Iridium constellation provides low-Earth orbit satnav service

January 12, 2017  - By
Iridium satellite, courtesy of Iridium.

Iridium satellite, courtesy of Iridium.

A strategic alliance announced on Dec. 15 between Orolia and Satelles includes product development and go-to-market activities of positioning, navigation and timing (PNT) solutions provided by the Iridium satellite constellation, independent of GPS/GNSS signals. The companies intend to provide PNT solutions to military, defense, government and commercial customers worldwide.

Orolia, the parent of GNSS-active companies Spectracom and Spectratime, among others, has formed a strategic alliance, including an equity investment, with Satelles Inc. to develop, market and sell PNT solutions based on Satelles’ satellite time and location (STL) signal technology.

STL is a unique space-based PNT technology that provides location and timing data independent from traditional GPS and other GNSS satellite signals. By using STL, Orolia’s Spectracom and McMurdo solutions will, according to the company, be less susceptible to vulnerabilities such as spoofing, interference and jamming that are associated with GPS/GNSS.

Based on the low-Earth orbit (LEO) Iridium satellite constellation, STL signals are up to 1,000 times stronger than GPS/GNSS; this signal strength, due in part to the constellation’s closer proximity to users, helps to prevent jamming and enables signal reach into buildings and other difficult locations. STL’s additional cryptographic security also ensures performance, productivity and security.

For further background on Iridium, see GPS World’s June 2016 Defense PNT column, “Iridium and GPS revisited: A new PNT solution on the horizon?” Projected applications and use cases include energy/utility grids, enterprise data networks including financial systems, maritime/aviation navigation, fleet/asset tracking management, search and rescue, and data center management.

Many highly sensitive military, defense, government and commercial applications and operations require accurate and reliable PNT data. Today, these applications rely on signals from GPS/GNSS satellites. There are instances, however, where GPS/GNSS signal strength and security are not sufficient and prone to signal disruption. For these cases, the companies jointly state, STL can be used as a secure signal of opportunity to complement GPS/GNSS, making the applications more accurate and secure, and less prone to interference and attack.

“There is a growing need for precise and robust positioning, navigation and timing information especially in business-critical, high-risk and life-saving operations,” said Jean-Yves Courtois, Orolia CEO. “By augmenting Orolia’s GPS/GNSS-based solutions with Satelles’ STL technology, we will have the industry’s first essentially fail-safe, resilient PNT solution. This breakthrough offering will be ideal for mission-critical applications in which the smallest discrepancy in PNT data accuracy, availability and stability can produce a network outage, a system crash or a loss of life.”

Signal strength, availability

The technical advantages provided by adding ranging satellites in low-Earth orbit (LEO) to the GNSS satellites in medium-Earth orbit (MEO) were explored in a 2012 Institute of Navigation paper by Per Enge, Bert Ferrell, David Whelan, Greg Gutt and David Lawrence. GPS World plans to publish an updated version of that paper, with key new material on current STL performance statistics, in an upcoming issue.

Briefly, the paper concluded that “Due to their proximity, signals received from LEO are approximately 30 dB stronger than the signals from MEO. Indeed, we show data collected inside an industrial-strength metal storage container. The power of a LEO signal received inside the container is approximately equal to the power of a GPS signal received under the open sky. On the other hand, LEO proximity also dictates that only a few Iridium satellites are in view of the ground-based user. We show typical examples where six to 11 GPS satellites are joined by one or two LEO satellites.”

The authors then examine the effect of the swift mean motion of LEO satellites, analyzing the ability to whiten multipath based on the rapid motion of the line-of-sight vectors from the user to the LEO satellites. In sharp contrast to MEO, the LEO satellites attenuate errors due to multipath solely based on satellite motion, and do not require user motion. They also analyze Doppler-based positioningvusing the rapid mean motion of the LEO satellites. The Doppler shift projects onto the line-of-sight vectors from the user to the LEO satellites. Over 100 or 200 seconds, this projection is a sharp function of the user location, and this connection enables Doppler-based positioning similar to the Transit satellite system. The authors’ analysis shows that position accuracies of 5 meters can be based on noncoherent code tracking of the LEO plus GPS signals.

This paper also discusses the broadcast of UTC time to sites with known locations, describing experimental results with absolute time accuracies of one microsecond. The broadcast of high-accuracy frequency from LEO would enable a high-accuracy hot clock to replace the relatively low-quality oscillator in GNSS receivers, allowing longer coherent and non-coherent averaging times and improving the sensitivity of GNSS receivers by several decibels. Many other navigation applications would benefit from one LEO satellite in view, the authors assert.

Market view from operator’s CEO

“We are a manufacturer and integrator of timing equipment,” Orolia CEO Jean-Yves Courtois told GPS World. Orolia is the parent company of GPS/GNSS product and service providers Spectracom, McMurdo and Spectratime. “This new STL service is not fully commercialized yet, but it’s operational and it can be tested. Receivers are available and can be integrated into our equipment.

“The timing signal is very accurate and close enough to GPS for most timing applications, although the positioning accuracy is lower than what GPS users are accustomed to. It is an augmentation for timing primarily, and secondarily for positioning,” Courtois continued.

“In terms of timing accuracy, it provides on the order of tenths of microseconds in accuracy, and this covers a lot of timing applications. This is an ideal timing backup or augmentation of GPS. In positioning it’s closer to 50 meters or more, much better for fixed objects than for mobile objects. The faster the vehicle, the lower the positioning accuracy. It’s not directly usable for GPS applications that require a few meters’ accuracy, but it can be associated with inertial navigation for much better results.

“The STL signal penetrates buildings well, it has unique features, and it performs at a high level. The signal is encrypted, so you have to subscribe to a service to receive a key, allowing access to the signal. Applications are developing based on equipment that will be STL-enabled. For the user it will be transparent. The user will have a different antenna.

“We are also active in tracking and emergency location devices, where this is also of interest. It has some authentication capability, to guarantee that the person who accesses the signal is in the location that he pretends to be.

“For customers to be able to use this service, there is some integration work to be done, some dedicated STL receivers to integrate into our current hardware set up, and software modifications. We are ready to work with government and defense organizations and other new clients. Our basic interest is to add some robustness to our equipment for our current customers, and then of course to develop new customers worldwide.”

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