MAVLink XML File Schema / Format

The format and structure of dialect files is formally defined in the XML Schema document: mavschema.xsd.

While this is the canonical reference, it is easier to understand the XML file by example (as shown in the following sections).

File Structure

The broad structure for MAVLink XML files is given below.

If you're creating a custom dialect file your file structure should be similar to the one below (but may omit any/all sections).

<?xml version="1.0"?>
<mavlink>

    <include>common.xml</include>
    <include>other_dialect.xml</include>

    <!-- NOTE: If the included file already contains a version tag, remove the version tag here, else uncomment to enable. -->
    <!-- <version>6</version> -->

    <dialect>8</dialect>

    <enums>
        <!-- Enums are defined here (optional) -->
    </enums>

    <messages>
        <!-- Messages are defined here (optional) -->
    </messages>

</mavlink>

The main tags are listed below (all are optional):

  • include: This tag is used to specify any other XML files included in your dialect.

    • Typically dialect files will include common.xml as shown above.
    • You can include multiple files using separate tags.
    • The path to included files can be relative to your dialect file. Note however that the project tests only cover the case where dialects are in the same folder.
    • Nested include of files is not supported (only files specified in the top level include are imported).
    • When building, generator toolchains will merge/append enums in all files, and report duplicate enum entries and messages.

  • version: The minor version number for the release, as included in the HEARTBEAT mavlink_version field.

    • For dialects that include common.xml the tag should be removed so that the version from common.xml is used (version from top level file will be used if specified).
    • For private dialects you can use whatever version you like.
  • dialect: This number is unique for your dialect. You should use: TBD
  • enums: Dialect-specific enums can be defined in this block (if none are defined in the file, the block is optional/can be removed).
  • messages: Dialect-specific messages can be defined in this block (if none are defined in the file, the block is optional/can be removed).

Enum Definition (enums)

Enums are used to define named values that may be used as options in messages - for example to define errors, states, or modes. There is also a special enum MAV_CMD that is used to define MAVLink commands.

Enums are defined within the <enums> ... </enums> blocks (as discussed in the previous section) using <enum> ... </enum> tags. Enum values are defined within the enum using <entry> ... </entry> tags.

For example, the definition of the LANDING_TARGET_TYPE message is given below:

<enum name="LANDING_TARGET_TYPE">
    <description>Type of landing target</description>
    <entry value="0" name="LANDING_TARGET_TYPE_LIGHT_BEACON">
        <description>Landing target signaled by light beacon (ex: IR-LOCK)</description>
    </entry>
    <entry value="1" name="LANDING_TARGET_TYPE_RADIO_BEACON">
        <description>Landing target signaled by radio beacon (ex: ILS, NDB)</description>
    </entry>
    <entry value="2" name="LANDING_TARGET_TYPE_VISION_FIDUCIAL">
        <description>Landing target represented by a fiducial marker (ex: ARTag)</description>
    </entry>
    <entry value="3" name="LANDING_TARGET_TYPE_VISION_OTHER">
        <description>Landing target represented by a pre-defined visual shape/feature (ex: X-marker, H-marker, square)</description>
    </entry>
</enum>

The main enum tags/fields are:

  • name: The name of the enum (mandatory). This is a string of capitalized, underscore-separated words.
  • description (optional): A string describing the purpose of the enum
  • entry (optional): An entry (zero or more entries can be specified for each enum)
  • deprecated (optional): A tag indicating that the enum is deprecated.

MAVLink Commands are defined in the MAV_CMD enum.

entry

The "normal" enum entry tags/fields are:

  • name: The name of the enum value (mandatory). This is a string of capitalized, underscore-separated words.
  • value (optional): The value for the entry (a number).
  • description (optional): A description of the entry.
  • deprecated / wip (optional): A tag indicating that the enum is deprecated or "work in progress".

An entry may also define the optional elements: param, hasLocation, isDestination, missionOnly. In practice these should only be used in the enum named MAV_CMD (described below).

