Procedural
Requirements
Effective Date: June 27, 2023
Expiration Date: June 27, 2028
|
|
NASA Procedural Requirements |
NPR 8079.1 Effective Date: June 27, 2023 Expiration Date: June 27, 2028 |
| | TOC | Preface | Chapter1 | Chapter2 | Chapter3 | Chapter4 | Chapter5 | AppendixA | AppendixB | AppendixC | AppendixD | AppendixE | AppendixF | ALL | |
D.1. The Orbital Collision Avoidance Plan (OCAP) is an implementation plan that describes how the design and planned operations of the space flight mission meet the intent of the NPR to protect the spacecraft asset and the space environment. The OCAP documents the structured study of aspects of mission design that affect close approach prediction and mitigation during mission operations. Its purpose is to ensure that all needed functionality is in place when the mission is put into operation. The project manager develops the OCAP for each project. An OCAP can cover multiple spacecraft if they are sufficiently similar.
D.2. The OCAP includes the results of study and analysis and the related design and operations considerations. The OCAP is developed iteratively between the project and the NASA Conjunction Assessment Risk Analysis (CARA) Program or the Johnson Space Center (JSC) Flight Operations Directorate (FOD). The project manager provides the initial inputs detailed below. CARA or JSC FOD responds with their analysis results, which provide the basis for discussion and decisions on the course of action for implementing the requirements of this NPR. Selected aspects of mission design need to be discussed and agreed upon between the project manager and CARA or JSC FOD early to avoid costly delays close to mission launch. The project manager documents the results of that discussion and iterative process and the resulting decisions in the OCAP and submits it to CARA or JSC FOD for review and approval. The completed NPR Compliance Matrix from Appendix C is attached when the OCAP is submitted.
D.3. The approval signature of the program manager and the concurrence of the CARA program manager or the JSC FOD interagency operations liaison indicates that the implementation chosen by the project either meets the NPR requirements or the requested tailoring is acceptable. Signature on the OCAP also indicates approval of the requested tailoring in the attached Compliance Matrix.
D.4. The project manager completes approval and baselines the OCAP prior to proceeding into the Implementation Phase. As changes occur, the project manager keeps the OCAP updated and notifies CARA or JSC FOD, who will assess the need to perform OCAP analyses again. In general, for projects that develop a CAOIA, the OCAP does not need to be updated after the CAOIA has been baselined.
D.5. Below are the required fields for the OCAP. If a section is not relevant or not applicable to a particular project, the project manager states that in the appropriate section and provides a rationale. For any section of the OCAP, backup information already documented in an alternate form (e.g., a previously approved OCAP from a comparable mission, a local procedure, or other configuration-controlled document) can be referenced. The project manager ensures that CARA or JSC FOD has access to the alternate form.
D.6 Orbital Collision Avoidance Plan Template
Table of Contents
1.0 PROJECT OVERVIEW
2.0 SPACECRAFT DESIGN
3.0 ORBIT SELECTION AND PLACEMENT
3.1 Spacecraft Colocation Analysis
3.2 Spacecraft Transit Burden
3.3 Close Approach Event Density
4.0 DEPLOYMENT, IMPROVING CATALOGING, AND ENHANCING TRACKABILITY
4.1 Cataloging
4.2 Trackability
4.3 Deployment
5.0 SPACECRAFT OPERATIONS
5.1 Ephemeris Generation
5.1.1 As-Flown State Information
5.1.2 State Estimation Parameters
5.1.3 Filter Tuning
5.1.4 Covariance Realism
5.1.5 Maneuver Execution Error
5.2 Conjunction Mitigation Options
5.3 Autonomous Maneuvering
5.3.1 Objectives and Reach
5.3.2 Control Algorithm Particulars
5.3.3 Ground Communication and Control
5.3.4 Autonomous Action Representation in Ephemerides
5.3.5 Halting Autonomous Actions
6.0 RISK ASSESSMENT PARAMETERS
Provide a short description of the planned project, its concept of operations, and its primary goals. This description should include the project's purpose, the basics of any data collection and analysis activities, or any other higher level project details that provide a sense of how the project's purpose drives design decisions. Such information typically appears elsewhere (although not always in a single location) in other project documents and literature; the intent here is to summarize only the relevant information in a single location to provide needed context for the reader of the OCAP.
Describe the physical design of the spacecraft by providing the information requested below. This information should be the best estimate of anticipated values as understood at the time of performing OCAP analyses:
Describe planned operations of the spacecraft from launch to disposal including, as applicable, transitional staging, orbit design, navigation, and propulsion system details such as:
Describe the selected orbital regime (including spacecraft altitude and inclination), rationale for the approach, and how the choices affect the mission objectives and performance. The description should include consideration of spacecraft colocation implications, the spacecraft transit burden, and the close approach event density. Estimate and describe the amount of spacecraft lifetime and propellant needed to accomplish the ascent, orbit maintenance, conjunction mitigation, and, if necessary, the disposal phase of the space flight mission. Ensure the spacecraft is adequately resourced and capable of implementing the plan.
