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Ask the Quexperts: What is dead reckoning?

Dead Reckoning (DR) traces its roots to ancient mariners using it to approximate their location when they could not see the stars or known features on land. DR was used then, as it is now, as a process for estimating the most recent position of a moving object by referencing a previously known position, and then estimating the change in position based on the object’s speed, direction of travel and the elapsed time since leaving the original position. DR in the historical sense was a best effort means of establishing the location of an object because it relied on approximation and was subject to significant errors introduced by unknown variables such as wind or drifting of a vessel on the ocean.

Today, DR has evolved in sophistication and is used to complement GNSS in cases where visibility of the GNSS satellites may be impaired or blocked. Obstructions of the low power signals may occur in urban canyons, highway underpasses, or areas in which tall buildings cause multipath effects that introduce errors into the ranging calculations. Underground parking and tunnels are particularly detrimental to GNSS signals and can create complete loss of positioning capability if not augmented by DR.

How modern dead reckoning works

In modern times, DR capable GNSS receivers incorporate a 6-axis IMU consisting of a 3-axis accelerometer and a 3-axis gyroscope. In the case of untethered dead reckoning (UDR), only the IMU is used and in the case of automotive dead reckoning (ADR), additional information can also be obtained from a wheel speed sensor and FWD/REV signals. The gyroscope provides angular velocity which provides orientation changes over time while the accelerometer provides linear acceleration from which distance travelled can be derived.

During periods of good GNSS visibility, the IMU data and optional wheel speed data is combined with the GNSS data internally by a Kalman filter that fuses the IMU reference frame to the GNSS reference frame and calibrates the sensors. The IMU can provide a higher update rate than the GNSS and can be used to extrapolate in between actual navigation updates to provide a higher resolution result.

On Quectel DR supporting GNSS devices the raw IMU data can also be output to the customers’ application processor, which is useful for behavior analysis or insurance telematics (harsh braking, harsh acceleration and erratic driving).

What dead reckoning is used for

The DR function provides for uninterrupted accurate tracking in the inner-city where regular GNSS blockages are common and where e-mobility scooter services are often required. The DR capability also has immense value when the rapid location of a device is required on startup. A typical example would be locating a ride-sharing vehicle inside a parking structure. DR is also being applied to fleet telematics systems in cars and trucks, as these vehicles need to maintain accurate information even while they travel through tunnels that can be several kilometers long.

Should the GNSS signal be impaired or blocked, the internal position engine will continue to provide estimates of the position, offset from the last known GNSS position until the positioning from the GNSS returns.

Dead reckoning is one of the methods used to underpin a wide range of use cases including automotive apps, track and trace of logistics shipments, ride-sharing and connected robotics such as lawn mowers and delivery bots and automated guided vehicles. DR, when combined with high precision RTK (real time kinematics) can deliver location accuracy down to the centimeter level.

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