New Developments

In our earlier writing, we predicted that the imminent availability of new navigation satellite signals and new markets for precision navigation (notably for autonomous vehicles) would bring survey-grade accuracy to mobile devices by 2022.¹ One year later our prediction looks conservative. The number and sophistication of navigation satellite constellations and signals is increasing, and dual-frequency chips are, for the first time, being mass manufactured for consumer applications. As of 2018, sub-meter accuracy is available for the mass market in the Xiaomi Mi-8 smartphone. Decimeter accuracy will be achievable soon, with centimeter accuracy to follow.

• Until the year 2000, the GPS civilian signal (L1) was intentionally degraded for reasons of national security. This policy, called Selective Availability limited the accuracy of single-frequency civilian GPS devices to approximately 50 meters.
• On May 1, 2000, the Clinton administration turned off Selective Availability, allowing users of the L1 signal to achieve accuracy of approximately 5 meters.
• In 2018 Xiaomi released the Mi-8, the first smartphone to incorporate a dual-frequency GNSS receiver. Using both the L1 and L5 signals, the Mi-8 achieved sub-meter accuracy in testing conducted by the European Space Agency.
• Survey accuracy requirements vary greatly by jurisdiction. For many applications, including rural boundary surveying, allowable errors are commonly measured by tenths of a meter.
• More stringent urban cadastral surveying standards may require centimeter-level accuracy.

As we have detailed before,² the declining cost of high-accuracy location capabilities is the result of several concurrent developments. One is the availability of new satellite constellations broadcasting new, open satellite navigation signals, notably the L5/E5 signals broadcast by Galileo and the new GPS Block IIF and Block III satellites. Progress has been rapid. On October 12, 2018, four new Galileo satellites went online.³ On November 1, the sixth of ten advanced navigation payloads was delivered for integration into forthcoming GPS III satellites. In its 2018 GNSS User Technology Report, the European Global Navigation Satellite Systems Agency (GSA) wrote that all of the major navigation constellations will reach full operational capability in the next five years.⁴ In layman's terms, in five years or less the globe will be covered by two different frequencies of free and open signal. Multi-constellation receivers using Russia’s GLONASS and China’s BeiDou satellites in addition to GPS and Galileo could eventually have access to more than 100 satellites.⁵ New satellite-based augmentation networks are also being deployed to improve the accuracy of location services:

The coming years will see two new GNSS (Galileo and BeiDou), and two RNSS (QZSS and NavIC), reach full operational capability. In parallel, the modernisation of existing GNSS (GPS and GLONASS) is also well underway. Thus, in just a few years there will be four global and three regional satellite navigation systems, and more than 100 satellites providing open access to more accurate and reliable PNT services, including through the use of multiple frequencies. Public augmentation systems, such as EGNOS, are also evolving to become multi-constellation and multi-frequency.⁶

These augmentation systems supply correction data needed to cancel out positioning errors, improving receiver accuracy to decimeter or centimeter levels. A particularly interesting augmentation method for our purposes is Precise Point Positioning (PPP).⁷ PPP systems allow dual- and multi-frequency receivers to achieve centimeter-level accuracy anywhere on the globe. The correction data can be supplied via satellite, meaning that this accuracy can be achieved without using differential techniques that require a second receiver and a communications channel. PPP is for the most part only available on a subscription basis, but there is increasing recognition of the value of making it accessible as a public, global utility. Japan’s QZSS constellation includes experimental sub-meter and centimeter-level PPP signals for East Asia and Oceania,⁸ and the EU is exploring upgrading its EGNOS augmentation system to support centimeter-level accuracy using the L1/E1 and L5/E5 signals.⁹ The International GNSS Service (IGS) launched a free, real-time global PPP service in 2013, streaming dual-frequency GPS corrections over the internet.¹⁰ This service allows sub-decimeter accuracy, but requires special firmware and only supports the GPS constellation.¹¹ Open access to PPP for all satellites would be a huge gain for the developing world, where the infrastructure required for other augmentation and correction systems is often lacking.¹²

The most notable recent advance in hardware has been the appearance of cheap dual-frequency chips for mobile devices. In 2018, we saw the launch of the Xiaomi Mi-8, the first cell phone equipped with dual-frequency GNSS. The dual-frequency Broadcom BCM47755 chip in the Xiaomi was the first of its kind to market, though numerous other chip makers are expected to follow suit.¹³

We should emphasize that our predictions concerned the convergence of consumer devices towards survey-grade accuracy. Standards vary by jurisdiction, but rural parcels often require decimeter accuracy, and accuracy on the order of one to five centimeters is often needed to meet more stringent urban requirements. Accuracy depends on many factors beyond the dual-frequency receiver, including antenna quality, signal processing software, and satellite availability.