Describe the expected insertion orbit at launch and any ascent sequence to achieve the final mission orbit after injection. If the spacecraft will maintain the planned orbit through orbit maintenance maneuvers, describe the expected frequency and size of maneuvers (See the CA2 Handbook sections 5.2 and 6.5 for more discussion and context.) Describe expected maneuvers at end of life for disposal.
Identify the ephemerides representing each of the orbits that the spacecraft will spend time in as well as launch vehicle trajectory predictions for determination of colocation in the injection orbit.
For spacecraft that take advantage of ridesharing arrangements, much of this information will not be known until late in the life-cycle. Under these conditions, it is understood that options for addressing analysis results may be limited.
Describe the chosen method of deploying the spacecraft. See Table D-1 for planning activities needed for different types of deployment. CARA or the JSC FOD trajectory operations officer (TOPO) can assist the project in analyzing and optimizing choices for deployment and will lead any discussions with the U.S. Space Command (USSPACECOM). (See the CA2 Handbook Section 5.1.)
For projects using rideshare arrangements, very little may be known at design time about the future orbit, let alone the manner of launching and orbit placement. Even though it may not be known at design time precisely which approaches may be used, planning and preparation can offer to the project the launch and/or deployment approaches that can be safely embraced. Failure to perform appropriate planning steps at design time could preclude the use of such launch and/or deployment approaches later should they be an integral part of an attractive launch and/or deployment option.
The design-time activities below ensure that needed information will be available to the launch cataloging agency if some of the more challenging launch and/or deployment mechanisms are employed.
Table D-1. Planning Activities for Deployment Scenarios
| Deployment Scenario | Necessary Planning Activities |
|---|---|
| Single payload | When the NASA spacecraft is the principal or the only payload for the launch and will be injected very close to its desired orbit location, no specific launch and/or deployment activities are required at design time. DOD tracking and cataloging of the launch is straightforward, and no additional information beyond what is contained in the "Ready minus 15" (R-15) standard launch alerting message is required. |
| Parent/child (deploy or jettison) | In a parent/child deployment, a payload is attached to (or entirely inside) another payload. The child spacecraft is released from the parent spacecraft after the parent spacecraft is deployed. Such scenarios can cause confusion during the identification and cataloging process if the details of this arrangement are not known.
If project spacecraft will deploy another spacecraft or jettison an object, the project manager will identify all spacecraft or objects involved in the parent/child situation, describe the deployment scenario and timeline, delineate the steps taken to attempt to ensure that any secondary deployments take place a safe distance from other spacecraft released in the primary deployment, and establish a communications protocol to be used to inform DOD about the arrangement, both in advance and in real-time as the different parts of the deployment scenario are actually executed. |
| Tethered | Because tethered spacecraft follow an orbit defined by the center of mass of the tethered system but in general can be tracked only at the extremities, they present challenges for orbit determination and spacecraft identification. The use of a tether for spacecraft deployment is a form of the parent/child scenario described above. When a project employs a tethered deployment mechanism, the project manager supplies CARA or JSC FOD with the information necessary for CARA/JSC FOD to determine whether the spacecraft can be identified and the orbit determined to a level of fidelity sufficient to enable conjunction assessment. The information needed includes the masses of the spacecraft involved, the length of the tether, the period of time during which the spacecraft will actually be operating as a tethered system, the orbit altitude, and the sizes and external materials of the two end objects to determine individual trackability. |
For multi-deployment scenarios, describe any method(s) of improving cataloging efficiency that will be employed by the project; for example, providing launch injection vectors, the rapid production of predicted spacecraft ephemerides, arranging for inter-payload deployment delays, or increasing the deployment velocities to increase payload separation.
If the launch will be a large-deployment rideshare or if the spacecraft does not meet the minimum trackability size criteria, describe the tracking enhancement measures that will be employed to improve trackability and spacecraft identification. CARA or JSC FOD will determine whether the spacecraft meets the SSN trackability requirements based on the dimensions from template Section 2. (For more context, see Sections 4.5 and 5.2 of the CA2 Handbook.)
If the spacecraft is deemed too small to track or has selected an orbit that is unlikely to obtain regular or reliable SSN tracking, this section will document the proven detectability enhancement or explain how satellite-predicted ephemerides will be generated and supplied. Such measures can include an on-board tracking radio beacon to provide position and identification, the use of corner cubes and an arrangement with a laser tracking facility to track an identified payload, coded light signals from a light source on the exterior of the spacecraft, radio frequency interrogation of an exterior Van Atta array, passive increase of albedo, or arrangement with a commercial tracking provider to provide specialized tracking and payload identification.
