BACKGROUND Loran-C and other radio-wave pulse train navigation systems have been designed to serve also to transmit and receive digital communication messages generally by pulse-position modulation (PPM) involving appropriate encoding of communication messages and logical multiplication and inversion of encoded signals prior to phase modulation of the Loran-C pulses, as described, for example, in U. S. Patents Nos. 4,800,391 and 4,821,038 of Megapulse Inc., the common assignee of the present application. With Loran-C 100K Hz carrier frequency systems, such modulation enables a digital bit rate of 70 bps.

A current example of such PPM is the Eurofix System wherein a seven-bit word is created using the last six pulses of an eight-pulse group. To enhance the Loran data bit rate capability, schemes such as increasing the number of pulses from 300pps to 500 pps (sometimes referred to as Supernumerary Loran) and using a three-level Intrapulse Frequency Modulation (IFM) on such system pulses are being discussed.

It would, however, be highly desirable, particularly with existing Loran-C transmitter stations world-wide, if the bit rate could indeed somehow be increased to 250 bps and higher with the current station equipment, and still without affecting the integrity of the navigation capability of the Loran-C system.

The present invention provides such a break-through the discovery of a novel modulation scheme readily implementable in current (and future) Loran-C (and similar) transmitters, wherein a novel type of frequency modulation is added on the Loran-C transmitted pulses.

OBJECTS OF INVENTION The primary object of the invention, accordingly, is to provide a new and improved method of and apparatus for expanding the digital bit-rate potential for message communication to be supplementarily carried on Loran-C radio navigation transmissions and the like, without, however, affecting the principal radio navigation capabilities thereof.

A further object is to accomplish such an end with electronic circuitry that lends itself to ready modification of the current worldwide Loran-C (and similar) transmitter equipments.

Other and further objects will be explained hereinafter and are delineated, also, in the appended claims.

SUMMARY In summary, from one of its important viewpoints, the invention embraces a method of expanding the digital bit rate potential for communication to be added to Loran-C and similar radio-frequency navigation system transmitter pulse trains and without affecting the navigation capability and integrity of the system, that comprises, generating such radio-frequency pulses centered on a predetermined carrier frequency and transmitting the same from antennas to enable users to receive and use the initial part of each transmitted pulse for navigation timing and location; during the remaining parts of each transmitted pulse, sweeping the generated carrier frequency thereof in opposite directions, first below or above, and then above or below the carrier frequency during successive further parts of each pulse, but within a predetermined band between predetermined lower and upper frequency limits on opposite sides of said carrier frequency; further, during said sweeping, maintaining the pulse energies below the lower frequency limit and above the upper frequency limit substantially equal for each pulse, and using the frequency modulation effected by said sweeping to provide communication bits.

Preferred and best mode implementations and designs for practicing the invention are hereinafter explained in detail.

DRAWINGS The invention will now be described in connection with the accompanying drawings, Figure 1 of which is a simplified circuit diagram of a current solid-state Loran-c transmitter (SSX), as of the type marketed by said Megapulse, Inc. under the nomenclature AN/FPN/64 ; Figure 2 is a waveform diagram illustrating the envelope of the antenna current in the transmitter of Figure 1, with and without the use of a"tailbiter"circuit; Figure 3 is a waveform diagram of the actual antenna current produced by the circuit of Figure 1 with"tailbiter" ; Figure 4A illustrates the antenna current Loran C pulse waveform of Figure 3 in timed relationship with the novel frequency modulation (Figure 4B) and concurrent phase modulation (Figure 4C) underlying the novel scheme of the present invention; Figure 5 is a schematic circuit diagram of the preferred transmitter circuit implementation of the invention for attaining the operation represented in Figures 4B and 4C; and Figure 6 is a composite wavefonn of an unmodulated and a frequency modulated Loran-C pulse which results in a phase modulation of-90°.

Figure 7 is a composite spectrum of the waveforms shown in Figure 6Cthe solid line is the modulated spectrum, the dotted line is the unmodulated spectrum.

Figure 8 is a composite waveforrn of an modulated and a frequency modulated Loran-C pulse which results in a phase modulation of +90°.

Figure 9 is a composite spectrum of the waveforms of Figure 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT (S) OF THE INVENTION Referring first to current-day radio pulse train solid state transmitters for purposes of contrasting with the present invention, and illustrating the invention as applied to the preferred Loran-C application, Figure I is a simplified schematic circuit diagram of a typical solid state Loran-C transmitter (SSX) generating Loran-C pulses, Figure 3, by impulse-charging a parallel tuned circuit Cc-Lc in a Coupling Network, so-labeled, and connected to the transmitting Antenna represented by its series inductance LA and capacitance CA, and parallel load resistance R. Both the Coupling Network and the Antenna are normally turned to the before-mentioned 100K Hz basic Loran-C carrier frequency. The Impulse Transmitter charges the Coupling Network which, in turn, transfers energy to the Antenna with the resulting antenna current waveform envelope shown in Figure 2. To avoid the oscillating tail of the resulting pulse, shown to the right in the waveform in the solid-line curve labeled"Without Tailbiter", a"tailbiter"circuit consisting of a solid-state switch ST in series with a resistor RT is shown in Figure 1, connected in parallel with the Coupling Network. Closing the switch ST when the coupling network voltage is zero, produces the desired exponentially decaying antenna current pulse shown by the dotted line "With Tailbiter"of Figure 2, generating the Loran-C radio-frequency (rf) current pulse of Figure 3 and 4A, all as described in said patents and the references cited therein.

