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Turnaround command effects on USB and SGLS satellite downlinks

Many satellites using commercial off-the-shelf USB and SGLS transponders such as the L3-Com CXS 2000 do not have turnaround command (TAC) suppression in their downlinks. Without TAC suppression, these satellites exhibit greater downlink service modulation losses for carrier, ranging, and telemetry. Depending on the selected uplink command modulation index and the turnaround ratio, these additional modulation losses could vary from 0.1 dB to 3 dB (for command mod indices less than 1 radian). They are due to the allocation of downlink power to the TAC, and partly to an increase in intermodulation (IM) power. For an uplink command modulation index of 0.3 radians, our calculations for both USB and SGLS signals show that the loss of downlink power to TAC and IM is less than 10%. However, when the uplink command modulation index is increased to a nominal operating value of 1.0 radian, the loss of downlink service power to TAC and IM becomes 40%. This large loss of downlink power to the TAC and IM increases the modulation losses for other downlink services, which could result in denial of services such as telemetry and ranging to ground users with small antennas. This paper shows that suppression of turnaround command will result in lower TAC and IM losses, which will in turn improve downlink services with higher link margins. These more robust downlink margins permit the use of a smaller and cheaper high-power amplifier (HPA) in the satellite transmitter12.