Title: VLF navigation system

Description: BACKGROUND OF THE INVENTION

The present invention has particular utility and important advantages as applied to aircraft navigation, and hence much of the discussion herein relating to the background of the invention and to the objects and advantages thereof, as well as the detailed description of a particular embodiment of the invention, will be directed toward the aircraft navigation aspect of the invention. Nevertheless, as will become apparent hereinafter, the present system and/or portions thereof are equally well suited for a variety of other types of navigation, and accordingly the present system and various inventive aspects thereof are not limited to use in connection with aircraft navigation.

Early aircraft navigation systems were principally of the "point source" type, and these point source navigation systems still are by far the most widely used type systems in aircraft navigation today. Point source navigation systems include such systems as "ADF" (automatic direction finding equipment); marker beacons; "VOR" (VHF omnirange stations); and "DME" (distance measuring equipment).

The ADF spectrum is generally from about 190 KHz to about 1750 KHz, and generally includes LF (low frequency) and MF (medium frequency) non-directional beacons and AM commercial broadcast stations.

Marker beacons set at 75 MHz have been operational for many years along U.S. federal airways, but are currently being phased out as an en route aid. They are still standard along instrument landing approaches.

VOR (sometimes referred to as "Omni" stations are internationally standardized for use by aircraft, in the frequency range of 108 MHz to 118 MHz. These stations provide bearing information relative to the respective stations, and the techniques have been greatly refined in recent years in an attempt to reduce site errors and ambiquities. VOR navigation is probably the most widely used form of aircraft navigation throughout the world today.

DME is a pulse-ranging system for aircraft, in the 960-1215 MHz band, and involves pulsed transmission from the aircraft and a return pulse from a ground station after a fixed delay.

VOR and DME are now frequency colocated to form a single site area coverage system, which is a Rho-Theta area coverage grid system. For each VOR frequency there is paired DME frequency, and a common channel selector is provided in compatible airborne equipment. The U.S. VORTAC, for example, is a VOR/DME system utilizing the DME function of the military TACAN (UHF tactical air navigational aid) system for distance measurement.

These point source type navigation systems, while useful in making approaches, are generally inadequate for en route navigation, and particularly for navigation on a worldwide basis which involves flying over great expanses of water and large primitive land masses.

Similarly, the area navigation combinations of such point source navigation systems, as for example VOR/DME, are generally inadequate, and involve serious problems. Thus, these systems utilize VHF and/or UHF, both of which are very limited in range, and are generally "line of sight". Accordingly, each such station only covers a very limited surrounding area, so that literally hundreds of stations are required to cover even the limited area of the United States. Even then, for reliability, due to the likelihood of station failure, even that number must be multiplied with backup equipment and overlapping station coverage. Such point source type area navigation equipment requires manual station selection, and frequent reselection because of the short range. Also, such equipment is generally subject to station errors and obstructions.

Nevertheless, for lack of a better system prior to the present invention, this VOR/DME type area navigation equipment is currently being widely implemented by the U.S. Government on a national basis.

One approach which has recently come into some use in commercial aviation to give more flexibility in area navigation is the inertial navigation system. This is a true "area" navigation system of the dead reckoning type, wherein an airborne computer provides position, ground speed, heading, time and distance to go to destination or a selected waypoint, and the like. The computer is fed inertial information from a series of accelerometers and gyros. Such inertial navigation systems are extremely expensive and are very heavy and bulky due to the nature of the sensors, and they are therefore only suitable for very large commercial and military aircraft of the intercontinental type. Inertial navigation systems also have the disadvantage of being subject to cumulative errors from the point of departure. Accordingly, such inertial systems appear to have only very limited usefulness in aircraft navigation.

Another group of navigation systems employed for area navigation is the "hyperbolic" group of navigation systems wherein navigational position fixes are made by time referencing signals from a plurality of widely spaced ground stations in a hyperbolic coordinate system related to the ground stations. Such hyperbolic-type navigation systems have the advantage of utilizing low frequencies, generally on the order of from about 10 KHz to about 130 KHz, which greatly extends the station range from hyperbolic systems as compared to point source area navigation systems, thereby permitting navigation over large areas with the use of only a very few stations. Hyperbolic navigation systems also have the advantage that they have great inherent accuracy because of the good frequency stability that can be produced in this low frequency range, although such accuracy depends entirely upon the capability of the system to produce close synchronization between transmitted signals and receiver reference signals. Hyperbolic navigational systems also have the advantage that, assuming the transmitted signals and receiver reference signals can be accurately synchronized, errors are generally not cumulative in nature.

One of the major problems in connection with prior art hyperbolic navigation systems is that very complicated and expensive hardware has generally been required in order to provide the necessary signal synchronization. Principal prior art hyperbolic systems include "Decca", "Loran", and "Omega" systems, all of which involve such complex and expensive hardware that they are primarily useful only for ship navigation, and not practical for aircraft navigation.

The Decca system requires a "master" and three "slave" ground stations for each area of coverage, with the accompanying undesirable multiplication of hardware. Decca is in the 70 KHz to 130 KHz frequency range, which is considerably above the 3 to 30 kilohertz VLF range and therefore does not have the advantage of great undistorted range of a VLF system.

Loran requires a master and two slave ground stations, all of which are atomic clock controlled. The Loran system utilizes a 100 KHz frequency, which is undesirably high, and in order to overcome the resulting ionospheric reflection problem, Loran utilizes a special pulsed waveform and gating.

The Omega system has the advantage of being in the true VLF frequency range, between about 10 KHz and about 14 KHz, but has the disadvantage of requiring an atomic clock on board which was initially started in synchronism with the ground station clocks.

These prior art hyperbolic navigation systems are intended to provide absolute positioning, rather than dead reckoning with initial and intermittent position fixing, and such absolute positioning requirement necessitates the excessive and expensive hardware such as atomic clocks, master and slave stations, and the like, employed in the systems.

