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Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E [email protected]
www.sematron.comMaking waves...
Application NoteRev A. Sept 11.
APPLICATION BRIEF
Better Undersea Sensing with theSA.45s Chip Scale Atomic Clock (CSAC)
KEY TAKEAWAYS
• No access to GPS means underseasensors must get time internally, usuallyfrom an OCXO
• The SA.45s CSAC’s much lower power
consumption can result in big costsavings on batteries and sensors
• The SA.45s CSAC’s better agingmeanssensors can be left unattended longerand still be accurate
• The SA.45s CSAC’s much smaller
temperature coefficientmeans CSACperforms better despite wide ambienttemperature swings
Sensors employed in undersea applications relyon precise timing to be effective. However,because time from GPS is unavailable under-water, these sensors have generally relied onOCXOs for stable and accurate time stampingwithin the sensor. Now those applications havea better option — The SA.45s Chip Scale AtomicClock (CSAC) from Symmetricom. Compared toOCXOs, the SA.45s CSACmaintains far higheraccuracy for far longer periods, uses much lesspower and maintains a much more stablefrequency despite the wide variations in temper-ature these sensors encounter.
WHY PRECISE TIMING MATTERS
One such undersea application is reflectionseismology (or simply “seismic”). In seismic,oil exploration firms place a grid of geophys-ical sensors (Figure 1) on the ocean floor tohelp determine likely spots where oil will belocated. The sensors can be dropped over theside of a ship or laid down by a remotelypiloted vehicle. The sensors can be inde-pendent or a cable can connect a row ofsensors. Each sensor typically includes ahydrophone, a geophone and an OCXO or aTCXO that is used to time stamp the data
received by the two other devices.Once the sensors are in place, a powerful airgun or array of air guns launches a sonicpulse from a ship. The ship moves in apattern that allows the air gun to be firedfrom many different angles relative to thesensor grid. Some of the pulse’s energyreflects off the ocean floor and back to thesurface, but the rest penetrates the oceanfloor, travels through the layers of rock andeventually reflects back to the sensors on theocean floor where it is detected and timestamped. Once the ship has finished itspredetermined pattern, the sensors areretrieved along with the time stamped data.
Because the sonic pulse travels at differentspeeds in different materials, the time ittakes to reflect back to the sensors off thevarious rock layers is different depending onwhich materials the pulse traverses. Whenthis timing data is post-processed, it createsa picture of the layers of rock and sedimentbeneath the ocean floor, showing which loca-tions likely hold oil or gas deposits. The moreprecise the timing, the more accurate thepictures of where oil and gas actually exist.
Sound Pulse
Ocean floor
30m to 2000 m depth
Regularly spaced
hydrophones or geophones
FIG 1 Since reflections of sonic pulses off undersea layers travel at different speeds through oil, gas, rocksand sediment, precise timing of these “bounce backs” can indicate oil or gas.
Symmetricom invented portable atomic
timekeeping with QUANTUM™, the
worldʼs first family of miniature and
chip scale atomic clocks.
Choose QUANTUM™ class for
best-in-class stability, size, weight
and power consumption.
Sandpiper House, Aviary Court, Wade Road, Basingstoke, Hampshire, RG24 8GX, UK
T +44 (0) 1256 812 222 F +44 (0) 1256 812 666 E [email protected]
www.sematron.comMaking waves...
Application NoteRev A. Sept 11.
©2010 Symmetricom. Symmetricom and the Symmetricom logo are registered trademarks of Symmetricom, Inc. All specifications subject tochange without notice. AB/Better Undersea Sensing with SA.45s CSAC/122910/PDF
SYMMETRICOM, INC.
2300 Orchard Parkway
San Jose, California
95131-1017
tel: 408.433.0910
fax: 408.428.7896
www.symmetricom.com
CSAC’S PERFORMANCE
ADVANTAGES
Symmetricom’s SA.45s CSAC (Figure 2) cangreatly improve the accuracy and reduce thecost of sensor systems, while maintaining amuch more constant frequency over timeand over wide shifts in temperature. With avolume of only 16 cm3, it is smaller thanmost OCXOs. In addition, the CSAC’s powerconsumption of 115mW is a reduction ofapproximately 10x to 20x compared to mostOCXOs, which typically consume 1.5 to 2Wsteady state.
As a true atomic clock, the SA.45s CSAC hasan aging rate of 3.0E-10/month. The unit canalso be programmed through an RS-232serial interface for control, calibration andstatus monitoring.
HOW USERS BENEFIT
These performance advantages translateinto key benefits for companies involved inseismic exploration:
• Improved accuracy over longer
deployments
During a typical deployment, sensors can beunderwater for several weeks at a time. Thisis because the ships and crews needed todeploy the sensors, take the measurementsand retrieve the sensors cannot always beoptimally scheduled. Bad weather can alsocause delays. Throughout the deployment,the OCXOs in the sensors are aging,producing a time stamping error that variesas the square of the time underwater. TheCSAC’s low aging rate — which can be1/100th of even a good OCXO — greatlyreduces these time stamping errors assensors are deployed for longer periods.
• Reduced power lowers battery and
sensor costs
Batteries are typically the biggest expense inthese underwater sensors — and thenumber of sensors in a typical grid isincreasing. Because the SA.45s CSACconsumes one-tenth to one-twentieth (oreven less, in some cases) of the power of anOCXO, it requires much less battery power— so sensors can be smaller and cost less.Alternatively, sensor manufacturers canchoose to retain the existing battery capacityand use the CSAC to create sensors formuch longer missions.
For even less power consumption, theSA.45s can also be programmed to operatein an ultralow power mode. In this mode, theCSAC’s physics package is turned off andthe unit operates as a free-running TCXO. Inthe ocean bottom’s isothermal environment,the TCXO’s drift will be almost entirely dueto aging. The physics package is thenperiodically (also programmatically) turnedback on and after warm-up (<100 sec) itredisciplines the TCXO. This mode enablesaverage power consumption levels wellbelow 50mW.
• Reduced effects from wide temperature
swings
Today most marine geophysical sensors arecalibrated to GPS on the deck of the boatbefore being dropped into the ocean.Because the water at the bottom of theocean is often just a few degrees abovefreezing, the sensor can see a temperaturechange of 30°C or more from its calibrationtemperature, causing a shift in frequencyand a linear error in time. Some sensorsuse software models to correct for thiserror, but the best approach is to minimizethe error to begin with. With a temperaturecoefficient of ±5.0x10-10 over its entiretemperature range, the SA.45s CSAC canoffer a 10x to 1000x improvement over OCXOor TCXO alternatives.
WHY SYMMETRICOM
Symmetricom® is the world’s leading sourceof highly precise timekeeping technologies,instruments and solutions. We providetimekeeping in GPS satellites, national timereferences and national power grids as wellas in critical military and civilian networks,including those that enable next generationdata, voice, mobile and video networks andservices. Our products include atomicclocks, hydrogen masers, timescalesystems, GPS instrumentation, synchronoussupply units, standards-based clients andservers, performance measurement andmanagement tools, and embeddedsubsystems that generate, distribute andapply precise frequency and time.
FIG 2 At only 16cm3, and with <115mW of powerconsumption, the SA.45s CSAC brings the benefitsof an atomic clock to portable applications for thefirst time.
