile parameters is an important enabling step toward constructing real time data‐assimilative models of the ionosphere. A robust and reliable feature extraction and fitting algorithm that supports both oblique and vertical incidence ionograms has recently been developed to enhance the regional ionospheric model for Australia's Jindalee Operational Radar Network. The so‐called DST‐IIP algorithm is an autoscaling technique based on a constrained quasi‐parabolic segment profile, which is modified from its climatological estimate to fit a set of key peak, edge, and line features in the ionogram image, representing the E, sporadic‐E (Es), and F2 layers. The F2 fitting, in particular, is performed in the ionogram domain, using analytic ray‐tracing and homing to produce a synthetic trace that reproduces the image features. Following thorough testing on a large database of midlatitude Australian ionograms, the DST‐IIP algorithm now runs routinely on‐board DST Group's Digital Oblique Receiving System. This paper provides an overview of the technique, along with sample results and performance statistics.

The problem addressed is that of high frequency (HF) single site location (SSL) using a linear array, where the signal of interest propagates beyond the line of sight via the sky-wave mode. The paper proposes a new geolocation method that exploits multipath to resolve the "coning" ambiguity in a linear array. For the first time, this opens up the possibility to perform meaningful HF-SSL using a linear array. The proposed method is also applicable to traditional HF-SSL systems based on two-dimensional apertures. In this case, the advantage is reducing estimation errors as opposed to resolving an ambiguity. Experimental data with known ground truth on a HF source is used to validate the method. Fig. 1 illustrates the classical HF-SSL concept, where a two-dimensional aperture is used to measure the azimuth and elevation of the signal direction-of-arrival. A real time ionospheric model (RTIM) derived from a vertical incidence sounder (VIS) allows the elevation angle to be converted to a ground range for source position estimation. Fig. 2 compares the characteristics of various geolocation methods. Trade-offs between the number of sites required, system complexity at each site, and knowledge demanded of the ionosphere and signal is summarized. A detailed treatment of HF-DF systems can be found in [1] and [2]. I. BACKGROUND AND MOTIVATIONIn particular, there is currently no operational HF-SSL system based on a linear array. This is not surprising given that a linear array cannot estimate bearing and elevation separately due to the coning ambiguity. This paper proposes and experimentally validates a new geolocation method that can be employed to enable HF-SSL using a linear array. Such results have significant implications for implementing HF-SSL on existing linear arrays. Potential applications include search and rescue, enforcement of HF spectrum regulations, and in military situations, as stated in [1].The key behind the approach is to exploit multipath propagation by simultaneously combining the information contained in all received modes. This is in contrast to many current HF-SSL systems, which employ wavefront testing procedures (WFT) in order to produce estimates only at times of quasi uni-modal propagation (QUMP). This is tantamount to regarding multipath as a nuisance rather than an opportunity. Such systems not only miss the opportunity to use the additional information contained in the ensemble of paths, but the severe pruning of data needed to satisfy QUMP conditions can also significantly limit the times at which measurements may be used. II. PROBLEM DESCRIPTION AND SCOPEThe HF-SSL system is assumed to be an antenna array with a multi-channel digital receiver. Particular emphasis is on a ULA, but the described method is applicable to general array geometries with minor modification. In the ULA case, the elements are assumed to have front-to-back directivity, such that the field of view is limited to ±90 degrees from boresight. The source is assumed to be at relatively long range such that propagation is vi...