There have been recent efforts to avoid the use of atomic clocks on board and to reduce the hardware requirement in a hyperbolic navigation system by having receivers with oscillator twelve adapted to be phase-locked to respective transmitter station signals. However, such efforts have generally resulted in equipment having a navigational capability that is unduly limited and uncertain. Such prior art efforts in this direction could not accommodate random area navigation, but required pre-selection of the particular route to be flown between two points, and then prior to take-off the time signals and differences to be utilized had to be looked up in a large book of tables. Only a fraction of the existing VLF transmitting stations could be used which had frequencies capable of being matched by binary division. Lock-on of the receivers to respective station signals was very slow, and under extreme temperature conditions lock-on would not occur at all. Such prior art equipment had no capability for station averaging or diurnal shift correction, or the like. Because of the restricted number of VLF stations that could be used in such system, and the requirement of a pre-selected route, it was not generally suitable for navigation on a worldwide basis.

A problem in connection with all of these prior art hyperbolic-type navigation systems has been to convert the hyperbolic lines of time/position into a meaningful display, and particularly to provide such conversion in a sufficiently compact package for widespread availability and so as to be suitable for use in other than ships or large aircraft; i.e., suitable for use even in small aircraft, and in ground vehicles.

The VLF frequency range is generally not subject to shadowing, and will permeate areas in the middle of numerous obstructions, as for example city streets despite the presence of tall buildings and other obstructions. Accordingly, the VLF frequency range is inherently particularly suitable for use in ground vehicle location and control (which is currently handled principally by VHF equipment that is particularly subject to interference from physical obstructions). However, the prior art VLF navigation systems have generally been much too cumbersome and expensive for such purpose.

SUMMARY OF THE INVENTION

In view of these and other problems in the art, it is a general object of the present invention to provide a novel navigation system of the area type which utilizes the advantages of the VLF electromagnetic frequency spectrum for accuracy, stability, and wide area coverage, yet which requires a minimum amount of mobile or airborne hardware.

Another object of the invention is to provide a novel VLF navigation system on either a hyperbolic or a Rho-Rho coordinate basis which embodies mobile or airborne receiver means that is compatible with any VLF frequency, whereby the system is capable of utilizing any and all available VLF transmitter stations regardless of the transmitted modulation. This capability enables the system to be utilized on a worldwide navigational basis utilizing currently available VLF communication signals and/or Omega timing signals.

Another object of the invention is to provide a novel VLF navigation system utilizing either hyperbolic or Rho-Rho coordinates wherein the lines of time/position are converted into a meaningful display that is continually upgraded by computer means, including such information as present longitude and latitude, distance and heading to destination or a waypoint, distance and reciprocal heading, groundspeed, and time to touchdown or a waypoint.

Another object of the invention is to provide a novel VLF area navigation system which has complete flexibility in route selection and in the selection of new routes while in transit.

Another object is to provide an area navigation system which, although utilizing hardware of modest extent, is extremely accurate, generally being capable of defining position accuracy (in terms of longitude and latitude) at any point around the world to within several hundred meters.

One factor contributing to this extreme accuracy is the use of VLF electromagnetic signals as the basis for navigation, preferably within the frequency range of from about 10 KHz to about 25 KHz, but if desired over the entire VLF frequency spectrum, which is considered to be from 3KHz to 30 KHz. In the VLF frequency range, there is no appreciable "shadowing" of the received signal, even in valleys behind hills, and even in areas with a multitude of obstructions, such as in cities. For example, VLF signals can be picked up without much degradation even inside of a building.

The ionosphere functions like a natural wave guide for VLF electromagnetic radiations, so that the VLF waves will go clear around the globe without reflecting, which causes ambiguities with higher frequencies.

Another factor contributing to the extreme accuracy of the present system is related to the capability of "locking on" to all VLF station frequencies within range. This allows a sufficient number of stations for station averaging to improve accuracy, and as a part of the averaging function any station outside of a reasonable average may be rejected to maintain accuracy. This accessibility to numerous station frequencies also enables station signals that are degraded by the diurnal shift ("night effect") to be programmed for correction in the computer forming a part of the apparatus.

Another factor contributing to the accuracy of the present system, as well as enabling the system to utilize any and all VLF frequencies, and also minimizing the hardware requirement, is novel receiver phase-locked loop means which normalizes all station frequencies to independent phase coherent data signals for a common working frequency. These phase coherent data signals for all VLF stations in range provide time difference data that is readily measured and converted to position data in the coordinates from the initial or an intermediate calibration point.

Still another factor in the accuracy of the system and the minimization of hardware is novel multiplexing and counting means, including logic circuitry, enabling a single counter to be rapidly sequentially employed by the computer of the system to make the time difference comparison for a series of station pairs.

Accuracy of the present system is maintained during a navigational sequence because errors tend to be random, both for the transmitter station coordinate system and for the phase-locked loop and time difference measuring portions of the system.

The extreme accuracy of the system is not rendered uncertain by any ambiguity or anomaly, since time difference information for a particular direction of travel is counted only up to a quarter cycle of the normalized data frequency. With the presently preferred normalized data frequency of 100Hz such quarter wave corresponds to a little over 400 nautical miles. The computer automatically recalibrates the phase coherent receiver data signals within this distance to avoid possible ambiguity or anomaly.

While the present invention is described herein primarily with respect to airborne navigational equipment for providing such navigational information as position, distance and heading (or reciprocal heading) to a target, groundspeed, and time to a target, it is to be understood that the present system and specific portions thereof are also readily adaptable for providing other navigational information, and for other general purposes than airborne navigation. For example, inputs can be added other than those specifically referred to herein to expand the functions, some examples being: (1) speed or velocity indication; (2) by the input of altitude information, glide slope information can be a product; (3) air data systems; (4) determination of wind and vectors; (5) can tie in with inertial systems and doppler and other navigation systems such as Loran and radar.

In addition to airborne navigation, it is contemplated that the present VLF system or portions thereof may also be utilized in some or all of the following: (1) collision avoidance, air and marine, above surface, surface, and subsurface; (2) marine navigation, surface and subsurface; (3) vehicular location and control (police, cabs, trains, and the like); (4) programmable automatic navigation; (5) Man pack (police, search, or the like); (6) geological survey; (7) reconnaissance; (8) remote position indicator; (9) airport ground control.

While the computer of the present system is described herein as part of the airborne equipment, it is to be understood that for some of the above additional uses it will be desirable to have one or more ground station computers which can perform all or some of the functions of the present airborne computer.

