GNSS Receivers

GNSS Receivers for UAVs, Drones and Autonomous Vehicles

What is GNSS and how does it work?

GNSS (Global Navigation Satellite System) is a standard term for any satellite-based navigation system that provides geospatial positioning data with global coverage. The most well-known GNSS is the United States-developed GPS (Global Positioning System). Other GNSS signals include Russia’s GLONASS, which is currently fully operational, and the European Union’s Galileo and China’s Beidou, which are both still currently under development.

A group of GNSS satellites, known as a constellation, consists of 18 to 30 medium earth orbit satellites, which are used as reference points to calculate position. These satellites contain a highly accurate atomic clock, used to synchronise the time between all satellites in the constellation. They broadcast signals towards Earth that contain time and orbital position information.

Trimble OEM GNSS Receivers for UAVs

Trimble OEM GNSS Receivers for UAVs

The broadcast time and receive times are compared to find the signal travel time, which is multiplied by the speed of light to find the distance of the satellite from the receiver. A GNSS receiver needs to “see” – i.e. track and receive signals from – at least four satellites in a constellation. Three distances from three reference satellites are used to find the satellite position via a process called trilateration, and a fourth satellite is required to compensate for the clock error between the accurate satellite atomic clocks and the less accurate ground receiver clock.

Once the position of the GNSS receiver has been determined relative to the satellites, this can then be translated to an Earth-based coordinate system to arrive at the final ground position.

GNSS Receivers

GNSS receivers receive the satellite data and process it in order to work out position, velocity and time. This information can be stored locally or sent to a remote monitoring or tracking station. GNSS receivers are mostly passive devices – an exception to this will be when the European Galileo system is completely functional. Galileo GNSS receivers will be equipped with an emergency function that will be able to broadcast information when activated.

Many GNSS receivers can track multiple GNSS constellations as well as many satellites simultaneously. In a typical GNSS receiver system, each signal from each satellite as assigned to its own dedicated channel. A multi-frequency GNSS receiver can handle signals broadcast by the satellite on multiple frequencies, such as L1, L2 and L5 for GPS. Dual antenna GNSS receivers provide greater heading accuracy, particularly for low dynamics situations.

A typical generic GNSS receiver architecture consists of the following blocks:

  • GNSS antenna – Captures L-band (1 to 2 GHZ radio frequencies) GNSS signals from the satellite
  • The front-end – filters, amplifies and digitizes the incoming signals
  • Signal processing – used to acquire and track the different signals
  • Applications processing – performs calculations on the signal information and presents results according to particular application
NovAtel OEMStar GNSS Receiver for UAVs

NovAtel OEMStar GNSS Receiver for UAVs

GNSS enclosures for drones and unmanned vehicles are available, which encapsulate a GNSS receiver board with additional connectivity options and functionality, and provide environmental protection.

GNSS receivers can be software-defined as well as dedicated hardware modules. Hardware GNSS receivers are generally more efficient as the hardware has been designed specifically around that particular application. However, software-defined GNSS receivers allow for greater flexibility, as it is easier to upgrade and add new features to software.

GNSS for unmanned and autonomous vehicles and drones

Many unmanned systems such as UAVs, UGVs and AUVs require the use of GPS/GNSS to provide them with a high degree of positioning accuracy for applications such as mapping, surveying, precision agriculture and search & rescue. A GNSS antenna is mounted somewhere on the vehicle, and satellite data is then usually fed into the avionics, autopilot or navigation systems.

In addition to navigation, unmanned vehicles may use GNSS to georeference gathered data, avoid collisions, or provide tracking capabilities. The GNSS data provides inputs to the control loop of a drone or other autonomous vehicle, allowing it to maintain position, return to home or follow a series of preset waypoints. This is particularly important for waterborne robots such as AUVs and ROVs whose positions can be significantly affected by tidal activity.

NovAtel SPAN on FlexPak6 GNSS INS Receiver

NovAtel SPAN on FlexPak6 GNSS INS Receiver

GNSS faces the limitation of needing to be within line of sight of at least four satellites in order to provide reliable navigation. In poor signal environments it can be advantageous to couple the GNSS with an Inertial Navigation System (INS), which uses rotation and acceleration information to calculate a relative position that can be used for navigation during loss of GNSS signal. In turn the GNSS can provide an external reference to the INS that helps reduce the effect of bias errors.

GNSS/INS receivers for UAVs and unmanned vehicles are particularly useful for urban or heavily wooded environments, or for missions where the vehicle is likely to traverse through tunnels or other GNSS signal obstructions.

GNSS Receivers: Frequently Asked Questions

GNSS receivers are devices that receive and process signals from satellite-based navigation systems such as GPS, GLONASS, Galileo and Beidou.

A GNSS constellation is the group of signal-broadcasting satellites used by a particular GNSS system.

GNSS satellites are used as reference points to calculate position, broadcasting signals that contain time and orbital position information. The broadcast time and receive times are compared to find the signal travel time, which in turn is used to find the distance of the satellite from the receiver. The position of the GNSS receiver relative to the satellites can be translated to an Earth-based coordinate system to calculate a final ground position.

GNSS stands for Global Navigation Satellite System.

GNSS receivers acquire and process GNSS satellite signals to work out ground position, velocity and precise time. They need at least four satellites within line of sight of the receiver’s antenna.

GNSS mapping and surveying uses the data acquired from GNSS satellites to precisely determine the positions of the targets under survey. This allows accurate distances and angles between objects to be calculated.

GPS (Global Positioning System) is a particular GNSS satellite constellation, developed by the USA. It is currently the world’s most utilised GNSS, consisting of up to 32 medium Earth orbit satellites in six different orbital planes.

Standard GNSS receivers can determine accuracy to within around two metres under optimal conditions. Additional corrections can be applied in order to improve this to centimetre-level accuracy.

Real-time kinematic (RTK) positioning uses two GNSS receivers, a moving “rover” and a stationary base station, to calculate more accurate position information that takes into account satellite movement and signal distortion. This correction data can be broadcast and applied to regular GNSS data to improve accuracy.