Interface Control Documents (ICDs) That Engineers Actually Use

An Interface Control Document (ICD) is the shared contract between two systems or teams. In ground station work, it is what prevents integration from turning into guesswork during a live mission. The most useful ICDs are not long “requirements novels.” They are practical, testable, and easy to reference when something breaks at 2 a.m. This guide explains what to put in an ICD so engineers actually use it, and how to keep it current as systems evolve.

Table of contents

1. What an ICD Is and Why It Matters
2. When You Need an ICD (and When You Do Not)
3. ICD Scope: Defining What Is In and Out
4. The ICD Sections Engineers Look for First
5. Data Interfaces: Formats, Identifiers, and Metadata
6. Control and Automation Interfaces: Commands, States, and Events
7. Network and Security Requirements That Do Not Get Skipped
8. Timing, Synchronization, and Coordinate Conventions
9. Error Handling, Retries, and Escalation Boundaries
10. Testability: How to Make Interfaces Verifiable
11. Versioning, Change Control, and Keeping the ICD Alive
12. Common ICD Failures and How to Avoid Them
13. ICD Template Checklist
14. Glossary: ICD Terms

What an ICD Is and Why It Matters

An ICD describes how two systems connect and what they must exchange to work together. It defines the interface in a way that is clear enough to build, test, and operate. In ground station environments, interfaces often span different organizations: station operators, mission teams, satellite manufacturers, and data processing groups. Even when everyone is competent, assumptions drift quickly without a shared reference.

A good ICD reduces time lost to basic questions:

• What exactly does the station deliver after a pass, and where does it go?
• What happens if delivery is late or incomplete?
• Which time standard is used in timestamps and logs?
• What is the naming scheme for products and pass IDs?
• Which party is responsible for retries, acknowledgements, and escalation?

When an incident happens, engineers do not want prose. They want crisp definitions and examples they can compare to what they are seeing.

When You Need an ICD (and When You Do Not)

An ICD is valuable whenever an interface crosses a boundary where misunderstandings are likely. That boundary might be a team boundary, a vendor boundary, or a technology boundary.

You almost always need an ICD when:

• Two organizations integrate: station operator to mission operator, or prime contractor to subcontractor.
• Automation relies on a stable contract: scripts and orchestration depend on predictable inputs and outputs.
• Data delivery must meet an SLA: latency, completeness, and auditability matter.
• Security boundaries exist: credentials, encryption, access methods, and network paths must be explicit.
• Operations must be repeatable: shift-to-shift consistency depends on documented behavior.

You may not need a full ICD when one team owns both sides and changes are frequent. In that case, a lightweight interface note can be enough, as long as it stays current and testable.

ICD Scope: Defining What Is In and Out

A common reason engineers avoid ICDs is that they are too broad. The fix is to define scope early and keep it narrow. An ICD should describe interfaces, not internal design.

A helpful scope statement answers:

• Which systems are involved: ground station, mission control, processing pipeline, storage, monitoring.
• Which direction the interface flows: station-to-mission, mission-to-station, or both.
• What is being exchanged: files, messages, commands, telemetry, logs, metrics.
• What is out of scope: internal RF design, internal database schema, or detailed UI design.

The best ICDs also define responsibilities at the boundary. If something fails, who owns the next action? That detail is often more important than the format.

The ICD Sections Engineers Look for First

Engineers usually open an ICD because they need a specific answer quickly. The top sections should be the ones that help troubleshooting and integration.

• Interface summary: what connects to what, and why.
• Message and data definitions: formats, required fields, and examples.
• Timing and sequencing: what happens first, and what depends on what.
• Success criteria: what counts as a successful delivery or successful command execution.
• Error handling: how failures are signaled, retried, and escalated.
• Security requirements: authentication, authorization, encryption, and network boundaries.

If these are easy to find, people will use the document. If they are buried behind long background sections, the document becomes shelfware.

Data Interfaces: Formats, Identifiers, and Metadata

For ground station integrations, the data interface is often the most important part of the ICD. It should describe what is delivered, how it is packaged, and how it can be validated. Engineers need to know exactly what to expect after a contact.

Define the deliverables

Start by listing each data product type and what it represents. Keep descriptions practical and focus on what the receiving system must do with it.

