What it the ROS way to represent invalid data in a message

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I have a sensor that outputs static or slowly changing data at a low rate and dynamically changing data at a higher rate.

I would like to have one message that aggregates this data together but doesn't have to wait for a 'static data' update before it publishes the first message (due to the lossy medium these messages come over it may never get one).

I'm looking for a way that a subscriber can tell which fields of the message are populated and which are not.

The options so far:

1. Have a bitfield that holds whether each field is valid or not.
2. Have pre-defined invalid values as default values for each field.
3. Represent each value as an array of the corresponding message type and publish an array of length 0 or 1.

Am I missing a ROS way to specify that a field in a message is optional or invalid?

Unfortunately it's not as simple as just having a seperate message for static vs. dynamic data because depending on the source of the data some fields in each of those messages may also be omitted...

For interest I am trying to integrate an AIS receiver ( http://en.wikipedia.org/wiki/Automati... )