The advent of affordable GPSpositioning and ubiquitouswireless communications isspurring interest and adoption

of tracking capabilities in diverse fields: fleetmanagement, personal location services, pets,electronic surveillance of parolees, cargo andother mobile assets. But the technical andoperational requirements for the varioustracking applications are not necessarily thesame. Power supply represents a key differ-entiator among tracking systems, which mustsupport positioning, microprocessor, andcommunications components and oftenother sensors.

A crucial question for system developersand end users revolves around this issue: canpower be obtained from a generation source,such as is available in vehicle-borne systems,or must the tracking equipment rely on bat-tery power?

Although the number of asset trackingdevices has increased dramatically in recentyears, few models have effectively solved theproblem of delivering an integrated and re-liable power-management solution. Indus-trial asset tracking exposes devices to harshvibration and environments and may requirethem to operate unattended for years. Theneed for tracking systems to effectively de-

termine and report location and local status,such as external temperature or alarm sen-sors, while operating without an outsidepower source has limited the scope and scaleof applications to date.

The power requirements for positioningand communications functionality are chal-lenging in a battery-powered device. Boththe location and transmit function must beoptimized for performance and low powerto enable asset management applications.This was the situation faced by Axonn LLC,a provider of wireless packet data solutionsfounded in 1985 and based in New Orleans,Louisiana, as it sought to develop a global-

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ASSET TRACKINGDESIGN CHALLENGE

Going the DistancePower Supply Management in an AssetTracking Device

Gary Naden Axonn LLC

Constant vibration, harshenvironments, and therequirement to operate unat-tended for years present radi-cally different challenges forindustrial tracking devices, asopposed to personal or in-carunits. This becomes quicklyapparent in the matter ofpower supply, which mustcome from batteries and nota vehicle-borne system, andmust support positioning,microprocessor, and commu-nications components, andoften temperature and alarmsensors as. A product design-er discusses how to optimizelocation and transmit func-tions for performance and lowpower.

footprint asset management device capableof multiyear operation. The result is the AX-Tracker introduced in June this year, whichintegrates a low-power GPS module, a satel-lite communications modem, and hostprocessor powered by innovative hybridlithium thionyl chloride primary batteries.

BackgroundAsset management is a critical part of anybusiness entity engaged in the transfer ofraw or finished goods. Such companies mustcarefully manage the resupply of raw mate-rials to ensure uninterrupted operations ofthe manufacturing or service element of theindustries in which they are involved. This,in turn, requires them to carefully managethe transport of finished goods so as to min-imize inventory held for sale. Businesses thatfail to optimize manufacturing and materi-als handling are at a significant competitivedisadvantage.

The uncertainty associated with raw ma-terials and finished goods in transit presentsa substantial problem in asset management.Companies generally operate with an ele-ment of uncertainty as to the exact time ofdelivery or the location of products and rawmaterials en route to their facilities. Un-foreseen conditions affecting the arrival oftruck, rail, or other vessel deliveries are im-possible to predict and difficult to model.However, companies can use real-time in-formation about material in transit to fore-cast deliveries, schedule manpower and othermaterials, and predict finished goods in-ventory supply.

Losses. The transportation industry esti-mates losses in excess of $40 billion a yeardue to the theft of cargo in transit alone. Or-ganized loss of cargo takes place in a widevariety of ways, from employee/driver theftto hijacking of entire fleets of trailers andrail cars. Other losses resulting from acci-dents, traffic congestion, road closures, andother delays add to this mammoth expense.

The transportation industry has strug-gled to limit such losses by means of radiocommunications for more than a decade.Cellular telephone service providers andhandset manufacturers have enabled a hostof communications products that are mak-ing an impact. More than $3 billion to datehave been invested in products and services

intending to mitigate asset managementproblems. These products provide manyfunctions from standard voice communica-tion data services such as Internet or E-mail,and real-time position reporting and sta-tus of vehicle operations such as speed, tem-perature, or brake conditions.

Tracking the OpportunityMarket demand for an untethered assetmanagement device is unsatisfied. The rawnumber of unpowered cargo containers andother platforms that represent prospectivetargets for mobile asset tracking is stag-gering: an estimated 250 million unpowered“platforms” traverse the United States car-rying raw and finished goods (see Table 1).