Describe how the ephemeris produced is consistent with the requirements in Section 4.2 of the NPR. For spacecraft that can change their orbit or trajectory or for spacecraft with highly eccentric orbits, describe the capability of the spacecraft for ephemeris generation that meets the requirements of Section 4.3 of the NPR. (See the CA2 Handbook Section 4.6 for more information.)
The description should encompass as-flown state information, state estimation parameters, filter tuning, covariance realism, and maneuver execution error.
5.1.1. As-Flown State Information
Indicate the method that will be used to understand the spacecraft's current position (e.g., telemetry, Global Positioning System (GPS) fixes) and the different observables or measurements that will be provided, which should include the rates at which measurements will be taken; the regularity at which the downlinking of such information will occur; and how the uncertainties of these data will be assessed, represented, and provided to CARA or JSC FOD.
5.1.2 State Estimation Parameters
Indicate the state and non-conservative force parameters that the orbit determination process will estimate. For the state estimation, indicate which conservative force parameters will be employed and at what fidelity (e.g., geopotential order, third body effects, solid earth tides). If atmospheric drag is to be estimated (and typically should be for low-Earth orbit (LEO) and high-eccentricity missions), indicate the atmospheric density model to be used and any additional drag-related prediction improvement approaches (e.g., debiasing methods, solar storm prediction models). If solar radiation pressure is to be modeled (and typically should be for missions with perigee heights above ~500km), provide a general description of the approach and features of the model that will be employed. The models required are functions of the orbit, but the following general guidelines can be stated:
5.1.3 Filter Tuning
Describe the orbit determination method to be deployed, the tunable parameters, and the approach that will be used to set these parameters both before launch and once on orbit. For example, if a batch filter is used, tuning parameters include the orbit determination fit span (and minimum data requirements), residual exclusion thresholds, and goodness-of-fit parameters used to determine whether an orbit determination is acceptable such as the weighted residual root-mean-square error and percent of residuals accepted. Nominal values for these parameters are indicated along with the rationale used for choosing those values. Similarly, appropriate parameters and associated values are given if a sequential estimator is selected.
5.1.4 Covariance Realism
While covariances can easily be obtained from orbit-determination engines and propagated to future time points, such covariances rarely provide a realistic statement of the actual state errors at those points without initial tuning, regular monitoring, and tuning refinement. Describe the process that will be used to evaluate the realism of produced covariances and the overall covariance-tuning and monitoring approach planned. A set of open-source covariance realism evaluation tools is posted on the CARA software repository.
5.1.5 Maneuver Execution Error
Maneuvers do not always occur exactly as commanded, and this execution uncertainty should be accounted for in the predicted post-maneuver state covariances that appear in owner/operator (O/O) ephemerides. Describe how spacecraft maneuver execution error will be determined and how it will be included in the covariances given in the predictive ephemerides. Previous approaches have included statistical characterization of actual maneuvers (requires a number of actual on-orbit maneuvers before the error characterization is meaningful) or Monte Carlo techniques that attempt to model all the possible process errors in maneuver execution.
Describe whether active orbit maintenance, including controlled deorbit, is possible for the spacecraft and what method will be used for this. Characterize spacecraft reorientation capabilities and their effects on mission conduct. Describe the mitigation capability selected and the rationale for its selection. (See the CA2 Handbook Section 4.6.)
If any level of on-board, autonomous maneuvering is planned, describe as outlined in the subsections below how the autonomous maneuver control functionality or paradigm will perform the needed notifications, fail-safes, and functionality to meet the requirements of Section 4.4 of the NPR. Include the objectives and reach, the control algorithm particulars, ground communication and control, autonomous action representation in ephemerides, and the mechanisms for halting planned autonomous actions and notifying CARA or JSC FOD.
5.3.1. Objectives and Reach
Describe the overall purpose of the automated maneuver control approach. For example, is it just for station keeping or does it also autonomously manage transit to the on-station position and active deorbit at the end of mission life? Will each spacecraft be autonomously managed as an independent unit or is there a "mother ship" that will autonomously coordinate and execute constellation reconfiguration among the member spacecraft? Is automated conjunction assessment included in the autonomous maneuver control? If so, provide a detailed description and algorithmic specifications of the conjunction assessment and risk analysis functionality as well as the expected input data and interfaces for receipt of such data.
5.3.2 Control Algorithm Particulars
Describe the overall control paradigm employed and its driving parameters and timelines such as look-ahead periods, automatic controller cycle/reevaluation time, and "freeze time" after which planned maneuvers or other activities are not revisited or altered.
5.3.3 Ground Communication and Control
Describe the methods and frequency with which autonomously planned control actions will be communicated to the ground and the associated timeline. Delineate which actions, if any, require ground approval before execution or whether there are other "fail-safe" methods that can abort any autonomously planned action.