Initial consumer testing of the Xiaomi revealed mixed results. This is likely attributable to uneven regional coverage of the L5/E5 satellites. In its own comprehensive testing, the GSA achieved static accuracy of 0.78 meters, and this can be improved upon with augmentation techniques like Precise Point Positioning and Real-Time Kinematic positioning (RTK).¹⁴ Broadcom claims to have successfully tested both RTK and PPP internally. Broadcom’s results have not been made public,¹⁵ but in May 2017 they provided a team of researchers from Trimble Inc. with a development kit for the dual-frequency BCM47755 chip found in the Mi-8.¹⁶ The researchers tested the Broadcom chip against a professional survey-grade receiver and found that both were able to achieve centimeter-level accuracy when using RTK and a professional-grade antenna.¹⁷ They were able to achieve similar results with the Broadcom chip and a cell-phone antenna, though with much longer convergence times. This was accomplished under ideal laboratory conditions, but their conclusion is nevertheless remarkable:

By connecting this next-generation GNSS chipset to a GNSS antenna typical of a cellular device and comparing the performance from a precision GNSS antenna, we’ve shown for the first time that it is possible to produce precision positions from a static cellular class GNSS device in ideal conditions at the centimeter level with both an RTK solution and a PPP solution.¹⁸

Since 2016, Google’s Android—the operating system for 88 percent of the world’s handheld devices, and even more in the developing world—has allowed access to raw navigation signal data, which is required for advanced signal processing techniques.¹⁹ Supporting innovation in the Android ecosystem is a key part of the European Space Agency's (ESA) strategy for promoting “better location performance in mass market applications.”²⁰ In 2017 ESA launched a Raw Measurements Task Force to “share knowledge and expertise on Android raw measurements and its use, including its potential for high accuracy positioning techniques.”²¹ The task force also organizes workshops, hackathons, and app development competitions.²² A September 2018 GSA presentation outlines the “four main areas of use…enabled by GNSS raw measurements:”

• “Scientific/R&D: As the observations are provided in a much more coarse form they can be used for testing hardware and software solutions and for new post processing algorithms e.g. for modelling ionosphere or troposphere.”
• “Integrity/Robustness: Access to raw measurements will offer new ways to detect RF interferences and to locate the interference source by combining the measurements from multiple devices (crowdsourcing), or verify the source (OS-NMA). SBAS corrections can be incorporated without the need for additional equipment.”
• “Increased Accuracy: Subject to hardware limitations, access to raw measurements means a developer can employ advanced positioning techniques (RTK, PPP) and create a solution currently only available in professional receivers. It results in a technological push to develop new applications.”
• “Testing, performance monitoring, and education: Raw measurements can be used for monitoring performance (data, accuracy, Rx clock), testing and to compare solution [sic] from single constellations, eliminate specific satellites or test for worst scenario performance. Education use for understanding GNSS, Signal [sic] processing or orbits in smartphone is not negligible too.”²³

It is difficult to overstate the R&D value of the Android raw measurements, especially in enabling mathematical signal processing advances which enhance location performance independently from the hardware. For example, a recent paper from the University of Otago in New Zealand demonstrated the potential for multi-constellation, single-frequency (L1) GNSS with RTK to compete with dual-frequency for accuracy:

By combining signals from four different Global Navigation Satellite Systems (GNSSs), Otago’s Dr Robert Odolinski and Curtin University colleague Prof Peter Teunissen, have demonstrated that it is possible to achieve centimeter(cm)-level precise positioning on a smartphone. “It’s all down to the mathematics we applied to make the most of the relatively low-cost technology smartphones use to receive GNSS signals, combining data from American, Chinese, Japanese, and European GNSS. We believe this new capability will revolutionize applications that require cm-level positioning,” Dr Odolinski says.²⁴

The “smartphone implementation” tested here consisted of a DataGNSS D302-RTK receiver with a list price of $1199.²⁵ Because it used only one frequency it remains sensitive to ionospheric interference. More importantly, multipath interference mitigation is inherently better with the L5 and E5 signals, which is critical in urban areas and places with heavy tree cover. But solutions like this, which leverage a profusion of signals to cancel out errors and mitigate the limitations of consumer grade antennas, could become even more powerful when used to enhance cheap dual-frequency receivers.