# getTrackVelocities

Obtain updated track velocities and velocity covariance matrix

## Syntax

## Description

returns a matrix of track velocities based on tracks and the velocity
selector.`velocities`

= getTrackVelocities(`tracks`

,`velocitySelector`

)

`[`

also returns the track velocity covariance matrices.`velocities`

,`velocityCovariances`

]
= getTrackVelocities(`tracks`

,`velocitySelector`

)

## Examples

### Find Velocity of 3-D Constant-Acceleration Object

Create an extended Kalman filter tracker for 3-D constant-acceleration motion.

`tracker = multiObjectTracker('FilterInitializationFcn',@initcaekf);`

Initialize the tracker with one detection.

```
detection = objectDetection(0,[10;-20;4],'ObjectClassID',3);
tracks = tracker(detection,0);
```

Add a second detection at a later time and at a different position.

```
detection = objectDetection(0.1,[10.3;-20.2;4],'ObjectClassID',3);
tracks = tracker(detection,0.2);
```

Obtain the velocity vector from the track state using the model name.

`velocity1 = getTrackVelocities(tracks,"constacc")`

`velocity1 = `*1×3*
1.0093 -0.6728 0

Obtain the velocity vector from the track state using the positin selector.

velocitySelector = [0 1 0 0 0 0 0 0 0; 0 0 0 0 1 0 0 0 0; 0 0 0 0 0 0 0 1 0]; velocity2 = getTrackVelocities(tracks,velocitySelector)

`velocity2 = `*1×3*
1.0093 -0.6728 0

### Velocity and Covariance of 3-D Constant-Acceleration Object

Create an extended Kalman filter tracker for 3-D constant-acceleration motion.

`tracker = multiObjectTracker("FilterInitializationFcn",@initcaekf);`

Initialize the tracker with one detection.

```
detection = objectDetection(0,[10;-20;4],"ObjectClassID",3);
tracks = tracker(detection,0);
```

Add a second detection at a later time and at a different position.

```
detection = objectDetection(0.1,[10.3;-20.2;4.3],"ObjectClassID",3);
tracks = tracker(detection,0.2);
```

Obtain the velocity vector from the track state using the model name.

`[velocity1,velocityCovariance1] = getTrackVelocities(tracks,"constacc")`

`velocity1 = `*1×3*
1.0093 -0.6728 1.0093

`velocityCovariance1 = `*3×3*
70.0685 0 0
0 70.0685 0
0 0 70.0685

Obtain the velocity vector from the track state using the velocity selector.

velocitySelector = [0 1 0 0 0 0 0 0 0; 0 0 0 0 1 0 0 0 0; 0 0 0 0 0 0 0 1 0]; [velocity2,velocityCovariance2] = getTrackVelocities(tracks,velocitySelector)

`velocity2 = `*1×3*
1.0093 -0.6728 1.0093

`velocityCovariance2 = `*3×3*
70.0685 0 0
0 70.0685 0
0 0 70.0685

## Input Arguments

`tracks`

— Object tracks

array of `objectTrack`

objects | array of structures

Object tracks, specified as an array of `objectTrack`

objects or an
array of structures containing sufficient information to obtain the track
velocity information. At a minimum, these structures must contain a
`State`

column vector field and a positive-definite
`StateCovariance`

matrix field. For a sample
track structure, see `toStruct`

.

`modelName`

— Motion model name

`"constvel"`

| `"constacc"`

| `"singer"`

| `"constturn"`

Motion model name, specified as one of these options:

`"constvel"`

— The function obtains the velocity states based on the state definition in the`constvel`

function.`"constacc"`

— The function obtains the velocity states based on the state definition in the`constacc`

function.`"constturn"`

— The function obtains the velocity states based on the state definition in the`constturn`

function.`"singer"`

— The function obtains the velocity states based on the state definition in the`singer`

(Sensor Fusion and Tracking Toolbox) function. The use of`singer`

model requires the Sensor Fusion and Tracking Toolbox™.

`velocitySelector`

— Velocity selection matrix

*D*-by-*N* real-valued matrix.

Velocity selector, specified as a
*D*-by-*N* real-valued matrix of ones
and zeros. *D* is the number of dimensions of the tracker.
*N* is the size of the state vector. Using this matrix,
the function extracts track velocities from the state vector. Multiply the
state vector by velocity selector matrix returns velocities. The same
selector is applied to all object tracks.

## Output Arguments

`velocities`

— Velocities of tracked objects

real-valued *1*-by-*D* vector | real-valued *M*-by-*D* matrix

Velocities of tracked objects at last update time, returned as a
*1*-by-*D* vector or a real-valued
*M*-by-*D* matrix.
*D* represents the number of velocity elements.
*M* represents the number of tracks.

`velocityCovariances`

— Velocity covariance matrices of tracked objects

real-valued *D*-by-*D*-matrix | real-valued
*D*-by-*D*-by-*M*
array

Velocity covariance matrices of tracked objects, returned as a real-valued
*D*-by-*D*-matrix or a real-valued
*D*-by-*D*-by-*M*
array. *D* represents the number of velocity elements.
*M* represents the number of tracks. Each
*D*-by-*D* submatrix is a velocity
covariance matrix for a track.

## More About

### Velocity Selector for 2-Dimensional Motion

Show the velocity selection matrix for two-dimensional motion when the state consists of the position and velocity.

$$\left[\begin{array}{cccc}0& 1& 0& 0\\ 0& 0& 0& 1\end{array}\right]$$

### Velocity Selector for 3-Dimensional Motion

Show the velocity selection matrix for three-dimensional motion when the state consists of the position and velocity.

$$\left[\begin{array}{cccccc}0& 1& 0& 0& 0& 0\\ 0& 0& 0& 1& 0& 0\\ 0& 0& 0& 0& 0& 1\end{array}\right]$$

### Velocity Selector for 3-Dimensional Motion with Acceleration

Show the velocity selection matrix for three-dimensional motion when the state consists of the position, velocity, and acceleration.

$$\left[\begin{array}{ccccccccc}0& 1& 0& 0& 0& 0& 0& 0& 0\\ 0& 0& 0& 0& 1& 0& 0& 0& 0\\ 0& 0& 0& 0& 0& 0& 0& 1& 0\end{array}\right]$$

## Extended Capabilities

### C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

In code generation, the

`tracks`

input must be specified as non-empty structures.

## Version History

**Introduced in R2017a**

### R2022b: Obtain velocity and covariance from tracks using motion model name input

You can now obtain velocities and associated covariances of tracks by specifying the motion model name as an input. For example,

[positions,covariances] = getTrackVelocities(tracks,"constvel")

`constvel`

function.## See Also

### Functions

### Objects

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