Axonn estimates that during the next 10years only about 15 percent of these will everbe configured to track assets based on busi-ness efficiencies as justification. Nonethe-less, this yields a capture market of approx-imately 37.5 million units in the UnitedStates alone for devices similar to the AX-Tracker. Homeland Security objectives havethe potential to alter these figures dramati-cally, especially in the container marketwhere interest is mounting to validate cargosecurity carried in containers from overseas.

Communications OptionsConventional tracking solutions typicallyrely on cellular communication systems orsatellite communica-tion systems. Existingapproaches that rely oncellular solutions gen-erally do not provideubiquitous coverage,which may be adequatefor urban and major in-terstate routes but be-comes unreliable inrural or sparsely pop-ulated regions. Addi-tionally, a cellular net-work implementedprimarily for voicecommerce is a poor so-lution for rail or vesseltransportation datacommunication. More-over, as cellular tech-nology advances, the

protocols implemented around the worldhave transitioned from analog to digital andnow to tri-band Global System for MobileCommunication (GSM). Thus, some com-munications systems developed only a fewyears ago have already become obsolete.

An additional factor in cellular commu-nication–based asset management systemsstems from their inherent two-way nature,which requires continuous line power foroperation. This type of system does not op-erate effectively on battery-only power with-out periodic reconnection to line powersources such as an automotive system.

Coverage. Satellite-based communicationsystems mitigate some of the coverage prob-lems associated with cellular asset manage-ment devices. Instead, the area of servicecorresponds to the “footprint” of the satel-lites in the system selected for use, whichsolves much of the availability problem ofrural and maritime coverage.

Trailers in service, U.S. 2,025,000Containers, U.S. 13,200,000Containers, U.S. from

abroad 230,000,000Rail cars in service, U.S. 4,500,000Total domestic “untethered”

opportunity 249,725,000

TABLE 1 U.S. market opportunity — Estimated total platforms

� FIGURE 1 AXTracker integrates a low-power GPS engine, aGlobalstar-compatible simplex satellite transmitter, GPS andGlobalstar antennas, and hybrid lithium thionyl chloride batter-ies. It has programmable transmission modes, up to four inde-pendently configurable alarm inputs, a serial interface port, andsmart sensor or user data input.

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ASSET TRACKING

Communications Power PlayCustomers prefer satellite asset manage-ment systems that match the communica-tion bandwidth with the requirements ofthe application. The data requirements forasset management are generally low-band-width in nature. However, many satelliteasset management systems are the succes-sors of cellular voice communications sys-tems and offer broadband feature sets suchas Internet and voice-over-Internet Proto-col. As a result, broadband satellite servicesare typically expensive and prone to com-

munication failures due to weather and ob-struction. Asset management systems thatemploy broadband satellite communicationsthus end up packaging other broadband ser-vices in order to recover the cost of the databandwidth. This drives up the cost of theassociated tracking services.

Another consideration for satellite com-munications-based tracking is the transmitpower required to communicate to geosta-tionary satellites and the constraints thatplaces on a remote asset management de-vice. Existing satellite asset management sys-tems generally must incorporate transmit-power amplifiers of up to 10 Watts tooperate adequately. As most satellite com-munication systems impose tightly con-trolled spectral masks, digital communica-tion systems must incorporate linear ornearly linear (Class A or Class AB) poweramplifier architectures to prevent spectral

regrowth. Spectral re-growth occurs in non-lin-ear (class C) amplifierswhen sine-waves areclipped producing digitalsignals that have Fouriercoefficients outside the al-lowable spectral band ofuse.

Linear amplifiers aremuch less efficient butpass complex transmissionsignals with minimumdistortion, thus preserv-

ing spectral utilization. As the band of useis set by the FCC and satellite channeliza-tion, the endpoint device is forced to usemore spectrally efficient but less power-efficient amplification. As a result, the trans-mit device must produce up to 10 Watts withamplifier architectures typically only 40 per-cent efficient. This creates difficult designlimitations, usually requiring sufficient linepower or bulky high-density battery sys-tems.