For example, in aircraft collision avoidance systems, the time difference information from the receiver phase-locked loops of the present system may be transmitted via radio signals to a ground station for processing and comparison with similar signals for other aircraft so as to determine the positions of the aircraft with respect to each other. Similarly, with vehicular location and control, it may be desirable to utilize a central computer which presents the vehicular position information to an operator at a fixed station.

Thus it is that the invention provides a navigation method in which two signals of different frequency are normalized or converted to two other signals of like frequency which are coherent. The phase difference between those two other signals is measured. It is measured by forming those two other signals as repetitive waves having similar shape and measuring the time that elapses between corresponding portions of the two waves. That has a number of advantages, one of which is that normalizing of both signals can be accomplished by using a common signal. That is accomplished in a preferred form of the invention by using dividers in the normalizing process and using the same signal to control operation of all dividers. The result is a high degree of accuracy. Phase is measured directly rather than indirectly by measuring error signal or the like.

The common normalizing signal may be derived from the on-board clock in the case where one of the "two other signals" is provided by such a clock. In addition to the method, the invention relates to apparatus with which to practice that invention and a preferred form of that method is described broadly as the method of converting an RF signal of one frequency into a normalized signal of another frequency, which comprises the steps of providing an oscillator signal for said RF signal which has a frequency that is a multiple of said RF signal frequency, said oscillator frequency being divisible by an integer to said normalized signal frequency, phase locking said oscillator signal to said RF signal, and dividing said oscillator signal by said integer to produce said normalized signal.

Another preferred form of the method is described as the method of making a phase difference comparison between first and second phase separated low frequency digital electrical waveforms having the same frequency, which comprises providing a string of high frequency electrical clock pulses, gating said clock pulses on by a leading edge of said first waveform, gating said clock pulses off by the corresponding leading edge of said second waveform, and counting the gated pulses.

The invention also envisions a preferred form of apparatus described broadly as comprising apparatus for converting an RF signal of one frequency into a normalized signal of another frequency, which comprises a phase-locked loop for said RF signal, said loop including an oscillator which has a frequency that is a multiple of said RF signal frequency, said oscillator frequency being divisible by an integer to said normalized signal frequency, divider means connected to said oscillator and arranged to divide the oscillator signal by said integer to produce said normalized signal.

It also envisions apparatus described broadly as comprising apparatus for comparing the phase difference between first and second phase separated low frequency digital waveforms having the same frequency, which comprises gating circuit means, first and second input connections to said gating circuit means applying the respective said first and second waveforms to said gating circuit means, a clock connected to said gating circuit means and providing a string of high frequency electrical pulses to said gating circuit means, an output from said gating circuit means, and counting means having an input connected to said gating circuit means output for counting gating clock pulses, said gating circuit means being adapted to gate said clock pulses to said counting means by a leading edge of said first waveform and to gate off said clock pulses from said counting means by the corresponding leading edge of said second waveform, whereby the number of said clock pulses counted by said counting means will be in direct proportion to said phase difference.

Further objects and advantages of the present invention will appear during the course of the following part of the specification, wherein the details of construction, mode of operation, and novel method steps of a presently preferred embodiment are described with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram illustrating the relationship of the computer, control head, and display portions of the present navigation system.

FIG. 2 is a plan view of a control head suitable for an airborne navigation system according to the invention.

FIG. 3 is a plan view of a display or readout unit also suitable for an airborne navigation system according to the invention, and particularly adapted for use in connection with the functions designated in the control head illustrated in FIG. 2.

FIG. 4 is a world map, in Mercator projection, indicating some currently available VLF transmitting stations suitable for use with the present invention.

FIG. 5 is a chart giving details of the VLF transmitting stations indicated on the world map of FIG. 4.

FIG. 6 is a graphical illustration of two independent lines of position and their respective hyperbolic lines of constant time difference for two VLF station pairs that are ideally located relative to each other.

FIG. 7 is a graphical view similar to FIG. 6, but illustrating independent lines of position and respective hyperbolic lines for two station pairs more typically located.

FIG. 8 is a graphical view similar to FIGS. 6 and 7, but illustrating two independent lines of position and respective hyperbolical lines of constant time difference for the minimum of three VLF transmitting stations.

FIG. 9 is a polar plot illustrating VLF transmitting station pairs available for navigation with the present system with a present position at the tip of Argentina.

FIG. 10 is a polar plot similar to FIG. 9 illustrating station pairs available for navigation with a present position at southwest Ontario, Canada.

FIG. 11 is a polar plot similar to FIGS. 9 and 10 illustrating station pairs available for navigation with the present system with a present position at southwest British Columbia, Canada.

FIG. 12 is an overall block diagram of the mobile or airborne portion of the present navigation system.

FIG. 13 is a general block diagram of an input portion of the system from antenna through receivers, the embodiment illustrated in FIG. 13 having 12 receivers adapted to receive and lock onto VLF signals from 12 respective transmitting stations.

FIG. 14 is a block diagram of input elements of the present system, including a complete block diagram of one of the receivers.

FIG. 15 is a detailed circuit diagram of a broadband preamplifier suitable for amplification of all of the incoming VLF signals, and preferably housed at the base of the voltage probe antenna employed to intercept the signals.

FIG. 16 is a circuit diagram of a transient suppressor through which all of the VLF signals pass.

FIG. 17 is a circuit diagram illustrating narrowband TRF amplifier, limiter, and high level AGC circuits employed in a particular receiver.

FIG. 18 is a combined detailed circuit diagram of phase-locked loop and signal present circuits, and logic diagram of programmable dividers employed in combination therewith.

FIG. 19 is a logic diagram for the phase-locked loop of the receiver for a particular VLF transmitting station frequency, indicating the logic of the two programmable dividers employed to divide a selected voltage controlled crystal oscillator frequency respectively to (1) a 100 Hz time reference signal and (2) a station frequency signal suitable phase shifted for loop locking and signal present designation.

FIGS. 20 to 27 are logic diagrams similar to that of FIG. 19, but applied to other currently available VLF station frequencies.

FIG. 28 is a block diagram of a multiplexer and time difference digitizer employed in the present system for determining time difference data sequentially from any number of transmitting station pairs selected by the computer.

