Object tracking

32 Geomatics World January / February 2012

The RFID tags used are passive, meaningthat they have no internal power source. Thepower used by the pebble’s microchip tag issupplied by inductive electromagneticradiation emitted from the RFID reader device.The Texas Instruments tags utilise half duplex(HDX) transmission technology, and operate ata low-frequency of 134.2 kHz. Ultra-highfrequency (UHF) tags are not suitable due tothe fact that if they are near a conductingmaterial such as metal (or in this case wetsalty ground) the range is greatly reduced.

There are three different sizes of RFID tagsbeing used (32 and 23mm glass transponders,and a 12mm plastic wedge transponder). The32mm tags have the greatest optimumdetection range at 1.0m, decreasing to 0.2mfor the 12mm tags. Only limited numbers ofthe 12mm wedge tags will be used initially totest the performance of resin-based artificialpebbles in simulating the movement of smallersized shingle particles.

RFID equipmentThe RFID detection equipment consists ofthree major components, an RFID control unit,an antenna load tuning box and an externalantenna. These components were assembledby Ohmex into two trial units, a portablehand-held unit and a towable large antennaconfiguration. During August 2010 a field testwas organised with Clive Moon (CoastalEngineer, Havant Borough Council) on HaylingIsland to trial the two assembled devices. Thehand-held antenna has an area of 0.2 × 0.2m,and recorded an optimum read range of 0.4mfor a 32mm transponder. The larger antenna

was 1.0 × 2.0m anddesigned to sit in a woodentray and be towed behind anall-terrain vehicle (ATV). Themaximum read range for a32mm transponder was only0.2m for this larger antenna.

The optimum read rangesduring the initial trial werelower than anticipated andprompted modifications tothe design of theequipment. The hand-heldantenna was re-wound usingheavier gauge wire. Duringsubsequent testing thisincreased the read range ofa 32mm tag to 0.8m, closeto the optimum range of1.0m suggested by themanufacturer.

The Havant, Portsmouth & GosportCoastal Partnership is a partnership oflocal maritime authorities responsible for

managing coast and flood erosion risk along136 kilometres of the East Solent coastline insouthern England. The partnership isundertaking a long-term study to track themovement of shingle along their coastline.The study, currently covering the open coaston Hayling and Portsea Islands, features thelong-term tracking of shingle pebbles thathave had a small radio frequency ID (RFID)tag embedded within them. The ultimatepurpose of the study is to improveunderstanding of the movement of beachmaterial along the coastline, which will inturn help to optimise future coastal erosionand flood protection schemes. Of specialinterest is the performance of the ongoingbeach nourishment operations at Eastoke,Hayling Island. SCOPAC, the StandingConference on Problems Associated with theCoastline, has also contributed financially tothe project, recognising the wider benefits ofdeveloping a new technique for trackinggravel sized material in the littoral zone.

Pebble TaggingTo accurately simulate the natural beachmaterial, individual pebbles are drilled usinghigh pressure water jets and a small RFID tag isplaced in the hole which is then filled with aplastic resin. Where possible the hole isorientated along the short axis of the pebble sothat in its natural repose position the majorityof tags will be pointing upwards, this being theoptimum orientation for RFID detection.

Measuring beachmovement is not aprecise or staticmeasurementproblem. Onesolution involvesinstalling 2400 tinyradio frequencyidentifiers inpebbles andtracking them overtime.

Pebble Tracking using RFIDBy Clive Moon and Ted Read

Figure 1: The study area,Hayling and PortseaIsland on the South Coastof UK

Object tracking

January / February 2012 Geomatics World 33

over a 12-month period to build up a pictureof the rate and direction of sedimenttransport. After 12 months, as many tracers aspossible will be recovered and re-measured toquantify the rate of attrition of the beachmaterial. This will help the partnershipimprove the accuracy of long-term volumetricpredictions for the fate of beach nourishmentmaterial placed along the coastline.

A two-week study of 300 tracers deployedat Eastoke, Hayling Island, in September 2010yielded good initial results. The averagedetection rate over the two weeks was 72%,with more than 95% of all the tags detectedat least once. After five months deploymentthe tracers had dispersed along threekilometres of beach, with the mean directionof drift westwards – as anticipated, based onthe current understanding of sedimenttransport in this area. The detection rate haddropped to 17% after five months – asignificant drop, but one of the key outputsfrom the project will be a clearerunderstanding of the detection ratesachievable over longer time periods.

For further information contact:Clive Moon – Havant, Portsmouth & GosportCoastal Partnership – clive.moon@havant.gov.ukTed Read – Ohmex Ltd – ted@ohmex.com

The towed antenna was redesigned toincorporate a multiplexer that can drive up tofour antennas, each 1.2 × 0.6m in area, againusing heavier gauge wire. The optimum readrange for a 32mm tag again increased to0.8m during subsequent testing. The newarray can cover a 3m wide strip of beach,increasing the survey efficiency by 50%compared to the original design.

Further testing is currently underway inwhich antennas are suspended from the backof an ATV rather than being dragged alongthe beach surface. If successful this willincrease the potential surveying speeds, whilstalso reducing wear and tear on the variousantenna components. Data from the antennaarray is logged and time-stamped using adatalogger. Position data is simultaneouslylogged using a Trimble R8 GNSS receiver withTSC2 datalogger, using corrections suppliedfrom a Trimble NetR5 base operated as part ofthe Southeast Strategic Regional CoastalMonitoring Programme. The data is post-processed to combine RFID tag numbers andlocations for each of the four antennas.

Initial ResultsAs part of the ongoing study 2,400 tracershave been deployed around the Hayling andPortsea Island open coast. Sixteen kilometresof beach is being scanned every three months

Figure 3: ATV mounted RFID antenna.

After fivemonthsdeployment thetracers haddispersed alongthree kilometresof beach. . .

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Figure 2: Tagged pebbles

Radio-frequency identification (RFID) isa technology that uses radio waves totransfer data from an electronic tag orlabel. Attached to objects, the RFID tag

enables a reader to identify and trackthe object. Some RFID tags can be readfrom several metres away and beyondthe line of sight of the reader. Basicallythere are two types of RFID tag: fixedand mobile. The latter are increasinglyused to track vehicles for toll chargeswhile the static have wide applicationin logistics and stock control as wellidentifying buried utility services.

The tag's information is storedelectronically. The RFID tag includes asmall RF transmitter and receiver. An

RFID reader transmits an encodedradio signal to interrogate the tag. Thetag receives the message and respondswith its identification information.Many RFID tags do not use a battery.Instead, the tag uses the radio energytransmitted by the reader as its energysource. The RFID system designincludes a method of discriminatingseveral tags that might be within therange of the RFID reader.

The day of the RFIDs has arrived