Simplex Transmitter. The simplex trans-mitter operating to a low earth orbit (LEO)

system saves power in two ways. First, be-cause it is simplex, the communications linkdoesn’t need to spend power listening or ne-gotiating with the satellite system. Second,because the satellites are in LEO, the dis-tance of transmission is shorter than a geo-stationary communication link requiringless power. The AXTracker transmits 250mW versus up to 10 W of transmit powerneeded for geostationary links. An additionalbenefit for LEO systems is that the satellitesare moving, mitigating the most commonproblem with geostationary links: shadingby stationary objects. Finally, as this satel-lite system is designed for packet data, it isvery well suited for millions of endpoint de-vices operating with low duty cycles. Usersdon’t needlessly pay for broadband satelliteuse to pass packet type data.

Currently, satellite-based asset manage-ment systems use duplex satellite architec-

tures, providing simultaneous two-waycommunications. To send data over a satel-lite, the remote device must generally ne-gotiate a data channel. Even if the data onlygoes one way, the communication modemmust contain both receive and transmit ca-pability to implement this negotiation. Re-mote asset management devices must bothlisten and transmit in order to facilitate datatransfer to and from a remote device.

Both cellular and two-way satellite assetmanagement systems require available linepower or extensive battery systems to op-erate. Even existing systems equipped withlow-power operational states must consumeexcessive power to manage two-way com-munications as well as transmit with suffi-cient energy to operate within the commu-nications infrastructure.

Other Operational ChallengesExisting asset management devices are gen-erally installed on the tractor-cab of thetruck, train locomotive, or vessel. This servesto locate the cargo while the load is attached.Unfortunately, when an accompanying load— such as the trailer, railcar, or barge — isdisconnected, the important location infor-mation that provides value for asset man-agement is lost. These “untethered assets”may become lost for hours or days or more,resulting in the total loss of perishable cargoor missed deadlines for loads that are non-perishable but time-critical. Inventory man-agement becomes difficult and highly labor-intensive to minimize misplaced loads.

Railcar tracking systems generally lagin capability behind trucking. While rail-cars remain on class 1 lines, the owners typ-ically know when the railcars have passedcheckpoints using barcode or visual identi-fication systems. Once the railcars reach class2 or class 3 lines, no real-time tracking gen-erally takes place. Class 1 lines are the main,long-haul rail systems that are generally op-erated by the rail company. Class 2 and 3lines are local spurs and company-ownedsidelines used to stage cars for shipment, orstore raw materials or finished goods. Oftenthe end customer loses the cargo once itleaves rail-carrier control, even though it ison his own sideline in the company yard.

Furthermore, customers often use rail-cars as temporary storage, delaying the

MORE THAN $40 BILLION a year is lost to the theft of cargoin transit that can include hijacking of entire fleets of rail carsand road trailers. Railcars on local spurs and sidelines used forstaging or storage generally receive no real-time tracking.

Asset management systems using BROADBAND SATELLITE COMMUNICATIONS package other services to recover thedata bandwidth cost, driving up tracking services costs.

DESIGN CHALLENGE

32 GPS World OCTOBER 2003 www.gpsworld.com

off-loading of goods and essentially main-taining inventory at the cost of the rail-fleetowner. The latter have a difficult time as-sessing demurrage charges because they maynot know if the railcar has been offloadedon schedule or even its current location. Asa result, rail fleet operators may resort to theexpensive solution of adding new cars to thefleet to satisfy logistic problems.

Barge and vessel owners generally are de-pendent on river pilots and deep-sea ves-sel operators for the location of goods usingvoice communication only. As such, com-modity traders usually maintain a staff of lo-gistics personnel to voice-track products asthey are moved. A radio-telemetry productthat works without a cellular infrastruc-ture and without the requirement of avail-able power could dramatically reduce thereliance of pilots and logistics staff.

Building a SolutionThe AXTracker mobile asset tracking de-vice is a ready-to-go, self-contained teleme-try unit capable of delivering long-term ser-vice life through the use of an advancedlow-power GPS engine coupled to a Glob-alstar-compatible simplex satellite trans-mitter. The units transmit real-time GPSlocation and other data via one-way simplexcommunications through the Globalstarsatellite system, a constellation of 48 lowearth orbiting (LEO) satellites that relaymessages to ground-based gateways, whichthen pass the call on to the terrestrial tele-phone network and the Internet. The track-ing device has a low-profile design, mea-suring just 9.2�6.45�1 inches. The trackingunits attach to the outside of cargo con-tainers, railcars, or trailers.