FIG. 29 is a logic diagram for the logic block in FIG. 28 that assures the data taken is a full, valid count and holds such data in the counter of FIG. 28 until a new "take data" request is received from the computer.

FIGS. 30 and 31 show time event waveforms for the logic diagram of FIG. 29, for two different input conditions to the logic diagram of FIG. 29 when the take data request is received from the computer.

FIG. 32 is a simplified schematic wiring diagram for the control head illustrated in FIG. 2, illustrating how the computer communicates back and forth with the control head which serves principally as an interface with the computer.

FIG. 33 is a computer algorithm flow chart.

FIG. 34 is a block diagram of a miniaturized computer suitable for performing the necessary computer functions for the present navigation system.

FIG. 35 is a graphical illustration of VLF lines of constant phase providing a Rho-Rho coordinate system for two VLF transmitting stations.

FIG. 36 illustrates modifications in the overall block diagram of FIG. 12 which adapt the system to the use of an on-board frequency standard.

FIG. 37 illustrates a modification of the input block diagram of FIG. 13 to include an on-board standard.

FIG. 38 illustrates a manner in which the multiplexer of FIG. 28 may be modified to adapt that portion of the system to the use of an on-board standard.

FIGS. 39, 40, and 41 graphically illustrate the algorithm for updating position en route with the on-board frequency standard providing one station of each station pair utilized in the computation.

DETAILED DESCRIPTION

The following detailed description will be directed particularly to a VLF navigation system according to the invention which is particularly adapted for airborne navigation, and wherein the computer portion of the system is a part of the airborne equipment. It is to be understood, however, that the principles of the invention are equally adaptable to other arrangements as indicated hereinabove, and that in some systems utilizing the principles of the invention the computer may be disposed in a fixed, central location and receive position information from one or more mobile units through radio communication.

FIG. 1 of the drawings provides a general indication of the relationship between the computer 10 and the two units with which the operator has direct communication, namely, the control head 12 and the display unit 14. The control head 12 is the interface between the operator and the computer 10. The control head 12 serves as the data input source to the computer, and it also informs the computer what information is to be displayed on the display unit or panel 14.

The control head 12 illustrated in FIG. 2 may be small and compact, but nevertheless includes all of the necessary switching as an interface for the computer 10 in a worldwide VLF navigation system according to the invention. Thus, the control head 12 includes a master on-off switch 16, which is a locking switch so that it cannot be inadvertently switched off and thereby inadvertently lose RAM (randome access memory) information stored in the computer 10 as a part of a particular navigational sequence.

A keyboard 18 on the control head 12 operates a switch matrix for entering data to the computer 10. When function selector switch 22 is in an enter data position, the enter data indicator light 20 flashes.

The function switch 22 also controls a switch matrix, and includes a series of positions for the selective entry of particular data, as well as a further series of positions designating en-route information desired by the operator to be displayed on the display or readout unit 14. When the function selector switch 22 is in one of the positions indicating en-route information, the en-route indicator light 24 is on.

The present navigation system is a dead reckoning type of navigation system with intermittent position fixing. The system requires the entry of initial position coordinates and time, and this initial positioning can be upgraded whenever the aircraft bearing the equipment passes over any known landmark. Then, based upon such initial coordinates and time, or upgraded coordinates and time, the system constantly determines .DELTA. coordinates from the initial or upgraded fix and thereby provides updated information to be displayed on the display unit 14 at the operator's discretion relating to position, left/right guidance information to destination or a waypoint, heading and distance to a waypoint or destination, ground speed, and time for arrival at destination or a waypoint.

Prior to start, the control head 12 is placed in the "enter data" mode by actuation of the function switch 22, and keyboard 18 is suitably actuated to enter the starting data. Thus, the function switch will be placed in the "start longitude" position, and the initial longitude will be entered into the computer by suitable actuation of the keyboard 18. The function switch 22 will then be placed in the "start latitude" position and the initial latitude will be entered by means of keyboard 18. Function switch 22 will be selectively positioned at the "date" and "Greenwich Mean Time" positions, and this informmation will likewise be selectively entered by means of keyboard 18. The coordinates of the destination will also be entered during this enter data mode, the function switch 22 being placed in the "destination longitude" position and the entry thereof being made through keyboard 18, and the function switch 22 being positioned in the "destination latitude" position and this information being entered through keyboard 18. During entry of the destination coordinates "waypoint" switch 26 is placed in position 5. In addition to such destination coordinates, up to four sets of waypoint coordinates may also be entered during this enter data mode by positioning a waypoint switch 26 in selected waypoint positions 1 through 4 and utilizing the keyboard 18 to enter the destination longitude and destination latitude coordinates of the respective waypoints.

Thus, the programming of the system is easily accomplished by placing the system in the enter data mode; and then, with the use of the keyboard 18 and function switch 22, simply "keying in" present position in longitude and latitude, the date and time, and destination, with desired waypoints.

Referring now to FIG. 3, the display or readout unit 14 includes a left/right guidance display 28 which provides left-right course deviation information to destination or to a waypoint. The display 28 illustrated in FIG. 3 is a digital type of display suitable for making direct use of the digital course deviation information provided by the computer. If preferred, this information may be transformed into analog information for a conventional meter-type indicator. The left/right guidance display 28 has a central, large window 30 indicating on course. A series 32 of spaced dots to the left of the central window 30 selectively illuminate to indicate the distance to the left of track to the destination or waypoint. Similarly, a series 34 of spaced dots to the right of central window 30 selectively illuminate to indicate the distance to the right of track to the destination or waypoint. With this arrangement, the dots of the left/right guidance display 28 are "flown" like a VOR needle; if one of the dots to the left of center is illuminated, the aircraft must be steered to the left to correct for the deviation; while if one of the dots to the right of center is illuminated, the aircraft must be steered to the right to correct deviation.

The left-right guidance display 28 will be in continuous operation during the "en-route" mode, and will indicate left/right positioning relative to the track to the final destination, unless one of the waypoints is selected by the waypoint selector switch 26, in which case the left/right indication will be for that waypoint. Typically, the series of waypoints will be selectively dialed in by the waypoint selector switch 26, and the left/right guidance display 28 will then apply to each waypoint in turn, and then to the final destination.

