Filter
Top Products
DIGITAL RADIO GUIDE SATELLITE TRANSMISSION - SIRIUS / XM
70
reduces the need for terrestrial repeaters and lowers the probability of outages from
blockage and foliage attenuation. Currently, Sirius employs approximately 100 repeater
sites and XM approximately 800.
5.2.1 Sirius Overview
The first Sirius spacecraft was launched on July 1, 2000. Exactly five months later, on
December 1, the third spacecraft was launched, completing the three satellite S DARS
(Satellite Digital Audio Radio Service) constellation. The three spacecraft are deployed in
inclined, elliptical, geosynchronous orbits, which allow seamless broadcast coverage to
mobile users in the contiguous United States. Terrestrial broadcast repeaters provide
service in urban cores. The system is in operation, providing the first ever S-DARS
service.
The constellation design results in satellite ground tracks over North America with two
satellites always above the equator. High elevation look angles from the mobile ground
terminals to the satellites minimize performance degradation due to blockage, foliage
attenuation and multipath.
The spacecraft were built by Space Systems/Loral using the 1300 bus modified for
operation in high inclination orbits. Each spacecraft was launched using a dedicated
Russian Proton booster. The satellite payload is a bent pipe repeater using 7.1 GHz for
the uplink and 2.3 GHz for the broadcast transmission. The repeater high power
amplification stage consists of 32 Traveling Wave Tube Amplifiers phase combined to
yield a total RF output power of nearly 4 kW at saturated operation. The satellite
antennas are mechanically steered to maintain the transmit beam centered on CONUS
(Contiguous United States) and the receive beam centered on the uplink earth station
located in Vernon Valley, New Jersey.
The satellite payload design and performance are described. The principal spacecraft
bus systems are described with emphasis on improvements made for operation in the
inclined, elliptical geosynchronous orbits.
The two active satellites transmit the same signal at different frequencies with a 4-second
delay between them, which is inserted at the uplink earth stations. In the urban core of
large cities where satellite blockage can be very high, terrestrial transmitters rebroadcast
the satellite signal. The satellites’ different orbital positions, transmission frequencies and
signal delay provide the diversity while the receivers’ equalizer and maximal ratio
combiner (e.g., sums the two satellite and terrestrial repeater signals) provide the other
listed techniques. Moreover, the achievement of high elevation angle is an extremely
important attribute, and its achievement required the adoption of a unique orbital
configuration.
Originally Sirius Satellite Radio had planned for geostationary satellites at 80° and 110°
West longitude. The resulting low elevation angles to mobile users in the northern third
of CONUS would cause service outages whose number and duration result in an
unsatisfactory quality of service irrespective of the diversity employed and the satellite
Effective Isotropic Radiated Power (EIRP) level. Satisfactory service might be achieved
by deploying an enormous number of terrestrial repeaters but this was judged impractical
given the economic and regulatory issues involved. Consequently, an orbital
constellation was designed by Sirius Satellite Radio and implemented by Space
Systems/Loral (SS/L) that provides high elevation angles over this critical area.