Telemetry, Tracking, and Command Link Performance Using the USB/STDN Waveform

The USB/STDN waveform (unified S-band/spaceflight tracking and data network) has been used to provide telemetry, tracking, and command (TT&C) services for many satellites in addition to NASA near-earth orbital missions. The tracking and command signals are used in the forward link, or the uplink (earth-to-space), while the spacecraft tracking and telemetry signals are used in the return link, or the downlink (space-to-earth). An uplink with a square-wave PRN ranging (pseudorandom noise) or tone-ranging signal is used to improve the carrier acquisition, and ranging performance. Typically the downlink signal is a heavily filtered signal that is needed to reject the side lobes or harmonics of the square waves to satisfy the NTIA (national telecommunications and information administration, a part of US department of commerce) emission spectrum requirements. This filtered downlink signal, sometimes called "sine-wave," intentionally reduces adjacent channel interference (ACI) at earth station receivers. In these space links, it is very important to analyze the modulation losses that affect the TT&C link performance. Currently available analyses assume an unfiltered forward link signal (uplink or earth-to-space link), and a heavily filtered return link (downlink or space-to-earth link). In this paper the modulation losses applicable to the USB/ STDN signal with and without spectral filtering are derived for both the forward link (FWD) and the return link (RTN). As examples of the use of the derived modulation loss expressions, four sets of link analyses for the USB link are given in this paper: unfiltered FWD/unfiltered RTN, unfiltered FWD/filtered RTN, filtered FWD/unfiltered RTN, and filtered FWD/ filtered RTN links.