Radiolocation systems now in public and military use employ a multiplicity of geographically distributed transmitters producing modulated signals. These transmitters are ground-based in older systems such as Loran. The newest systems, such as the Global Positioning System (GPS) developed by the U.S. Department of Defense, employ multiple earth satellites in known, controlled orbits. At times even these GPS satellites are supplemented by ground-based transmitters called "pseudolites." The basic principle of radiolocation is straightforward: (1) measure the time required for the signal from each transmitter to arrive, and (2) by application of geometry to the set of distances from the satellites, determine the location of the receiver. If the receiver and satellites were all equipped with zero-error time standards, signals from only three (non-coplanar) satellites would be required to locate the receiver instantaneously upon measurement of the signal delays. Timing uncertainty at the receiver adds a fourth dimension to the problem, which can be solved by computing with four remote signals.
In the GPS system, satellite signals are controlled by highly precise (atomic) clocks in the satellites, and are further adjusted by data transmitted periodically from ground control stations. Each satellite transmits, at a low data rate, ground-computed departure from its planned orbit, and ground-observed bias in the satellite clock. Since the original design of the system was based on the requirement that at least four satellites be continuously in view, on or just above the receiver's horizon, a large part of the earth's surface, there are often more than four satellites in radio contact (that is, not eclipsed by the earth). Indeed, the Russian navigation satellites, GLONASS, may also be in view of the receiver and used in the position computation in addition to GPS. There are expected to be 24 GLONASS satellites in orbit.
While a receiver in space would require four satellite signals to locate itself, ground, sea or air based vehicles have additional information about altitude above the earth's surface that assists in obtaining a location solution when fewer than four satellites are within view. For example, a receiver on the ocean surface has a good estimate of altitude above mean sea level.
My invention relates to design of receivers for use in estimating position from signals derived from multiple remote transmitters. Although the invention is preferably for use in connection with GPS, it should be understood that the principles and the invention are more broadly useful.
Glossary of terms as used herein:
Delay-lock loop (DLL): A delay lock loop is an electronic circuit which operates to maintain two signals with the same known time-sequence, in time-synchronism. Typically, a digital pseudo-noise signal is received by radio communication and reference signal with the same binary sequence is generated locally. Because of: uncertainty in time between the remote and local sources, relative motion of the remote and local signal source, transmission delay, and slight differences in timing (clock) frequency, the time difference by which the reference signal must be shifted is not accurately predictable. A delay lock loop.sup.1 includes a correlator (which may also be referred to as a discriminator), and the output of this electrical circuit is minimum when the two input signals are in synchronism. The correlator output goes positive for one direction of time shift between the two signals and negative for the other. It will usually be small unless the two signals are less than about one-half chip interval from exact synchronism. The output of the correlator, usually with high frequency components filtered out, can be used to control the time delay in the reference signal generator by using it to control a number controlled oscillator. In a delay lock loop, the circuit is closed to maintain synchronism. The time delay between the two signals, as indicated by the amount of shift in the local reference generator, can be multiplied by the velocity of light to represent a pseudo-range between the remote source and the local system. FNT .sup.1 (a name originated by the inventor) Reference - J. J. Spilker, Jr., Digital Communications by Satellite, Prentice Hall, Englewood Cliffs, N.J. 1977.
Pseudo-noise (PN) signal or sequence: The signal produced by a reference generator, usually in the form of a time series of binary digits (bits: zeros or ones). The length of the sequence is the number of bits (chips) in the sequence before it repeats.
Chip: One bit-period of a pseudo-noise signal.
Number controlled oscillator: A frequency synthesizer whose clock phase and rate can be shifted in time by inputting a binary number. If a positive digital value advances the sequence by a corresponding number of steps, a negative value will retard it accordingly.
PN Generator: A pseudonoise sequence generator usually comprised of a feedback shift register with feedback taps and other logic designed to give the proper pseudorandom sequence of binary numbers. The PN generator clock rate and phase can be controlled by an NCO.
Coherent and incoherent signals: In radiolocation terminology, the signal from a remote transmitter is coherent if the timing of the PN signal and that of the carrier frequency are derived from a common timer (clock). If the signal is not coherent, it is usually necessary to maintain separate DLLs in the receiver to track the carrier frequency and the PN modulation of the carrier.
Carrier-tracking loop: With coherent or incoherent signals, a separate loop may be used to track carrier frequency, locking on the intermediate frequency carrier in the receiver. The carrier frequency shift signal is a relative-velocity output that can be separately fed into a location estimator.
Coherent and noncoherent detection and tracking: With coherent tracking the received carrier phase is estimated and used as part of the detection/tracking operation. In noncoherent detection knowledge of carrier phase is not required and envelope or square-law detectors are employed. Noncoherent detectors often are slightly inferior in performance to coherent detectors. However, in some instances coherent detection is not feasible.