As is further well-known, the Loran-C navigation system uses the first three cycles of the rf pulse to determine the time-of-arrival for position-fixing. Adding communication modulation, as explained in said patents, accordingly, must not disturb these cycles in phase or in frequency.

Phase of frequency modulation can start, however, at 30 p. sec. or later into the pulse. By changing the normally tuned 100 kHz resonant frequency of the Antenna, the radio frequency of the antenna current will correspondingly change. In the Loran-C navigation system, however, the specifications require that the spectrum of the Loran-C rf pulse stay within the predetermined frequency band of about 90 to 110 kHz, and further that the energy both below 90 kHz and above 110 kHz limits be less than 0.5% of the total pulse energy.

Frequency modulation of +2. 5 kHz, as shown in Figure 4B, generates a phase shift at time 160 psec into the pulse of +90° or-90°, as shown in Figure 4C.

To accomplish this modulation, the invention uses fast, highly efficient solid-state switches Si-84 shown in the schematic diagram of Figure 5. These switches are connected across capacitor Cs"Cs2 and inductors Ls3 and Ls4-all in series with the Antenna. As earlier described in connection with Figure 4, the pulse is divided into three intervals. In Interval 1, the pulse frequency is 100 kHz. In Interval 2, pulse frequency is either 97.5 kHz or 102.5 kHz. In Interval 3 pulse frequency is either 102.5 kHz or 97.5 kHz. Just before the end of Interval 3 pulse frequency is again 100 kHz. The pulse has three states which are designated as the Zero State, the Plus State, and the Minus State. In the Zero State all switches are open except the Tailbiter Switch, ST, that closes at time 70 usec into the pulse and opens before the start of the following pulse. In the Plus State the switch positions are: <BR> Af<BR> 0 Interval 1 All switches open Interval 2 ST and S3 closed. The closing of S3 decreases the series inductance which, in turn, increases pulse frequency. The closing of the Tailbiter Switch, ST, shapes the tail as shown in Figure 2. +2.5 kHz Interval 3 ST, Si, S2, and S3 closed. The closing of S 1 and S2 increases the antenna capacitance thus decreasing the Antenna frequency.-2.5kHz In the Minus State the switch positions are: Af 0 Interval 1 All switches open Interval 2 ST and S2 closed-2.5 kHz Interval 3 ST, S2, S3 and S4 closed +2.5 kHz At high power levels it is easy to close a solid state switch but it is difficult to open the switch. In the switching scheme described all the switch closings take place at high power levels while switch openings take place at low power levels, i. e. end of the pulse tail.

The effects on the pulse waveform and spectrum of the frequency modulation of Figure 4 are shown in Figures 6-9. The effects of a positive phase shift of 90° are shown in Figures 6 and 7. The solid line in Figure 7 is the spectrum of the modulated pulse while the dotted line is that of an unmodulated pulse. As can be seen from Figure 7 this has slightly increased for the modulated pulse-thus meeting the spectrum requirement of 1% out-of-band intact. The effect of a negative phase shift of 90° are shown in Figures 8 and 9. Again the spectrum meets the out-of- band requirement.

The invention thus, in, for example, introducing and removing inductance between the transmitter output and the antenna to shift the frequency from 100 kHz toward 97.5 kHz, and then sweeping toward 102.5 kHz and then back to 100 kHz, all within the critical restrictions earlier discussed, maintains the center of the frequency spectrum for each pulse close to 100 kHz, and equalizes the energies below and above 100 kHz. This frequency modulation can serve as communication bits as explained in said patents.

Through this technique of the invention, thus, it is now possible substantially to increase the digital data bit rate communication modulation potential of Loran-C pulse navigation transmitters of current and future form, (and of other pulse train communication systems and the like), without in any way affecting the integrity of the navigation capability and integrity thereof.

This modulation scheme, as earlier mentioned, has been found to permit a bit rate of over 250 bps on the Loran-C signal without affecting the navigation capability of the Loran-C navigation operation.

Further modifications will also occur to those skilled in this art, including different values of frequency shifting within the above-defined requirements and specifications (though for Loran-C operation, of the order of about the illustrative frequency values presented); and, as earlier pointed out, the approach of the invention may also be useful with other types of navigation and pulse train communication system C such being considered to fall within the spirit and scope of the invention as defined in the appended claims.