In designing the product, Axonn took intoconsideration factors such as product size,service life between battery replacements,and the ability to transmit data reliably underextreme environmental conditions. AX-Tracker technology integrates an inexpen-sive simplex modem, GPS and Globalstarantennas, and primary batteries employinga hybrid lithium thionyl chloride technol-ogy. It features programmable transmissionmodes, up to four independently config-urable alarm inputs, a serial interface port,smart sensor or user data input, and up to63 user-definable “geofencing” detection

regions. Geofencing creates virtual fencesbased on location coordinate boundaries,which an asset may not cross without trig-gering an alarm or violation notice. Thisprevents an asset being taken out of a spec-ified zone or entering a prohibited area with-out alerting system monitors.

GPS On BoardAxonn’s mobile asset tracking system uses a12-channel L1 C/A-code receiver mountedon a motherboard with the GPS antenna,host processor, satellite modem, and Glob-alstar transmitter. TheGPS was selected forthree primary reasons: acold-start time to first fixaveraging 45 seconds, alow-power state usingonly a few micro-ampsof current while main-taining battery-backupRAM, and a moderatelylow run current averag-ing about 70 mA.

To achieve long ser-vice life, the AXTrackerdevice enters an ex-tremely low power state,drawing less than 10 uAwhich is lower than theself-discharge current ofthe battery. The devicewakes on a prepro-grammed schedule or by external alarm/sen-sor trigger and powers on the GPS moduleto ascertain location. Next, the GPS mod-ule is disabled and the simplex modem isturned on to transmit the location and localstatus information. The unit then resumesthe low-power state until the next wakeinterval.

Only the required sections of the unit arepowered on as needed. Low duty cycles andlow transmit current enable the use of pri-mary batteries embedded in the device. Dur-ing active transmission mode, AXTrackerrequires high current pulses of up to 1.2 Afor intervals of up to 1.3 seconds. This cre-ates a difficult requirement for a battery tech-nology to provide high pulse current, lowleakage current, wide environmental op-eration and years of service life.

Communications LinkThe mobile tracking device incorporatesnew simplex satellite communications tech-nology that enables transmission of packet-switched data. The unit leverages Axonn’score RF spread spectrum data communica-tions technology created by Axonn for theautomated meter reading (AMR) marketand other applications. The simplex modemutilizes the same custom ASIC developedby Axonn to digitally spread the satellite sig-nal. This is the same basic technology in-stalled over the past 10 years in more than

12 million AMR simplex devices, many ofthem using primary cells for power.

Simplex communications offer the mostefficient power management method pos-sible because it does not require the deviceto continually operate a receive capability.The modem, also manufactured by Axonn,operates as a one-way conduit to the satel-lite system drawing most of the 1.2 A of peakcurrent during transmit. When not in use,it is shut off by the AXTracker host boardprocessor. This enables low-power endpointhardware and low-cost network interface,suiting the unit to provide location infor-mation for assets lacking available power.

Power Supply AlternativesA primary consideration was the need forAXTracker to work with any form of asset,including automotive trailer, ocean cargo

THE AXTRACKER (white square) on a container in a transityard. 250 million unpowered cargo containers and other plat-forms travel the United States carrying raw and finished goods.

DESIGN CHALLENGE ASSET TRACKING

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container, as well as freestanding assets. Useof solar panels with photo-voltaic cells torecharge batteries was ruled out because itadds cost to the cell and requires additionalrecharging circuitry. Also, due to the harsh,rugged environment of many trailer and railapplications, solar panels were not consid-ered reliable for this application because theyare prone to failure as a result of shock, im-pact, and dirt accumulations.

Rechargeable batteries were also prob-lematic because they are designed to workwith only certain types of assets, notablythose in the automotive sector. When usedin automotive applications, rechargeablebatteries typically draw power from thelighting or brake systems, which requiresexternal wiring and installation expense. De-sign engineers generally prefer not to ex-tract power from these vital safety systems.Also, if an asset is untethered for extendedtime and the rechargeable batteries cannotbe recharged, system failure could result.

