Limits on Unintended Radio Emission from Geostationary and Geosynchronous Satellites in the SKA-Low Frequency Range

TL;DR

This study searches for unintended radio emissions from geostationary/geosynchronous satellites in the SKA-Low band (72-231 MHz) using the GLEAM-X MWA dataset. By stacking residual images around predicted GEO positions derived from space-track TLEs and applying beam corrections, the authors obtain $4\sigma$-level upper limits on EIRP well below 1 mW, with best cases around $10\ \mu\mathrm{W}$. Although three $4\sigma$ candidates appear in preliminary analysis, refined positional uncertainties indicate these are consistent with random fluctuations rather than true satellite emissions. The results establish a vital baseline for GEO RFI monitoring relevant to SKA-Low operations and EMC comparisons, suggesting GEO UEMR currently poses minimal detected risk at present sensitivity, while highlighting the need for continued monitoring as SKA-Low improves in sensitivity.

Abstract

We search data from the GLEAM-X survey, obtained with the Murchison Widefield Array (MWA) in 2020, for the presence of radio frequency interference from distant Earth-orbiting satellites, in the form of unintended emissions similar to those recently seen from objects in Low Earth Orbits (LEO). Using the GLEAM-X Dec = 1.6 degree pointing, which is stationary in azimuth (on the local Meridian) and elevation (near the celestial Equator), the very wide field of view of the MWA maintains custody of a large number of satellites in geostationary and geosynchronous (GEO) orbits in this direction for long periods of time. We use one night of GLEAM-X data in the 72 - 231 MHz frequency range to form stacked images at the predicted coordinates of up to 162 such satellites, in order to search for unintended radio emission. In the majority of cases, we reach 4 sigma upper limits of better than 1 mW Equivalent Isotropic Radiated Power (EIRP) in a 30.72 MHz bandwidth (dual polarisation), with the best limits below 10 uW. No convincing evidence for unintended emissions at these detection thresholds was found. This study builds on recent work showing an increasing prevalence of unintended emissions from satellites in LEO. Any such emission from objects in GEO could be a significant contributor to radio frequency interference experienced by the low frequency Square Kilometre Array and warrants monitoring. The current study forms a baseline for comparisons to future monitoring.

Figures (3)

• Figure 1: MWA imaged field of view in azimuth and elevation coordinates (blue line), with the MWA beam response as a colour grayscale. Red points are the coordinates of individual satellites at individual times. Frequencies are 88 MHz to 216 MHz, from top left to bottom.
• Figure 2: The detection stacks for NORAD 43432 at 88 MHz (top left), 15236 at 118 MHz (top right), and 27683 at 154 MHz (bottom). Shown in red are the 4 arcminute error region adopted in the search process and the specific error regions estimated for the individual satellites (shown using the larger of the RA and DEC region values in Table \ref{['err-reg']} as a circular error region.)
• Figure 3: EIRP upper limits ($4\sigma$) at 88 MHz to 216 MHz, from top left to bottom right. The horizontal axis is labelled "Order", which is simply the order in which TLEs for satellites were returned from the space-track.org query.