Abstract:
The DME pulse pair is followed by a third, steep-edge pulse. The time marks are divided from the third pulse. To ensure that no random pulses are evaluated as the third pulse, the pulse pairs are still evaluated for decoding and to control tracking, while the third pulse can be evaluated only during a given time after the reception of the pulse pair. The novel interrogators and transponders can also cooperate with existing interrogators and transponders, in which case only the improvement in accuracy is lost.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a two-way ranging system wherein an interrogator transmits pulse pairs as interrogation signals to a transponder, the transponder, after a fixed delay following the reception of the interrogating pulse pair, transmitting a pulse pair as a reply signal to the interrogator if the received interrogating pulse pairs have a predetermined spacing and the distance between the interrogator and the transponder is derived in the interrogator from the time difference between the transmission of the interrogating pulse pair and the reception of the reply pulse pair, taking into account any built-in equipment delays. 
     Such a two-way ranging system is described in a book by E. Kramar entitled &#34;Funksysteme fur Ortung und Navigation&#34;, Verlag Berliner Union GmbH, Stuttgart, 1973, on pages 147 to 159. 
     A two-way ranging system which has been introduced worldwide is the ICAO (International Civil Aviation Organization) standard DME (distance-measuring equipment). The operating data for DME, including the maximum bandwidth of the carrier wave of the DME pulse pairs and the shape of the DME pulse pairs, are specified in Annex 10 to &#34;Aeronautical Telecommunications of the ICAO&#34;. 
     Even with an optimum shape of the pulse pairs both in the interrogator and in the transponder, there is a maximum measurement accuracy in practice. For given applications, however, it is desirable to further improve this accuracy. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a two-way ranging system having a measurement accuracy higher than the measurement accuracy previously obtained. 
     A feature of the present invention is the provision of a two-way ranging system comprising: an interrogator to transmit an interrogation signal to a transponder, the transponder transmitting a reply signal to the interrogator after a fixed delay following the reception of the interrogation signal, the interrogation signal and the reply signal each including a pulse pair having a predetermined spacing therebetween and a third pulse following the pulse pair after a predetermined time interval, the third pulse having a leading edge steeper than the leading edge of the pulses of the pulse pair; and evaluating means disposed in the interrogator responsive to the third pulse in each of the interrogation signal and the reply signal to provide a distance measurement between the interrogator and the transponder, the distance measurement and the reply signal being accomplished only if the pulse pair has the predetermined spacing and the third pulse is spaced from the pulse pair the predetermined interval. 
     Compared with the conventional DME, measurement accuracy is improved considerably. The novel interrogators and transponders can also cooperate with existing DME interrogators and transponders, in which case only the improvement in accuracy has to be traded in, so that existing equipment can be used. 
     A particular advantage is that the DME decoding and the DME search and track modes are still used to recognize the interrogation and reply signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which: 
     FIG. 1 is a block diagram of a conventional DME interrogator; 
     FIG. 2 is a block diagram of a conventional DME transponder; and 
     FIGS. 3 and 4 are block diagrams of parts added to the conventional DME interrogator of FIG. 1 to provide the improved two-way ranging system in accordance with the principles of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First, the conventional DME interrogator will be described with the aid of FIG. 1. An interrogation generator 1 controls a pulse shaper and modulator 2. The signal provided by the pulse shaper and modulator 2 is the video signal, which is the envelope of the DME interrogation signal. In a transmitter 3, the carrier wave of the DME pulse pair is modulated and amplified. The DME interrogation signal is passed through a transmit-receive switch 4 to an antenna 5, from which it is radiated. 
     The DME reply signal picked up by the antenna 5 is applied via the transmit-receive switch 4 to a receiver 6. A decode and trigger device 7 checks whether the two pulses of the receiver&#39;s video output signal have the prescribed spacing. Only then will the video signal be released for further evaluation. From the leading edge of the first DME pulse, the device 7 derives a trigger pulse which indicates the instant of reception of the DME reply signal. This trigger pulse as well as the trigger pulse from the interrogation generator 1, which indicates the instant of transmission of the DME interrogation signal, are passed to an evaluating device 8. To store the instant of transmission, the trigger pulse from the interrogation generator 1 starts a counter in the evaluating device 8, which is stopped by the trigger pulse from the decode and trigger device 7, whereby the instant of reception is stored, too. Evaluating device 8 derives the distance between interrogator and transponder from the time difference between the instants of transmission and reception, taking into account any built-in equipment delays in the interrogator and the transponder. 
     The distance is displayed on an indicator 9. 
     The conventional DME transponder will now be explained with the aid of FIG. 2. 
