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The Problem - How can you measure range using low cost transponders

RFID systems comprise a tag and a reader. As operating range increases, the number of tags in the reader zone is likely to increase and multiple tags need to be read at the same time.

Before the arrival of RFID-radar, readers have been able to read the identity of multiple transponders in a zone at one time, but they have not been able to locate where in the zone the transponders actually are physically.

As the performance of readers and transponders improve, operating ranges increase giving one reader the ability to monitor larger areas meaning location information in addition to the identity information becomes very important.

RFID-radar works with very low cost transponders and relatively cheap reader systems.

Rfid radar kit with Patch antennas
Trolley Scan's new RFID-radar kit with High gain patch antennas

The invention

  • Trolley Scan have developed a reader system capable of fixing the location of many passive transponders in a reading zone
  • The invention can monitor a zone up to 100 meters deep, to an accuracy of less than 0.5 meters.
  • The scan time to range all tags in the zone is a couple of seconds.
  • Once located individual transponders can be tracked.
  • The transponders are low cost Tag-Talks-First type devices, capable of being produced at a few US cents.
  • The reader has only one array of antennas
  • The reader is relatively low cost to produce.
  • The reader is able to map out the location of all the transponders in the field in 1D, 2D or 3D space, processing all the transponders in its reading zone in parallel.
  • The reader uses only 10kHz of bandwidth, operating at UHF frequencies, allowing many readers to operate in close proximity with minimal interference.
  • The reader provides a range measurement to each tag as well as a pointing vector with an accuracy better than 1 degree.
  • Position updates are given at 1 second intervals
  • The system can be implimented in all countries of the world
  • This is a world first base technology, allowing companies involved in its development the ability to deliver very advanced low cost RFID systems for many years.

    Physical location of transponders

    Placement of twelve transponders in front of a radar for a test showing capabilities

    Rfid-radar with transponder
    Trolley Scan's new RFID-radar front panel showing simple controls for 2D scanning


    Twelve transponders are plotted. The yellow zone is the field of view of the radar. The axes are in meters.

    The equipment

    The system comprises tags, readers and evaluation software.

    The reader

    The reader provides an energy field to power up the transponders, and measures the distance the signal travels from the transponder back to the reader. For 2D and 3D applications, two or three receiver channels are included in the reader to allow angle of arrival to be determined at the reader site by comparing the range from each of the reader channels.

    The reader contains a fast processor module making up to 10000 range measurements per second to allow the tracking of each of the tags in the zone with update rates at one second intervals.

    The reader interfaces to other computer networks via RS232.

    The reader has one array of adjacent antennas measuring range from that array to the tags. The array contains one transmit antenna for energising the passive transponders and one antenna for each receiver (2 antennas total for 1D,3 antennas for 2D and four for 3D).

    The reader can be set to operate in any frequency range from 860 to 960 Mhz, using 10 kHz of spectrum for the energising signal. Two reader systems can operate within 4 meters of each other. The reader processor consolidates the huge amount of information retrieved by the reader sections. It can report

    1. Identity of tags in the field.
    2. Identity, range and pointing vector to the tag in 1D, 2D or 3D space
    3. Identity and track of movement of tags in the reader zone, updated at 1 second intervals.
    The reader has a location accuracy of better than 0.5 meters, a pointing accuracy of 1 degree and will eventually cover a maximum range of 40 meters depending on the tag used with the reader.

    The tags

    Typical transponders in a credit card sized format and stick format

    These are passive backscatter Ecotag UHF transponders. They can be packaged in different antenna forms. Operating range depends on the sensitivity of the transponder, with the 5 uW versions operating up to 40 meters, and the 200uW versions working out to 10 meters.

    Operating range is a function of two processes

    The radar algorthm has a practical operating range for measurement of 40 meters with better than 0,5 meter accuracy.
    The current 200uW Ecochiptags with the current energiser has a maximum operating range of 10 meters and the 5uW stick tags have a range of 40 meters. We currently have also developed other forms of transponders with a maximum range of fifty meters. As we develop more sensitive transponders so this range can increase.


    Accuracy range
    50 centimeter in absolute mode & millimeter resolution in relative mode
    Max no of dimensions
    3 (2 in evaluation version)
    Pointing accuracy
    0.2 degree
    Maximum range of reader
    40 meters (practical) Energising dependant on transponder type- 40 meters for current 5uW tags
    Transponder technology
    Passive TTF (Tag talks first) protocol
    Multiple transponders
    Up to 50 in the energising zone
    Interfacing to computer network
    Field of view
    64 degrees- 80 degrees
    Operating frequency of reader
    860 to 960 MHz
    Operating bandwidth of reader
    Interference zone with second reader on close frequency
    4 meters
    Multipath discrepancies
    Range corrected for multipath

    Competitive technologies

    Ranging beacons
    These are very sophisticated transponders that are attached to expensive military hardware in warehouses for locating where the "Military tank is parked". The transponders cost more than US$1000 each and the warehouse has to be fitted with ranging beacons all over the warehouse to provide triangulation.
    GPS units fitted in each transponder with the position data communicated via WiFi networks. Transponders are very expensive and are major user of battery energy for the communication and position location.
    The site where the transponders are to be used has to be mapped before use showing power fluctuations of signals on multiple receivers caused by a single source. Susceptible to other objects being moved into the site after survey impacting the survey, multipath variation due to reflections and needing multiple receivers. Very crude positioning technique.


