Ookla Maps Size of Starlink’s Direct to Cell 4G Mobile Satellite Beams

Network benchmarking giant Ookla (Accenture), which collects data from consumers via their popular broadband and mobile Speedtest.net service, has cleverly used data collected from Smartphones using Starlink’s 4G mobile satellite data connectivity to map the size of their individual Direct-to-Cell (DTC) beams in the real-world.

Granted, this is probably more of interest to those with a strong technical interest in such things, but if that’s you then read on. Firstly, a bit of context. At present mobile operators that harness Starlink’s network of DTC satellites in Low Earth Orbit (LEO), such as O2 Satellite in the UK, use the service to deliver basic 4G mobile data connectivity via supporting apps to some of the remotest parts of the country.

As part of this each of Starlink’s first generation DTC satellites are equipped with three downlink (download) antennas and one uplink (upload) antenna. Each antenna is capable of supporting 8 beams (wireless signals sent to an area on the ground) and 2 polarizations, resulting in a total capacity of 48 downlink beams sent down toward the Earth and 16 uplink beams directed back up.

However, up until now the only measurements we’ve had of the beam sizes (i.e. how much area they cover on the ground) has come from an estimate modelled from regulatory filings, whereas what Ookla have just done is use real-world tests from Smartphones to actually observe and measure the D2C beam footprint at global scale. Nobody has done this before.

Luke Kehoe, Lead Analyst at Ookla, explained:

“When two phones in our background signal scans report the same decoded satellite ID and beam index at the same time (i.e., decoded from the broadcast LTE cell identity, which embeds the satellite ID and a trailing beam index), we treat them as being inside the same D2C beam. We independently verify each satellite assignment against archived orbital data (via Space-Track) at the time of observation. Thousands of these matches across thirteen markets let us measure the beam footprint directly.

As a robustness check, scrambling the beam indices (within the same satellite and 10 min window, preserving timing/geography) increases the median separation between matched phones from under 1 km to about 192 km.

Unsurprisingly, our measured footprint follows the expected geometry here. Its effective radius is about 44 km at a low satellite elevation of ~30°, tightening to around 19 km when the satellite is nearly overhead (with little change from February through August this year). This near-overhead result is close to the roughly 15–22.5 km cell radius implied by SpaceX’s -3 and -5 dB contours in its 2023 FCC filing.

Note that the wider footprint at low elevation reflects the beam being projected onto the ground at an angle.”

Beam size is one of the key capacity levers for such networks, since smaller footprints improve link margin and allow the same spectrum to be reused more densely (i.e. this is also why small cells are so effective for urban densification in terrestrial mobile networks, despite not covering a wide area).

The next generation of DTC satellites from Starlink are expected to combine a phased-array antenna five times larger than the first gen, offering four times more bandwidth per beam, thousands of spatial beams, new S-band spectrum, MIMO and 5G technology. Starlink has previously said that these will start to reach orbit during 2027 and should be ready to go live by the end of that year (here), offering mobile broadband speeds of around 150Mbps per user.