MAVLink Commands (enum MAV_CMD)

Individual entry values in the enum named MAV_CMD are use to define MAVLink Commands. Each command has a value (its "command number") and specifies up to 7 parameters.

These parameters are encoded in MISSION_ITEM or MISSION_ITEM_INT messages (Mission Protocol), or COMMAND_INT or COMMAND_LONG messages (Command Protocol).

For example, see MAV_CMD_NAV_PAYLOAD_PLACE:

<enum name="MAV_CMD">
....
    <entry value="94" name="MAV_CMD_NAV_PAYLOAD_PLACE">
        <description>Descend and place payload. Vehicle descends until it detects a hanging payload has reached the ground, the gripper is opened to release the payload</description>
        <param index="1">Maximum distance to descend (meters)</param>
        <param index="2">Empty</param>
        <param index="3">Empty</param>
        <param index="4">Empty</param>
        <param index="5">Latitude (deg * 1E7)</param>
        <param index="6">Longitude (deg * 1E7)</param>
        <param index="7">Altitude (meters)</param>
</entry>
...
</enum>

MAV_CMD entry value elements may additionally define these tags/fields:

  • param (optional): Up to 7 param tags, numbered using an index attribute.
  • One (but not both) of:
    • hasLocation (optional): A boolean (default true) that provides a hint to a GCS that the entry should be displayed as a "standalone" location - rather than as a destination on the flight path. Apply for MAV_CMDs that contain lat/lon/alt location information in param 5, 6, and 7 values but which are not on the vehicle path (e.g.: MAV_CMD_DO_SET_ROI_LOCATION).
    • isDestination (optional): A boolean (default true) that provides a hint to a GCS that the entry is a location that should be displayed as a point on the flight path. Apply for MAV_CMD that contain lat/lon/alt location information in their param 5, 6, and 7 values, and that set a path/destination (e.g.: MAV_CMD_NAV_WAYPOINT and MAV_CMD_NAV_LAND).
    • missionOnly (optional): Apply with value true if the MAV_COMMAND only "makes sense" in a mission. For example, the fence mission commands could not possibly be useful in a command.

param

The <param> tag is used in the MAV_CMD enum as part of defining mission commands. Each entry value may have up to 7 params declared, with index values from 1-7.

A param must include the following attribute:

  • index - The parameter number (1 - 7).

A param should have:

  • description: Parameter description string (tag body)

A param should also include the following optional attributes where appropriate (which may be used by a GUI for parameter display and editing):

  • label - Display name to represent the parameter in a GCS or other UI. All words in label should be capitalised.
  • units - SI units for the value.
  • enum - Enum containing possible values for the parameter (if applicable).
  • decimalPlaces - Hint to a UI about how many decimal places to use if the parameter value is displayed.
  • increment - Allowed increments for the parameter value.
  • minValue - Minimum value for param.
  • maxValue - Maximum value for the param.
  • multiplier - Multiply by this value to get the unscaled original value. This is primarily intended for specifying any scaling applied to unitless values, where scaling is not encoded in the units.
  • reserved - Boolean indicating whether param is reserved for future use. If the attributes is not declared, then implicitly reserved="False".

    See Defining XML Enums/Messages > Reserved/Undefined Parameters for more information.

  • default - Default value for the param (primarily used for reserved params, where the value is 0 or NaN).

Message Definition (messages)

Messages are defined within the <messages> ... </messages> block using <message>...</message> tags. Individual fields to be encoded in the message payload are defined using <field> ... </field> tags. Every message must have at least one field.

For example,the definition of the BATTERY_STATUS message is given below (this message was chosen because it contains many of the main fields and attributes.

