The ESBT passenger is a "spread-spectrum" transponder (i.e. a combined transmitter and receiver) operating in the 2-GHz band, also known as the S-band. The ESBT was produced by Alcatel Espace under an ESA contract.

ESBT is an experimental prototype designed to test intersatellite links, in this case between SPOT 4 in low-earth orbit and the Artemis telecommunication satellite in geostationary orbit. The package will provide experimental telemetry and command links between SPOT 4 and the Artemis ground segment in Redu, Belgium.

The ESBT passenger comprises:

• the transponder proper transmitting telemetry and receiving commands on a 2-GHz carrier at 4 kbit/s,
• a tracking antenna offering hemispherical coverage, mounted on the satellite's anti-earthward panel, for optimal link efficiency,
• a command and control interface controlling ESBT operations.

The ESBT is independent of the host satellite's mission, its only direct contact with the Spot system being through the SPOT 4 operations control centre, or CMP. Commands uplinked to SPOT 4 by the CMP via the ESBT are not actually used by the satellite; nor is the telemetry downlink used as a nominal, operational capability. Rather, the ESBT's main mission is to assess the quality of the low-data-rate (4 kbit/s) forward and return links (i.e. command uplink and telemetry downlink, respectively) between the experimental Artemis ground segment, acting as a dummy CMP, and SPOT 4. These experiments will be entirely managed by the ESA station in Redu, Belgium, without the least involvement of the SPOT 4 CMP, save for the programming of orbital slots during which the ESBT may be switched on. The Redu station will also evaluate the quality of range data generated using the ESBT.

In addition to these experiments, the ESBT will, however, be used operationally to relay SPOT 4 housekeeping telemetry via Artemis to the SPOT 4 CMP, the Artemis downlinked being received by the SRIP (station for receiving imagery via Pastel) facility at Aussaguel, 20 km from Toulouse.

This capability will significantly enhance the SPOT 4's housekeeping telemetry coverage. Whereas each station in the CNES TT&C network provides just 10 to 15 minutes' contact time per pass, the ESBT will offer 30 to 40 minutes continuous contact as a result of the considerably longer periods of contact between SPOT 4 and Artemis (and considerably longer periods of contact between SPOT 4 and Artemis). This will be a major step forward, particularly when monitoring onboard operations that must be watched closely and continuously (e.g. during recovery operations after a period in safe mode).

The spread-spectrum technique involves mixing a useful digital signal (here at 4 kbit/s) with a complex pseudo-random code (in this case at 3 Mbit/s).

Advantages include:

• elimination of interference with other systems operating in the same frequency band at high power levels,
• protection (in this case of command and telemetry links) against interference by other links of the same type (whether spread-spectrum or not) thanks to a pseudo-random code assigned specifically to this link and which can only be decoded using the appropriate key.

The interface with the ground segment uses a modulator/demodulator, or modem, similar to those used to connect a computer to a telephone line, produced by Finnish company Elektrobit. Modems have been installed at both the Redu station in Belgium and at the SRIP station at Aussaguel.

The ESBT technology is expected to find important applications in manned space programmes (shuttles, capsules, space stations, etc.) where reliable, long-term links are essential and even vital.

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page updated on the 00-06-06