From Liam Gumley:

Terra Level-0 MODIS data (APID 64) does include real-time ephemeris and attitude data from the spacecraft. This is what allows Terra MODIS data to be processed in real-time with no additional information.

Aqua MODIS Level-0 data (APID 64) does not include any real-time ephemeris or attitude data. This is for historical reasons due to the fact that the Terra and Aqua spacecraft were manufactured by different vendors. However, to allow real-time processing of Aqua data, a predicted set of ephemeris and attitude data is uplinked to Aqua on a regular schedule, and this information is sent back down as part of the direct broadcast stream. This information is known as GBAD (ground based attitude determination) and is stored in Aqua Level-0 APID 957. This is a separate Level-0 file which is created when Aqua data is received. To use the GBAD data, you must run it through the "GBAD Processor" created by the NASA Direct Readout Lab which turns it into the same format as the definitive attitude and ephemeris files. So to process Aqua MODIS data in real-time, you can supply MODISL1DB with these predicted attitude and ephemeris files. In normal conditions, the prediction is usually very good (earth location within 50 meters of what is obtained using the definitive data). However in periods of enhanced upper atmospheric activity, the prediction can be in error by up to several hundred meters compared to definitive. The Rapid Response group monitors this every day at:

  http://rapidfire.sci.gsfc.nasa.gov/ephdiff/

For this reason, it is recommended that anyone using MODIS data for location sensitive applications (e.g., land cover change or burned area mapping) should wait for the definitive ephemeris and attitude. However, for most other applications using the GBAD data in real-time is just fine.

So to summarize, Aqua DB sites can use the GBAD data (from DB) and the GBAD Processor (from DRL) to create attitude and ephemeris files which are supplied to MODISL1DB (or SeaDAS).