    Any Ecochiptag,Ecowoodtag, laundrytag, Ecosportstag, Ecofarmtag or the new 5 microwatt long range tag can be used with the RFID-radar. The reader has two receiver channels to measure the range and one energising channel for powering the transponders. The reader uses a 10kilohertz bandwidth, allowing many readers to operate in close proximity.

    The radar measures the distance the signal travels from the various transponders to the radar antenna. By comparing the range on two receivers, the angle of arrival can be measured and so a direction and range can be shown. The reader can process about 50 transponders in a zone at a time, at the rate of 10000 range measurements per second, and can operate up to ranges of 40 meters with suitable transponders. Only one set of antennas is needed for a full fix with one reader. A third receiver channel can be added to give 3D coverage rather than 2D coverage (or a second evaluation system fixing the same transponders from a slightly different position and solving mathematically). A major feature of the system, besides its accuracy in measuring the range, is that the interference generated by this radar for other similar users and for other RF users, such as cell phones, is very low. Many RFIDradar systems can operate in close proximity without interfering with each other.

    Another feature of the radar system is the ability to track moving targets. By measuring the range many times per second of each transponder, the reader can plot the path of a moving object in its field of view.

    The reader outputs location and identity information at regular intervals, which can be displayed on a laptop running a Visual Basic program to show a radar type display.

    The radar can also locate and process transponders attached to wood and metal.

    Importance of this invention

    "The RFID-radar has the performance of a US$1million battlefield radar, but at the price of an RFID reader"

    When read ranges of RFID systems are small, then location information is not important as it is always in close proximity to the reader. As technology improves and the transponders need less and less power to operate, so the read range starts increasing, meaning the uncertainty in the exact location of the transponder increases. This improved sensitivity transponder technology needs increased information from the reader to assist in the speedy location of the transponder and here the technology detailed in this brochure becomes important.

    This invention is the only solution to the RFID-Radar problem that uses low cost transponders.

    The arrival of RFID-radar has stimulated the development of lower powered UHF backscatter transponders to increase operating range. While conventional RFID readers typically would want maximum read ranges of about ten meters, RFID-radars ability to report location means that ranges of 40 meters from one reader are being requested.


    A reader that can generate position information of all tags in its aperture, and provide updates of their movement, generates a lot of data.

    Trolley Scan are one of the world leaders in UHF RFID developments and manufacture leading edge designs which they export currently to 53 countries.

    Trolley Scan are now delivering

    Specifications of commercial version now being supplied

    The specifications of the system are: Trolley Scan will upgrade any supplied model to any later versions when available at a cost of the parts, a small service fee and provided the client takes care of freighting and duty costs in returning systems to South Africa for upgrade.

    How does RFID-radar work?

    The radar makes two measurements on each signal received from each transponder in its receiving zone - namely a range measurement and an angle of arrival. It is the ability to measure range with narrow bandwidth that make this an outstanding unique RFID instrument. The system uses the same transponders that are used by conventional RFID readers allowing RFID-radars and RFID readers to monitor the same transponder simultaneously.

    Light and radio waves travel at the speed of light, namely 300 000 000 meters per second. RFID systems need to operate in a crowded RF spectrum, where other RFID systems, cell phones, radio stations and other communication users have to share the available radio spectrum. RFID-radar uses just 10 kilohertz of bandwidth to operate, meaning it can detect time differences only as small as 0.1 milliseconds, or 0.0001 seconds. In this time the radio wave will travel 30 kilometers, or 30 000 meters or 3 000 000 centimeters Yet the radar is able to determine the range of the transponder based on its received transmission to an accuracy of a few centimeters, or nearly 1 million times better than its basic time measuring properties. If we used a conventional military radar approach and wanted to get centimeter precision, then we would have to measure the time of flight to 0.3pico seconds (.3*10-12) which would use 300GHz of radio spectrum.

    RFID-radar pay for this million times improvement in the timing accuracy of the basic system by taking a longer time to determine the exact position. As a result it is well suited to a static situations where transponders are relatively stationary. However developments are in progress to address the accurate tracking of movement by adapting some of the principles of operation.

    We have also now added features that allow movements as small as one millimeter to be measured in static situations.This allows simultaneous measurements from the reader to up to 50 transponders and can be used to detect movement in structures such as bridges, dam walls, buildings, mine walls and roofs, and other fixed structures that might be stressed by weather, traffic, water pressure, wind etc. Here one lowcost instrument can be installed to permanently monitor the integrity of structures and send warnings before damage or failure. These measurements are instantaneous.

    The angle of arrival measurement is virtually instantaneous and used in conjunction with range gives a 2D positioning system from a single measuring location.

    "RFID-radarTM", "ClaymoreTM" and "Better than a tape measure!TM" are the trademarks of Trolley Scan (Pty) Ltd

    RFIDradar is a development of Trolley Scan (Pty) Ltd (South Afica), one of the world's leading developers of UHF RFID technology.

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