The transponder-based time-of-flight and time-distance-of-arrival schemes require excessive, nanosecond synchronization on both ends to maintain accuracy, said Hans Schantz, chief scientist at Q-Track (Huntsville, Ala.). Leading the industry in this approach is WhereNet, which operates on the 2.45-GHz band, said Schantz. "They have 100-MHz channels but can't locate to better than 10 feet — and it only gets worse with multipath interference in enclosed offices." Other systems measure signal strength to calculate distance and location, but suffer from the fact that signal amplitude can vary greatly, depending on the environment.

While Schantz was originally a UWB antenna designer and had established a long track record in that field, the Federal Communications Commission's marginal and constrictive allotment to UWB led him to pursue an alternative technology for real-time locating systems that would not face the power limits of UWB.

Schantz zeroed in on the fact that when radio signals first leave the antenna, their electric and magnetic fields are "phase quadrature," or 90° out of phase. However, by the time the signal has traveled about a half wavelength, that difference goes to zero and the fields become synchronous. Because most RF systems operate in the far-field mode with transmitters and receivers typically many wavelengths apart, this behavior has been more or less ignored.

Distance measuring
Working with colleague Bob DePierre, Schantz devised a simple algorithm that allows Q-Track to compute the distance between the transmitter and receiver, based on the transmitting frequency and the degree to which the fields have converged. He patented the technology and trademarked the name NFER.

Operating under FCC Part 15 (5.219) rules, the system can occupy the bands from 510 to 1,705 kHz. The low frequency of operation translates to good range, said Schantz, while also allowing greater penetration of interior walls. "We also don't face the same multipath interference issues as higher-frequency devices." The simplicity of the system and the low operating frequencies also allow the use of standard, off-the-shelf components, he added. The prototype was implemented using Philips 605 chips for the front end and a Xilinx Inc. FPGA for the digital processing. "We will have evaluation kits by early next year and expect to get the size down to that of a cigarette box," he said.

Applications range from container and asset tracking to monitoring firefighters in real-time. The company is currently in contract with the U.S. Department of Homeland Security and also is being funded by a Small Business Innovation Research grant. Schantz sees the biggest potential for NFER, however, in supply chain management.

In the 1,300-kHz band, the wavelength is approximately 750 feet, so the system has been shown to have an accuracy of 1 foot out to 250 feet. "This goes up to 400 feet in the 800-kHz band," Schantz added. "But we're bounded by the transmit power and the efficiency of the antenna." Under FCC Part 15 rules, Schantz said the transmitter can send no more than 100 milliwatts to the antenna.

At such low frequencies, however, the most efficient antenna would be too large to be practical. Hence, much of the company's R&D is going into improving that efficiency to minimize the antenna size.