What criteria are used to classify an airport as a satellite airport?
The answer is found in §91.130 and AIM 3-2-4.

A satellite airport is any airport under the class C shelf outer are of 20NM.


  §91.130   Operations in Class C airspace.
  
  (a) General. Unless otherwise authorized by ATC, each aircraft operation in Class C airspace must be conducted in compliance with this section and §91.129. For the purpose of this section, the primary airport is the airport for which the Class C airspace area is designated. A satellite airport is any other airport within the Class C airspace area. 
  
  (b) Traffic patterns. No person may take off or land an aircraft at a satellite airport within a Class C airspace area except in compliance with FAA arrival and departure traffic patterns.


What does Class C airspace area mean?


  AIM 3-2-4 Class C Airspace Sub Paragraph 3 Note 4
  
  Though not requiring regulatory action, Class C airspace areas have a procedural Outer Area. Normally this area is 20 NM from the primary Class C airspace airport. Its vertical limit extends from the lower limits of radio/radar coverage up to the ceiling of the approach control's delegated airspace, excluding the Class C airspace itself, and other airspace as appropriate. (This outer area is not charted.)




I believe the answer above is correct.  There is also another way of looking at it.


  §71.51   Class C airspace.
  The Class C airspace areas listed in subpart C of FAA Order 7400.11D (incorporated by reference, see §71.1) consist of specified airspace within which all aircraft operators are subject to operating rules and equipment requirements specified in part 91 of this chapter. Each Class C airspace area designated for an airport in subpart C of FAA Order 7400.11D (incorporated by reference, see §71.1) contains at least one primary airport around which the airspace is designated.


That document shows the boundaries of Class C Airspace Area and does not include the outer area.


  ASO SC C  Myrtle Beach, SC Myrtle Beach International Airport   (lat. 33°40'47"N., long. 78°55'42"W.) 
  
  That airspace extending upward from the surface to and including 4,000 feet MSL within a  5-mile radius of the Myrtle Beach International Airport, and that airspace extending upward from 1,200 feet MSL to and including 4,000 feet MSL within a 10-mile radius of the Myrtle Beach International Airport.  This Class C airspace area is effective during the specific dates and times of operation of the Myrtle Beach Approach Control facility, as established in advance by a Notice to Airmen.  The effective date and times will thereafter be continuously published in Airport/Facility Directory. 
  
  AMENDMENTS  03/27/97  62  FR  6698  (Added) 


Since the information from the FAA conflicts with one another, I would ask for a legal interpretation.