Limits on radio emission from meteors using the MWA

Abstract

Recently, low frequency, broadband radio emission has been observed accompanying bright meteors by the Long Wavelength Array (LWA). The broadband spectra between 20 and 60 MHz were captured for several events, while the spectral index (dependence of flux density on frequency, with S ν ∝ ν α ) was estimated to be −4 ± 1 during the peak of meteor afterglows. Here we present a survey of meteor emission and other transient events using the Murchison Widefield Array (MWA) at 72-103 MHz. In our 322-hour survey, down to a 5σ detection threshold of 3.5 Jy/beam, no transient candidates were identified as intrinsic emission from meteors. We derived an upper limit of -3.7 (95% confidence limit) on the spectral index in our frequency range. We also report detections of other transient events, like reflected FM broadcast signals from small satellites, conclusively demonstrating the ability of the MWA to detect and track space debris on scales as small as 0.1 m in low Earth orbits.

The development of non-coherent passive radar techniques for space situational awareness with the Murchison Widefield Array

“…• undertaken a detailed analysis of the apparent FM reflections from LEO RFI detected in Zhang et al (2018); • developed an analysis using a quantity we call the Dynamic Signal to Noise Ratio Spectrum (DSNRS), that helps classify signals as originating from objects in orbit from terrestrial transmitters; • used the DSNRS to analyse three signals found to be FM reflections from obejcts in orbit (up-to a maximum range of 2298 km) and signals identified as out-of-band transmissions from two CubeSats; • detected short duration signals at FM frequencies that do not coincide with satellite locations predicted using the TLE catalog; • identified position off-set between the actual and predicted TLE position, thus demonstrating MWA's potential to be used for catalog maintenance; • performed a study completeness analysis, considering the reasons why some satellites in the field-ofview went undetected during the observations used in this work.…”

“…Some satellites considered in this work are within a few hundred kilometers of the MWA, which puts them into the near field as seen by the MWA's longest baselines (3 km). Consequently we restrict our analysis to baselines shorter than 500 m in order to avoid near-field affects (Zhang et al, 2018). Natural weighting was also used for all objects.…”

“…The data reduction pipeline used in this work incorporates the difference imaging technique that was found to work effectively by Zhang et al (2018). Once imaged, difference images were formed at each time step t by subtracting the image at time step t − 1 from the image at time step t. The difference images remove the persistent celestial sources, along with their side-lobes, thus greatly reducing the side-lobe confusion noise in the difference images.…”

“…Using similar techniques, the MWA has also been used to search for meteors (Zhang et al, 2018), based on possible intrinsic radio emission, as seen at lower frequencies by the Long Wavelength Array Ellingson et al, 2013;Obenberger et al, 2016), or the reflection of FM radio waves from the ionisation trails left by meteors. In Zhang et al (2018), the static celestial sources in their images were removed using difference imaging techniques, leaving non-static meteors visible as transient signals in the difference images. During these observations, some apparent FM reflections from satellites were also detected (of objects much smaller than the ISS) but were not investigated in detail.…”

“…During these observations, some apparent FM reflections from satellites were also detected (of objects much smaller than the ISS) but were not investigated in detail. The current paper undertakes a comprehensive examination of these detections and extends the difference imaging techniques of Zhang et al (2018), to improve the detection of objects in Earth orbit for the purposes of SSA using non-coherent passive radar techniques with the MWA.…”