BARNARD MICROSYSTEMS LIMITED

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reference

Small Unmanned Aircraft Status Message syntax

In this section, we detail a binary Unmanned Aircraft Status Message Syntax that is only 600 bytes in size. This 4,800 bit message can be BYTE multiplexed at a rate of 1 message per second with 4,800 bps CELP encoded digitised Tower or ATC voice. The Tower or ATC centre voice together with one status message per second can be transmitted using the modem capability of an inexpensive satellite phone to a low earth orbit satellite.

• Message length
• Status Message keywords
• IMU section
• FCU section
• Power
• Warnings
• Communication systems
• GPS
• S+A System
• PAYLOAD system

We calculate a Status Message length for a well equipped Unmanned Aircraft with:

• three engines
• three battery units
• one generator
• five Power Supply Units
• four altimeters (3 ultrasonic + 1 LIDAR)
• five communication systems (3 VHF + 1 satellite modem + 1 GSM modem)
• a Sense and Avoid system
• four cameras

• BLOCK 0 has a 4 Byte unsigned integer checksum, but is not FEC encoded since the Payload Status is not considered to be "mission critical" from a command and control perspective
• BLOCK 1 is FEC encoded to 255 Bytes, as is BLOCK 2, using Reed Solomon RS(255,223) coding.
• If the data in BLOCK 1 or in BLOCK 2 is less than 233 Bytes, then the data is padded with NULL_0101 to 233 Bytes. NULL_0101 = 0101 0101, i.e. an alternating sequence of ONEs and ZEROs.
• BLOCK 0, 1 and 2 data may be encrypted using an algorithm that does not increase the message size. Any encryption of BLOCK 1 and BLOCK 2 data is performed prior to FEC encoding.
• The Payload BLOCK 0 + FEC encoded BLOCK 1 + FEC encoded BLOCK 2 = 600 Bytes = 4,800 bits.
• This Status Message is BYTE multiplexed with the 4,800 bps digitised voice channel to end up with a 9,600 bps digital data signal that can be transmitted via a satellite phone modem.

Compatibility with STANAG 4586

"The length shall be a 32-bit unsigned integer of the number of bytes in the “Message Data”.  The length shall be any number between 1 and 538.

Note the UDP protocol under IPv4 has a guaranteed minimum datagram size of 576 bytes that must be supported by all implementations.  Subtracting the IPv4 header size of 20 bytes and the UDP header size of 8 bytes, leaves 548 bytes as the maximum amount data that can be sent in a datagram that will guarantee interoperability.  Therefore, no message or multi-message datagram shall exceed this data limit.  Subtracting the message wrapper size of 24 bytes, gives 524 bytes as the maximum message length of a single message with no room for another message in the datagram.  Extra care should be taken when packing multiple messages in the same datagram." - STANAG 4586

One can comply with the message size recommended in STANAG 4586 by sending the BLOCK 0 data (90 Bytes) in one datagram, and the BLOCK 1 + BLOCK 2 data (510 Bytes) in a second datagram. Both BLOCK 0 and BLOCK1 contain the same message number, so the Ground Control Station can correctly reunite the data on receipt of the two datagrams.

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Status Message keywords

Example

Note that the BEGIN HEADER and the END Item_IDs are not included to save message length.

• HEADER binary message section length = 28 Bytes

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IMU section

Example

• IMU binary message section length = 52 Bytes

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FCU section

FCU section: engine

FCU engine binary message section length = 15 Bytes per engine

FCU section: flaps

FCU flaps binary message section length = 18 Bytes

FCU section: general