Technology: System Overview | Spacecraft | Earth Stations

Technology

Spacecraft

On-Board Baseband Processing (BBP) and Switching

ACTS Hopping Spot Beams The multibeam antenna was designed to provide dynamic coverage with hopping spot beams that can be programmed to cover a sequential set of spots and dwell long enough to communicate with users in each spot. This allows several users to transmit and receive at the same frequency on a time-shared basis allowing efficient frequency reuse. This time division, multiple access (TDMA) technique requires a switching system on board the spacecraft to interconnect the beams and route messages.

One method on ACTS to connect users is a Baseband Processor (BBP). The BBP is a high-speed digital processor on the satellite that provides on-demand circuit switching to efficiently route traffic among small user terminals in a full mesh network. By using a programmable switch onboard the satellite, the system can easily support rapidly changing traffic loads. The BBP mode could support integrated voice, video, and data with information throughput rates of 1.79 Mbps (28-64kbps channels) using 1.2 meter user terminals.

The BBP uses on-board stored, baseband-switched TDMA. During one TDMA frame time, the beams hop to many locations dwelling long enough to pickup or deliver the required traffic. On the satellite, the received signals are demodulated, decoded as required, temporally stored in memory, routed on a 64-kbit individual circuit basis, remodulated, encoded if required and transmitted in the proper downlink spot beam. Two uplink and two downlink beams operate simultaneously and independently to provide flexible communications assigning bandwidth on-demand to each small user terminal in the network.