Ultra-Low Power Time Synchronization Using Passive Radio Receivers

Yin Chen† Qiang Wang* Marcus Chang† Andreas Terzis†

†Computer Science DepartmentJohns Hopkins University

*Dept. of Control Science and EngineeringHarbin 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 Launch Time

MSF Britain 60 kHz 1966

BPC China 68.5 kHz 2007

TDF France 162 kHz 1986

DCF77 Germany 77.5 kHz 1959

JJY Japan 40, 60 kHz 1999

RBU Russia 66.66 kHz 1965

WWVB USA 60 kHz 1963

LF Time Signal Radio Stations

Radio Controlled Clocks & Watches

This work will test DCF77 and WWVB

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 1012

– 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

60 seconds

Bit value = 0 Bit value = 1 Marker bit

• Each frame lasts 60 seconds• Each bit lasts 1 second

2010-5-2406:11:00 UTC

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

• 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 timeReading bits & Writing to the uart

Drop-in replacement of GPS

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• For DCF77, the distance is 700 km (indoor), mainly

ground wave

Near the edge of the coverage map

Outdoor Experiment

Availability

WWVB OutdoorWWVB 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

Motes were placed together under a tree.

Avg Hourly Variation

Max Hourly Variation

Indoor 0.09 ppm 0.67 ppm

Outdoor 0.36 ppm 6.68 ppm

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

FTSPTime signal

receiverSync error in

milliseconds range

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