For normal en-route operation, each dot in the display 28 indicates a certain increment of distance off course. For example, each of the dots in the display 28 may normally indicate a 1200 foot increment off course. As an example of this, if the third dot to the left of the central window 30 is illuminated in the series 32, with the increment being 1200 feet per dot, this would indicate that the track to the destination or waypoint is approximately 3600 feet to the left of the present course. However, the accuracy of the present system is suitable for use in an approach mode, wherein a more refined left/right readout is desirable. Accordingly, an approach switch 36 is provided on the control head 12, and actuation thereof provides a different order of magnitude for the digital readout on the left/right guidance display 28, as for example 120 feet per dot for the series 32 and 34 of spaced dots. When the approach switch 36 is actuated on the control head 12, an approach mode light 38 will be illuminated on the display unit 14 to indicate the order of magnitude of the increments indicated by the dots in the display 28.

The display or readout unit 14 also includes a pair of digital display-type readouts 40 and 42, which are driven by shift register type decoders that are responsive to serial pulse data from the computer. The readout 40 has a sevensegment digital display, the seven digits being required to display longitude information in degrees, minutes, and hundredths of a minute (or seconds if desired). On the other hand, a six-digit digital display is sufficient for the decoder 42, which is used to display latitude information in degrees, minutes, and hundredths of a minute (or seconds if desired).

In the en-route mode, with the function switch 22 in the longitude-latitude position, the readout 40 will display present longitude, and readout 42 will display present latitude. This information will be constantly upgraded by the computer approximately once per second or faster. Associated with the readout 40 are east and west lights 44 and 46, respectively, which will illuminate according to whether the longitude reading is east or west. Similarly, associated with the readout 42 are north and south lights 48 and 50, respectively, which will illuminate according to whether the readout 42 is north latitude or south latitude.

In the en-route mode, with the function switch 22 turned to the distance-heading position, the readout 40 will have a readout in nautical miles to destination, or to a waypoint indicated by waypoint switch 26, and the readout 42 will have a readout in true heading to destination or such waypoint. These readouts for readouts 40 and 42 will be respectively indicated by "nautical mile" and "heading" lights 52 and 54.

In the en-route mode, with the function switch 22 in the distance-reciprocal heading position, the readout 40 will likewise give the distance in nautical miles to destination or a selected waypoint, as indicated by the light 52; while the readout 42 will indicate the reciprocal heading to destination or such waypoint, as indicated by a reciprocal heading light 56.

In the final position of function switch 22, which is the speed-time to target (or touchdown) position, the readout 40 will display the present groundspeed in knots, as indicated by the light 58; while the readout 42 will display the remaining time to touchdown at the destination, or time to a selected waypoint, in hours, minutes, and seconds, as indicated by illumination of the "time to target" light 60.

Accordingly, in the en-route mode, at the operator's discretion navigational information is constantly updated and displayed which includes left/right guidance information to waypoint or destination, which may be in a refined approach mode if desired; longitude and latitude of present position; distance and heading to waypoint or destination; distance and reciprocal heading to waypoint or destination; and groundspeed and time to waypoint or touchdown at destination.

In the event that sufficient station receivers at any time fail to be "locked" onto respective VLF station signals, it is assumed that present course and speed will be maintained, and on such basis the same navigational information will be updated and displayed on the display unit by the computer according to the position of the function switch 22. In this case, a "dead reckoning" light 62 will illuminate on the display unit 14 to indicate that insufficient VLF stations are presently in acquisition for the normal navigation mode.

Referring now to FIGS. 4 and 5 of the drawings, ten currently available VLF transmitting stations suitable for the present system are indicated on the world map of FIG. 4, with details thereof set forth in the chart of FIG. 5. The VLF frequency range is generally considered to be from 3 KHz to 30 KHz, which represents a wave length range of from about 10.sup.5 meters to about 10.sup.4 meters. All of the currently available stations designated in FIGS. 4 and 5 are in the 10 KHz to 25 KHz range, which is therefore the presently preferred VLF range. Nevertheless, any RF station frequency may be utilized in the present system, provided its frequency is very stable. All of the VLF stations indicated in FIGS. 4 and 5 are frequency-controlled by atomic clocks, so as to be stsble to within an accuracy of on the order of about one part in 10.sup.-.sup.11. Frequency and hence phase stability of the VLF transmitting signals with this general order of accuracy enables any such VLF transmitting stations to be utilized in the present system even though none of these transmitting stations are synchronized relative to each other. Utilization of a separate receiver in the present system for each such station, and provision of a phase-locked loop in each receiver which accurately locks onto the respective transmitted signal, enables these unsynchronized but accurate transmitted VLF signals to be utilized in the present system for time difference measurements without requirement of special waveforms or pulses. Additionally, the present system embodies in each receiver novel programmable dividers capable of dividing by any integer and not limited to conventional binary counter type division, so that all received VLF frequencies may be reduced to a common operational frequency in the system (e.g., 100 Hz).

Accordingly, the present system is the only VLF navigation system of which the applicants are aware that is capable of utilizing all of the atomic clock controlled VLF transmitting stations.

The ten VLF transmitting stations identified in FIGS. 4 and 5 are sufficient in coverage so that navigation may be accomplished with the present system on a worldwide basis with only these ten stations if desired. Additionally, a sufficient number of these 10 stations can normally be locked onto at any point in the world to permit station averaging so as to increase overall accuracy and to average out any known frequency varying factors such as the diurnal shift.

It is preferred to utilize VLF com (communication) stations because of the CW (continuous wave) signals transmitted thereby. However, intermittent transmitting stations may also be used, such as the Omega stations. If Omega stations are used, it is preferably to utilize Omega timing signals, which are on approximately 60% of the time; the receiver phase-locked loops in the present system being sufficiently stable so that there will not be any appreciable phase shift during the 40% off time.

Eight of the 10 currently available stations designated in FIGS. 4 and 5 are VLF com stations, including Maine, Maryland, Panama, Washington, Hawaii, Australia, Great Britain, and Japan; while two of the designated currently available stations are Omega stations, including Trinidad and Norway.

It is contemplated that two additional stations will be used in equipment according to the invention, and accordingly spaces for two alternate stations are included in the chart of FIG. 5.

