From The RadioReference Wiki
ASTRO is the trademark name for Motorola's Digital Communications Solutions.
The original Motorola ASTRO implementation uses the standard Type II SmartZone 3600 baud data channel, and the digital voice solution is based on the Motorola proprietary VSELP vocoder. There are only a handful of original VSELP ASTRO systems in use today, it is no longer marketed by Motorola.
These days, ASTRO refers to Motorola's use of an open standard digital voice trunking or conventional solution. This implementation uses the standard Type II SmartZone 3600 baud data channel and the digital voice solution is APCO-25 Common Air Interface (CAI) using the the IMBE vocoder.