Reliability. Rechargeable battery technol-ogy also has significant reliability issuesfor industrial applications requiring morethan three years of service life. Recharge-able batteries such as nickel-cadmium(NiCd) or Lithium-Ion (Li-Ion) often suf-

fer from limited “memory,” longevity, andtemperature range of operation. NiCd willlose full capacity over time based on thepoint of recharge. For this reason, manu-facturers of NiCd devices suggest the unitbe fully discharged before recharging. It’s along-known technology problem withNiCd. Li-Ion has similar problems withlongevity. Consider how long a cell-phonebattery lasts in a benign environment. Pri-mary battery power, by contrast, offershigher energy densities with greater tem-perature range of operation. Energy den-sity refers to the amount of energy stored ina given cell volume.

To make the tracking unit a true stand-alone and self-contained device, Axonn se-lected a hybrid lithium thionyl chloride bat-tery for integration into the unit. Of all of

the available lithium battery chemistries,bobbin-type Li/SOCL2 cells were selectedbecause they offer advantages such as highenergy density, high capacity (19.2 amphours), low self-discharge rates, excellentsafety characteristics, and extended tem-perature range of �40°C to �85°C. Thelast feature is critical, as roofs of contain-ers and tractor trailers are often subject toextreme temperature variations. This tech-nology also extends service life due to its lowself-discharge rate of 1 to 2 percent per year.

Battery Pack. To accommodate the com-

pact, low-profile design, The manufacturerdeveloped a custom battery pack of eightAA-cells plus a hermetically sealed hybridlayer capacitor (HLC). Use of the batterypack enables the unit to deliver two locationreports a day for seven years of uninter-rupted service between battery replacements.More frequent updates will reduce servicelife depending on device configuration.

The batteries are ideally suited for theunit’s high current pulse requirements (upto 1.2 A for up to 1.3 seconds). This uniquecombination enables hybrid lithium cells todeliver their full capacity under higher loadsfor short-duration events. The hybrid bat-tery configuration affords ready access tohigh pulse current without passivation froma battery chemistry not known for pulse ca-pability. Passivation is the phenomenon

where batteries lose energy density due touneven deposition of mineral plating in use,caused by high pulse current. The HLC de-vice provides the peak current and essen-tially gets recharged from the lithium thionylchloride batteries during the low-powerstate.

ConclusionThe Axonn AXTracker has put together theright combination of integral componentsto enable the global management of un-powered assets. By combining the latestGPS technology with the latest Globalstarsatellite simplex technology and advancedbattery design, the mobile asset tracking de-vice delivers a place-and-play solution, with-out the need for harnesses or external powerand antennas. It attaches using screws, ad-hesive tape or industrial glue in seconds. Thedevice is less than three months old andalready being used by more than 50 prod-uct integrators and data service providers.

Gary Naden is vice-president of engineering andbusiness development of Axonn, L.L.C. His expertise is indesign engineering of low-cost, high volume digitalradio. He is the author of more than 16 U.S. and interna-tional patents issued and pending in the field of digitalcommunications systems. His team designed the satellitemodem used by all Globalstar simplex data radio trans-mission unit (RTU) devices and has integrated thatmodem into the AXTracker. Prior to joining Axonn heserved in design and systems engineering capacity at E-Systems deploying airborne communications interceptequipment. �

ManufacturersThe AXTracker mobile asset tracking deviceproduced by Axonn LLC (New Orleans,Louisiana) uses a 9543LP GPS module fromLeadTek (Chung-Ho, Taipei Hsien, Taiwan)incorporating a SiRFstarII/LP GPS chipsetfrom SiRF Technology (San Jose, California).The GPS ceramic patch antenna PA25-1575-008SA is from Spectrum Control Inc.(Fairview, Pennsylvania). The hybrid lithiumthionyl chloride batteries are PulsesPlus fromTadiran Lithium Batteries (Port Washington,New York). Axonn manufactures the sim-plex transmitter compatible with GlobalstarLP’s (San Jose, California) satellite system.

THE TRACKING UNITS (white square)attach to the outside of cargo containers,railcars, or trailers.

RECHARGEABLE BATTERY TECHNOLOGY has significant reliability issues for industrial applications requiring morethan three years of service life.