• Raw recordings: what is recorded, when recording starts and stops, and what file boundaries mean.
• Decoded payload: what counts as “decoded,” and which error correction and framing steps are assumed.
• Supporting metadata: pass context, station ID, antenna, frequency plan, and timing references.

Naming and identifiers

Consistent IDs are what allow automation and troubleshooting to scale. The ICD should define identifiers and ensure they are stable across systems.

• Pass ID: unique identifier tied to a scheduled contact window.
• Product ID: unique identifier for each delivered artifact.
• Spacecraft identifier: what naming convention or ID is used in all records.
• Station identifier: stable station name or code used in logs and delivery paths.

Metadata fields that matter in practice

Metadata is where integrations usually fail. Specify which fields are required, which are optional, and what defaults apply.

• Time tags: time standard, resolution, and whether times represent planned or observed events.
• Orbit context: what orbit source is used for pass prediction, and how it is referenced.
• RF context: frequency, polarization, bandwidth, symbol rate, and any applied offsets.
• Quality indicators: lock status, packet loss indicators, or completeness measures.

Integrity and completeness checks

Define how the receiver verifies that data is complete and uncorrupted. This avoids “we received something, so it must be fine” failures.

• Checksums: what is provided and at what granularity.
• Expected counts: expected file count or expected product list for a pass when applicable.
• Minimum required metadata: what must exist for a product to be considered deliverable.

Control and Automation Interfaces: Commands, States, and Events

Control interfaces are where misunderstandings can create operational risk. The ICD should define which system is allowed to request actions, how those actions are acknowledged, and what states the station exposes.

Command definitions

For each command or request, specify the intent, the parameters, and the expected response. Keep it simple and precise.

• Command name: stable name used by both sides.
• Inputs: required parameters, allowed ranges, and defaults.
• Outputs: success response, failure response, and status codes.
• Side effects: what changes in the station when the command is accepted.

States and events

Automation depends on shared understanding of state. If one system thinks the station is “ready” and the other thinks it is “initializing,” you get missed passes.

• Station state: idle, preparing, tracking, receiving, delivering, error, maintenance.
• Pass state: scheduled, pre-pass, AOS, acquired, in-contact, LOS, post-pass, complete.
• Events: “acquisition achieved,” “recording started,” “delivery complete,” “delivery failed.”

Sequencing

Define the expected order of operations and what “late” means. Small timing assumptions can create large operational differences.

• When pre-pass configuration must be applied.
• How far before AOS tracking begins.
• What happens if acquisition does not occur by a certain time.
• When delivery is expected to start and complete.

Network and Security Requirements That Do Not Get Skipped

Security sections are often written as generic text and then ignored. Engineers use security requirements when they are concrete: what is allowed, what is not, and how the interface is protected. The ICD should describe security in a way that can be implemented and tested.

Access and authentication

• Authentication method: how systems prove identity.
• Authorization model: which actions are allowed for which roles or systems.
• Credential handling: how credentials are stored, rotated, and revoked.

Network boundaries

Define which networks are involved and what connectivity is required. When this is unclear, teams often “open everything temporarily” and the temporary change becomes permanent.

• Allowed ports and protocols: only what is required for the interface.
• Source and destination constraints: which systems can talk to which systems.
• Segmentation expectations: which zone the interface crosses and what controls are expected.

Logging and auditability

When something goes wrong, logs are the interface’s truth. Define what must be logged and how long it must be retained.

• Authentication events: successful and failed access attempts.
• Command history: who requested actions, when, and with what parameters.
• Delivery audit: what was delivered, to whom, and whether integrity checks passed.

Timing, Synchronization, and Coordinate Conventions

Timing and coordinate mismatches create subtle bugs that can take weeks to diagnose. ICDs should spell out conventions rather than assuming everyone uses the same defaults.

Time conventions

• Time standard: which standard is used in timestamps and logs.
• Format: how timestamps are represented and whether time zones appear anywhere.
• Event times: whether “AOS time” is predicted or observed, and how that is labeled.

Coordinate conventions

If the interface includes pointing, pass geometry, or orbit context, define coordinate frames and units clearly.

• Azimuth reference: where azimuth zero is defined and direction of increase.
• Elevation reference: definition and whether negative values are possible.
• Units: degrees vs radians, meters vs kilometers, Hz vs kHz.

These details may feel boring, but they prevent misinterpretation that looks like “random” failures in operations.

