Pseudolite Network for Space Users

1. Pseudolite Network for Space Users A GPS Augmentation Study for NASA Headquarters Code M-3 Thomas R. Bartholomew Kevin L. Brown Al Gifford

2. Outline • Introduction • Purpose of GPS Augmentation Study • Potential joint timekeeping infrastructure • Proposed Architecture • Benefits to NASA & LORAN • Overview of Pseudolite Network • Progress to Date • Future Plans Bartholomew, Brown & Gifford

3. Introduction • At the ILA 2002 Conference, an idea for a Joint NASA/LORAN Satellite Time Transfer Service was described • The idea was to exploit the potential LORAN independent timekeeping capability and the NASA capability of measuring clocks globally from the International Space Station (ISS) • From the NASA perspective the idea is on hold due to flight schedule uncertainties of ISS clock experiments • However, the potential of the distributed timekeeping capability of the LORAN system remains of interest to NASA • This presentation describes an on-going study which explores potential synergy between the resources of NASA and LORAN for timekeeping and dissemination Bartholomew, Brown & Gifford

4. Study Purpose • Explore potential synergy between NASA and the LORAN community in the area of distributed timekeeping • Synergetic combined timekeeping architecture • Benefits to NASA: Enables a GPS augmentation pseudolite network for NASA application above the GPS constellation • Potential benefits to the LORAN community Bartholomew, Brown & Gifford

5. Synergetic Distributed Timekeeping • NASA provides the ability to measure clocks across all participating sites to the 1 nanosecond level • Near-term: Time transfer utilizing the Telemetry Data Relay Satellite System (TDRSS) • Far-term: Time transfer via the International Space Station • LORAN provides distributed timekeeping infrastructure • Clocks upgraded to Agilent 5071s (approximately 100) • Clock measurement capability upgraded to state-of-the-art; includes and GPS receiver for cross-site measurement • Clock management includes ensembling of site clocks and a possible calculation of a system wide distributed time scale Bartholomew, Brown & Gifford

6. TDRSS Provides Time Sync Service to Host Sites LORAN sites are optimum hosts because of good clocks and robust operations Bartholomew, Brown & Gifford

7. Benefits to NASA • Enables pseudolite network for augmentation of GPS services to space users • Provides access to timekeeping resources which will support future missions beyond GPS orbits • Near-term: Improved ephemeris for TDRSS satellites • Provides robust GPS global monitoring and communication system for JPL real-time data services Bartholomew, Brown & Gifford

8. NASA Global Pseudolite Network (supporting satellites outside GPS constellation) GPS III GPS III GPS III Up to 60 Sites Hosting NASA Pseudolite Package requirements for host sites are good clocks with a known offset to GPS Time Bartholomew, Brown & Gifford

9. Benefits to LORAN • Provides significantly improved stand-alone (GPS independent) capability • Inter-site synchronization at the 1 ns level • Robust independent timekeeping & dissemination capability • Direct source of UTC will support applications requiring 100ns wrt UTC • Frequency stability will support comms applications • Additional use of LORAN facilities – support to NASA Bartholomew, Brown & Gifford

10. Overview of Pseudolite Network • NASA operated pseudolite network, on-time with GPS, broadcasting time, range & other data to NASA satellites - including TDRSS for relay to other users in space • A subset of the pseudolites located at LORAN sites would have the benefit of access to the local (and potential LORAN system-wide) clock ensemble • The pseudolite system would, in addition to the listed functions, also provide the capability of synchronizing the LORAN sites via two-way satellite time/frequency transfer (TWSTFT) using TDDRS Bartholomew, Brown & Gifford

11. Components of Pseudolite Package • Reconfigurable transmitter • A spread spectrum transmitter providing a GPS-like ranging signal • TDRSS S-band frequencies will be utilized in test and demo phase • Eventual frequencies near GPS L1, L2 and L5 are under study • GPS monitoring Subsystem • Raw pseudorange • Local ionospheric maps required for single frequency pseudolite • Two-way time transfer subsystem • Provides time comparison between host sites and TDRSS ground stations • GPS data collected at host site will be transmitted in real-time to TDRSS ground station Bartholomew, Brown & Gifford

12. Subcomponent Status Bartholomew, Brown & Gifford

13. Progress to Date • Development of concept; initial studies, ILA paper in 2002, PTTI Paper in Dec 2002, Frequency Control Paper in Apr 2003, STS107 experiment (LPT), measurements, and future experiment plans • Ionospheric mitigation demonstrated at S-band frequencies • On-going discussions with NIST and industry (TSC) regarding timescale management and distribution Bartholomew, Brown & Gifford

14. Future Plans • Demo pseudolite concept thru TDRSS in Nov/Dec 03 and time-transfer thru TDRSS Jul/Aug 04 • Process of identifying hosts (with good “geometry”) • USCG, NIST, USNO, etc. – would like to expand that dialog to include others • Invitation from NASA management to USCG for support in the continuing study of utilization of National and International LORAN assets • Study to include near-term TDRSS time transfer experiments Bartholomew, Brown & Gifford