 GPS uses many more satellites than were used with Transit, allowing the system to be used continuously, while Transit provided a fix only every hour or more.After 1996, the satellites were kept in use for the Navy Ionospheric Monitoring System (NIMS).The satellites (known as OSCAR or NOVA satellites) used in the system were placed in low polar orbits, at an altitude of about 600 nautical miles (1,100 km), with an orbital period of about 106 minutes.