    <message id="147" name="BATTERY_STATUS">
      <description>Battery information. Updates GCS with flight controller battery status. Smart batteries also use this message, but may additionally send SMART_BATTERY_INFO.</description>
      <field type="uint8_t" name="id" instance="true">Battery ID</field>
      <field type="uint8_t" name="battery_function" enum="MAV_BATTERY_FUNCTION">Function of the battery</field>
      <field type="uint8_t" name="type" enum="MAV_BATTERY_TYPE">Type (chemistry) of the battery</field>
      <field type="int16_t" name="temperature" units="cdegC" invalid="INT16_MAX">Temperature of the battery. INT16_MAX for unknown temperature.</field>
      <field type="uint16_t[10]" name="voltages" units="mV" invalid="[UINT16_MAX]">Battery voltage of cells 1 to 10 (see voltages_ext for cells 11-14). Cells in this field above the valid cell count for this battery should have the UINT16_MAX value. If individual cell voltages are unknown or not measured for this battery, then the overall battery voltage should be filled in cell 0, with all others set to UINT16_MAX. If the voltage of the battery is greater than (UINT16_MAX - 1), then cell 0 should be set to (UINT16_MAX - 1), and cell 1 to the remaining voltage. This can be extended to multiple cells if the total voltage is greater than 2 * (UINT16_MAX - 1).</field>
      <field type="int16_t" name="current_battery" units="cA" invalid="-1">Battery current, -1: autopilot does not measure the current</field>
      <field type="int32_t" name="current_consumed" units="mAh" invalid="-1">Consumed charge, -1: autopilot does not provide consumption estimate</field>
      <field type="int32_t" name="energy_consumed" units="hJ" invalid="-1">Consumed energy, -1: autopilot does not provide energy consumption estimate</field>
      <field type="int8_t" name="battery_remaining" units="%" invalid="-1">Remaining battery energy. Values: [0-100], -1: autopilot does not estimate the remaining battery.</field>
      <extensions/>
      <field type="int32_t" name="time_remaining" units="s" invalid="0">Remaining battery time, 0: autopilot does not provide remaining battery time estimate</field>
      <field type="uint8_t" name="charge_state" enum="MAV_BATTERY_CHARGE_STATE">State for extent of discharge, provided by autopilot for warning or external reactions</field>
      <field type="uint16_t[4]" name="voltages_ext" units="mV" invalid="[0]">Battery voltages for cells 11 to 14. Cells above the valid cell count for this battery should have a value of 0, where zero indicates not supported (note, this is different than for the voltages field and allows empty byte truncation). If the measured value is 0 then 1 should be sent instead.</field>
      <field type="uint8_t" name="mode" enum="MAV_BATTERY_MODE">Battery mode. Default (0) is that battery mode reporting is not supported or battery is in normal-use mode.</field>
      <field type="uint32_t" name="fault_bitmask" display="bitmask" enum="MAV_BATTERY_FAULT">Fault/health indications. These should be set when charge_state is MAV_BATTERY_CHARGE_STATE_FAILED or MAV_BATTERY_CHARGE_STATE_UNHEALTHY (if not, fault reporting is not supported).</field>
    </message>

The main message tags/fields are:

  • message: Each message is encapsulated by message tags, with the following attributes
    • id: The id attribute is the unique index number of this message (in the example above: 147).
      • For MAVLink 1:
        • Valid numbers range from 0 to 255.
        • The ids 0-149 and 230-255 are reserved for common.xml. Dialects can use 180-229 for custom messages (provided these are not used by other included dialects).
      • For MAVLink 2:
        • Valid numbers range from 0 to 16777215.
        • All numbers below 255 should be considered reserved unless messages are also intended for MAVLink 1.

          IDs are precious in MAVLink 1!

    • name: The name attribute provides a human readable form for the message (ie "BATTERY_STATUS"). It is used for naming helper functions in generated libraries, but is not sent over the wire.
  • description: Human readable description of message, shown in user interfaces and in code comments. This should contain all information (and hyperlinks) to fully understand the message.

  • field: Encodes one field of the message. The field value is its name/text string used in GUI documentation (but not sent over the wire). Every message must have at least one field.

