While speed describes how fast an object is moving, velocity provides the speed and the direction in which an object is moving relative to true north. NovAtel's velocity solutions determine:
- The speed of an object in a given direction
- The relative speed of objects
Velocity solutions from GNSS receivers determine the direction of the motion of the GNSS antenna relative to the ground, not the direction in which the vessel, aircraft or vehicle is pointed or “headed”. For example, a boat pointing north but drifting south at 2 m/s will show a velocity of 2 m/s to the south.
GNSS derived velocity is used in many industries:
- Auto steer systems
- Precision navigation
Speed determination applications
- Variable rate used in distribution of agriculture inputs such as fertilizers, seeds, chemicals
- Vehicle testing
- Speed control and monitoring systems
- Real time and post-mission processed velocity
- GNSS and GNSS/INS velocity are well suited for high dynamic applications
- Output available in numerous formats and rates.
- For GNSS velocity derived from averaging, averaging period can be configured
Velocity Determined from GNSS Measurements:
Instantaneous Doppler Velocity
Velocity estimated from instantaneous Doppler frequency measurements. As a Doppler frequency measurement is phase rate, no averaging time interval is used in this type of velocity estimation. The latency in the Doppler velocity output is based on an analysis of the receiver's tracking loops to determine the lag inherent in the Doppler measurement.
Instantaneous Doppler velocity has low latency associated with the measurement and is suited for high dynamic applications where the velocity of an object changes quickly but is noisier than other velocity determination methods.
Successive Position Averaged Velocity (available in RTK or L-Band enabled models only)
Average velocities are computed from the average change in position over a set time interval or extracted from a low latency positioning filter. As such, it is an average velocity based on the time difference between successive position computations and not an instantaneous velocity tagged at a particular time.
The velocity latency to be subtracted from the time tag is normally half the time between filter updates. For example, a logging rate of 10 Hz would translate into a velocity latency of 0.05 seconds. The latency value is related to the output rate of a velocity log request.
Velocity Determined from GNSS/INS Measurements:
An Inertial Navigation System (INS) is a self-contained navigation technique used to track an object's position and orientation relative to a known starting point, orientation and velocity. It consists of an Inertial Measurement Unit (IMU) that has three accelerometers and three gyroscopes to measure linear acceleration and angular velocity respectively.
High accuracy velocity, position and orientation of an object can be determined when combining GNSS and INS measurements. GNSS/INS solutions can be output up to 200 Hz (based the IMU data rate).
Post-Processed Velocity Measurements:
GrafNav and Inertial Explorer® post-processing software take advantage of both forward and backward data computations to generate the most accurate combined solution. This is especially useful to increase solution accuracy and availability and to provide quality assessment of data collected under challenging environments. Both GNSS only velocity and GNSS/INS velocity measurements can be computed in post mission processing software.
When choosing a velocity solution, consider the following needs of your application:
- Solution accuracy
- Signal availability
- Solution rate
- Latency of solution
- Type of motion
- Continuity of solution updates
- System exportability
- System price
- System installation constraints
- Real time versus post-processed solution
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