5.3.4 Autonomous Action Representation in Ephemerides
Describe the methods that will be used to represent autonomously selected actions in circulated spacecraft-predicted ephemerides, namely, how and at what frequency planned maneuver information will be obtained from the spacecraft so that it can be represented in the distributed ephemerides. Because ephemerides are the mechanism by which a spacecraft's intended future positions will be represented both in conjunction assessment screenings against the space catalog and to other O/Os, it is important that the actions of autonomous control systems are relayed so that they can be made available to the screening and position deconfliction processes. Describe the way these ephemerides that include modeling of planned maneuvers will be made available in near-real-time and with sufficient lead-time to enable the conjunction assessment process.
5.3.5 Halting Autonomous Actions
Describe the mechanisms for ground personnel to halt planned autonomous actions when necessary and the way CARA or JSC FOD will be notified of such situations.
6.0 Risk Assessment Parameters
Describe the approach selected for the payload hard-body radius (HBR) value to use for operational conjunction assessment and risk analysis and summarize the rationale.
In the table below, each of the required areas of the template after Project Overview is explained in terms of inputs from the project, analysis by CARA/JSC FOD, and the context for the analysis. The purpose is to describe the needed input and the process involved in deciding on the approach for the project that will be documented in the OCAP.
Table D-2. Inputs, Analyses, and Context for OCAP Fields
| Topic | 2.0 SPACECRAFT DESIGN |
|---|---|
| Project Manager Inputs | Describe the physical design of the spacecraft (best estimate of anticipated values as understood at the time of performing OCAP analyses) including:
Describe planned operations of the spacecraft from launch to disposal including, as applicable, transitional staging, orbit design, navigation, and propulsion system details such as:
|
| CARA/JSC FOD Analyses and Context | Input to other analyses. |
| Topic | 3.0 ORBIT SELECTION AND PLACEMENT |
| Project Manager Inputs | The spacecraft orbit (or orbits) selected, rationale for that choice, and how strongly those choices affect mission performance (flexibility for adjustment).
|
| CARA/JSC FOD Analyses and Context | Certain orbits present greater close approach safety risks and challenges than others. The purpose of this group of analyses is to elucidate the difficulties that a particular orbit will engender and determine whether the orbit as proposed is problematic for conjunction assessment. Results will be analyzed to determine whether minor adjustments to the proposed orbit could notably improve the spacecraft safety profile. If the chosen orbit presents considerable close approach challenges, additional safety capacities and activities may be necessary to mitigate the risk if the orbit selection cannot be modified. The goal of these analyses is to ensure that the spacecraft is properly equipped for handling the conjunction assessment implications of the selected orbit. Note: "Rideshare" spacecraft that do not determine the target orbit provide orbit selection information as soon as possible, which may not be until after PDR. Under these conditions, it is understood that the available options for addressing analysis results may be limited. If rideshares do not have information about their injection orbit until very close to the launch date, it is critical that project managers deliver this information as soon as possible to allow sufficient time for CARA to perform this analysis. Three aspects of the selected orbit and orbit placement approach must be investigated to determine the conjunction assessment and collision avoidance burdens imposed: spacecraft colocation analysis, spacecraft transit burden, and close approach event density. |
| Subtopics | 3.1 Colocation |
| Project Manager Inputs | Whether the spacecraft will maintain the planned orbit through orbit maintenance maneuvers. If so, the expected frequency and size of maneuvers (See the CA2 Handbook sections 5.2 and 6.5 for more discussion and context.) |
| CARA/JSC FOD Analyses and Context | Active spacecraft routinely come into regular contact with debris objects, but the threat posed by any such objects is transient because, in most cases, these objects are decaying while the active spacecraft remain in stable or maintained orbits. However, in encountering actively maintained spacecraft (and other spacecraft placed in or objects finding themselves in frozen orbits), "systematic" conjunctions can arise; that is, conjunctions that regularly reappear over a long period of time. Such situations are both a safety hazard themselves and a particular nuisance because of the expanded O/O contact and coordination requirements for conjunctions between active spacecraft. Note that "colocation" in this context is defined as being in the vicinity of another operational spacecraft close enough that systematic conjunctions occur.
