Detection and characterisation of Starlink VHF TT&C emissions at 137.055 MHz with the 21 CentiMeter Array

Abstract

Satellite emissions at low frequencies can interfere with astronomical observations. Observations with the EDA2 have revealed recurrent Starlink emissions near 137~MHz, attributed to very high frequency (VHF) telemetry, tracking, and command (TT\&C) transmissions. We aim to characterise Starlink VHF emissions detected with the 21 CentiMeter Array (21CMA) to better assess their potential impact on future low-frequency radio astronomical observations and inform the development of methods to mitigate the resulting interference. We searched five coordinated VHF channels in observations obtained on 15 April 2025 (UTC) and demodulated and decoded candidate signals. Two valid packets are detected at 137.055~MHz, each lasting 1.027584~s and separated by 99.987~s. A blind comparison with 11\,305 active satellites identifies STARLINK-32838 (NORAD ID 62827) as the transmitter, with differential Doppler measurements linking both events to the same spacecraft. Orbital analysis shows that the satellite was undergoing orbit raising and that the limited number of detections was partly attributable to its being below the horizon for most of the observing time. Re-analysis of EDA2 detections indicates that such emissions are not confined to orbit raising. The stronger event's estimated flux density is 9.23~MJy at 1000~km, consistent with previous measurements at the order-of-magnitude level. Signal demodulation and decoding, combined with orbital analysis, can improve the identification and characterisation of satellite-related radio frequency interference. The detection of similar VHF emissions across different orbital phases indicates that their potential interference with low-frequency radio astronomy should not be considered solely in the context of orbit raising.