by Marco Fortunato, NSL

6 July 2018

Xiaomi-Mi-8-696x435Figure 1 Xiaomi-Mi-8

During May 2018, the GNSS community waited with bated breath for the release of the first Android dual-frequency smartphone, the Xiaomi MI 8, finally seeing its launch on the last day of that month. This new generation of mobile devices is embedded with a Broadcom BCM47755 chip, which can record GNSS signals on both the L1/E1 and L5/E5 frequencies.

This device is an important stepping stone to the revolution that is sweeping the world of GNSS mass-market receivers, which are responding to the newly available signals of Galileo and GPS that allow improved positioning and signal tracking. Google has even released a new Android GNSS Raw Measurement API that is available on some Smartphones providing access to the embedded receivers’ raw data that allows the creation of the traditional GNSS measurements of pseudorange and carrier phase. The FLAMINGO ( initiative is championing this revolution with a consortium of Europe's top space companies. Utilising Android raw GNSS measurements and advances in GNSS hardware, systems are being developed for smartphones and IoT to achieve positioning under 50 cm.

 Whilst the Google API is not yet available for dual-frequency mobile devices, the performance of Xiaomi MI 8 may still be assessed using the logged NMEA data files, produced from internal position-velocity-time (PVT) calculations. To evaluate the capabilities of the Xiaomi MI 8, we compare the accuracy of the internal PVT solution to that of a Samsung S8, embedded with the older single frequency Broadcom 4774 chipset, and from a geodetic class GNSS receiver (Septentrio PolaRx5e). These devices are placed in a static scenario for a period of 10 minutes.

xiaomiFig1SmFigure 2 Scatter plot (enlarge)

Figure 2 shows the position scatter in a local reference frame for each device. The origin is fixed at the Ground Control Point (GCP), graphically representing the accuracy of the distance from the origin. An R95, i.e. the radius containing 95% of solutions, is presented for each dataset in order to illustrate the GNSS precision.

Of course, the highest accuracy and precision is achieved by the Septentrio PolaRex5e. Considering the mass-market devices, the GNSS dual-frequency chipset, as stated by Broadcom, improves both accuracy and precision. This is clearly presented in the figure. The horizontal precision, for example, almost halves: decreasing from 4.92 to 2.75 is observed in the R95. The overall accuracy is presented in Figure 3, where the Root Mean Square Error (RMSE) is considered for the East, North and Up directions. Calculations are made for each individual epoch, with reference to the GCP. These results highlight the significant improvements obtained by the new chipset in the vertical component: a reduction from 12.63 m to 5.11 m is observed between the Samsung S8 and Xiaomi MI 8 respectively. As expected, the geodetic class receiver is still far superior to the performance of both smartphone device. The lower RMSE in the N direction for the Xiaomi MI 8 originates from the greater, with respect to the Septentrio’s solutions, the standard deviation of the internal PVT results. This leads to a different mean distance from the precise coordinates.

xiaomiFig1SmFigure 3 RMSE in a local reference system for the dataset collected from Septentrio PolaRx5e, Xiaomi Mi 8 and Samsung S8

A test was also performed in a dynamic scenario in order to evaluate the improvements that can be obtained in real-time navigation. Measurements were taken by a Samsung S8 and Xiaomi MI 8, placed in the pocket of a cyclist’s backpack for 10 minutes.

Figure 4 and 5 illustrate the results in two slightly different environments. The pink and white markers represent the Samsung S8 and Xiaomi MI 8 respectively.

XiaomiGig3smFigure 4 (enlarge)

xiaomiFig4SmFigure 5 (enlarge)

Figure 4 demonstrates that in a slightly closed environment (with proximity to buildings), the Samsung S8 provides more reliable results. Alternatively, in a more open environment (Figure 5), the track reconstructed by the Xiaomi MI 8 is much more representative. This may highlight that the Xiaomi is placing more emphasis on the GNSS PVT solution than the Samsung S8 which may be placing more dependence on other sensors within its navigation solution. These results clearly illustrate the potential of dual-frequency receivers. Eventually, the Android raw measurements API will be made available for dual-frequency smartphones, allowing a more comprehensive analysis. Currently, we can only catch glimpses of the potential of this new GNSS technology. The future will be exciting.