THE SYSTEM DESCRIBED IN CONNECTION WITH STATION PAIR SELECTION FOR USE WITH HYPERBOLIC COORDINATES

FIG. 6 illustrates two station pairs A,B and C,D which are ideally located to provide a hyperbolic gridwork of the most uniform possible configuration. In this ideal system, axes A--B and C--D are at right angles to each other and of equal length. Hyperbolic lines 64a, 64b, 64c, etc., are lines of constant time difference between the stations A and B; while the hyperbolic lines 66a, 66b, 66c, etc., are lines of constant time difference between the stations C and D. Thus, the two independent lines of position A--B and C--D provides a hyperbolic gridwork from which a time difference of .DELTA. T will be defined between a start point P.sub.1 and a destination or waypoint P.sub.2 for each set of hyperbolic lines. Inasmuch as the .DELTA. T for each set of hyperbolic lines corresponds to a certain distance (one nautical mile corresponds to a .DELTA. T of approximately 6.8 microseconds) the hyperbolic time coordinates for the destination or waypoint P.sub.2 may be determined by time difference information, and accordingly the longitude and latitude coordinates may be determined for the waypoint P.sub.2. The direction of the line P.sub.1 -- P.sub.2 on the hyperbolic coordinates indicates heading to waypoint or destination. The rate of change of .DELTA. T for each set of hyperbolic coordinates provides groundspeed information.

It will be understood that the station pairs A,B and C,D are all of different frequencies, and are not synchronized or phase locked with each other. They are, however, stabilized by atomic clocks, such as cesium beam clocks, and therefore in fact contain phase information which remains extremely stable relative to each other, and is utilized in the present invention to define the aforesaid .DELTA. T information. Briefly, this is accomplished in the present system by locking a separate receiver in the system to each of the stations A, B, C, and D, with respective phase-locked loops in the receivers which provide loop signals that are in effect phase coherent with the respective transmitter station signals; and then normalizing all of the receiver loop signals to a common working frequency and thus rendering these working frequency signals for the various receivers phase coherent; i.e., having a steady or fixed phase relationship relative to each other. These phase coherent working signals of common frequency in the receivers then define the .DELTA. T information for the hyperbolic coordinates of each station pair such a A,B and C,D in a high frequency crystal controlled clock counter.

By this means the hyperbolic lines of position are converted in the present invention to an accurate and meaningful display, which has heretofore been a major problem in the art.

FIG. 7 illustrates the hyperbolic lines of constant time difference, or lines of position, for station pairs A', B' and C', D' which are more typically located. It will be seen that the two station pairs A'B' and C'D' still provide a good gridwork of hyperbolic coordinates with their lines of constant time difference. With both the ideal hyperbolic gridwork of FIG. 6 and the more typical hyperbolic coordinate system of FIG. 7, it will be seen that the best resolution is obtained generally in the central part of the grid. Station pair selection is made on the basis of both good geometry and sufficiently close range for good signal reception. Nevertheless, a station is not selected which is too close, because of the distortion in the hyperbolic lines which occurs close to a station.

Station pair selection is made by the computer, according to a station pair algorithm stored in the computer ROM (read only memory) chips. For this purpose, the range and bearing (heading) to each transmitter is calculated from the present position periodically. Then, based upon the receiver position at any time, the computer software (ROM) sets up the computer to select stations according to acceptable geometry and range conditions. Alternatively, for this purpose the world may be divided into a large number of small zones, as for example 1800 small zones; and a small number of major zones of irregular shape, as for example 48 major zones. Then, based upon the receiver position at any time, the computer software (ROM) sets up the computer to select stations according to the particular area in which this position is located.

The receivers for the respective stations are all arranged to provide a "signal present" acquisition signal to the computer, which is a go or no-go signal indicating whether or not a particular signal is phase-locked onto its respective station.

As an example, assume that for the particular position, the computer software determines that the computer wants to select Trinidad and Washington as a station pair. The computer then goes to the receivers for Trinidad and Washington, and if both have a signal present indication, the computer will utilize this signal pair. However, if it receives a no-go or signal not present from the Trinidad receiver, then the computer will eliminate every Trinidad pair and will proceed to select station pairs utilizing other stations.

Assuming that there are N stations available to the computer at a particular position (i.e., N stations giving a go or signal present indication from their respective receivers), then the number of unique station pairs available to the computer is N-1. In other words, there are N-1 independent lines of position available from N stations. Since the minimum number of independent lines of position required for navigation is two, then the minimum number of stations required for navigation is three. FIG. 8 illustrates VLF hyperbolic lines of constant time difference for three stations A", B" and C", in an example of typical three-station geometry.

However, with the 10 presently available VLF stations indicated on FIGS. 4 and 5, normally six unique station pairs will be available for navigation at any point around the world. Accordingly, several station pairs may be generated by the computer in the station pair selection routines for each designated position or zone around the world, and it is contemplated that the six best pairs of these several pairs will be utilized by the computer, other pairs being alternates that may be used by the computer if one or two of the other pairs are not locked onto by their respective receivers. The computer station pair selection routines are to be arranged for selection of the best station pair geometry, as well as range.

The presently contemplated use of six station pairs for determining a position enables three independent determinations of positions to be made, since it requires only two station pairs for one position determination.

Statistically, if the errors in these three separate position determinations are truly random errors, and if the accuracy of each determination is to within .DELTA. position, then averaging of the three positions can improve .DELTA. by 1/.sqroot.3.

The mode of operation for station pair averaging in the computer is as follows: The computer takes a first station pair, determines the latitude and longtitude therefrom, and stores this information. The computer then does the same for a second station pair, and then it does the same for a third station pair. Thus, the computer now has stored three independent positions of longitude and latitude. It adds the three longitude positions and divides by three, and adds the three latitude positions and divides by three, to determine an average longitude position and an average latitude position. The computer is programmed to reject any data outside of a reasonable average. Thus, for example, if data from one station pair should be caused by some interference or other factor to be considerably outside of a reasonable average, then such data is rejected, and the computer will then select one of the alternate station pairs, and the new group of three station pairs will again be averaged, and will be accepted if all data is within a reasonable average.

