Short for “Acknowledge” or “Acknowledgement”. In the flight software world, it means that when you send a data packet you’re expecting the receiving point to “Acknowledge” that it received it correctly. (It verifies data integrity with things like Cyclic Redundancy Checks.) Your code might spin and do nothing while waiting for the “Ack” or “Acks” to come back before you send the next file. Some things require “Acks” but it can slow things down, especially for bad connections where you simply never hear the Ack. For this reason, not all transmissions require Acknowledgements and instead send a stream of data without caring if the data is fully verified.

Short for “Application Programming Interface”. In other words, it’s the Interface between two sets of software. It’s how you get your code to talk with somebody else’s code. Each Interface can have different rules, commands, methods of initialization, and more.

This one is tricky because different functions and organizations use this to mean different things.

CCSDS Definition: Short for “Application Process Identifier”. It’s the field in a packet’s primary header that uniquely identifies a stream of packets (indicates source, destination, or type). It helps route data, commands, telemetry, etc. to the correct locations.

Core Flight Software Definition: Short for “Application Identification”. This is a unique reference assigned to Apps you install on top of Core Flight Software.