Format proposal, with covariance as an example

[David-OConnor]

Currently, implementation of DSDL types if often complicated. The causes manifest in these related ways:

• Bit-level alignment
• Strings used as enums
• Variable field sizes
• Variable message sizes
• Non-constant, and/or non-documented indexes.

This makes writing firmware to read from or write to DroneCAN devices more complicated than for other common communications protocols. I recommend something like this instead. This is from the UBLOX M10 interface description:

Note that it uses byte-level alignment, an explicit length, and explicit indexes. It lists the fields in a standard format, without mixing enums, comments, and fields. This is substantially easier to code for than DroneCAN messages. I propose considering something like this for future messages, and adding new messages for existing functionality that uses something like this. This specific example could be used as a replacement for this in the Fix2 message:

#
# Precision
#
float16[<=36] covariance    # Position and velocity covariance. Units are
                            # m^2 for position, (m/s)^2 for velocity and
                            # m^2/s for position/velocity.

float16 pdop

#
# Position and velocity solution in ECEF, if available
#
ECEFPositionVelocity[<=1] ecef_position_velocity

Note how this, from the DSDL uses bit-level alignment due to its length field, changes the index of future values, and is effectively useless for describing what values go in it.