These are the available options that allow a vehicle to estimate its position without a GPS. Once enabled this allows all autonomous and semi-autonomous modes just as they do would a GPS is available.
The low cost IMUs (accelerometers, gyros, compass) used in most autopilots drift too quickly to allow position estimation without an external velocity or position source. In other words, low-cost IMUs on their own are not sufficient for estimating position.
A board with more than 1MB of flash is required to run non-GPS navigation, except for Vicon as 1MB boards still support the GPS_INPUT message, although they don’t support the GLOBAL_VISION_POSITION_ESTIMATE so they have to be run using the GPS_INPUT message. See Firmware Limitations for details.