It will be understood that the ten VLF transmitting stations referred to in FIGS. 4 and 5 of the drawings are presently available stations, and that other and perhaps better stations will probably be established, and that perhaps some of these presently available stations will terminate. Accordingly, it is preferred to employ a separate ROM chip in the computer for each separate station, and to arrange these station ROM chips to be replaceable so as to take into account such contemplated station changes.

FIG. 9 is a polar plot, with north at the top of the page, and present position at the pole or center, which is at the tip of Argentina, with a longitude of 66.97 and a latitude of 56.99. The radial distance between adjacent concentric circles on the plot is 1,000 nautical miles. Eight usable station pairs from the aforesaid currently available stations are indicated below the plot. For this position, the computer is programmed to make three independent position calculations from (1) the two station pairs Hawaii-Great Britain and Maine-Australia; (2) the two station pairs Great Britain-Australia and Australia-Washington; and (3) the two station pairs Great Britain-Washington and Panama-Australia. Alternate station pairs programmed to be selected by the computer in the event of either a no-go from a receiver in one of the preferred selected station pairs or in the event data is rejected as being outside of a reasonable average of the three independent sets of data, are Panama-Hawaii, and Hawaii-Australia. The geometry for all of the selected station pairs is good.

While Trinidad is well located as to geometry, it is not at present sufficiently powerful for reliable reception beyond about 4,000 miles, and since it is approximately 4,600 miles from present position, it is not included in the station pairs. The stations are thus picked both for geometry and for range. In this regard, it is to be noted that Japan is located approximately 10,600 miles from the present position, and is likewise not one of the selected stations.

FIG. 10 is a polar plot similar to FIG. 9, but with less detail. The plot in FIG. 10 also has north at the top of the page, and is for a present position at southwest Ontario, Canada, having a longitude of 94.88 and a latitude of 47.21. For the zone in which this position is located, the computer is programmed to make the three position determinations for averaging based upon (1) the two independent lines of position provided by Panama-Japan and Hawaii-Great Britain; (2) the two independent lines of position provided by Japan-Trinidad and Panama-Great Britain; and (3) the two independent lines of position provided by Washington-Maine and Panama-Washington. Alternate station pairs or independent lines of position are Hawaii-Maine and Trinidad-Washington.

It will be noted from the plot of FIG. 10 that the closest stations to the present position (at the center of the plot), Washington and Maryland, are approximately 1,000 miles away. While stations considerably closer than this can normally be employed, it will be apparent from the typical examples of station geometry set forth in FIGS. 7 and 8 that if the range to a station is too close, the hyperbolic grid geometry tends to become considerably distorted for stations substantially offset from the station pair line. Accordingly, the computer is programmed to not use stations that are so close as to cause such distortion, which tends to introduce errors in the calculations.

On the other hand, for strong transmitting stations ranges up to about 10,000 nautical miles may be satisfactory. With the currently available stations indicated on FIG. 4 and 5, the desired station pairs involve stations within 10,000 miles for most navigational zones throughout the world.

FIG. 11 is a polar plot similar to FIGS. 9 and 10 for a present position in southwest British Columbia, Canada, with a W longitude of 129.25 and an N latitude of 47.21. In this case first position data is obtained from the two station pairs Australia-Trinidad and Great Britain-Hawaii; second position data is obtained from the two station pairs Japan-Panama and Hawaii-Maryland; and third position data is obtained from the two station pairs Hawaii-Trinidad and Hawaii-Japan. Alternate station pairs are Australia-Maine and Maine-Hawaii.

THE OVERALL SYSTEM

FIG. 12 is a block diagram of an overall mobile or airborne system embodying the principles of the present invention, which is adapted for navigation utilizing VLF stations like those heretofore referred to on a worldwide basis.

The computer 10, which is the heart of the system, is preferably a miniaturized type computer suitably compact for convenient placement and for use in small aircraft. Such computer, and the required chips therefor, are currently available from Intel Corporation, of 3065 Bowers Avenue, Santa Clara, California 95051. A suitable computer 10 to accommodate 12 contemplated stations, and hence 12 separate receivers, may include 16 ROM (read only memory) chips of 1,024 bits each; 16 RAM (random access memory) chips of 320 bits each; and one CPU (central processor) chip including one accummulator and 16 registers. While the various computer algorithms will be described in more detail in a later section, they briefly include input and output control algorithms; and en-route algorithms including present position, station pair selection, diurnal correction, left/right steering track, distance to waypoint, true heading, speed, and time to touchdown or waypoint.

The control head 12, which is the interface between operator and computer, includes (as best shown in FIG. 2) the input keyboard 18, function switch 22, waypoint 26, on/off switch 16, and a display test switch forming part of the keyboard 18. Control head 12 has connection 68 with the computer which includes four control head interrogation lines that are scan lines from the computer; connection 70 consisting of four keyboard input data lines to the computer; and connection 72 consisting of four function/waypoint input lines to the computer.

The display unit 14, shown in detail in FIG. 3, includes the left/right guidance course display 28, the seven-digit readout 40, the six-digit readout 42, and the various function indication lights 38, 44, 46, 48, 50, 52, 54, 56, 58, 60, and 62. The display unit 14 has a connection 72 with the computer including two data lines for the left/right digital display; and a connection 74 with the computer including nine data lines for the readouts 40 and 42. Function indication is provided to the display unit 14 through connection 75 from control head 12, consisting of five indicator control lines.

Signal input from all stations into the system is through a single antenna generally designated 76, which consists of a voltage probe-type antenna and a broadband preamplifier housed in or near the antenna base. The signals from the antenna 76 are fed to all of the 12 receivers, which are tuned to their respective stations, the receivers as a group being designated 78. Connection 80 from the receivers 78 to the computer 10 includess twelve input lines to the computer, one for each receiver, indicating receiver signal lock-on. Time data from the receivers 78 is provided to the computer 10 through a multiplexer 82; connection 84 from receivers 78 to multiplexer 82 including twelve input data lines (one for each receiver) for the "normalized" receiver data output frequency (e.g., 100Hz). The multiplexer 82 includes multiplex logic, a high frequency crystal controlled clock (e.g., 10 MHz), and a stop/start counter for counting the clock pulses. Connection 86 from the computer 10 to multiplexer 82 is a take data request line. Connection 88 from computer 10 to multiplexer 82 includes 24 start/stop station pair selection control lines. Connection 90 from multiplexer 82 to computer 10 includes 20 bit input time difference data lines which are from the start/stop counter (a binary coded decimal counter) of the multiplexer.

