Digital mission engineering for Low Earth Orbit
Abstract
Most decisions about a Low Earth Orbit mission are made on a computer long before anything is built: the orbit, the manoeuvres, the propellant, how close the spacecraft will come to others. Digital mission engineering is the practice of making those decisions in a simulation whose behaviour is understood. This paper sets out what a mission team needs from such a tool, describes how KnownX MPT is organised to provide it, walks through one planning task from start to finish, and summarises the evidence for the tool's propagation. It is written for engineers, researchers and programme leads deciding how to equip a team.
1. The planning problem
A mission in Low Earth Orbit raises the same questions at every stage of its design. Where will the spacecraft be at a given time? How does its orbit change over days and weeks? What does a burn of a given length do, and is there enough propellant for it? How close will the planned path come to another object?
Each question can be answered with existing methods. The difficulty for a team is practical. The answers often come from separate programs and spreadsheets that do not share a model, so a change in one place is not carried to the others. A figure in a report cannot easily be traced back to the run that produced it. And it is not always clear how far a given number can be relied on.
Teams at universities, research institutes and new space programmes feel this most, because they have the fewest people to maintain a chain of tools.
2. What a planning tool has to provide
| Need | Why it matters |
|---|---|
| One model for everything | The picture on screen, the prediction and the exported data must come from the same physics, or they will disagree. |
| Known standing of each result | The user must be able to tell which capabilities have been checked against a reference and which have not. |
| Fast iteration | Planning is a series of "what if" questions. The effect of a change should be visible at once. |
| Traceable outputs | An exported result should carry the conditions that produced it. |
| A short path to competence | A new team member should be productive in hours, not weeks. |
3. The approach in KnownX MPT
3.1 One force model
KnownX MPT sums gravity from every body in the scenario, the Earth's J2, J3 and J4 terms, atmospheric drag and thrust in a single force model. The interactive workspace, the trajectory prediction, the conjunction assessment and the validation comparisons all use it. There is no separate, simplified model for display.
3.2 Status beside every capability
Each capability carries a status that is shown in the workspace while it is in use: Validated when an automated check compares it with a reference in every release, and Implemented when it is working and tested but not yet checked against an external reference. The same status is published in the help.
3.3 Plans that can be replayed
A plan is written as a mission sequence: timed steps that set attitude, engine and throttle. Because the plan is data, the prediction and the conjunction assessment can replay it, and show where it leads before it is flown.
3.4 Comparison with real satellites, in the product
The Validation Center lets the user import the published elements of a real satellite and compare KnownX's propagation with SGP4. The user does not have to take the vendor's figures on trust.
4. From scenario to result
| Step | What the engineer does | Where |
|---|---|---|
| 1. Describe | Start from a template or build a scenario: bodies, spacecraft, orbit, active models. | Mission Launcher, Scenario Composer |
| 2. Plan | Script the manoeuvres as timed steps. | Mission Sequence |
| 3. Predict | See the path the plan produces and the closest approach to other vehicles. | Trajectory Map |
| 4. Run | Fly the plan and watch each step execute. | Workspace, Mission Timeline |
| 5. Record | Record telemetry and export it. | Plots |
| 6. Check | Compare propagation with published data for a real satellite. | Validation Center |
At any step the engineer can ask the Mission Companion a question about the mission, or ask it to prepare a change.
5. A worked example
The scenario template conjunction-demo places two vehicles on crossing 600 km orbits. As loaded, the conjunction assessment reports a closest approach of about 6 km.
- The engineer opens the Mission Sequence and adds one step: at 90 seconds into the run, point prograde, engine on, full throttle, for 45 seconds.
- As soon as the step is saved, the assessment replays the plan. The reported miss distance opens from about 6 km to about 240 km.
- The engineer runs the scenario. The step moves from planned to active to complete on the Mission Timeline, and the spacecraft follows the predicted path.
- Telemetry for the run is recorded and exported for the record.
The whole exercise takes a few minutes. The point is not the particular numbers but the loop: change the plan, see the consequence, decide.
6. The evidence behind the numbers
Point-mass gravity and the J2 term are validated against analytic references. For real objects, KnownX's numerical propagation has been compared with SGP4 for three satellites in different orbit regimes:
| Object | Regime | Difference after 24 h | Difference after 72 h |
|---|---|---|---|
| ISS (Zarya) | Low orbit | 1.7 km | 2.3 km |
| Landsat 9 | Sun-synchronous | 1.0 km | 9.9 km |
| GPS BIIR-5 | Medium orbit | 0.8 km | 4.3 km |
The differences are mostly along the orbit: the two propagations follow nearly the same path, slightly out of step. The method, the full tables and the conditions that apply are in technical note KX-TN-001.
7. Working with the Mission Companion
KnownX MPT includes an AI assistant, the KnownX Mission Companion, in a panel beside the simulation. It is given the state of the open mission and can answer questions about it or prepare changes: a sequence step, a throttle setting, a change of time rate. Each proposal appears as a card, and nothing changes until the engineer applies it. The Companion is the subject of white paper KX-WP-002.
8. Where it fits
| Well suited to | Outside this release |
|---|---|
| Orbit and manoeuvre studies in Low Earth Orbit over hours to days | Long-term propagation in medium and geostationary orbits |
| Planning and rehearsing a sequence of burns | Automatic targeting and optimisation of manoeuvres |
| Screening a plan for close approaches | Probability of collision |
| Teaching and operator training | Re-entry analysis |
KnownX MPT is a planning and analysis tool, and is not intended as the sole basis for a safety-critical operational decision.
9. Getting started
KnownX MPT runs on Windows and is licensed per seat. A free trial gives full access for evaluation. The three tutorials in the help take a new user from a first orbit to an avoidance burn and a comparison with a real satellite in about half an hour.
10. Further reading
- KNOWN Corporation, Propagation accuracy: validation method and results, KX-TN-001.
- KNOWN Corporation, An assistant in the loop: how the KnownX Mission Companion works with the engineer, KX-WP-002.
- KNOWN Corporation, KnownX MPT Help: Capabilities and status.
- Vallado, D. A., Fundamentals of Astrodynamics and Applications, 4th ed., Microcosm Press, 2013.