Effects of turnaround command on SGLS down carrier, ranging and telemetry services

This paper deals with command and control of a satellite with the effects of turnaround-command signal (TAC) and intermod products (IMs) in the downlink services. Unlike previous papers [2, 3, 5], this paper will show a complete uplink and downlink performance with and without the effects of TAC in the downlink services. For a small uplink command modulation index of 0.3 radians, our analyses for both USB and SGLS signals show that the loss of downlink power to TAC and IM is less than 10% or about 0.5 dB. However, when the uplink command modulation index increases to a nominal operating value of 0.9 radian, the loss of downlink service power to TAC and IM increases to 40% which is equivalent to a loss of about 2.2 dB. This increase in the TAC and IM powers will reduce the power to all desired downlink services, which in turn could result in reduced or denial of services to many ground users. For simplicity in derivation of modulation losses, and for adjacent channel interference mitigation, the uplink and downlink signals are assumed to be filtered.