SpaceX Starlink Satellites and Unintended Consequences

A second unintended consequence.  The first was that they interfere with a lot of astrophotography by being high enough to be illuminated in the first and last hour or so of night on the surface and therefore bright enough to show up as streaks in pictures taken of the night sky.  

This unintended consequence is that they can be used as an approximation to the Global Positioning System satellites, allowing position determination to within about 8m (26 feet).  For contrast, GPS accuracy is generally between 0.3 and 5m.  The story is about a couple of researchers from Ohio State University and published in the OSU News.  I got the tip via Ars Technica who printed a bit more details.   

This technology won't replace your smartphone's map application any time soon, and this initial experiment apparently required 13 minutes of tracking six Starlink satellites to pinpoint a location on Earth. But researchers were able to achieve the locational feat without any help from SpaceX, and they say the test proves the method could be used for navigation.

"The researchers did not need assistance from SpaceX to use the satellite signals, and they emphasized that they had no access to the actual data being sent through the satellites—only to information related to the satellite's location and movement," an Ohio State News article said.

"We eavesdropped on the signal, and then we designed sophisticated algorithms to pinpoint our location, and we showed that it works with great accuracy," Zak Kassas, the director of CARMEN (Center for Automated Vehicle Research with Multimodal AssurEd Navigation), a US Department of Transportation-funded center at Ohio State University, said in the article. "And even though Starlink wasn't designed for navigation purposes, we showed that it was possible to learn parts of the system well enough to use it for navigation."

The main reason this isn't going to replace your smartphones' app is that the Starlink satellites don't transmit in or even near the same frequency band as the GPS satellites.  GPS uses a couple of frequencies near 1.5GHz, specifically 1227.6 and 1575.42 MHz, both of which are close to 1700 and 1900 MHz where a couple of very heavily-used cellphone bands are located.  Starlink uses a band quite a bit higher in frequency, called Ku ("Kay-you") band.  The serendipity of having been able to add GPS reception relatively easily won't extend to adding Ku band and Starlink.  

During the experiment, "a stationary National Instruments (NI) universal software radio peripheral (USRP) 2945R was equipped with a consumer-grade Ku antenna and low-noise block downconverter (LNB) to receive Starlink signals in the Ku-band," they wrote. "The sampling bandwidth was set to 2.5 MHz and the carrier frequency was set to 11.325 GHz, which is one of the Starlink downlink frequencies."

The researchers recorded Starlink signals for 800 seconds, or about 13.3 minutes. "During this period, a total of six Starlink SVs transmitting at 11.325 GHz passed over the receiver, one at a time," they wrote. Researchers stored samples of the Ku signals "for off-line processing."

The Big Idea here is what's called Signals of Opportunity, in which any signals from known sources can be used for navigation.  SpaceX has launched over 1700 satellites but has talked about putting up over 40,000.  Far, far more than the GPS constellation. By then there will always be Starlink satellites available and the algorithms should get improved accuracy,

..."Signals from LEO Space Vehicles are received with higher power compared to medium Earth orbit (MEO) where GNSS [Global Navigation Satellite System] SVs reside. Moreover, LEO SVs are more abundant than GNSS SVs to make up for the reduced footprint, and their signals are spatially and spectrally diverse."

Another advantage of LEO satellites is that "they do not require additional, costly services or infrastructure from the broadband provider."...

Conceptual illustration of a small portion of the Starlink constellation.  Illustration from Getty Images

The work was done as a collaboration between Ohio State and University of California at Irvine, and was funded by grants from the US Office of Naval Research, the National Science Foundation, and the Department of Transportation.  Their paper is titled The First Carrier Phase Tracking and Positioning Results with Starlink LEO Satellite Signals and was published last week in the journal IEEE Transactions on Aerospace and Electronic Systems.   There's much more at Ars Technica.