OpNavCamera applies to a satellite-based tracking instrument and is a subset of attributes under OpticalProperties. The nested attributes described below determine the behavior and modeling of a pinhole camera used in optical navigation measurements between the satellite and any target bodies. For the mathematical details of the models of OpNav measurements and bias formulations, see Optical Navigation Description.
Focal length
Enter a distance value for the FocalLength attribute. This distance is the focal length of the camera, measured from the image plane to the focal point of the camera.
Pixel size
Enter a distance value for the PixelSize attribute. The assumption is that the camera produces square pixels. The pixel size represents the length and width of each individual pixel when mapped from pixel space to image space.
Point observation method
Use the PointObservationMethod attribute to specify the approach in which a point-based optical navigation method identifies the location of the target body relative to the tracking instrument.
| Observation Method | Description |
|---|---|
| Astrometric | In this case, point observations have a star background. ODTK uses the orientation of the star background to determine the orientation of the camera in inertial space. ODTK takes measurement noise from the associated measurement statistics. |
| Image Processing | In this case, processing an optical image creates the point observations. This introduces additional biases that would not be present in an astrometric observation. ODTK computes measurement noise from the TrackingInstrument PixelSpaceDirectionalNoise setting. |
Image extent in pixels
ODTK uses the nested attributes of the ImageExtentInPixels attribute to define the size, in pixels, of the image produced by the optical navigation camera. It then uses these extents to determine if the target is in frame and calculates the magnitude of certain optical biases.
| Extent | Description |
|---|---|
| X Axis | Enter a positive integer value for the XAxis attribute. This represents the number of pixels that occur in any row of the image produced by the optical navigation camera. |
| Y Axis | Enter a positive integer value for the YAxis attribute. This represents the number of pixels that occur in any column of the image produced by the optical navigation camera. |
Camera pointing model
Use the CameraPointingModel attribute to specify where the camera is pointing through the duration of the scenario.
| Camera Pointing Models | |
|---|---|
| Model | Description |
| AlignWithTargetBodyVector | The camera always points so that the vector from the camera to the target body aligns with the offset axis. This setting is useful for preflight analyses, when the attitude of the spacecraft is not yet known. |
| FixedInSatelliteBodyFrame | The camera always points in a fixed direction relative to the satellite body. The align axis determines the direction of the camera and the constrain axis should define the positive Y-axis direction of the camera frame. This setting is preferred for the processing of real observations, where the spacecraft attitude is known. |
Offset Axis (only for Align With Target Body Vector)
Use the X, Y, and Z attributes to set the components of the offset axis. You do not need to normalize this vector. It defines the axis in the camera frame along which the target body lies. A vector entirely in the Z axis indicates that the target body lies perfectly along the optical axis of the camera, with a positive value indicating the object is ahead of the camera.
Align Axis (only for Fixed In Satellite Body Frame)
Use the X, Y, and Z attributes to set the components of the align axis. You do not need to normalize this vector. It defines an axis in the satellite body frame that aligns with the camera’s optical axis.
Constrain Axis (only for Fixed In Satellite Body Frame)
Use the X, Y, and Z attributes to set the components of the constrain axis. You do not need to normalize this vector. It defines an axis in the satellite body frame that aligns with the camera’s optical axis. If the input constrain direction is not orthogonal to the align direction, the constrain direction is projected into the plane perpendicular to the align axis to provide the needed orthogonal direction during calculation.
UV bias
Use the UVBias nested attributes to model the behavior of biases occurring in the pixel coordinates. UV biases affect nonastrometric bearing measurements (RA/Dec).
| Bias Parameter | Description |
|---|---|
| Estimate | Select true or false. If true, ODTK applies biases in the u and v pixel coordinate directions. |
| U Bias Model | The UBiasModel attributes define bias corrections in the u value of pixel coordinates for any optical navigation measurements. |
| V Bias Model | The VBiasModel attributes define bias corrections in the v value of pixel coordinates for any optical navigation measurements. |
Optical scale bias
Use the nested attributes to model the behavior of a scaling bias applied to the pixel coordinates. A positive scale bias signifies that the image is scaled larger than expected. The optical scale bias affects nonastrometric bearing measurements (RA/Dec).
| Bias Parameter | Description |
|---|---|
| Estimate | Select true or false. If true, ODTK applies a multiplicative bias to the scale of the image. A larger scale magnifies the offset of a target's pixel coordinates from the optical axis. |
| Bias Model | The BiasModel attributes define bias corrections applied to the optical scale for any optical navigation measurements. |
Image processing bias
Use the nested attributes to model the behavior of an image processing bias applied to the pixel coordinates. This bias arises from inaccuracies that occur when raw images are processed and converted to optical navigation measurements prior to their introduction into ODTK. The image processing bias affects limb-based bearing measurements (RA/Dec).
| Bias Parameter | Description |
|---|---|
| Estimate | Select true or false. If true, ODTK applies a bias to the optical navigation measurement. |
| Bias Model | The BiasModel attributes define bias corrections applied to optical navigation measurements. |
Target scale bias
Use the nested attributes to model the behavior of a target scale bias applied to range measurements calculated with the optical camera. This bias represents an incorrect scale in the calculation of the diameter of, and subsequently derived range to, the target body. The target scale bias affects range measurements.
| Bias Parameter | Description |
|---|---|
| Estimate | Select true or false. If true, ODTK applies a multiplicative bias to the target scale. |
| Bias Model | The BiasModel attributes define bias corrections applied to the optical scale for any optical navigation measurements. A positive value indicates a larger estimate target body diameter and results in a shorter range measurement than actuality. |
Pixel space directional noise
Enter the pixel value for the PixelSpaceDirectionalNoise attribute. This represents noise as a result of image processing in the pixel coordinate space. ODTK uses this value to compute observation noise for nonastrometric bearing measurements (RA/Dec).
Limb roughness
Enter the distance value for the LimbRoughness attribute. This represents noise along the limb of the body being captured in the image. Noise in the limb roughness contributes to inaccuracies in the estimate of target body size, resulting in errors in optical navigation range estimates.
Image processing bias sine component
Enter the numeric value for the ImageProcBiasSineComponent attribute. This represents the exponent of the sine component that occurs in the image processing bias. For a description of the model for the image processing bias, see Optical Navigation Description.
Add Attitude Uncertainty Deweighting
The Boolean attribute AddAttitudeUncertaintyDeweighting specifies whether or not ODTK processing considers attitude uncertainty to deweight Op Nav measurements. The default is false.
You can specify an amount of spherical attitude uncertainty in the satellite attitude properties. ODTK processing converts the attitude uncertainty to right-ascension and declination uncertainty, which is added to the measurement covariance. For details about the conversion process, see Technical Manuals.