Second-hand satellites may gain new voice

Line-Of-Sight (LOS) measurements
Antenna mounted on van to measure signal reception
5 February 2003

ESA engineers are proposing a technique to enable a digital satellite radio service for European drivers - without the need to launch a single new satellite into orbit.

Commercial digital satellite radio is already a reality in the United States, using a costly set of dedicated satellites.

The rival American services allow subscribing drivers to choose between numerous near-CD quality radio channels without tune-out or static.

Two parallel ESA studies have examined a lower-cost method of providing in-car Europe-wide satellite radio, along with supplemental text, pictures and video data. ND Satcom AG (prime contractor), DLR, IMST and SES Astra conducted one of the studies while Alcatel Space (prime contractor), Frauenhofer Gesellschaft, Skygate and SES Astra performed the other.

"This service would include music and voice data," said ESA engineer Rolv Midthassel from the Technology Projects Division of the Telecommunication Department. "Plus additional data could be displayed on-screen such as information on songs, traffic and weather forecasts, and other services dedicated to car drivers."

It promises to be much cheaper to set up than US satellite radio, because it requires no new expensive satellite launches. Instead the proposal is to reuse existing TV satellites nearing the end of their operating life.

Once in position, 35,000 km away in space, TV satellites will remain in orbit forever, but their useful life amounts to 15 years or less. Onboard thrusters must keep each satellite pointed precisely in geostationary orbit so they stay lined up with fixed-position Earth-based receivers.

However once the thrusters' propellant runs out the satellites drift out of correct orbit, and are left useless for TV broadcast applications. But further life can be squeezed from a low-propellant TV satellite switched over to mobile digital radio broadcasting where precision position control is less important.

Most thruster propellant is expended correcting satellite attitude in the north-south direction. But if station-keeping is limited to the east-west axis then satellite lifetime could be extended by some five years.

The satellite's position would oscillate across the sky by a few degrees. But vehicle-mounted digital radio antennas would keep track of the satellite as it moves, just as they would maintain contact with it as the car bearing the antenna moves across the landscape.

Antenna
The antenna

"Satellite reception is frequently shadowed by trees, rain, or cloud or blocked altogether by mountains, tunnels or tall buildings," explained Midthassel. "In urban areas the studies indicate that the signal is blocked an average 30 per cent of the time, maybe for several minutes at a time."

These reception gaps make real-time broadcasting impracticable. Instead the service would operate on a cache system – data files are stored by the receiver for later playback.

"Sophisticated interleaving, data coding and large signal margins should enable the useful data to be reconstructed even when some of the signal has disappeared," said Midthassel.

Interleaving is a method whereby the burst of erroneous data arising from blockages and shadowing will be spread out in time and mixed with correctly received data bits. And coding data will then enable the receiver to interpret it in the correct context.

Studies indicate a useful data rate of a million bits per second per satellite transponder. With each satellite having several transponders, this makes the system performance comparable or even superior to US services.

"The receiver cache will contain files like songs, news flashes, and discussion programmes," Midthassel added. "Based on a pre-set user profile an individually-tailored radio programme could be constructed out of available files."

"Alternatively the user could choose which service he wants from the cache – so you could play the latest news then and there instead in a half-hour's time. Exiting the car to fill up with petrol, you could halt the news then restart it once you're back."

Test reception measurements have been carried out by both teams monitoring signals from SES Astra satellites.

The next steps come this year, with plans to design a low-cost fully electronically steerable antenna as well as develop a suitable gateway transmitter and user receiver which will serve as a system demonstrator for extended field testing. An optimised demonstration system will be complete in about two-and-a-half years leading to a commercial prototype in about five years from now.

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