Researchers use Starlink satellites to pinpoint location, similar to GPS

A SpaceX Falcon 9 rocket flying upward just after being launched.
Enlarge / The SpaceX Falcon 9 rocket with 60 Starlink satellites is launched from the Kennedy Space Center on October 6, 2020, in Cape Canaveral, Florida.

Signals from broadband SpaceX Starlink satellites can be used to locate 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 on the use of signals from Low Earth Orbit (LEO) satellites for navigation, similar to how GPS works.

This technology won’t replace your smartphone’s mapping app anytime soon, and this initial experiment apparently required 13 minutes of tracking six Starlink satellites to locate a location on Earth. But the researchers were able to accomplish the location feat without the help of SpaceX, and they say the test shows that the method could be used for navigation.

“The researchers did not need the help of SpaceX to use the signals from the satellites, and they emphasized that they did not have access to the real data that is sent through the satellites, only to the information related to the location and movement of the satellite.” added. Ohio State News article said.

“We eavesdrop on the signal, and then we design sophisticated algorithms to locate our location, and we show that it works with great precision,” Zak Kassas, Director of CARMEN The Center for Multimodal Assured Navigation Automated Vehicle Research, a center funded by the US Department of Transportation at Ohio State University, said in the article. “And although Starlink was not designed for navigation purposes, we showed that it was possible to learn parts of the system well enough to use it for navigation.”

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

Kassas said his “team has used similar techniques with other low-Earth orbit satellite constellations, but with less precision, pinpointing locations within about 23 meters,” according to the Ohio State News article. “The team has also been working with the US Air Force to identify high-altitude aircraft locations; they were able to get within 5 meters using ground cellular signals,” Kassas said. GPS provides signals with average errors of less than one meter.

The document is titled “First Carrier Phase Tracking and Positioning Results with Starlink LEO Satellite Signals” and was published last week in IEEE Transactions on Aerospace and Electronic Systems magazine. The researchers also presented their findings at a conference at the Institute of Navigation. His work was supported by grants from the United States Office of Naval Research, the National Science Foundation, and the Department of Transportation.

“Signs of opportunity”

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

“With SpaceX launching more than a thousand space vehicles (SV) in LEO, a renaissance in LEO-based navigation has begun,” they wrote. “LEO SV signals are received with higher power compared to Medium Earth Orbit (MEO) where GNSS [Global Navigation Satellite System] The SVs reside. Additionally, LEO SVs are more abundant than GNSS SVs 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 expensive additional services or infrastructure from the broadband provider.” But that does not mean that the task for the researchers was easy. “However, broadband providers often do not disclose the structure of the transmitted signal to protect their intellectual property. As such, one would have to dissect LEO SV signals to draw navigation observables,” they wrote.

the abstract researchers from the conference presentation noted that broadband providers could change their protocols to support browsing. But the researchers argue that their own third-party approach is more viable despite requiring “more sophisticated receiver architectures.”

“[T]Improving existing protocols to support navigation capabilities requires significant changes to existing infrastructure, which may cost private companies such as OneWeb, SpaceX, Boeing and others, who are planning to launch tens of thousands of broadband Internet satellites in LEO. , are not enough. willing to pay, “they wrote.” Also, if these companies agree to that additional cost, there will be no guarantee that they will not charge users for additional navigation services. In these circumstances, exploiting broadband LEO satellite signals opportunistically becomes a more viable approach. “

A new algorithm

The researchers previously considered a “cognitive approach to tracking the Doppler frequency of unknown LEO SV signals,” but said in their most recent paper that this method “cannot estimate the carrier phase, nor can it be adopted here, as it requires knowledge of the period of the beacon within the transmitted signal, which is unknown in the case of Starlink LEO SV. ” To overcome that barrier, “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’s downlink signals or its air interface in general, except for channel frequencies and bandwidths. A receiver cannot easily be designed to track Starlink signals with the aforementioned information just because a deeper understanding of the signals is needed. 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 to sampling Starlink signals: (i) the signals are transmitted in Ku / Ka bands, which are beyond the carrier frequencies that most commercial SDRs can support, and (ii) the Downlink channel bandwidths can be increased to 240 MHz, which also exceeds the capabilities of current commercial SDRs. The first challenge can be solved by using a downconverter / mixer between the antenna and the SDR. However, the sampling 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 (eg decoding of telemetry or ephemeris data or synchronization with a certain preamble). Therefore, the goal of the receiver is to get enough of the downlink signal 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 National Instruments (NI) 2945R Universal Software Radio Peripheral (USRP) was equipped with a consumer grade Ku antenna and a low noise block down converter (LNB) to receive Starlink signals in the Ku band, “he wrote. “The sampling bandwidth was set at 2.5 MHz and the carrier frequency was set 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 receptor was estimated using a weighted non-linear least squares (WNLS) estimator. The result was 25.9 meters from the actual location, but the error was reduced to less than eight meters by “equipping the receiver with an altimeter (to know its altitude).”

The conclusion of the article read:

This chart showed the first results of carrier phase tracking and positioning with real Starlink LEO SV signals. The transmitted signal from Starlink SV was modeled and an adaptive KF (Kalman filter) based carrier phase tracking algorithm 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 resulting positioning performance was: 2 – D error of 7.7 m when the receiver altitude is known, and error 2 – D 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. Presumably, these additional satellites would 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 possibilities of using Starlink satellites to get location results in something closer to real time, and we also hope to ask them how they envision LEO-based systems being used for navigation when the methods and technology are more advanced. We will update this article if we get a response.

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