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

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?

How did we disseminate time information in history?

Time Ball

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

Current Day Time Sources

Station Country Frequency

MSF BPC TDF DCF77 JJY RBU WWVB Britain China France Germany Japan Russia USA 60 kHz 68.5 kHz 162 kHz 77.5 kHz 40, 60 kHz 66.66 kHz 60 kHz LF Time Signal Radio Stations

Launch Time

1966 2007 1986 1959 1999 1965 1963 This work will test DCF77 and WWVB Radio Controlled Clocks & Watches

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

WWVB Radio Station

• • Located near Colorado, operated by NIST Covers most of North America

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

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

WWVB Code Format

• • Each frame lasts 60 seconds Each bit lasts 1 second 60 seconds 2010-5-24 06:11:00 UTC Bit value = 0 Bit value = 1 Marker bit

Time Signal Receiver Design

• Requirements – Low power consumption – High accuracy – Low cost – Small form factor

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 Most of the time Reading bits & Writing to the uart Costs (as of 2010) • CME6005: $1.5

• • PIC16LF1827: $1.5

Antenna: $1 • Total: $4 Drop-in replacement of GPS Time in NMEA format & 1-pulse-per-second

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

Power Consumption

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 Near the edge of the coverage map For DCF77, the distance is 700 km (indoor), mainly ground wave

Outdoor Experiment

Availability

WWVB Outdoor WWVB Indoor DCF 77 Indoor

•

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

Comparison with FTSP

• FTSP sync accuracy depends on resync frequency – Because clock frequency varies over time

Clock Frequency Variations

Indoor Outdoor Avg Hourly Variation 0.09 ppm 0.36 ppm Max Hourly Variation 0.67 ppm 6.68 ppm Motes were placed together under a tree.

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 Sync error in milliseconds range FTSP Time signal receiver

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

Applications

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

Synchronous MAC Protocols

• Modify LPL – Sleep interval is divided into slots

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

Thank you!

Signal Generator

• 50 meters coverage