1. 10/7/2015 1 Innovative Solutions International Satellite Navigation Division ION NTM 01 Capabilities of the WAAS and EGNOS For Time Transfer SBAS, an Alternate to GPS for Time Bill Klepczynski Innovative Solutions International CGSIC 29 March 2001

2. 10/7/2015 2 Innovative Solutions International Satellite Navigation Division ION NTM 01 Outline F Timekeeping Introduction & Terminology F Time Distribution F Time Transfer  Time Synchronization  Time Coordination F SBAS Capabilities in Time Distribution F SBAS Capabilities in Time Transfer F Conclusions

3. 10/7/2015 3 Innovative Solutions International Satellite Navigation Division ION NTM 01 Timekeeping Introduction F Navigation systems have always been related and associated with timekeeping ever since Harrison developed his No. 4. F WAAS and EGNOS are the latest in the evolution of navigation systems. They are Satellite Based Augmentation Systems (SBAS) to GPS. F SBAS have some interesting properties that make them potential leading contributors to timekeeping even in their early stages of development.

4. 10/7/2015 4 Innovative Solutions International Satellite Navigation Division ION NTM 01 Timekeeping Introduction F Timekeeping is concerned with Time Distribution and Time Transfer. F Time Distribution = Satellite and 1 User  User interested in getting time F Time Transfer = Satellite and 2 Users  Users interested in comparing clocks F Both Time Distribution and Time Transfer imply Time Synchronization and Time Coordination

5. 10/7/2015 5 Innovative Solutions International Satellite Navigation Division ION NTM 01 Time Synchronization F In Synchronization, users utilize information gained from their observations and from other users to physically set their clock to some reference time scale. F GOAL - Users’ clocks are set running identically to some reference clock. F Information to check the synchronized clock needs to be gathered at regular intervals. F Systems may run off from reference, but the degree of run off depends on precision and accuracy of updates and quality of clock.

6. 10/7/2015 6 Innovative Solutions International Satellite Navigation Division ION NTM 01 Time Coordination F In Coordination, users only note the difference between their local clock and some reference time scale. F Users then use this information to relate other measurements to the reference time scale. F Again, an information link must be maintained to verify and check the behavior of the users local reference clock with respect to the reference time scale.

7. 10/7/2015 7 Innovative Solutions International Satellite Navigation Division ION NTM 01 Time Distribution F How does clock on wall get its time? F Navigation system distributes its reference time scale (GPS Time) and, sometimes, its relation to other time scales (UTC). F Users can set their clocks to the navigation system reference time or simply note the difference.

8. 10/7/2015 8 Innovative Solutions International Satellite Navigation Division ION NTM 01 Time Transfer F User works with one or several other users. F Each users note the difference between their local clocks and the navigation reference time scale. F Users must make their observations at the same instant of time and use the same satellite in order to minimize common errors.

9. 10/7/2015 9 Innovative Solutions International Satellite Navigation Division ION NTM 01 SBAS and Timekeeping F WAAS or a Satellite Based Augmentation System (SBAS) can contribute in all timekeeping areas F SBAS still in its early stages of development F Will show current capabilities of an SBAS

10. 10/7/2015 10 Innovative Solutions International Satellite Navigation Division ION NTM 01 Advantages of SBAS in Timekeeping - I F Main advantage of an SBAS is that it remains at approximately same point in sky, i.e., at same elevation and azimuth. Can use high gain antenna, such as Feed Horn or Dish (see earlier paper). F 2 users can simultaneously track phase of GEO for long periods of time. This allows detection of pathological behavior in timed systems in real time (see earlier paper). F Can make observations throughout the night when ionosphere is at a minimum of activity !

11. 10/7/2015 11 Innovative Solutions International Satellite Navigation Division ION NTM 01 Advantages of SBAS in Timekeeping - II F A world wide timed GNSS (network of SBAS) will dramatically change some traditional timing concepts. F Can do continuous “code - carrier phase” tracking. F Several different satellites that can be used. F Several different upload stations F Signals are under independent control from GPS

12. 10/7/2015 12 Innovative Solutions International Satellite Navigation Division ION NTM 01 Current GPS Data F From USNO Time Service web site, one can get recent GPS timing data for comparison with WAAS and EGNOS data. F Data is obtained with a single frequency timing receiver. F Klobuchar Ionospheric model is basis for data reduction. F All satellites are plotted on the graph.

13. 10/7/2015 13 Innovative Solutions International Satellite Navigation Division ION NTM 01 USNO MC - GPS Time (all satellites) JAN 2001  = 7.7 ns.

14. 10/7/2015 14 Innovative Solutions International Satellite Navigation Division ION NTM 01 Time Transfer between 2 Labs NRCUSNO WAAS EGNOS USNO - WAAS = X NRC - WAAS = A USNO- NRC = X - A USNO - EGNOS = Y NRC - EGNOS = B USNO- NRC = Y - B

15. 10/7/2015 15 Innovative Solutions International Satellite Navigation Division ION NTM 01 NRC - USNO (6/06/2000) using WAAS, midnight, 30 minutes, relative, s.e.= 1.3 ns

16. 10/7/2015 16 Innovative Solutions International Satellite Navigation Division ION NTM 01 NRC - USNO (6/06/2000) using EGNOS, midnight, 30 minutes, relative, s.e. = 2.1 ns.

17. 10/7/2015 17 Innovative Solutions International Satellite Navigation Division ION NTM 01 Time Transfer between 2 SBAS NRCUSNO WAAS EGNOS USNO - WAAS = X USNO - EGNOS = Y WAAS - EGNOS = Y - X NRC - WAAS = A NRC - EGNOS = B WAAS - EGNOS = B - A

18. 10/7/2015 18 Innovative Solutions International Satellite Navigation Division ION NTM 01 WAAS - EGNOS (06/06/2000) using USNO, 30 minutes, s.e. = 1.2 ns.

19. 10/7/2015 19 Innovative Solutions International Satellite Navigation Division ION NTM 01 WAAS - EGNOS (06/06/2000) using NRC, 30 minutes, s.e. = 1.0 ns.

20. 10/7/2015 20 Innovative Solutions International Satellite Navigation Division ION NTM 01 Discussion F Keep in mind that WAAS is semi-operational F EGNOS does not transmit continually F EGNOS transmissions are for testing different segments of the program F There are some problems with early available software that will be overcome with time.

21. 10/7/2015 21 Innovative Solutions International Satellite Navigation Division ION NTM 01 Conclusions F Even though the data presented here is selective, an SBAS appears to be a significant improvement over GPS. F Several different timekeeping labs could be continuously tracking several GEOs, thus allowing for better coordination of different national time scales. Data seems to indicate that sub-nanosecond time transfer should be achievable between laboratories on a continuous basis. F Several stations, with the proper choice antenna, can track several different GEOs. This would allow either synchronization or coordination of different SBAS. Data seem to indicate that nanosecond level coordination between SBAS is achievable.