Research for Science Art and Technology (RFSAT) Limited

Economic sub-centimeter accuracy GNSS receiver

A generic high-precision GNSS receiver with sub-centimeter accuracy in RTK mode and sub-meter one in stand-alone mode, while remaining very affordable for consumer markets, has been developed by RFSAT in the H2020-AgriBIT project. The final prototype integrates multiple augmentation options, from autonomous operation to SBAS and custom PPP-RTK corrections over both the mobile (3G and 4G, with 5G+ compliance being validated), wireless (Wi-Fi) and (optionally) wired (Ethernet) networks. The augmentation service has proven to be able to reach centimeter accuracies in RTK mode (subject to the quality of the 3rd-party services) and 30cm in SBAS mode.

Its modularity enhances customization capabilities to specific needs of a given application, minimizing the complexity as well as the cost to customer for such custom deployments. The receiver supports L1/L2/L5 GNSS frequencies and can be deployed locally on site for providing RTK correction factors to receivers both stationary and mobile ones, including high-speed vehicles. The prototypes have been constructed in modular way, to offer easy customization to diverse applications, minimizing the complexity as well as the cost to customer for such custom deployments.

The core of the design is an ESP_F9x board, a centimeter accuracy positioning device with RTK and dead reckoning functionality. It contains the uBlox ZED-F9R multifrequency GNSS RTK receiver for acquiring position and applying the RTK fix, the Quectel LTE modem with GPRS fallback (connecting to the internet to fetch RTCM stream), an ESP32-WROOM microcontroller module (running NTRIP client) and logging the output of the GNSS receiver (NMEA messages) to an SD card.

The GNSS module on its own provides accuracy of up to 30cms, hence to achieve 1cm, RTK corrections need to be injected in order to calculate the desired high precision position. This module contains a 32-bit microcontroller with Wi-Fi and Bluetooth connections running the NTRIP client as a Rover and feeds the RTCM stream to the GNSS module. Its ESP_RTK firmware is capable of creating the rover or base station function from the RTK model for the GNSS receivers capable of applying the RTCM stream in order to achieve sub-centimetre level accuracy.

ESP32-DevKitC V4 board

The LTE modem connects to the NTRIP caster and creates a channel for the RTCM stream to be delivered to the ZED-F9x chipset from uBlox. It supports also the slot for the SD card used for logging the chosen variety of data.

ESP_BG95 LTE modem

The LTE modem module connects the device to the Internet in areas with certain LTE and GPRS bands, thus making the device operational also in areas where Wi-Fi coverage is not available. For this purpose, a nano-SIM card slot has been provided. This module also contains a micro-SD card slot to record the positions into a log file. The ESP-F9x module supports also achieving decimeter levels of accuracy through satellite-based augmentation (SBAS).

ESP_F9x module

Functionalities

A suitable multiband GNSS active antenna may be connected to the SMA connector on the ESP_F9x module. In the case that the LTE module is not required only the ESP32-DevKitC and ESP_F9x modules are stacked together hence the RTCM stream is provided via a nearby Wi-Fi router. In the event that Wi-Fi is not available the ESP_BG95 module is stacked between ESP32-DevKit and ESP_F9x module. The SIM card with a data plan is inserted into the SIM slot with a suitable LTE antenna connection to the SMA or uFL connector on the ESP_BG95 module.

The NMEA data is sent via micro-USB port or via Bluetooth COM port connection to another device as well as being saved on the onboard SD card. The final unboxed design is shown below. In order to incorporate the RTK GNSS module from u-Blox several PCB boards were developed to test in different scenarios such as areas with Wi-Fi coverage, areas with cellular networks coverage and standalone function.

Final version of the prototype GNSS receiver from RFSAT

All the tests carried out so far in open sky as well as city areas, have successfully demonstrated compliance with the initial speculations. Development targeted a very small form factor, suitable for use for general purpose applications in diverse domains.

The GNSS receiver is shipped to each customer/partner in a pre-configured condition, allowing it to be used out-of-the-box. In case that any additional steps are required (e.g. inserting a SIM card or configuring 3rd-party RTK service), those would be communicated on per-case bases.

In order to operate the RFSAT GNSS Receiver, the user needs to only connect the provided UBlox GNSS antenna to the available F-SMA socket and plug a 5V (min 1A) micro-USB connector to the middle port on the receiver case. The upper micro-USB port can be used to connect to e.g. a PC for monitoring NMEA message sent over the COM port. The bottom micro-USB port is NOT normally used, but in exceptional cases could be also used for powering up the device.

Once the antenna and the USB power is connected, the receiver boots up its ROM and initializes its hardware and connects to external services (if applicable). The whole start-up process may take few minutes, especially when connecting to the mobile network. During normal operation, the device will display a status of its operation on the embedded LCD screen:

  • Mobile signal power (up to 5 bars), if 3G/4G/LTE modem is used for connecting to Internet
  • Wi-Fi signal strength (up to 5 bards), if Wi-Fi model has been configured for access to Internet
  • Bluetooth status icon, if device was configured to transmit NMEA messages via Bluetooth
  • Device ID (in the presented case, being AJG03)
  • Confirmation of NTRIP data transmission, with indicative diagram of data paths configuration

Embedded WEB server

The receiver can be fully configured using a WEB browser connected to its embedded WEB server. To enter the embedded WEB server, the top button needs to be pressed for five (5) seconds. The screen will switch from displaying status screen to the Wi-Fi Hotspot screen.

Screed of the GNSS receiver in WEB server mode

Afterwards the WEB server can be accessed by connecting to the Wi-Fi device “ESPrtk : <ID>” where the ID is displayed in the second line of text on the LCD screen of the GNSS receiver. In the example used the ID is “AJG03”. The WEB server can be then accessed via WEB browser by connecting to: https://192.168.4.1

References for Further Reading