engleski

Performance Assessment of Commercial GNSS Receivers

Commercial GNSS receivers became part of almost all communication devices. They offer relatively high speed in position fix with different systems for increased precision. All integrated, and relatively cheap, receivers are single-frequency receivers and offer accuracy in position estimation of around 5 m or less in horizontal plane. Modern smartphones are, besides GNSS receivers, equipped with Assisted GPS (A-GPS), WiFi-based positioning, or cellular network positioning system. These systems do not offer increase in accuracy but allow quicker fix for the GNSS receiver. Besides these capabilities, GNSS receivers use different constellations and most of manufacturers provide parallel usage of GPS, GLONASS, Galileo, and BDS. For performance assessment of single-frequency receivers we have assembled an UAV and equipped it with a possibility of autonomous flying using only data obtained from GNSS receivers. For testing we used six different GPS receivers with the following chipsets: u-blox 6 UBX-G6000-BT, u- blox Neo-6M, u-blox UBX-G7020, u-blox Neo-M8, Venus638, and MediaTek MT3339. All of these chipsets are built as System-on-Chip (SoC) and the difference between them is in architecture and number of (tracking/acquisition) channels. Sensitivity of the tested chipsets in tracking mode varies from -161 dBm for Venus638 to -167 dBm for u-blox Neo-M8, while the number of channels for acquisition varies from 51 for Venus638 to 72 for u-blox Neo-M8. Another important factor in receivers' capabilities is the navigation update rate, which allows receiver to calculate actual speed correctly. Tested chipsets allowed position update rate between 5 Hz and 18 Hz but final results depended on hardware implementation of each receiver. Horizontal positioning accuracy was 2.5 m to 3 m for all chipsets without usage of SBAS. Preliminary testing was set-up as ground- based in order to allow precise guidance of UAV along the straight line with constant speed. Since UAVs are unable to fly in a perfectly straight line, it is impossible to obtain quality measurements of GNSS receiver's accuracy in position and speed estimation. To avoid misguidance as much as possible we have placed operation UAV on a mobile robot which has the possibility to travel with a constant speed along a straight line. Results of measurement show that all devices performed with similar accuracy and maintained error bias constant. This means that location measurements from all devices had slight offset which was stable but difficult to interpret. The UAV was equipped with closely positioned antennas, on the area of around 100 cm2 which might introduce antenna coupling and result in measurement inaccuracy. To resolve this issue we plan to repeat measurements using only one system at a time on the same location using the same satellites.

GNSS receiver ; UAV ; location accuracy

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