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Aug 282010

It’s been a busy week, so here is yet another RTLS/RFID update:

Aug 272010

Ranger’s US 1,639,667 (1927) ranging system requires precisely synchronized oscillators at transmitter and receiver and determines distance from a starting point by counting the number of half wavelength beats in the rectified signal.

Conventional RTLS typically relies on one of three general approaches:

  • Direction Finding (DF): originated by John Stone Stone in 1902,
  • Amplitude Ranging: originated by Lee de Forest in 1904, and
  • Time of Flight (Time Difference of Arrival (TDOA), or Transponder Ranging are typical approaches): orignating in the 1920′s and 1930′s.

As DF came of age after the First World War, inventors sought techniques other than DF and Amplitude Ranging to implement RF-based location systems. Inventors began to develop Time-of-Flight based systems. But they also explored a wide variety of other less conventional approaches. This post will review four novel approaches to RF-based location all invented in the 1920′s and 1930′s:

  • Synchronous Ranging: an early attempt at a ToF-based system,
  • Differential Attenuation: a comparison between two different frequency signals with differing rates of attenuation to determine range,
  • Multipath Interference Ranging: seeking an interference between a direct and reflected ray for maritime collision avoidance, and
  • Transponder Ranging: a more sophisticated ToF-based system.

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The Second World War spurred further innovation in RF-based navigation. The first wide-scale deployment of an RF-based location system was the Long Range Navigation or “LORAN.” LORAN was an evolution from a shorter range British system called “GEE.” Like GEE, LORAN employed synchronized impulse signal transmissions from paired transmit towers. By measuring the Time Difference of Arrival (TDOA) of the paired signals, users could determine which of a family of hyperbolic curves passed through their location. The intersection of two such curves defines the user’s location. The Figure illustrates two overlapping families of hyperbolic curves. Each curve denotes a line of constant TDOA for signals from a pair of stations. Two measurements of TDOA yield the 2-D position of the receiver.

Alfred L. Loomis invented the Long-Range Navigation or "LORAN" system in 1942. Source: U.S. Patent 2,884,628.

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Croyden Aerodrome Control Tower with mast supporting direction finding aerial, circa 1930.

By the 1920’s direction finding was well-advanced, and DF techniques began to see everyday use in both marine and aerial navigation. [1] The Figure shows a DF array from around 1930 deployed at Croyden Aerodrome in the UK [2]. Another good example of a sophisticated and relatively simple to use DF system from the period is the one developed by the noted French engineer, Henri Busignies (1905-1981) in 1927. [3] Adcock type antennas were used to mark out routes for aviation. By sending out complementary signals, such as “A” (dot-dash) and “N” (dash-dot), the signal heard on the equi-signal line yields constant dash tones. This provided a simple aural signal to keep pilots on a desired path and provide feedback if they happened to stray. [4, 5]

[[1]]      John H. Morecroft, Principles of Radio Communication, 2nd ed. (New York: John Wiley & Sons, Inc., 1927), pp. 884-894.

[[2]]     R. N. Vyvyan, Wireless Over Thirty Years, (London: George Routledge & Sons, Ltd., 1933), p. 158-159.

[[3]]     Henri Busignies, Radio direction finder, Hertzian compass, and the like, US Patent 1,741,282, December 31, 1929.

[[4]]     Frederick Emmons Terman, Radio Engineering, 1st ed. (New York: McGraw-Hill Book Company, 1932), pp. 588-597.

[[5]]     Frederick Emmons Terman, Radio Engineering, 2nd ed. (New York: McGraw-Hill Book Company, 1937), pp. 722-733.

Jul 202010

Frank Adcock's UK Patent 130,490 direction finding antenna system carefully used only vertical elements for reception and horizontal transmission lines. This made his DF array sensitive only to vertically polarized signals, significantly improving night-time DF performance.

Under optimal conditions during daylight hours, direction finding (DF) accuracy could be as good as one to two degrees (300m at 10km range). [[1]] At night, however, the ionosphere reflects distant signals from over the horizon. The resulting “skywave” signals have a mix of vertical and horizontal polarization components that can confound DF systems by introducing phase offsets. DF error could increase to as much as 30 or even 90 degrees at night. [[2]]

In 1918, a British Army Officer, Frank Adcock (1886-1968), devised a solution to this problem “in which the aerials, which have identically the same dimensions, are so mounted and connected that only the vertical parts are effectively influenced by the electromagnetic radiation, the horizontal parts, or those parts having a horizontal component, being so arranged that the effect on them is eliminated or reduced to a minimum.” [[3]] The Figure shows two embodiments of Adcock’s array. Later testing by the Marconi company indicated that a well designed Adcock array could achieve three degree accuracy, even at night. [[4]]

Interestingly, Adcock was a classicist working as a cryptographer when he devised this common-sense configuration. He went on to teach classical history at Cambridge and was co-editor of the Cambridge Ancient History for which he wrote ten chapters.

