I believe Sun-synchronous and Heliosynchronous may share the same meaning - Here is a DOD definition "(DOD) An orbit in which the satellite's orbital plane is at a fixed orientation to the sun, i.e., the orbit precesses about the earth at the same rate that the earth orbits the sun. It has the characteristics of maintaining similar sun angles along its ground trace for all orbits, and typically has an inclination from 96 to 98 degrees, depending on the orbit altitude and orbit shape (eccentricity)."  http://www.answers.com/topic/heliosynchronous-orbit