Defense & aerospace · Interface control document

ICD-GCS-004 Excerpt: Air Vehicle Status Message

Interface: KESTREL-4 Air Vehicle (AV) → Ground Control Station (GCS)
Document: ICD-GCS-004, Revision C (fictional)
Section: 3.2.1

About this sample

Layout verified

Audience

Ground-station software engineers implementing the message parser, and the integration team verifying it against the air vehicle.

Document type

Interface control document excerpt: a byte-level message layout with units, scale and range, an enumeration, a bitfield, requirements traceability, and a revision history.

How this sample was made and checked

Approach

The layout table carries units, scale and range in the same row as the byte offsets, so an engineer can write the decoder in one pass, and reserved values and bits say what the receiver must do with them.

How it was checked

A test checks the byte layout on every change: the offsets are contiguous, each size matches its type, the fields total 32 bytes, and the CRC covers every byte before it.

Sample content

This interface and all values are fictional and written for demonstration only.

3.2.1.1 Overview

The Air Vehicle Status Message (AV_STATUS) reports position, attitude, and power state. The AV transmits AV_STATUS at 10 Hz over the primary datalink.

Property	Value
Message ID	0x21
Direction	AV → GCS
Rate	10 Hz (± 0.5 Hz)
Length	32 bytes (fixed)
Byte order	Little-endian
Integrity	CRC-16/CCITT-FALSE over bytes 0–29

3.2.1.2 Message Layout

Byte	Field	Type	Units	Scale	Range	Description
0	msg_id	uint8	—	—	0x21	Message identifier
1	seq	uint8	—	—	0–255	Sequence counter; wraps to 0
2–5	time_ms	uint32	ms	1	0–4,294,967,295	Time since AV power-on
6–9	lat	int32	deg	1e-7	−90.0 to +90.0	WGS-84 latitude
10–13	lon	int32	deg	1e-7	−180.0 to +180.0	WGS-84 longitude
14–17	alt_msl	int32	m	0.01	−500.0 to 6,000.0	Altitude above mean sea level
18–19	roll	int16	deg	0.01	−180.00 to +180.00	Roll angle, right wing down positive
20–21	pitch	int16	deg	0.01	−90.00 to +90.00	Pitch angle, nose up positive
22–23	heading	uint16	deg	0.01	0.00 to 359.99	True heading
24–25	batt_mv	uint16	mV	1	0–30,000	Flight battery voltage
26	batt_pct	uint8	%	1	0–100	Estimated remaining capacity
27	mode	uint8	—	—	See 3.2.1.3	Flight mode enumeration
28–29	flags	uint16	—	—	Bitfield	See 3.2.1.4
30–31	crc	uint16	—	—	—	CRC-16 of bytes 0–29

3.2.1.3 Flight Mode Enumeration

Value	Mode	Description
0	STANDBY	Motors disarmed
1	MANUAL	Pilot-in-command direct control
2	WAYPOINT	Autonomous mission plan navigation
3	LOITER	Holding pattern at current position
4	RTB	Return to base
5–255	Reserved	GCS shall treat as UNKNOWN

3.2.1.4 Status Flags

Bit	Name	Meaning when set (1)
0	GPS_VALID	GPS fix is 3D and HDOP < 2.0
1	LINK_DEGRADED	Datalink packet loss > 10% over the last 5 s
2	BATT_LOW	batt_pct ≤ 20
3	GEOFENCE_BREACH	AV is outside the mission geofence
4–15	Reserved	Transmitted as 0; GCS shall ignore

3.2.1.5 Requirements Traceability

Req. ID	Requirement	Verification
GCS-REQ-118	The GCS shall discard any AV_STATUS message with an invalid CRC.	Test
GCS-REQ-119	The GCS shall alert the operator within 1 s when BATT_LOW is set.	Test
GCS-REQ-120	The GCS shall declare link loss if no valid AV_STATUS is received for 2 s.	Test
GCS-REQ-121	The GCS shall display reserved mode values as UNKNOWN.	Inspection

Revision History

Rev	Date	Description	Approved
C	2026-08-14	Added GEOFENCE_BREACH flag (bit 3)	CCB-26-031
B	2026-03-02	Changed alt_msl scale from 0.1 to 0.01 m	CCB-26-009
A	2025-12-10	Initial release	CCB-25-044