navigation_errors¶
- navigation_errors … end_navigation_errors¶
navigation_errors gps_status ... # GPS Error specifications gps_in_track_error ... gps_cross_track_error ... gps_vertical_error ... gps_degraded_multiplier ... # INS Error specifications ins_accelerometer_bias_error ... ins_gyroscope_bias_error ... ins_random_walk_error ... ins_vertical_error ... ins_x_errors ... ins_y_errors ... ins_z_errors ... ins_xy_errors ... # Other parameters ins_scale_factors ... randomness ... show_status_changes ... time_history_path ... end_navigation_errors
Overview¶
The Navigation_Errors block is a subcommand of the platform command. The primary function of this block is to define the errors that exist between where a platform thinks it is located and where it is actually located. The error difference is represented as a ‘location error vector’. These errors are used to affect the sensor reports by altering the reporting location and any absolute reported locations within the report.
This model provides the capability to produce different errors depending on the availability of GPS. If GPS is defined to be available then GPS errors are used. If GPS is not available then the INS errors are used. Script methods are available to query and change the GPS availability state.
At each update_interval, the model computes the errors or drift rates to be used for the next update interval.
If GPS is active, it computes in-track, cross-track and vertical errors using the following:
in_track_error = gaussian(gps_in_track_error) cross_track_error = gaussian(gps_cross_track_error) vertical_error = gaussian(gps_vertical_error)
Additionally, if gps_status is set to 2 (degraded) each of the above errors will be multiplied by the gps_degraded_multiplier.
If GPS is not active, it computes the in-track and cross-track drift rates and vertical error using the following:
heading_error = gaussian(ins_heading_error) perceived_speed = true_speed + gaussian(ins_velocity_error) in_track_drift_rate = perceived_speed * abs(cos(heading_error)) - true_speed cross_track_drift_rate = perceived_speed * sin(heading_error) vertical_error = gaussian(ins_vertical_error)
During each mover update, the model does the following:
If GPS is not active, the in_track and cross track_errors are updated:
dt = time-since-last-mover-update
in_track_error = in_track_error + dt * in_track_error_drift_rate
cross_track_error = cross_track_error + dt * cross_track_error_drift_rate
Then, regardless of the GPS state, the current in_track_error, cross_track_error and vertical_error are converted from the local body frame to the world frame for use as the current location error vector.
Note¶
This is not a full-fledged navigation error model (i.e., it will not directly affect the motion of the platform. Instead, it is used to introduce small errors in the sensor detection reporting process that arise because the sensing platform may not know EXACTLY where it is located).
Commands¶
- gps_cross_track_error <length-reference>¶
- gps_in_track_error <length-reference>¶
- gps_vertical_error <length-reference>¶
Define the standard deviation of a Gaussian distribution, which is used to create the in-track, cross-track and vertical components of the location error vector when GPS is available.
Default: 0.0 m
- gps_degraded_multiplier <real-value>¶
Specify a value by which computed GPS location errors will be multiplied when gps_status is degraded (2).
- gps_status <integer-value>¶
Set the initial operating state:
-1: GPS is not available. Only INS errors will be computed.
0: The model is disabled. Neither GPS or INS errors will be computed.
1: GPS is available. Only GPS errors will be computed.
2: GPS is available with degraded capability. Only GPS errors will be computed, and these will be multiplied by gps_degraded_multiplier.
3: GPS errors can be set from an external source (e.g., using WsfPlatform.SetPerceivedLocationErrorWCS); otherwise, the model is disabled.
Default: 1 (GPS is available)
- ins_accelerometer_bias_error <angle-reference>¶
Define the standard deviation of the accelerometer error used when GPS is not available.
Default: 0.0 m/s^2
- ins_gyroscope_bias_error <angle-rate-reference>¶
Define the standard deviation of the gyroscope bias error used when GPS is not available.
Default: 0.0 deg/hour
- ins_random_walk_error /variable <real-value>¶
- ins_random_walk_error <real-value> deg/sqrt-hr¶
Define the standard deviation of the random walk error used when GPS is not available.
Default: 0.0 deg/sqrt-hr
- ins_scale_factors <real-value> <real-value> <real-value>¶
- ins_vertical_error <length-reference>¶
Define the standard deviation of a Gaussian distribution, which is used to create the vertical component of the location error vector when GPS is not available.
Default: 0.01 m
- ins_x_errors coefficient <real-value> [ exponent <real-value> ]¶
- ins_y_errors coefficient <real-value> [ exponent <real-value> ]¶
- ins_z_errors coefficient <real-value> [ exponent <real-value> ]¶
- ins_xy_errors coefficient <real-value> [ exponent <real-value> ]¶
Specify an custom error model for each axis in the platform’s entity coordinate system (ECS), applicable when gps_status is set to “INS” (-1). Each entry for a given axis represents an term of a polynomial with the specified coefficient and exponent.
Default
- time_history_path <dir-name>¶
This enables the writing of a time history file that shows the true position and error components as a function of time. <dir-name> is the name of the directory to which the file will be written. The file name will be:
<dir-name>/<platform-name>.neh
If this command is omitted then no time history data will be written.
Default: not specified (no time history will be written).
- randomness <boolean-value>¶
Specifies whether location errors are computed as gaussian random numbers. If so, the location errors (gps_in_track_error, gps_cross_track_error, and gps_vertical_error) are used as the standard deviations in the random number generation. If disabled, the location errors are interpreted as constant offsets.
Default: enabled
- show_status_changes <boolean-value>¶
Script Interface¶
The WsfPlatform script class has methods to query and change the GPS availability status and to query the location error vector.
The script methods can be used to change the GPS availability based on any criteria the user desires.
