An isochronous satellite is one that orbits the earth in one
day. So it orbits the earth as the same speed as the earth
rotates.
Most satellites that are isochronous are infact geostationary
satellites as they also stay in the same position above the earth.
However these satellites occupy a very specific orbit above the
equator.
It is possible to have isochronous satellites that aren't
geostationary however getting them to remain in orbit would prove
difficult.