A reversed solar illumination dependence of unintended emission from Starlink Direct-to-Cell satellites at 72–234 MHz with the EDA2

Abstract

Second-generation Starlink Direct-to-Cell (DTC) satellites carry an additional payload for direct cellular phone connectivity whose unintended electromagnetic radiation (UEMR) at sub-300 MHz frequencies has not been individually characterised. We reanalyse

detections from

1 806 1 806 $1\,806$

Starlink satellites observed with the Engineering Development Array version 2 (EDA2) at 21 frequencies between

72.685 72.685 $72.685$

and

234.375 234.375 $234.375$

MHz (Grigg et al. 2025, A&A, 699, A307), separating 175 DTC and

1 623 1 623 $1\,623$

Ku-only v2-Mini comparison satellites via the McDowell General Catalogue (McDowell 2020, ApJL, 892, L36). Among catalogued detections, DTC satellites have a per-satellite median range-corrected flux density

1.45 times 1.45 × $1.45\times$

that of the Ku-only comparison (Cliff’s

delta equals plus 0.30 δ = + 0.30 $\delta=+0.30$

,

p equals 4.1 times 10 Superscript negative 11 p = 4.1 × 10 − 11 $p=4.1\times10^{-11}$

). At

230.469 230.469 $230.469$

MHz the XX linear-feed detection fraction reaches

0.811 0.811 $0.811$

against a

0.481 0.481 $0.481$

pooled v2-Mini baseline (event-level

p tilde 10 Superscript negative 274 p ∼ 10 − 274 $p\sim10^{-274}$

), and the excess remains in a satellite-cluster-robust sensitivity check across 84 satellites. The detected DTC population is brighter in eclipse than in sunlight (illuminated/eclipsed flux density ratio

0.47 0.47 $0.47$

) while the Ku-only comparison shows the opposite sense (

1.18 1.18 $1.18$

); the population-level contrast persists across altitude, sub-satellite latitude, frequency, and launch-epoch matching. The opposite trends disfavour a simple state-independent UEMR component that scales monotonically with instantaneous solar photocurrent and are consistent with an active on-board contribution whose detected flux changes between illumination states; the catalogue alone does not identify the subsystem or establish a uniform within-satellite response. Within the

230.469 230.469 $230.469$

MHz coarse channel, fine-channel inspection isolates the excess to a single

tilde ∼ $\sim$

24 kHz bin near

230.627 230.627 $230.627$

MHz, tail-driven and absent at five control channels. Three mechanism-oriented checks show that this feature has no unique coincidence with the Low-Frequency Array (LOFAR)-resolved (Bassa et al. 2024, A&A, 689, L10) clock fundamentals, is unresolved at the EDA2 24 kHz resolution, and is not solely driven by the three satellites with the largest per-satellite ratios.