[[1]]      R. N. Vyvyan, Wireless Over Thirty Years, (London: George Routledge & Sons, Ltd., 1933), p. 115.

[[2]]     R. N. Vyvyan, Wireless Over Thirty Years, (London: George Routledge & Sons, Ltd., 1933), p. 128, 164.

[[3]]    Frank Adcock, Improvement in Means for Determining the Direction of a Distant Source of Electro-magnetic Radiation, UK Patent 130,490, August 7, 1919.

[[4]]    R. N. Vyvyan, Wireless Over Thirty Years, (London: George Routledge & Sons, Ltd., 1933), p. 164.

Jul 142010

W.G. Wade of the Bureau of Standards uses a direction-finding (DF) loop antenna (pioneered by Lee de Forest) at a Kennsington Maryland field station in 1919. Similar systems were employed by the Allies and Central Powers during the First World War. Source: NIST Photographic Collection

In 1902, a reporter asked Guglielmo Marconi about the vulnerability of wireless signals to interception. Marconi reassured the journalist, “It isn’t possible without a special installation and without guessing the frequency.” [[1]] The First World War demonstrated the magnitude of Marconi’s error.

The Russian Army began the war sending signals in plaintext. Thus, the Germans were often better informed on Russian dispositions than the Russians themselves. This signal intelligence bonanza enabled the Germans’ decisive victory at Tannenberg in 1914. [[2], [3]]

In 1916, Romania attempted to join the Allies and launch a surprise attack upon the Central Powers. However, Romanian units used:

“…fixed call signs in plaintext; gave the names, numbers, and designations of troop units and formations; sent back all reconnaissance reports in plaintext; and received their instructions and commands in the clear. Of course they used cipher for a large part of their radiograms, but this was done with so little skill, and the number of plaintext messages gave the Austrian cryptanalysts so many clues, that the cryptographic system was solved in a very short time.” [[4]]

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Jul 022010

Ettore Bellini (1876-1943) and Alessandro Tosi devised a much improved direction-finding system in 1907. [[1]] Their scheme deployed two orthogonal arrays similar to those of Stone. The key advantage of the Bellini-Tosi direction finder was a rotating transformer coupling. Rather than rotate a potentially large antenna system, the Bellini-Tosi system uses fixed orthogonal antennas with a rotatable transformer. The Figure (below) shows the Bellini-Tosi array and details of the rotating transformer or “goniometer” coupling. Bellini developed many improvements to their invention including a version with a cardiod pattern (with Tosi) [[2]], and a capacitive goniometer [[3], [4]].

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Jun 292010

A prolific inventor, Lee de Forest not only invented some of the first direction finding (DF) antenna systems, but also deserves the credit for having invented the first RF ranging system. Realizing that signal strength declines with distance, de Forest proposed inserting a variable resistor into the RF circuit to enable a measurement of signal strength. Given a previously determined table of signal strength versus range, the range may be determined from this measurement of signal strength. [[1]] The Figure shows two embodiments of de Forest’s 1904 “Wireless range finder.”

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Jun 242010

John Stone Stone (1869-1943) patented the first effective direction finding system in 1902. [[1], [2]] Stone’s scheme involved a two element antenna with a first element (V) arranged no more than a half wavelength away from a second element (V’). The Figure below shows Stone’s invention. The two elements are arranged so that their respective signals add up 180 degrees out of phase with respect to each other. Thus, a signal incident in a direction normal to the plane containing the two elements their combined action is nil. Stone also embellished upon his invention in later years. [[3], [4]]

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Kitsee and Wilson’s US 651,014 (1900) direction finding antenna relied on shielding of a spherical capacitive end load to “shadow” signals.

Communications may have been the first commercial application of wireless technology, but Real-Time Location Systems (RTLS)  were close behind. In the first few years of radio, a variety of aggressive inventors recognized the problem of RTLS and leaped to offer solutions. Some of their ideas illustrated the inventors’ misunderstanding of the behavior of radio waves. Inventors assumed (erroneously) that long wavelength RF signals would cast sharp shadows in an optical fashion. Isidor Kitsee and Charles E. Wilson, for instance, proposed a spherically end-loaded antenna with a shield to block signals from a particular direction. [[1]] The Figure (left) shows the Kitsee-Wilson antenna.

Ladd’s US 733,910 (1903) direction finding antenna employed a rotating slit intended to allow the antenna to be illuminated only if the slit were aligned with a distant transmitter.

Hermon W. Ladd similarly proposed a whip antenna with a rotatable shield. [[2]] In Ladd’s proposed system, a narrow slit in a rotating shield is supposed to allow the antenna to be illuminated only when the slit is aligned with the direction of incidence of the signal. Both these DF antennas fail to work, because the low frequency signals (typically <300kHz) they aimed to detect have wavelengths too long (typically >1km) to be shadowed by such a small shield or to illuminate such a small slit. The Figure (right) shows Ladd’s direction finding system.

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