Some spacecraft use a single nominal orbit, but others use different orbits for different mission stages. CARA or JSC FOD performs an analysis, using tools developed in-house, to determine whether any known or existing spacecraft present a systematic conjunction likelihood with the proposed spacecraft orbit. If any such spacecraft are identified, CARA or JSC FOD communicates to the project small changes to the spacecraft's orbit that could obviate the systematic conjunctions or, if an orbit change is not desired, the additional infrastructure and communications requirements needed to manage the systematic conjunction situation. If colocations cannot be eliminated by choosing an alternative orbit, or if it is not desirable to do so for space flight mission reasons, then the project manager develops a plan to coordinate orbit placement and/or sharing with the other affected operators. (See the CA2 Handbook Section 4.3.) |
| Subtopic | 3.2 Spacecraft Transit Burden |
| Project Manager Inputs | The project forwards its post-launch-injection ascent plans and if applicable, its end-of-life active descent plans to CARA or JSC FOD for transit burden analysis. Such information should be communicated by a trajectory ephemeris. If this is not possible, the project manager coordinates with CARA or JSC FOD on other documentation methods. |
| CARA/JSC FOD Analyses and Context | The purpose of this analysis is to determine what conjunctions a spacecraft may encounter during its ascent from the injection orbit to the mission orbit and similarly, from the mission orbit to the disposal orbit.
CARA or JSC FOD will determine the existing (and, if known, planned) active spacecraft orbits through which the spacecraft will be transiting, what existing communications and management protocols exist for the non-NASA spacecraft, and therefore, what active coordination activities and capabilities will be necessary for the NASA project to accomplish these transits safely. CARA will also ensure coordination with JSC FOD for transiting through the International Space Station (ISS) or any other human-space-flight (HSF) orbits, if applicable. If the CARA or JSC FOD recommendation is for implementation of a communications or other operational process, that will be documented in the CAOIA for non-HSF projects or other document specified for HSF projects. Communication with other operator(s) should begin as soon as possible, preferably for non-HSF projects before the CAOIA is written. During ascent and disposal, the expectation is that the spacecraft will yield right-of-way to on-station active spacecraft. The OCAP describes how, such as through risk mitigation maneuvers or trajectory alterations. If yielding is not possible, the project manager and CARA document the alternative plan. (See the CA2 Handbook Section 4.3 for more information.) |
| Subtopic | 3.3 Close Approach Event Density |
| Project Manager Inputs | Provide on-station orbits. |
| CARA/JSC FOD Analyses and Context | Different orbits contain different object densities and as such, they generate different numbers of conjunctions and high-interest close approach events. It is important to estimate the number of routine conjunctions that a mission will encounter, the number of high-interest close approach events that will require explicit planning and management engagement, and the number of actual conjunction risk mitigation actions that will be needed so that appropriate operations plans, mission staffing, and fuel expenditure and/or mission interruption due to conjunction mitigation actions can be determined and incorporated explicitly into the project's planning.
Drawing on its historical conjunction database, CARA or JSC FOD will take the project's provided on-station orbits and run appropriate tools to estimate the number of conjunctions, high-interest close approach events, and mitigation actions required over the mission's expected lifetime. CARA will furnish this information to the project so that it can be used in refining operations plans and staffing levels (documented in the CAIOA) and establishing the amount of fuel required and anticipated mission disruption for risk mitigation actions (needed for spacecraft design). Note: For spacecraft using rideshare arrangements, an event density analysis can be conducted once the injection orbit is known. Although changes in fuel budget, etc., cannot be pursued, staffing and operations planning adjustments are possible. |
| Topic | 4.0 DEPLOYMENT, IMPROVING CATALOGING, AND ENHANCING TRACKABILITY |
| Subtopic | 4.1 Deployment |
| Project Manager Inputs | The project describes the chosen method of deploying the spacecraft. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will assist in determining whether the method enables cataloging by the U.S. space surveillance network (SSN). CARA or the JSC FOD trajectory operations officer (TOPO) can assist the project with this analysis and will lead any discussions with the U.S. Space Command (USSPACECOM). (See the CA2 Handbook Section 5.1.)
Single-payload, parent/child, and tethered deployment scenarios require planning at design time. The easiest approach to complying with deployment scenario requirements is by adopting the protocols used during similar previous launches that met the requirements. If requested, CARA or JSC FOD can assist the project manager in identifying such previous launches. Table D-1 summarizes the planning activities needed for each deployment scenario. |
| Subtopic | 4.2 Improving Cataloging |
| Project Manager Inputs | For multi-deployment scenarios, describe any method(s) of improving cataloging efficiency that will be employed by the project; for example, providing launch injection vectors, the rapid production of predicted spacecraft ephemerides, arranging for inter-payload deployment delays, increasing the deployment velocities to increase payload separation. |
| CARA/JSC FOD Analyses and Context | It is now common under rideshare conditions for large numbers of spacecraft to be deployed, often en masse, as part of a single launch. Such launches make it difficult for the DOD cataloging authority as they try to positionally separate and catalog large clusters of essentially identical spacecraft. Large rideshare launches can require several weeks to be fully cataloged, which delays the on-orbit collision avoidance process for such spacecraft and adds risk for those spacecraft that begin their transit out of the cluster to other orbital locations.
