Communications: directive radio wave systems and devices (e.g. – Directive – Position indicating
Reexamination Certificate
2001-01-19
2002-12-10
Issing, Gregory C. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Position indicating
Reexamination Certificate
active
06492945
ABSTRACT:
1. BACKGROUND OF THE INVENTION
1.1. Field of the Invention
This invention relates to improved techniques for determining position by radio, more particularly for determining position instantaneously, from carrier-wave phases of radio signals received from transmitters having different carrier-wave frequencies. In a preferred embodiment of the invention, these carrier waves also have random phases, and the position of a receiver is determined by reference to the phases of signals received by another, reference, receiver. The invention includes resolving, or reducing, the position ambiguity stemming from the integer-cycle ambiguity inherent in an observation of the phase of a periodic wave such as a carrier wave. Resolving ambiguity enables practically instantaneous position-determination, which is a substantial improvement over prior-art techniques based on sustained, continuous, carrier phase tracking.
1.2. U.S. Pat. No. 4,667,203; GPS; GLONASS
In U.S. Pat. No. 4,667,203 one of the present inventors (Counselman) discloses methods and systems for determining position from carrier-wave phases of radio signals received from a plurality of transmitters aboard earth-orbiting satellites such as those of the Global Positioning System (GPS). The disclosure of Counselman '203 includes methods and systems for resolving position ambiguity caused by the integer-cycle ambiguity of carrier phase. In a preferred embodiment of Counselman '203, the transmitted signals have random phases, and the position of a receiver is determined by reference to the phases of signals received at another, reference, receiver. However, Counselman '203 requires the transmitted signals to have suppressed carriers and/or wide, overlapping, spectra.
Counselman '203 does not teach or render obvious a technique for determining position from carrier waves transmitted with different frequencies. Counselman '203 does not teach or render obvious an instantaneous positioning technique. On the contrary, Counselman '203 teaches determining position by combining observations spanning a significant period of time, such as several thousand seconds.
In the Russian GLONASS system, which is otherwise very similar to GPS, different satellites transmit signals with slightly different implicit carrier frequencies. (Both the GLONASS and the GPS satellites transmit substantially overlapping spread-spectrum signals whose carriers are actually suppressed.) These slight carrier-frequency differences are intended to facilitate the separation of different satellites' signals in a receiver. An instantaneous radiopositioning method using combined GPS and GLONASS observations that includes resolving implicit carrier-phase ambiguities despite the GLONASS frequency differences is disclosed in a paper entitled “Single-epoch integer ambiguity resolution with GPS-GLONASS L1 data,” by M. Pratt et al., appearing in the Proceedings of the Institute of Navigation Annual Meeting in Albuquerque, N. Mex., June 1997, pp. 691-699.
1.3. MITES
In an article entitled “Miniature Interferometer Terminals for Earth Surveying (MITES)”, appearing in Bulletin Geodesique, Volume 53 (1979), pp. 139-163, by Charles C. Counselman III and Irwin I. Shapiro, there is proposed a system for determining position by measuring carrier-wave phases of multi-frequency radio signals received from a plurality of earth-orbiting satellites. The reason for having multiple frequencies is to resolve ambiguity and determine position instantaneously. However, the MITES scheme requires each one of the plurality of transmitters to emit the same multiplicity of different-frequency carrier waves. (Small frequency shifts and/or modulation are used to mitigate interference, as in the above-mentioned GLONASS system.) In other words, although Counselman and Shapiro teach the use of multiple frequencies, they teach that the multiple frequencies must be transmitted by each single transmitter; and that all transmitters should transmit the same frequencies.
1.4. Differential Measurements
The idea that all transmitters utilized in a radio-positioning technique should transmit the same or nearly the same frequencies is fundamental to many radiopositioning systems, including Loran and Omega (discussed below) in addition to GPS, GLONASS and MITES (discussed above). It is desired for all transmissions to be the same frequency band, or “channel,” not merely to conserve spectrum, but fundamentally to facilitate differential measurements, i.e., measurements of differences between signals received from different transmitters.
1.5. Loran
The Loran system is described in an article by W. O. Henry, entitled “Some Developments in Loran,” appearing in the Journal of Geophysical Research, vol. 65, pp. 506-513, February 1960. The current version of Loran, known as Loran-C, employs several-thousand-kilometer-long chains of synchronized transmitters stationed on the surface of the earth, with all transmitters having the same implicit carrier frequency, 100 kiloHertz, but with each transmitter emitting a unique time-sequence of short pulses. This sequence, which includes polarity reversals of the pulses, enables a receiver to distinguish between signals from different transmitters. A suitable combination of observations of more than one pair of transmitters can yield a determination of the receiver's position on the surface of the earth. Basically, a receiver observes the difference between the times of arrival of pulses from a pair of transmitters. Since the transmitters are synchronized, a time-difference-of-arrival (TDOA) observation implies that the receiver is located somewhere along a particular hyperbolic curve having vertices at the transmitters. (The locus of points having a given difference between their distances from two vertices is an hyperbola.) Observing TDOA for additional pairs of transmitters provides additional hyperbolic constraints on the receiver's position, and enables a unique position to be determined.
1.6. Omega
The Omega system is described in an article by Pierce, entitled “Omega,” appearing in IEEE Transactions on Aerospace and Electronic Systems, vol. AES-1, no.3, pp. 206-215, December 1965. Omega, like Loran and conventional GPS, is an hyperbolic positioning system. In the Omega system, the phase difference between the radio waves received from different transmitters is measured rather than (principally) the time difference (TDOA) as in the Loran-C system. To facilitate resolution of phase ambiguity, Omega transmitters transmit plural frequencies. However, different transmitters transmit the same frequencies. Again, this is done to facilitate differential measurements.
1.7. Utilizing Signals of Opportunity
It is known in the radiopositioning art, i.e., the art of determining position by radio, to utilize signals of opportunity, by which we mean signals emitted by uncooperative transmitters. Typically such transmissions are not intended for positioning; different transmitters operate on wholly different frequencies; they are not synchronous; and their carrier-wave phases are random. Lack of synchronization or instability in time, frequency, and/or phase prevents many radiopositioning methods from being usefully employed.
An example of radiopositioning by utilizing signals of opportunity is determining position by radio direction finding (RDF) observations of commercial broadcast signals in the medium-frequency, amplitude-modulated (AM) broadcast band from about 550 to 1700 kHz. These signals are transmitted for purposes other than positioning, but the transmitters are marked and identified on nautical charts to facilitate their use as radio beacons, for navigation by RDF. Different AM broadcast transmitters within any given region of the country (or world) are assigned to completely separate, disjoint, frequency channels to avoid interference.
Another prior-art radiopositioning technique utilizing signals of opportunity tracks the phases of the carrier waves of signals received from commercial broadcasters in the medium-frequency AM broadcast
Counselman, III Charles C.
Hall Timothy D.
Choate Hall & Stewart
Issing Gregory C.
Massachusetts Institute of Technology
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