Researchers use Starlink satellites for precise positioning, similar to GPS

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The SpaceX Falcon 9 rocket soared upward immediately after launch.
enlarge / The SpaceX Falcon 9 rocket carrying 60 Starlink satellites was launched on October 6, 2020 at the Kennedy Space Center in Cape Canaveral, Florida.

Engineering researchers report in a new peer-reviewed paper that signals from SpaceX Starlink broadband satellites can be used to pinpoint positions on the earth to within 8 meters. Their report is part of a growing body of research that uses signals from low earth orbit (LEO) satellites for navigation, similar to how GPS works.

This technology will not replace your smart phone’s map application anytime soon, and this preliminary experiment obviously requires 13 minutes to track six Starlink satellites to determine the position on the earth. But the researchers were able to achieve the feat of positioning without any assistance from SpaceX, and they said the test proved that the method can be used for navigation.

“Researchers don’t need SpaceX’s help to use satellite signals. They emphasize that they can’t get the actual data sent via satellites-they can only get information related to the satellite’s position and movement,” a Ohio News Report Said.

“We eavesdropped on the signal, and then we designed a complex algorithm to determine our location, and we proved that it was very accurate,” Zach Casas, Carmen The Ohio State University US Department of Transportation-funded Center (Autonomous Vehicle Research Center with Multi-Mode Guaranteed Navigation) said in the article. “Although Starlink is not designed for navigation purposes, we have shown that it is possible to learn various parts of the system well to use it for navigation.”

The research was conducted by Kassas with Joe Khalife (a postdoctoral researcher at the University of California, Irvine) and Mohammad Neinavaie (a doctoral student at the University of California, Irvine). Kassas is also a professor and director of the Autonomous Systems Perception, Intelligence, and Navigation (ASPIN) Laboratory at the University of California, Irvine, of which Khalife and Neinavaie are members. Their experiment was conducted using antennas on the campus of the University of California, Irvine.

According to Ohio News, Casas said that his “team uses technology similar to other low-Earth orbit satellite constellations, but with lower accuracy and can accurately locate positions within about 23 meters.” “The team has also been working with the U.S. Air Force to determine the location of high-altitude aircraft; they can use land-based cellular signals to enter within 5 meters,” Casas said. The average error of the signal provided by GPS is Less than one meter.

The title of the paper is “The first carrier phase tracking and positioning result using Starlink LEO satellite signal”, and Publish Published last week in the IEEE Aerospace and Electronic Systems Trading Journal. The researchers also presented their findings at the Navigator Society meeting. Their work was funded by the U.S. Naval Research Office, the National Science Foundation, and the Department of Transportation.

“Signs of Opportunity”

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

They wrote: “As SpaceX launched more than a thousand spacecraft (SV) to LEO, LEO-based navigation has begun to revive.” Higher power reception [Global Navigation Satellite System] SV resides. In addition, 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 broadband providers to provide additional, expensive services or infrastructure.” But this does not mean that the task of the researcher is easy. “However, broadband providers generally do not disclose the structure of the transmitted signal to protect their intellectual property rights. Therefore, people must analyze the LEO SV signal to map navigation observations,” they wrote.

this Generalize The researchers’ presentation at the conference pointed out that broadband providers can change their protocols to support navigation. But the researchers believe that despite the need for a “more complex receiver architecture,” their own third-party approach is more feasible.

“[T]Using existing protocols to support navigation functions requires major changes to existing infrastructure, and private companies such as OneWeb, SpaceX, and Boeing are planning to launch tens of thousands of broadband Internet satellites to LEO, the cost of which may not be willing to pay,” they wrote. “In addition, if these companies agree to additional fees, there will be no guarantee that they will not charge users for additional navigation services. In this case, opportunistic use of broadband LEO satellite signals becomes a more feasible method. “

A new algorithm

Researchers have previously considered a “cognitive method for tracking the Doppler frequency of an unknown LEO SV signal”, but in their recent paper, this method “cannot estimate the carrier phase, nor can it be used here because it requires Understand the beacons in the transmitted signal, which is unknown in the case of Starlink LEO SV.” To overcome this obstacle, they “developed[ed] A carrier phase tracking algorithm for Starlink signals without prior knowledge of its structure. “

The newspaper said:

Apart from the channel frequency and bandwidth, little is known about the Starlink downlink signal or its air interface. Due to the need for a deeper understanding of the signal, it is not easy to design a receiver to use the above information to track the Starlink signal. In this case, software-defined radios (SDR) will come in handy because they allow sampling of frequency bands of the radio spectrum. However, there are two main challenges in sampling Starlink signals: (i) the signal is transmitted in the Ku/Ka band, which exceeds the carrier frequency that most commercial SDRs can support, and (ii) the downlink channel bandwidth can be increased to 240 MHz, This also exceeds the current commercial SDR capabilities. The first challenge can be solved by using a mixer/downconverter between the antenna and the SDR. However, the sampling bandwidth can only be as high as the SDR allows. Generally, opportunistic navigation frameworks do not require much information from communication/navigation sources (for example, to decode telemetry or ephemeris data or to synchronize to a certain preamble).Therefore, the goal of the receiver is to utilize enough downlink signals to be able to [to] Generate raw navigation observations (for example, Doppler and carrier phase).

Track 6 satellites for 800 seconds

During the experiment, “the fixed National Instruments (NI) Universal Software Radio Peripheral (USRP) 2945R is equipped with a consumer-grade Ku antenna and a low-noise block downconverter (LNB) to receive Ku-band Starlink signals,” they Write. “The sampling bandwidth is set to 2.5 MHz, and the carrier frequency is set to 11.325 GHz, which is one of the Starlink downlink frequencies.”

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

Use a weighted non-linear least squares (WNLS) estimator to estimate the location of the receiver. The result is 25.9 meters away from the actual position, but after “equipped with an altimeter for the receiver (to know its height)”, the error dropped to less than 8 meters.

The conclusion of the paper says:

This letter shows the first result of carrier phase tracking and positioning using real Starlink LEO SV signals. Established a model of Starlink SV transmitting signal, and developed a carrier phase tracking algorithm based on adaptive KF (Kalman filter) to track Starlink signal. The experimental results show that the carrier phase tracking time of 6 Starlink LEO SVs is about 800, and the positioning performance is: when the receiver height is known, the error is 7.7 m 2-D, and when the receiver height is unknown, 25.9 m 2-D Error and 33.5 m 3-D error.

SpaceX has launched more than 1,700 satellites, but the plan is eventually Launch tens of thousands To expand the capacity and availability of broadband services. These additional satellites will probably also make it easier to construct the kind of navigation system envisaged in the new research.

We contacted researchers today to ask about the prospects of using Starlink satellites to obtain positioning results in a closer to real-time manner, and how they envision the use of LEO-based systems for navigation with more advanced methods and technologies. If we receive a response, we will update this article.

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