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
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
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 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
• 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
Most of the time
• 600 nA in sleep mode with a 32 KHz timer active
Reading bits & Writing to the uart
• 800 uA when running at 4 MHz
• 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 &
Near the edge of the coverage map
outdoor), mainly sky wave
• 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%
Indoor
80%
90%
< 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
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