Analysis and pre-planning ensure that needed information will be available to the launch cataloging agency if some of the more challenging launch and/or deployment mechanisms are employed. |
| Subtopic | 4.3 Enhancing Trackability |
| Project Manager Inputs | The project manager will describe the particular tracking enhancement measures that will be employed to improve trackability and spacecraft identification if the launch will be a large-deployment rideshare or if the spacecraft does not meet the minimum trackability size criteria. Such measures can include an on-board tracking radio beacon to provide position and ID, the use of corner cubes and an arrangement with a laser tracking facility to track and identify the payload, coded light signals from a light source on the exterior of the spacecraft, radio frequency interrogation of an exterior Van Atta array, passive increase of albedo, or arrangement with a commercial tracking provider to provide specialized tracking and payload identification. (See the CA2 Handbook Section 4.5 for more details.) |
| CARA/JSC FOD Analyses and Context | Based on the dimensions provided in Section 1.0 of the OCAP, CARA or JSC FOD will determine whether the spacecraft meets the SSN trackability requirements. (For more context, see Sections 4.5 and 5.2 of the CA2 Handbook.) CARA or JSC FOD will evaluate the proposed tracking enhancement measures or how satellite-predicted ephemerides will be generated and supplied. |
| Topic | 5.0 SPACECRAFT OPERATIONS |
| Subtopic | 5.1 Ephemeris Generation |
| Project Manager Inputs | Describe how the ephemeris produced is consistent with the requirements in Section 4.2 of the NPR. For spacecraft that can change their orbit or trajectory or for spacecraft with highly eccentric orbits, describe the capability of the spacecraft for ephemeris generation that meets the requirements of Section 4.3 of the NPR. (See the CA2 Handbook Section 4.6 for more information.) |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review and determine whether it believes the O/O orbit determination approach is adequate for providing O/O predicted ephemerides that can serve as the basis for conjunction assessment decisions. CARA or JSC FOD will recommend needed enhancements or upgrades.
Given the different propulsion methods in use, more and more conjunction assessment activities are being performed directly from O/O ephemerides, so it is extremely important that these products provide accurate predicted states and realistic covariances. (Note that the common TLE [two-line element] format does not include the necessary covariance information to enable conjunction assessment.) Risk assessment for spacecraft close approaches is based on precise predictions of the two spacecrafts' states and state uncertainties at the time of closest approach. Because most secondary objects are not active spacecraft that cooperatively provide ephemeris data, a state solution derived from non-cooperative tracking needs to be used for these objects; typically, this solution comes from the DOD space catalog. However, the solution for the protected asset can frequently be improved over the DOD solution by using an O/O ephemeris. The following aspects of O/O ephemerides enable them to be more accurate than the DOD solutions:
Details of the time, place, and manner of ephemeris construction and delivery to CARA or JSC FOD are documented in the CAIOA. Additionally, the actual evaluation of O/O ephemerides in terms of prediction error and covariance realism must be performed after the spacecraft achieves on-orbit operations and is generating real ephemerides, although simulated activities before launch are encouraged to ensure the proper operation of the orbit determination process and exercise tuning practices. At design time, however, it is important to ensure that the assembled orbit determination process will be able to produce the ephemeris products needed for conjunction assessment. |
| Subtopic | 5.1.1. As-Flown State Information |
| Project Manager Inputs | Indicate the method that will be used to understand the spacecraft's current position (e.g., telemetry, Global Positioning System (GPS) fixes) and the different observables or measurements that will be provided: the rates at which they will be taken; the regularity at which the downlinking of such information will occur; and how the uncertainties of these data will be assessed, represented, and provided to CARA or JSC FOD. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.1.2 State Estimation Parameters |
| Project Manager Inputs | Indicate the state and non-conservative force parameters that the orbit determination process will estimate. For the state estimation, indicate which conservative force parameters will be employed and at what fidelity (e.g., geopotential order, third body effects, solid earth tides). If atmospheric drag is to be estimated (and typically should be for LEO and high-eccentricity missions), indicate the atmospheric density model to be used and any additional drag-related prediction improvement approaches (e.g., debiasing methods, solar storm prediction models). If solar radiation pressure is to be modeled (and typically should be for missions with perigee heights above ~500km), provide a general description of the approach and features of the model that will be employed. |
| CARA/JSC FOD Analyses and Context | The models required are functions of the orbit, but the following general guidelines can be stated:
|
| Subtopic | 5.1.3 Filter Tuning |
| Project Manager Inputs | Describe the orbit determination method to be deployed, the tunable parameters, and the approach that will be used to set these parameters both before launch and once on orbit. For example, if a batch filter is used, tuning parameters include the orbit determination fit span (and minimum data requirements), residual exclusion thresholds, and goodness-of-fit parameters used to determine whether an orbit determination is acceptable, such as the weighted residual root-mean-square error and percent of residuals accepted. Nominal values for these parameters are indicated, along with the rationale used for choosing those values. Similarly, appropriate parameters and associated values are given if a sequential estimator is selected. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.1.4 Covariance Realism |