    • type: Similar to a field in a C struct - the size of the data required to store/represent the data type.
      • Fields can be signed/unsigned integers of size 8, 16, 23, 64 bits ({u)int8_t, (u)int16_t, (u)int32_t, (u)int64_t), single/double precision IEEE754 floating point numbers. They can also be arrays of the other types - e.g. uint16_t[10].
    • name: Name of the field (used in code).
    • enum (optional): Name of an enum defining possible values of the field (e.g. MAV_BATTERY_CHARGE_STATE).
    • units (optional): The units for message fields that take numeric values (not enums). These are defined in the schema (search on name="SI_Unit")
    • multiplier (optional) - Multiply by this value to get the unscaled original value. Allowed values are defined in the schema (search on name="factor"). For example, GPS_STATUS is a value in degrees (0-360) sent in a byte field (0-255). To get the original value, scale by multiplying the value with the multiplier (360/256). This is currently only used for values where scaling is not encoded in the units.
    • display (optional): This should be set as display="bitmask" for bitmask fields (hint to ground station that enum values must be displayed as checkboxes).
    • print_format (optional): TBD.
    • default (optional): TBD.
    • increment - Allowed increments for the increment value.
    • minValue - Minimum value for field.
    • maxValue - Maximum value for the field.

    • instance: If true, this indicates that the message contains the information for a particular sensor or battery (e.g. Battery 1, Battery 2, etc.) and that this field indicates which sensor. Default is false.

      This field allows a recipient automatically associate messages for a particular sensor and plot them in the same series.

    • invalid: Specifies a value that can be set on a field to indicate that the data is invalid: the recipient should ignore the field if it has this value. For example, BATTERY_STATUS.current_battery specifies invalid="-1", so a battery that does not measure supplied current should set BATTERY_STATUS.current_battery to -1.

      Where possible the value that indicates the field is invalid should be selected to outside the expected/valid range of the field (0 is preferred if it is not an acceptable value for the field). For integers we usually select the largest possible value (i.e. UINT16_MAX, INT16_MAX, UINT8_MAX, UINT8_MAX). For floats we usually select invalid="NaN".

      Arrays represent multiple elements, some (or all) of which may need to be marked as invalid. The following notation is used to specify the values that indicate elements of the array are invalid:

      • invalid="[value]": Array elements that contain value are invalid.
      • invalid="[value:]": All array elements are invalid if the first array element is set to value.
      • invalid="[value1,,value3,]": Array elements are invalid if they contain the value specified in the corresponding position of the comma separated list. If the a position in the list is empty, there is no way to indicate the corresponding array element is invalid. The example above indicates that elements 1 and 3 are invalid if they contain value1 and value3, respectively. For element 2 and any elements >4 the invalid property of the field cannot be set.
      • invalid="[value1,]": The first array element is invalid if it contains value1: the invalid property cannot be set for all other elements.

  • deprecated / wip (optional): A tag indicating that the message is deprecated or "work in progress".

  • extensions (optional): This self-closing tag is used to indicate that subsequent fields apply to MAVLink 2 only.
    • The tag should be used for MAVLink 1 messages only (id < 256) that have been extended in MAVLink 2.

Common Tags

This section lists a number of tags can be used in a number of other types - e.g. messages and enums.

deprecated

The <deprecated> tag can be used in an enum, enum entry (value) or message to indicate that the item has been superseded. The tag attributes indicates the time of deprecation and the replacement item, while the element may (optionally) contain a string with additional information about the deprecation.

The generator toolchain can be configured to conditionally build messages omitting the deprecated entries.

An entity should be marked as deprecated only when the main users have had an opportunity to update to the new method.

As a concrete example, below we see that SET_MODE is deprecated and replaced by MAV_CMD_DO_SET_MODE on 2015-12.

    <message id="11" name="SET_MODE">
      <deprecated since="2015-12" replaced_by="MAV_CMD_DO_SET_MODE">Use COMMAND_LONG with MAV_CMD_DO_SET_MODE instead</deprecated>

The deprecated attributes are:

  • since: Year/month when deprecation started. Format: YYYY-MM.
  • replaced by: String of entity that supersedes this item.

wip

The <wip> tag can be used in an enum entry (value) or message to indicate that the item is a "work in progress". The element may (optionally) contain a string with additional information about the new item.

The generator toolchain can be configured to conditionally build messages omitting the wip entries.

Most commonly, the tag is used as shown:

<wip />

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