Power supply 92 is connected to computer 10 through power input lines 94; to control head 12 through power input lines 96; to display unit 14 through power input lines 98; to receivers 78 through power input lines 100; and to multiplexer 82 through power input lines 102. Connection 104 between control head 12 and power supply 92 includes on/off control lines associated with the on/off master switch 16 of the control head, whereby the master switch 16 enables or disables power from the power supply 92 to all components of the apparatus through the various power input lines 94, 96, 98, 100, and 102.

INPUT PORTION OF THE SYSTEM

FIG. 13 is a general diagram of the input portion of the system from the antenna generally designated 76 through the receivers generally designated 78. The antenna 76 comprises a voltage probe antenna, and housed proximate the base structure thereof is a broadband preamplifier. The VLF signals from all stations in receiving range are received by the antenna 76 and amplified by the broadband preamplifier housed therewith, and are then passed through a transient suppressor generally designated 106, and are fed in parallel to all of the receivers in the group 78 of receivers. Since it is contemplated that VLF signals will be utilized from twelve separate transmitting stations throughout the world, 12 receivers are illustrated in this parallel array in FIG. 13, being designated receiver No. 1, receiver No. 2, ... receiver No. 12.

Each of the separate receivers numbers 1 to 12 includes as a part thereof its own phase-locked loop, sometimes referred to herein as "PLL"; and accordingly, PLLs numbers 1, 2, ... 12 are illustrated in FIG. 13 associated with their respective receivers numbers 1, 2, ... 12. A calibrate line 108 from the computer is connected to all of the PLLs, in a parallel connection arrangement, to provide a common "calibrate" signal from the computer to all of the PLLs so as to calibrate all of the normalized (e.g., 100 Hz), phase coherent data signals from the PLLs to a phase synchronous relationship.

Each PLL has a "signal present" line 110 to the computer. Each PLL also has a "normalized data" line 112 to the multiplexer 82, these data lines 112 being included in the connection 84 designated in FIG. 12.

Reference will now be made to FIG. 14, which includes a complete block diagram of one of the receivers, and which also discloses the relationship of input elements thereto. In FIG. 14 the antenna means 76 heretofore disclosed as a single block in FIGS. 12 and 13 is broken down into separate units including voltage probe antenna 114 and RF preamplifier 116, which is a broadband preamplifier and includes impedance matching circuitry. As indicated hereinabove, the preamplifier 116 is preferably housed within the base of the antenna structure of antenna 114. For the presently preferred VLF range of about 10 KHz to about 25 KHz, the broadband preamplifier will preferably have a bandpass of from about 7 KHz to about 40 KHz. This is sufficiently wide to allow the frequencies of interest to pass with negligible phase shift.

Optimum signal is obtained by physically locating the preamplifier 116 proximate the antenna 114. This is because antenna impedance at these VLF frequencies is so high that any stray capacitances due to cable would tend to severely degrade the signal.

Output of the preamplifier 116 is fed through a cable to a common equipment housing (not shown) and in particular to the transient suppressor 106 therein. This common equipment housing contains not only the transient suppressor 106 but all of the receivers 78, the computer 10, the multiplexer 82, and the power supply 92.

The transient suppressor 106 is employed because signals in this VLF frequency spectrum have high level transients due to atmospheric conditions, including lightning and other atmospheric effects, as well as local disturbances such as 60 Hz harmonics. The transient suppressor 106 clips such transients at a prescribed level, and thereby keeps these disturbances from disrupting the circuitry in the remainder of the system, and in particular removes high level transients which would cause distortion in the RF signal such as would make "lock-on" in any of the receiver phase-locked loops difficult to achieve.

Circuit details of a suitable broadband RF preamplifier 116 and of a suitable transient suppressor 106 are shown in FIGS. 15 and 16, respectively, and described in connection therewith.

The output of transient suppressor 106 is fed in parallel to the inputs of all of the receivers, including the input element of the complete receiver, designated receiver No. 1, illustrated in FIG. 14. This input element is a narrowband TRF (tuned radio frequency) amplifier designated 118. This narrowband TRF amplifier 118 is tuned to the respective VLF transmitted frequency, and the output thereof is processed through a limiter 120. A high level AGC (automatic gain control) circuit 122 is associated with the narrowband amplifier 118 to prevent overloading. The output of limiter 120 is then fed to an amplifier 124 which is the input element of the phase-locked loop which is generally designated 126.

Suitable detailed circuitry for the narrowband TRF amplifier 118, limiter 120, and high level AGC circuit 122 is shown in FIG. 17 of the drawings.

RECEIVER PHASE-LOCKED LOOP

The particular VLF station frequency isolated in the receiver of FIG. 14 is designated in that figure as F.sub.c. The F.sub.c output of amplifier 124 may be considered to be the analog (sine wave) data signal for the phase-locked loop 126. This F.sub.c analog data signal is fed simultaneously to two phase detectors, a signal present phase detector 128, and a PLL phase detector 130. The connection from amplifier 124 to signal present phase detector 128 is designated 132, while the connection from amplifier 124 to the loop phase detector 130 is designated 134.

A digital reference signal is provided for the two phase detectors 128 and 130 by means of a temperature compensated voltage controlled crystal oscillator 136 sometimes hereinafter referred to as a TCVCXO, or simply as a VCO. The TCVCXO 136 is selected to have a preferred frequency in the range of from about 4 MHz to about 5 MHz, wherein such oscillators are available having a high degree of frequency stability. The actual frequency selected for the TCVCXO 136 is a multiple of the analog data signal frequency F.sub.c, and the multiplication factor "M" from the analog signal frequency F.sub.c to the TCVCXO frequency that is in the approximately 4 MHz to 5 MHz range is preferably a factor that is divisible by four for phase shifting purposes as will become more apparent hereinafter. Thus, the digital reference signal provided by the TCVCXO 136 equals Mf.sub.c.

By having the voltage controlled oscillator 136 in the preferred frequency range of about 4 MHz to about 5 MHz, and having it a temperature compensated crystal oscillator in addition to being voltage controlled, it can be held to an accuracy of within a couple of