AWAC wave data and Signature wave data both contain a parameter called AST quality. This parameter gives an indication of the quality of your altimeter data collected with the AST method (see What is the difference between AST and LE?). The definition of the parameter is different between AWACs and Signatures: 

AWAC: 

For AWACs, the quality parameter is a result of the match filter process used when detecting the surface, and is a dimensionless value. As a rule of thumb, the data quality is considered okay if the quality parameter is over 100. If it is over 200 the data is considered very good, and if the quality parameter is below 100 it indicates that there are some issues with the data set that you should investigate further. 

Signature:  

For Signatures, the quality parameter has a more physical meaning, and is the amplitude from the point where the altimeter has detected the surface using the AST method. The amplitude is given in steps of 0.01 dB; for example, a quality value of 8000 means that the altimeter amplitude is 80 dB where it detects the surface, which is considered good.  

In your Signature data, you will also find a parameter called Altimeter quality. This parameter gives an estimate of the quality of the altimeter data collected with the LE method and must not be confused with the AST quality parameter. Altimeter quality is also a dimensionless value that is a result of the match filter process used to detect the surface. The limits for what is considered good data depends on the instrument, but if the quality is below the limit for your instrument, the LE distance will be set to zero.