Researchers use Starlink satellites to pinpoint location, like GPS

Enlarge / The SpaceX Falcon 9 rocket carrying 60 Starlink satellites is launched from the Kennedy Space Center on October 6, 2020 in Cape Canaveral, Florida.

Signals from SpaceX Starlink broadband satellites can be used to pinpoint locations on Earth with an accuracy of eight meters, engineering researchers reported in a new peer-reviewed paper. Their report is part of a growing body of research into using signals from low earth orbit (LEO) satellites for navigation, the same way GPS works.

This technology won’t replace your smartphone’s mapping app anytime soon, and that first experiment apparently took 13 minutes of tracking six Starlink satellites to pinpoint a location on Earth. But the researchers were able to perform the localization feat without any help from SpaceX, and they say the test proves the method could be used for navigation.

“The researchers did not need SpaceX’s help to use the satellite signals, and they stressed that they did not have access to the actual data sent from the satellites, only the location-related information and to the movement of the satellite, “said a Ohio State News Article noted.

“We listened to the signal, then we designed sophisticated algorithms to locate our location, and we showed that it worked with great precision”, Zak Kassas, director of CARMEN (Center for Automated Vehicle Research with Multimodal AssurEd Navigation), a center funded by the U.S. Department of Transportation at Ohio State University, said in the article. “And while Starlink was not designed for navigation purposes, we have shown that parts of the system can be learned well enough to use it for navigation.”

The research was conducted by Kassas with Joe Khalife (a postdoctoral fellow at the University of California, Irvine) and Mohammad Neinavaie (a doctoral student at UC-Irvine). Kassas is also a professor at UC-Irvine and director of the Autonomous Systems Perception, Intelligence, and Navigation (ASPIN) lab, while Khalife and Neinavaie are members of the lab. Their experiment was conducted using an antenna on the UC Irvine campus.

Kassas said his “team used similar techniques with other constellations of low-earth orbiting satellites, but with less precision, locating locations within 23 meters,” according to the Ohio State article. News. “The team also worked with the US Air Force to pinpoint the locations of the planes at high altitude; they were able to approach within 5 meters using ground cellular signals,” Kassas said. The GPS provides signals with average errors of less than a meter.

The article is titled “The first results of carrier phase tracking and positioning with Starlink LEO satellite signals” and was published last week in the IEEE Transactions on Aerospace and Electronic Systems journal. The researchers also presented their results at an Institute of Navigation conference. Their work was funded by grants from the US Office of Naval Research, the National Science Foundation, and the Department of Transportation.

“Signals of Opportunity”

The researchers’ paper said that “various theoretical and experimental studies” have considered the possibility of using “signals of opportunity” from LEO broadband satellites for navigation.

“With SpaceX having launched over a thousand space vehicles (SVs) in LEO, a renaissance of LEO-based navigation has begun,” they wrote. “Signals from LEO SV are received with higher power compared to Medium Earth Orbit (MEO) where GNSS [Global Navigation Satellite System] The VS reside. Additionally, SV LEOs are more abundant than SV GNSS to compensate for the reduced footprint, and their signals are spatially and spectrally diverse. “

Another advantage of LEO satellites is that “they do not require additional and expensive services or infrastructure from the broadband provider”. But that doesn’t mean the researchers’ job was easy. “However, broadband providers generally do not disclose the structure of the transmitted signal to protect their intellectual property. As such, LEO SV signals would have to be dissected to draw navigation observables,” they wrote.

The summary of the conference presentation the researchers noted that broadband service providers could modify their protocols to support browsing. But the researchers argue that their own third-party approach is more viable despite the fact that they require “more sophisticated receiver architectures”.

“[T]Adapting existing protocols to support navigation capabilities requires significant changes to existing infrastructure, including the cost of private companies such as OneWeb, SpaceX, Boeing and others, who plan to launch tens of thousands of broadband Internet satellites in LEO, may not be willing to pay, ”they wrote. “Moreover, if these companies accept this additional cost, there will be no guarantee that they will not charge users for additional navigation services. Under these circumstances, opportunistic exploitation of broadband LEO satellite signals becomes a more viable approach. “

A new algorithm

The researchers previously considered a “cognitive approach to follow the Doppler frequency of unknown LEO SV signals”, but stated in their most recent paper that this method “cannot estimate the phase of the carrier, nor can it be adopted here. because it requires knowledge of the beacon period in the transmitted signal, which is unknown in the case of Starlink LEO SV. ” To overcome this obstacle, they “develop[ed] a carrier phase tracking algorithm for Starlink signals without prior knowledge of their structure. “

The newspaper said:

Little is known about Starlink downlink signals or their air interface in general, except for frequencies and channel bandwidths. One cannot easily design a receiver to track Starlink signals with the aforementioned information alone because a deeper understanding of the signals is required. Software Defined Radios (SDRs) are useful in such situations as they allow bands of the radio frequency spectrum to be sampled. However, there are two main challenges for sampling Starlink signals: (i) the signals are transmitted in the Ku / Ka bands, which is beyond the carrier frequencies that most commercial SDRs can support, and (ii) downlink channel bandwidths may be high. at 240 MHz, which also exceeds the capabilities of current commercial SDRs. The first challenge can be solved by using a mixer / converter between the antenna and the SDR. However, the sample bandwidth can only be as high as the SDR allows. In general, opportunistic navigation frames do not require much information from the communication / navigation source (for example, decoding telemetry or ephemeris data or synchronizing with some preamble). Therefore, the goal of the receiver is to exploit the downlink signal sufficiently to be able to [to] produce raw navigation observables (eg, Doppler and carrier phase).

Track six satellites for 800 seconds

During the experiment, “a Stationary Universal Software Radio Device (USRP) 2945R from National Instruments (NI) was fitted with a consumer grade Ku antenna and a low noise block down converter (LNB) for receive Starlink signals in the Ku band, “they wrote. “The sample bandwidth was set at 2.5 MHz and the carrier frequency at 11.325 GHz, which is one of Starlink’s downlink frequencies.”

The researchers recorded the Starlink signals for 800 seconds, or about 13.3 minutes. “During this period, a total of six Starlink SV transmitting at 11.325 GHz passed through the receiver, one at a time,” they wrote. The researchers stored samples of the Ku signals “for offline processing.”

The position of the receiver was estimated using a weighted nonlinear least squares (WNLS) estimator. The result was 25.9 meters from the actual location, but the error fell to less than eight meters by “fitting the receiver with an altimeter (to find out its altitude)”.

The conclusion of the article said:

This letter showed the first results of carrier phase tracking and positioning with real Starlink LEO SV signals. A signal model transmitted by Starlink SV was formulated and an adaptive carrier phase tracking algorithm based on KF (Kalman filter) was developed to track the Starlink signal. The experimental results showed the carrier phase tracking of six Starlink LEO SVs over a period of approximately 800.The positioning performance obtained was as follows: 2-D error of 7.7 m when the receiver altitude is known, and 2-D error of 25.9 m and 3-D error of 33.5 m when the receiver altitude is unknown.

SpaceX has launched more than 1,700 satellites but plans to eventually launch tens of thousands to expand the capacity and availability of broadband service. These additional satellites would probably also facilitate the construction of navigation systems of the type envisioned in the new research.

We reached out to the researchers today to ask them about the prospects for using Starlink satellites to achieve closer real-time location results, and how they envision using LEO-based systems for research. navigation when methods and technology are more advanced. We will update this article if we get a response.

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