Cassiopeia was a constellation visible from Earth.
After the Krynoid on Earth was destroyed, the Fourth Doctor tried to take Sarah Jane Smith on a vacation to Cassiopeia. However, the TARDIS instead materialised in Antarctica as the Doctor had forgotten to reset the coordinates. (TV: The Seeds of Doom)
A Charged Vacuum Emboitment that shunted entropy from N-Space into E-Space existed in this constellation. When the universe was hanging by a thread and entropy was taking over, the Fourth Doctor and the Tremas Master sent the reboot program from the Pharos Project on Earth to the CVE in Cassiopeia using a light speed overdrive. The signal arrived and restarted the CVE, stopping the entropy. (TV: Logopolis)
Many billions of years later, the CVE in Cassiopeia finally closed. (PROSE: Timewyrm: Apocalypse)
The Eleventh Doctor suggested Cassiopeia as a family vacation spot. He described it as "nice every time of year." (COMIC: Summer Wholiday)