CPAW Workstation: System Models

In this section:

CPAW System Models

Collection planning is enabled by a set of models that simulate and predict the physical environment, spacecraft subsystems, targets, and ground stations. With accurate vendor provided models, CPAW generates effective collection plans that maximize the quality and quantity of imagery collected. Deployed operational systems and the quality of collected imagery confirm the accuracy and effectiveness of the software system modeling.

Physical Environment

CPAW uses STK to model the spacecraft's orbit and the positions of the sun, moon, stars and planets as well as ground facilities and specific targets and areas of interest. CPAW generally uses a predicted ephemeris file provided by a mission flight dynamics system that provides position data at 1 minute increments. STK software within CPAW interpolates between these data points.

Weather Processing

Weather is a critical factor in the quality of an optical image. Generally, if there is significant cloud cover over a target of interest, the image will be of marginal quality. CPAW use global cloud cover forecast files, such as those provided commercially by MDA Federal, to predict the percentage of cloud cover as it puts together the optimized collection plan. Files are generally converted in Portable Gray Map files for processing and visualization. These files, which include true cloud cover and snow cover, are shown below:

3D Globe with Grayscale Cloud Cover

2D Map with Snow

Spacecraft Power

CPAW models power consumption and battery charging for the duration of each planning window. A power allocation associated with each planning window is used to ensure spacecraft power constraints are not violated. In cases where a manually-planned colletion plan has a net power consumption (sum battery charge minus net imaging consumption) that is greater than the allocation, the user is notified that there is a violation in the plan.

Satellite-specific power consumption computations are defined by system and event-driven rules. The battery charging formula is based on a baseline charging rate (a configurable parameter) and the angle profile between solar array normal and the sun vector over the duration of the planning window. The result is the sum battery charge for the planning window. The attitude profile for an planning window is driven by the selection of included scans and the required slews to maneuver between the scans.

Spacecraft Data Storage

CPAW models data generation rates during imaging. Each scan generates an amount of data equal to the data generation rate for the specific imaging mode (using configurable parameters) multiplied by the duration of the scan. The total data generated during a planning window equals the sum of data generated by all scans included in the collection plan for that window.

Data storage modeling results are valid as long as the total data generated is less than the data storage allocation for the planning window. In cases where a user-defined collection plan generates more data than the allocation for the window, a violation is generated to warn the user.

High-Gain Antenna and Ground Terminal Access

This model only applies to collection plans for "real-time" planning windows (those plans generated for planning windows during ground station contacts with simultaneous real-time downlinks).

Ground terminal locations (LAT/LON/HEIGHT) and horizon masks (minimum elevation angle) are modeled in STK and internally by the CPAW engine. CPAW's computations are used to verify ground terminal constrained access for the duration of any planning window that is flagged as a "real-time" planning window.

The High-gain antenna Az/El pointing profile is checked against a blockage table to ensure no structural blockage (antenna pointing through spacecraft structure while tracking the ground terminal) occurs during data downlink for the planning window. High-gain antenna slewing rates and accelerations are also computed to verify that maximum rate and acceleration limits (both configurable) are not exceeded during the planning window. For manually-planned collection plans, the user is notified if any of these access, blockage, or slew limits are exceeded.

Target Access

Target areas that are non-rectangular or larger than can be imaged with a single scan are automatically split into 4-corner rectangular strips with width determined by the miminum sensor width determined by planning-window-specific and target-specific minimum off-nadir angle.

Target access windows are computed by CPAW based on target corner points. Access start equals the time when both start side corner points are within the satellite Field of Regard and satisfy all target-defined constraints (including min/max incidence angle, min/max azimuth angle, and max GSD).

Sample Splitting of a Target Area

Restricted Access

Restricted areas are defined in STK and displayed on the map display with a bold red outline. CPAW verifies that the sensor footprint does not impinge on any defined restricted area during imaging times defined in the collection plan. If any incursions are found, a violation is generated to warn the operator.

Sample of Restricted Area

Camera

The CPAW camera model is provided to help determine the appropriate camera settings for images. Parameters and setting vary by imaging mode and can include line rates, TDI settings, and other camera parameter types generally driven by sun angle and other similar computable values. The camera model is an option that may be disabled in lieu of a configurable default settings or target-defined settings. The operator may override automatically-selected camera settings at any time (although the software will always warn the operator if the selection is contrary to the camera model).

Scan

Based on operator, model, or algorithm-defined target scan start point, scan start time, and imaging mode parameters, CPS computes the attitude profile of the spacecraft necessary to scan the target area. Orbit Logic can customize baseline scan code for any spacecraft imaging mode, or integrated a spacecraft-vendor-provided scan model. Scan attitude profiles are the boundaries for slews within the planning window.

Attitude

Attitude slews are computed using a plug-in slew model (generally provided by the spacecraft vendor) from the planning window starting attitude to the start of the first scan, between scans, and from the end of the last scan back to the default safe spacecraft attitude (generally a sun-pointing attitude).

Every time a scan is modified or a new scan added during the CPAW collection plan definition process, two new slews (slew to scan start and slew from scan end) must be verified and computed. Inputs and outputs from the slew functions generally include the delta slew time and the starting/stopping quarternion, rate, and acceleration.

Sensor Exclusion

Spacecraft star trackers and cameras generally have bright object pointing constraints defined by spacecraft and camera vendors. Some of these constraints are "hard" and must be avoided in any collection plan, and some are "soft" in that they are allowed but may degrade image quality or geolocation accuracy. CPAW predicts bright object incursions based on sensor models, spacecraft position data, bright object (sun and moon and sun glint point on the Earth) position propagation computations, defined minimum off-boresight angle, and the slew/scan attitude profile over the duration of planning windows.

CPS makes these predictions AFTER each scan is "included" in the collection plan (only then is a partial or complete planning window attitude profile known). There are no pre-computed violations since these would be target-specific and start time specific and it would be inefficient to precompute violation-free scan start windows for thousands of targets within the satellite field-of-regard during a planning window. Violations are resolved by changing the scan start time or removing that target from the plan (either manually or via automated scheduling algorithm).