| Project Manager Inputs | While covariances can easily be obtained from orbit-determination engines and propagated to future time points, such covariances rarely provide a realistic statement of the actual state errors at those points without initial tuning, regular monitoring, and tuning refinement. Describe the process that will be used to evaluate the realism of produced covariances and the overall covariance-tuning and monitoring approach planned. A set of open-source covariance realism evaluation tools is posted on the CARA software repository. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.1.5 Maneuver Execution Error |
| Project Manager Inputs | Maneuvers do not always occur exactly as commanded, and this execution uncertainty should be accounted for in the predicted post-maneuver state covariances that appear in O/O ephemerides. Describe how spacecraft maneuver execution error will be determined and how it will be included in the covariances given in the predictive ephemerides. Previous approaches have included statistical characterization of actual maneuvers (requires a number of actual on-orbit maneuvers before the error characterization is at all meaningful) or Monte Carlo techniques that attempt to model all the possible process errors in maneuver execution. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.2 Conjunction Mitigation Options |
| Project Manager Inputs | Provide information concerning whether active orbit maintenance, including controlled deorbit, is possible for the spacecraft and what method will be used for this, as well as characterizations of spacecraft reorientation capabilities and their effects on mission conduct. (See the CA2 Handbook Section 4.6.) |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations for the mitigation capability of the spacecraft.
CARA's mission statement regarding orbital safety is "To take prudent measures, at reasonable cost, to improve safety of flight, without imposing an undue burden on mission operations." The word "prudent" here is operative: the NASA position does not translate into an absolute requirement for all missions to perform active collision risk mitigation. Indeed, many low-cost missions that use extremely small spacecraft with very short orbital lifetimes may not require any active risk mitigation methods at all. That said, "prudent measures" usually does mean that missions must avail themselves of any mechanisms they have available that can meaningfully reduce the collision likelihood of high-risk close approach events. Generally, missions that have the ability to adjust the orbit of their spacecraft must employ that mechanism to mitigate collision risk for high-risk close approach events. This ability is typically applied to chemical or electric propulsion systems, but other methods are also effective in reducing conjunction risk. One such additional method to perform orbit adjustment is "differential drag," in which the spacecraft reorients itself to change its ballistic coefficient and thus use the change in its drag acceleration to modify its orbit. While the effective ?V that this method can apply is considerably smaller than what is possible using chemical propulsion, both directed studies and practical experience with spacecraft that employ this method have shown that it can be an effective means of conjunction risk mitigation when the lead times used are long enough. Another conjunction risk mitigation method is "attitude realignment," in which the smallest (or at least a smaller) cross-sectional presentation of the spacecraft is aligned to be perpendicular to the relative velocity vector of the two objects that are in conjunction. This approach reduces the protected asset's exposed area facing the oncoming secondary objects and thus reduces the probability of collision. For some spacecraft, attitude realignment does not render a substantial reduction in the probability of collision and thus is not mandated as a mechanism to be employed for conjunction risk mitigation, even if a spacecraft has no other mitigation techniques at its disposal. However, some spacecraft achieve a meaningful reduction in collision risk by employing this realignment method. Analysis is required to determine whether this method will be effective for a given spacecraft design. If the spacecraft does not have chemical or electric propulsion, CARA or JSC FOD will consider provided data concerning the spacecraft orbit maintenance methods and attitude control mechanisms to determine whether these additional methods can be used to appreciably reduce collision risk. CARA or JSC FOD will also assist in the development of a mission conjunction assessment concept of operations that includes the use of the recommended mitigation capability. The capability selected and the rationale for its selection will be documented in the OCAP, and for non-HSF projects, the conjunction assessment concept of operations that makes use of it will be documented in the CAIOA. |
| Subtopic | 5.3 Autonomous Maneuvering |
| Project Manager Inputs | If any level of on-board, autonomous maneuvering is planned, describe how the autonomous maneuver control functionality or paradigm will perform the needed notifications, fail-safes, and functionality to meet the requirements of Section 4.4 of the NPR, including the objectives and reach, the control algorithm particulars, ground communication and control, autonomous action representation in ephemerides, and the mechanisms for halting planned autonomous actions and notifying CARA or JSC FOD. |
| CARA/JSC FOD Analyses and Context | CARA or FOD will review the responses to the above and determine whether the as-designed autonomous maneuver control approach will integrate properly with the collision avoidance paradigm or whether design changes will be necessary.
