From the beginning of Argos service, locations have been classified according to the following criteria:
Regardless of the number of messages received during a satellite pass, an estimated error is calculated. For classification purposes, the error is assumed to be isotropic and hence characterized by a single number called the radius of error. It corresponds to one standard deviation (sigma) of the estimated location error. The location class is attributed based on the radius of error. The location class and associated error is sufficient for many applications.
Still, the location error is not strictly isotropic and hence is better approximated by an ellipse than by a circle. Users wishing to use an improved description of the location error, for example to assimilate positions into an animal movement model, will now have access to this information (see Chapter 3.4.4). Note however, that the classical error description based on location class and radius of error will remain available to all users.
Class 
Type 
Estimated error* 
Number of messages received per satellite pass 

Least Squares 
Kalman Filter 
Least Squares 
Kalman Filter 

G 
GPS 
< 100m 
1 message or more 

3 
Argos 
< 250m 
4 messages or more 

2 
Argos 
250m < < 500m 
4 messages or more 

1 
Argos 
500m < < 1500m 
4 messages or more 

0* 
Argos 
> 1500m 
4 messages or more 

A 
Argos 
No accuracy estimation 
Unbounded accuracy estimation 
3 messages 

B 
Argos 
No accuracy estimation 
Unbounded accuracy estimation 
messages 
1 or 2 messages 
Z 
Argos 
Invalid location (available only for Service Plus/Auxiliary Location Processing) 

* Class 0 locations are available by request only.