What is an inter-satellite link?
An inter-satellite link (ISL) is a communication channel established directly between two satellites, bypassing the need for ground stations to relay traffic. In a LEO constellation without ISLs, data from a user in a remote area must travel: user → satellite → nearest ground gateway → internet backbone → destination. With ISLs, data can travel: user → satellite A → satellite B → satellite C → ground gateway nearer the destination, potentially skipping continents of terrestrial infrastructure.
Laser ISLs: the Starlink example
Starlink Gen1.5 and Gen2 satellites use optical (laser) ISLs operating in near-infrared wavelengths. As of 2024, Starlink's ISL network transmits over 42 petabytes per day at 5.6 Tbps aggregate capacity. Each Gen2 satellite supports up to four simultaneous ISL connections to neighbouring satellites. Laser links offer higher bandwidth, lower interference risk, and better security than RF ISLs, but require precise pointing between satellites separated by hundreds or thousands of kilometres.
Latency implications
Light travels in vacuum at 299,792 km/s — about 47% faster than in optical fibre (where the refractive index slows propagation). A London-to-Tokyo packet via Starlink ISLs has a theoretical one-way latency of ~60 ms; the same route via submarine fibre cable takes ~230 ms. For financial trading, video conferencing, and latency-sensitive enterprise applications, this difference is commercially significant.
RF ISLs
Radio frequency ISLs in Ka or V band are simpler to implement than laser links but offer lower bandwidth. They are used in older constellations like Iridium (L-band ISLs) and are being considered for hybrid LEO-GEO systems where precise pointing for laser links is impractical.