Error Handling, Retries, and Escalation Boundaries

Engineers often open ICDs during failures. That means error handling and escalation must be explicit. A practical ICD states what happens when things do not go well and who owns the next step.

Define failure modes

• Acquisition failure: no lock achieved during the expected window.
• Partial pass: lock achieved late or lost early.
• Delivery failure: products captured but not delivered successfully.
• Integrity failure: delivery succeeded but checks indicate corruption or incompleteness.

Retries and timeouts

Retries are useful, but only when bounded. Define how many retries are attempted, how long to wait between attempts, and what happens when retries are exhausted.

• Retry policy: count limits and delay strategy.
• Timeouts: when an action is declared failed.
• Fallback: what degraded mode looks like when normal delivery is not possible.

Escalation boundaries

Escalation rules prevent confusion. Define when to alert the mission team, when to create an incident, and when to pause automation.

• Severity levels: what qualifies as minor, major, or critical.
• Ownership: which team acts first for each type of failure.
• Stop conditions: what failures should halt further attempts until reviewed.

Testability: How to Make Interfaces Verifiable

Interfaces that cannot be tested become arguments. Engineers trust ICDs when the document provides a way to verify compliance. Practical ICDs include a small test plan or acceptance criteria that can be checked repeatedly.

Acceptance criteria that are easy to validate

• Contract checks: required fields exist and follow the defined types and ranges.
• Sequence checks: events and state transitions occur in the documented order.
• Integrity checks: checksums match and completeness rules pass.
• Timing checks: time tags are consistent and appear in the expected format.

Include “known good” examples. Even a small set of examples helps engineers compare live outputs to expected behavior during troubleshooting.

Versioning, Change Control, and Keeping the ICD Alive

An ICD is only useful if it stays current. Interfaces evolve: new product types, new fields, and new failure handling. If the document lags reality, engineers stop trusting it.

Practical versioning habits

• Assign a version: every ICD release should have a version identifier and date.
• Track changes: keep a short change log that says what changed and why.
• Use backward compatibility rules: define what counts as a breaking change.
• Coordinate rollouts: specify when both sides must upgrade together versus independently.

A simple rule works well: the interface is not “changed” until the ICD is updated and the change is tested. That discipline prevents quiet drift.

Common ICD Failures and How to Avoid Them

Most ICD failures are preventable. They happen when documents are written to satisfy a process rather than to support engineering work.

• Too much background: long history sections hide the actual contract details.
• Ambiguous requirements: words like “should” and “typically” replace testable statements.
• Missing ownership: nobody knows who retries, who alerts, or who approves changes.
• No examples: teams interpret formats differently without concrete examples.
• Ignored failure modes: documents describe success but not what happens when steps fail.
• No change control: the interface changes but the ICD does not, eroding trust.

The fix is consistent: keep the ICD focused on the boundary, make it testable, and write it as a tool for the person on call.

ICD Template Checklist

Use this checklist when creating or reviewing an ICD. It is designed to reflect what engineers actually need during integration and operations.

• Interface overview: systems involved, direction of flow, and responsibilities.
• Deliverables: product types, minimum content, and success criteria.
• Identifiers: pass ID, product ID, station ID, spacecraft ID conventions.
• Metadata: required fields, optional fields, defaults, and units.
• Timing conventions: time standard, formats, and predicted vs observed labeling.
• Sequence: expected state transitions and event order.
• Error handling: failure modes, retries, timeouts, and escalation paths.
• Security: authentication, authorization, encryption expectations, and logging.
• Acceptance tests: what to verify and how to confirm compliance.
• Versioning: change log, compatibility rules, and rollout expectations.

Glossary: ICD Terms

Interface Control Document (ICD)

A document that defines how two systems connect and what they exchange, serving as a shared contract.

Interface

The boundary between systems where data, commands, or signals are exchanged.

Deliverable

An artifact produced by one system and provided to another, such as a file, message, or report.

Acceptance criteria

Testable statements that define what “correct” behavior looks like for an interface.

Breaking change

An interface change that requires the other side to update in order to continue working correctly.

Backward compatible change

An interface change that adds capability without breaking existing consumers.

Ownership boundary

The point where responsibility shifts from one team or system to another for actions like retries and escalation.

Sequence

The expected order of interface actions and events, often critical for automation and operations.