1. Ultra-Low Power Time Synchronization Using Passive Radio Receivers Yin Chen † Qiang Wang * Marcus Chang † Andreas Terzis † † Computer Science Department Johns Hopkins University * Dept. of Control Science and Engineering Harbin Institute of Technology

2. Motivation Message passing time synchronization – Requires the network be connected – Requires external time source for global synchronization Is there a low-power and low cost solution?

3. How did we disseminate time information in history?

5. Time Ball

7. Since half a century ago, we started to use RF time signals.

8. Current Day Time Sources StationCountryFrequency Launch Time MSFBritain60 kHz1966 BPCChina68.5 kHz2007 TDFFrance162 kHz1986 DCF77Germany77.5 kHz1959 JJYJapan40, 60 kHz1999 RBURussia66.66 kHz1965 WWVBUSA60 kHz1963 LF Time Signal Radio Stations Radio Controlled Clocks & Watches This work will test DCF77 and WWVB

9. Contributions Ultra-low power universal time signal receiver Evaluations on time signals availability and accuracy in sensor network applications Applications using this platform The antenna is 10 cm in length Smaller ones are available but we have not tested on our receiver

10. WWVB Radio Station Located near Colorado, operated by NIST Covers most of North America

11. WWVB Time Signal 60 kHz carrier wave Pulse width modulation with amplitude-shift keying NIST claims – Frequency uncertainty of 1 part in 10 12 – Provide UTC with an uncertainty of 100 micro seconds

12. WWVB Signal Propagation Part of the signal travels along the ground – Groundwave : more stable Another part is reflected from the ionosphere – Skywave : less stable At distance < 1000 km, groundwave dominates Longer path, a mix of both Very long path, skywave only

13. WWVB Code Format 60 seconds Bit value = 0 Bit value = 1 Marker bit Each frame lasts 60 seconds Each bit lasts 1 second 2010-5-24 06:11:00 UTC

14. Time Signal Receiver Design Requirements – Low power consumption – High accuracy – Low cost – Small form factor

15. Core Components CME6005 40-120 kHz, can receive WWVB, DCF77, JJY, MSF and HBG less than 90 uA in active mode and 0.03 uA when standby PIC16LF1827 600 nA in sleep mode with a 32 KHz timer active 800 uA when running at 4 MHz Costs (as of 2010) CME6005: $1.5 PIC16LF1827: $1.5 Antenna: $1 Total: $4 Time in NMEA format & 1-pulse-per-second Most of the time Reading bits & Writing to the uart Drop-in replacement of GPS

16. Decoder Loop Every second – MCU decodes the next bit from the signal receiver Every minute – MCU decodes the UTC time stream – MCU sends the time stream to the uart

17. Power Consumption

18. Experiment Configurations Multiple motes, each connected to a receiver One master mote All motes are wired together – Master mote sends a pulse through a GPIO pin every 6 seconds – All motes timestamp this pulse as the synchronization ground truth For WWVB, the distance is 2,400 km (indoor & outdoor), mainly sky wave For DCF77, the distance is 700 km (indoor), mainly ground wave Near the edge of the coverage map

19. Outdoor Experiment

20. Availability

21. WWVB Outdoor WWVB Indoor DCF 77 Indoor

22. Accuracy The differences of the time readings at the motes when the master mote sends the pulses Clock frequencies vary more in outdoor experiment 50%80%90% Indoor< 1.3 ms< 2.8 ms< 3.9 ms Outdoor< 1.4 ms< 3.0 ms< 4.3 ms

23. Comparison with FTSP FTSP sync accuracy depends on resync frequency – Because clock frequency varies over time

24. Clock Frequency Variations Motes were placed together under a tree. Avg Hourly Variation Max Hourly Variation Indoor0.09 ppm0.67 ppm Outdoor0.36 ppm6.68 ppm

25. Power Consumption What happens as sync interval T increases? Schmid et al. observed that FTSP syncs in the millisecond range when using T = 500 seconds interval FTSP Time signal receiver Sync error in milliseconds range

26. Qualitative Observations Steel frame buildings completely shield the time signal Brick buildings allow signal reception Laptops (when using AC power), oscilloscopes can easily interfere the time signal within a few meters – We used a portable logic analyzer connected to a laptop running on its battery

27. Applications Synchronous MAC Protocols Latency Reduction Sparse Networks Drop-in Replacement for GPS Network-Wide Wakeup Failure-Prone Sensor Networks

28. Synchronous MAC Protocols Modify LPL – Sleep interval is divided into slots

29. Summary Lower power consumption in the millisecond range Support sparse networks Provides an appropriate solution to the milliseconds and seconds range – GPS is an overkill – RTC drifts a few minutes per year even with temperature compensation

30. Thank you!

31. Signal Generator 50 meters coverage