The RT Logic TDRS Link Test System (TSIM) generates all the spread spectrum signals used by the NASA Tracking and Data Relay Satellite System (TDRSS). The TSIM was built for NASA as an economical alternative to the TDRSS User RF Test Set (TURFTS) to test payloads and transponders that will access TDRSS while orbiting. RT Logic can easily modify the basic TSIM for other applications.
The TSIM receives command data and generates the TDRSS spread spectrum forward link signal. The forward link signal contains the command data modulo-2 added to a command PN sequence and a ranging PN sequence. The two PN sequences are modulated in an unbalanced QPSK format. The TSIM receives the return link RF signal, and despreads and demodulates either the Data Group 1 (Mode 1, Mode 2, or Mode 3) return link PN codes, or demodulates Data Group 2. The return link (Data Group 1 Mode 1, 2, or 3) signal is demodulated in a staggered QPN format known as Staggered Quadrature PN (SQPN), and allows data to be placed on both the I and Q phases. The TSIM recovers this data and outputs data and clock for both I and Q streams. Epochs from the forward and return links are used for range measurements. The return link (Data Group 2) signal is demodulated in a Staggered QPSK (SQPSK), and allows data to be placed on both the I and Q phases. The TSIM recovers this data, and outputs data and clock for both I and Q streams or interleaves the I and Q streams into a single data stream. A Doppler simulator allows any Doppler profile to be easily simulated on the forward link.
As a fully integrated system (typically rack-mounted), the TSIM comes with a GUI for easy configuration. Ethernet control is also provided.