NAPLES, Italy — Sirius XM Radio will need to install more than 600 new ground terminals around North America as it prepares for the retirement of three satellites in highly elliptical orbit and their replacement by two satellites in conventional geostationary orbit, the man who designed the elliptical-orbit system said Oct. 4.

The highly elliptical satellites offer a better look angle over North America, particularly the northern latitudes, than do satellites operating from geostationary orbit 36,000 kilometers over the equator. That is the reason for the unusual orbit selected in the first place.

The three elliptical-orbit satellites require only 130 terrestrial repeaters to assure signal strength in urban canyons and other areas where mobile users do not have a line-of-sight link to the satellites.

A two-satellite constellation in geostationary orbit will need 800 repeaters to provide the same service reliability of 99.95 percent, said Robert D. Briskman, now technical executive at New York-based Sirius XM Radio Inc., who is credited with being most responsible for Sirius’ choice of elliptical orbit.

Addressing the 63rd International Astronautical Congress here, Briskman said the switch to geostationary orbit would happen around 2013, when the three elliptical-orbiting satellites, all launched in 2000, are to be retired.

The satellite radio customers on board will be transferred to what should be two geostationary-orbit satellites that should be operational then.

The Sirius FM-6 satellite, built by Space Systems/Loral of Palo Alto, Calif., is scheduled for launch next spring aboard an International Launch Services Proton rocket from Russia’s Baikonur Cosmodrome in Kazakhstan. It follows the June 2009 launch of the FM-5 satellite. FM-6 will serve as an in-orbit spare until the elliptical satellites are retired.

Briskman said the three elliptical-orbit spacecraft may be kept as in-orbit spares, or may be put to another use that he did not disclose.

When work first began on FM-6, it was expected to result in a satellite to be flown in the same highly inclined elliptical orbit as the three first Sirius satellites. But following the merger of XM Satellite Radio with Sirius, the decision was made to modify FM-6 for geostationary operations.

Peter B. de Selding was the Paris Bureau Chief for SpaceNews.