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Center Independent Research & Development: GSFC IRAD

TASS Signal Specification

Completed Technology Project

Project Description

Conceptual image of on-orbit reception of TASS beacon and GPS signals on the International Space Station (ISS).

The Tracking Data Relay Satellite System (TDRSS) Augmentation Service for Satellites (TASS) is a unique mission enabling service that could be provided by the current Space Network (SN) or the future Space Mobile Network (SMN). TASS will provide a global S-band beacon to disseminate global GPS differential corrections, GPS constellation status, Earth orientation parameters, space weather alerts, low rate forward commands, TDRS ephemeris, and a GPS compatible radiometric source to allow missions that receive TASS to improve the accuracy and reliability of on-board navigation and to allow for automation of many aspects mission operations. To date, work remains to finalize the TASS signal specification and to develop the necessary software/firmware to receive and process the signal.

Preliminary development of the TASS system concept and design included establishing mature ICDs with the various data sources, developing a TASS Data Integrator (TDI) unit to subscribe and collate the various data into the TASS message, and to combine the GSFC Navigator GPS receiver firmware with Space Network compatible TDRSS transponder firmware in a single FPGA platform. 

This FY16 IRAD will build on the current success to further develop the end-to-end TASS system by meeting the following objectives:

  1. Execute a dynamic TASS link budget in order to finalize the TDRS MAF spoiled beam configuration and determine the maximum achievable data rate on the TASS beacon
  2. Finalize I/Q power ratio choice to enable weak-signal (low ES/N0) tracking necessary for exploitation of the TASS LDPC ½ error correcting code as well as enabling fast-acquisition of TASS signal, either via command channel (I) or dataless pilot channel (Q)
  3. Finalize code division multiple access (CDMA) spreading codes as part of the TASS signal specification. Demonstrate suitable auto correlation properties and cross correlation properties against NASA CDMA (SNIP) codes
  4. Design and code (in software or firmware) appropriate circular correlation or massively parallelized acquisition scheme to acquire TASS signal with zero a-priori information (no Doppler aiding or forward Doppler compensation)
  5. Use software radio device to modulate TASS signal at desired RF frequency in lab (2106.4 MHz)
  6. Use software radio device to demonstrate acquisition engine performance over a range of incident C/N0s and dynamics

Finalization of the TASS signal specification is necessary to proceed with further development of TASS. Once the GPS and TDRSS firmware is integrated into a common platform, the next step will be modification of the TDRSS waveform to both receive the TASS signal and to process the TASS data message. At the end of this FY16 IRAD, the TASS system (including both ground and user elements) will be considered TRL6.

This work is applicable to the S-band (2.0 to 2.2 GHz) forward and return space-to-space spectrum allocations held by NASA.

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