# US 8,786,488 B2 · System and method for microwave ranging to a target in presence of clutter and multi-path effects

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- Number: US 8,786,488 B2
- Document ID: US8786488B2
- Status: Granted
- Kind: B2
- Application number: 13/163,005
- Filed: 2011-06-17
- Published: 2011-12-22
- Granted: 2014-07-22
- Earliest claimed: 2010-06-17
- Assignees: Amtech Systems, LLC
- Applicants: Kelly Gravelle; Jeremy Landt; Patrick W. Lunsford
- Inventors: Kelly Gravelle; Jeremy Landt; Patrick W. Lunsford
- Web page: https://patentgrep.com/patent/US8786488B2

## CPC

- G01S 13/765: {with exchange of information between interrogator and responder}
- G01S 7/2813: {Means providing a modification of the radiation pattern for cancelling noise, clutter or interfering signals, e.g. side lobe suppression, side lobe blanking, null-steering arrays (specially adapted to secondary radar systems G01S13/762; aerials or aerials systems H01Q21/29, H01Q25/00)}
- G01S 7/285: Receivers
- G01S 13/20: whereby multiple time-around echoes are used or eliminated
- G01S 13/28: with time compression of received pulses
- G01S 13/751: {wherein the responder or reflector radiates a coded signal}
- G06K 7/10297: {arrangements for handling protocols designed for non-contact record carriers such as RFIDs NFCs, e.g. ISO/IEC 14443 and 18092 (cryptographic protocols H04L9/00; network security protocols H04L63/00; real-time communication protocols in data switching networks H04L65/00; network protocols for data switching network services H04L67/00)}
- G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G06K: GRAPHICAL DATA READING (image or video recognition or understanding G06V); PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS

## Abstract

A system for measuring the range to an RFID tag including situations containing high clutter and multi-path signals is disclosed. The system includes an RFID reader; an RFID tag; and a coordinated pulse compression radar system. In the system the RFID reader causes the tag to respond to received signals in a first backscatter state at a first time and a second backscatter state at a second time. The pulse compression radar system transmits short pulses coordinated by the backscatter state of the RFID tag and the system creates a differential signal comprised of the differences between radar signals obtained during the first and second states of the tag to obtain an uncorrupted measure of a round trip time of flight of said radar pulses between the pulse radar system and the RFID tag.

## Claims

### Claim 1

A method for measuring distance between an RFID tag reader and an RFID tag, the RFID tag having a plurality of backscatter reflection states; the method mitigating detrimental effects of noise, clutter and multi-path signals, comprising the steps of:
transmitting an RF signal to the tag;
receiving a first backscattered RF return signal from the tag corresponding to a first backscatter reflection state of the tag;
receiving a second backscattered RF return signal from the tag corresponding to a second backscatter reflection state of the tag;
signaling by the tag reader to a pulse radar timing of said first and second backscatter reflection states;
transmitting a first radar signal by said pulse radar during said first backscatter reflection state of the tag;
transmitting a second radar signal by said pulse radar during said backscatter reflection state of the tag;
receiving reflected return radar signals by said pulse radar from the tag during said first and second backscatter reflection states of the tag;
processing each of said received reflected radar signals using a fast Fourier transform (FFT) to produce complex signal spectrums;
forming a complex conjugate of the FFT of one of said transmitted radar signals;
multiplying said complex conjugate of the transmit signal with the FFT of each received signal individually to create first and second products;
producing a correlation for each received signal individually by performing an inverse FFT of each of said first and second products;
subtracting the correlation obtained with the tag in said first backscatter reflection state from the correlation with the tag in said second backscatter reflection state to produce a signal having a peak;
determining round trip time delay to the RFID tag from the time of said peak;
calculating the distance between the RFID tag and the reader based on said round trip time delay.

### Claim 2 (depends on 1)

The method of claim 1, wherein said calculating of distance is based on a plurality of said determined round trip time delays.

### Claim 3 (depends on 1)

The method of claim 1, further comprising commanding all but one RFID tag to be silent.

### Claim 4 (depends on 1)

The method of claim 1, wherein said first and second radar signals are chirp radar signals.

### Claim 5

A system for measuring distance between an RFID tag reader and an RFID tag, the RFID tag having a plurality of backscatter reflection states; the system mitigating detrimental effects of noise, clutter and multi-path signals, the system comprising:
an RFID tag reader comprising a transmitter configured to transmit an RF signal to the tag and to transmit a timing signal indicating backscatter reflection states of the tag;
a pulse radar configured to receive said timing signal from said RFID tag reader indicating backscatter reflection states of the tag; to transmit a first radar signal during a first backscatter reflection state of the tag; to transmit a second radar signal during a second backscatter reflection state of the tag and
to receive a first reflected radar signal from the tag during said first backscatter reflection state of the tag and a second reflected radar from the tag signal during said second backscatter reflection state of the tag;
said pulse radar further configured to:
compress said first and second received reflected radar signals using a signal compression process;
the system further comprising a signal processor configured to:
process each of said received reflected radar signals using a fast Fourier transform (FFT) to produce complex signal spectrums;
form a complex conjugate of the FFT of one of said transmitted radar signals;
multiply said complex conjugate of the transmit signal with the FFT of each received signal individually to create first and second products;
produce a correlation for each received signal individually by performing an inverse FFT of each of said first and second products;
subtract the correlation obtained with the tag in said first backscatter reflection state from the correlation with the tag in said second backscatter reflection state to produce a signal having a peak;
determine a round trip time delay to the RFID tag from the time of said peak; and
calculate the distance between the RFID tag and the reader based on said round trip time delay.

### Claim 6 (depends on 5)

The system of claim 5, wherein said processor is further configured to calculate distance based on a plurality of said determined round trip time delays.

### Claim 7 (depends on 5)

The system of claim 5 wherein said transmitter is further configured to command all but one RFID tag to be silent.

### Claim 8 (depends on 5)

The system of claim 5, wherein said first and second radar signals are chirp radar signals.