Every attempt will be made to integrate autonomously controlled missions into the conjunction analysis and mitigation process with a minimum of disruption. However, the fact remains that collision avoidance, especially when two active spacecraft are in conjunction, is of necessity a communicative activity. Autonomous control must provide mechanisms for appropriate conjunction and maneuver intention data exchange to make safe spacecraft operation possible. |
| Subtopic | 5.3.1. Objectives and Reach |
| Project Manager Inputs | Describe the overall purpose of the automated maneuvering. For example, is it just for station keeping or does it also autonomously manage transit to the on-station position and active deorbit at the end of mission life? Will each spacecraft be autonomously managed as an independent unit or is there a "mother ship" that will autonomously coordinate and execute constellation reconfiguration among the member spacecraft? Is automated conjunction assessment included in the autonomous maneuver control? If so, provide a detailed description and algorithmic specifications of the conjunction assessment and risk analysis functionality as well as the expected input data and interfaces for receipt of such data. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.3.2 Control Algorithm Particulars |
| Project Manager Inputs | Describe the overall control paradigm employed and its driving parameters and timelines such as look-ahead periods, automatic controller cycle/reevaluation time, and "freeze time" after which planned maneuvers or other activities are not revisited or altered. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.3.3 Ground Communication and Control |
| Project Manager Inputs | Describe the methods and frequency with which autonomously planned control actions will be communicated to the ground and the timeline associated with this. Delineate which actions, if any, require ground approval before execution, or whether there are other "fail-safe" methods that can abort any autonomously planned action. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.3.4 Autonomous Action Representation in Ephemerides |
| Project Manager Inputs | Describe the methods that will be used to represent autonomously selected actions in circulated spacecraft-predicted ephemerides, namely, how and at what frequency planned maneuver information will be obtained from the spacecraft so that it can be represented in the distributed ephemerides. Because ephemerides are the mechanism by which a spacecraft's intended future positions will be represented both in conjunction assessment screenings against the space catalog and to other O/Os, it is important that the actions of autonomous control systems are relayed so that they can be made available to the screening and position deconfliction processes. Describe the way these ephemerides that include modeling of planned maneuvers will be made available in near-real-time and with sufficient lead-time to enable the conjunction assessment process. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Subtopic | 5.3.5 Halting Autonomous Actions |
| Project Manager Inputs | Describe the mechanisms for ground personnel to halt planned autonomous actions when necessary, and the way CARA or JSC FOD will be notified of such situations. |
| CARA/JSC FOD Analyses and Context | CARA or JSC FOD will review the information and make recommendations. |
| Topic | 6.0 RISK ASSESSMENT PARAMETERS |
| Project Manager Inputs | Describe the approach selected for the payload hard-body radius (HBR) value to use for operational conjunction assessment and risk analysis and summarize the rationale.
The project will furnish CARA or JSC FOD with dimensioned schematics for their spacecraft. |
| CARA/JSC FOD Analyses and Context | Based on this information, CARA or FOD will produce a set of possibilities for a payload HBR value and document the analysis in this section. From this set of possibilities, CARA or JSC FOD and the project will then agree on the HBR value to use for operational conjunction assessment and risk analysis.
Of the risk assessment parameters used in the risk evaluation, the one associated with a particular mission payload is the HBR. The probability of collision (Pc), which is the principal conjunction risk assessment calculation, determines the likelihood that the actual miss distance will be smaller than a specified threshold. This threshold is usually set to represent the combined sizes of the two spacecraft in conjunction so that the Pc truly does give the probability of collision because a collision can be presumed to occur if the miss distance between the two objects is smaller than their combined size. This combined size is historically realized as a sphere that encapsulates both objects if they are placed adjacent to each other. The radius of this sphere is called the HBR. In each conjunction, estimates of the size of the secondary object are computed by CARA or JSC FOD. This estimate is based either on published spacecraft dimension data or, lacking that, on spacecraft skin-track signature data such as radar-cross section or visual magnitude measurements. Because the actual dimensions of the protected asset are known, determinations of its size are more straightforward, but there is a range of options for representing the three-dimensional size as a single numeric value from very conservative to risk tolerant. The selection of a size representation methodology is governed largely by the risk posture of mission personnel. It is not beneficial to employ an approach that is unnecessarily conservative as this generates excessive false alarms and results in unneeded work for both the mission and CARA or JSC FOD. |
| TOC | Preface | Chapter1 | Chapter2 | Chapter3 | Chapter4 | Chapter5 | AppendixA | AppendixB | AppendixC | AppendixD | AppendixE | AppendixF | ALL | |
| | NODIS Library | Program Management(8000s) | Search | |
This document does not bind the public, except as authorized by law or as incorporated into a contract. This document is uncontrolled when printed. Check the NASA Online Directives Information System (NODIS) Library to verify that this is the correct version before use: https://nodis3.gsfc.nasa.gov.