     The DME interrogation signal is picked up by an antenna 21 and passed through a transmit-receive switch 22 to a receiver 23. Signal processing is similar to that in the DME interrogator. The video output signal of the receiver 23 is checked in a decode and trigger device 24 as to whether the pulses have the prescribed spacing, and a trigger pulse is derived which indicates the instant of reception. The trigger pulse is applied to a delay line 25 which controls a pulse shaper and modulator 26 for a transmitter 27 in such a way that 50 μs (microsecond) elapse between the reception of the DME interrogation signal and the transmission of the DME reply signal. The DME reply signal is applied from the transmitter 27 through the transmit-receive switch 22 to the antenna 21 and radiated from there. As mentioned earlier, the accuracy attainable with conventional DME interrogators and transponders is limited. 
     The following explains how the accuracy can be enhanced in accordance with the principles of the present invention. 
     The interrogation and reply signals of the novel two-way ranging system contains, in addition to the DME pulse pair, a third pulse having steeper edges and a broader bandwidth than the DME pulse pair. The shape of the third pulse may be rectangular or trapezoidal. Other pulse shapes are also possible. Since the third pulse has a steep leading edge, trigger pulses can be derived therefrom which indicate the instant of transmission or reception very accurately. This broadband and steep pulse cannot be processed by conventional DME equipment because the bandwidth available during signal processing is too small. 
     The third pulse will have the following characteristics: 
     Pulse duration: ˜1 μs 
     Rise time: &lt;100 ns (nanoseconds) 
     Bandwidth: 5 MHz 
     Spacing from the DME pulse pair: 200 μs. 
     The frequency of its carrier wave may be, for example, (1) a fixed frequency in the secondary-radar frequency band, (2) a fixed frequency in the C band, (3) a fixed frequency in the L band, (4) the respective DME operating frequency, or (5) different frequencies of the DME frequency band (frequency hopping). 
     Since the higher accuracy is generally required only at a short distance from the transponder, e.g., within 30 km (kilometers), it suffices to radiate the third pulse with a lower power than that of the DME pulse pairs. At a given distance, it is then possible to switch, by hand or automatically, from the mode &#34;pulse pairs&#34; to the mode &#34;pulse pairs pulse third pulse&#34;. The respective mode is indicated to the user. 
     The generation of the novel interrogation signal will now be explained with the aid of FIG 4. Analogous remarks apply to the generation of the reply signal. 
     Like in the conventional DME interrogator of FIG. 1, the interrogation generator 41 controls the pulse shaper and modulator 2, and the transmitter 3 transmits the DME pulse pair. In the novel DME interrogator, the interrogation generator 41 controls an additional pulse shaper and modulator 42, which modulates, in an additional transmitter 43, an additional carrier wave which may differ from the carrier wave in the first transmitter 3. The additional pulse shaper and modulator 43, which generates the steep third pulse, is controlled by the interrogation generator 41 in such a way that the third pulse is separated from the DME pulse pair by the desired spacing. The interrogation signal radiated from the antenna of the novel interrogator is composed of the output signals of the transmitters 3 and 43. The interrogation generator 41 (e.g., a computer) is adjustable so as to drive either both pulse shapers and modulators 2 and 42 or only pulse shaper and modulator 2 for generating the pulse pairs. It may also be adjusted automatically as a function of distance. 
     If the third pulse is radiated in addition to the DME pulse pair, the instant of transmission is the instant at which the third pulse is radiated. This information is transferred to the evaluating device. 
     As seen in FIG. 3, the reply signal received by the interrogator is applied by the transmit-receive switch 4 to the first receiver 6, which is followed by the decode and trigger device 7, and to a second receiver 36. If the DME pulse pair and the third pulse have carrier waves of equal frequency, one common RF unit will suffice. In the following it will be assumed that the carrier waves have the same center frequency. Consequently, both the pulse pair and the third pulse are processed in both receivers. Because of the small bandwidth in the first receiver 6, the third pulse is distorted there, so it is unsuitable for further evaluation. At the output of the second receiver 36, the third pulse is provided undistorted. When the decoder 7 has recognized that the pulse pair is a DME pulse pair, a gate circuit 37 following the second receiver 36 is opened by the trigger pulse output of decoder 7 only during the time the third pulse should be present. This prevents the evaluation of any pulses which are not the third pulse. The trigger pulse from the decode and trigger device 7, which indicates the instant of reception of the reply signal when no third pulse is present, is fed to the evaluating device 8. The third pulse is fed to an additional evaluating device 38. This same technique is used in the transponder to determine the time for transmitting the reply signal. 
     Depending on the mode of operation selected (only &#34;DME pulse pair&#34; or &#34;DME pulse pair plus third pulse&#34;), the result determined by the first evaluating device 8 or the second evaluating device 38 is passed on to the indicator 9. 
     As already mentioned in connection with the interrogation generator 41, the changeover may also be effected automatically as a function of distance. 
     The gate circuit 37 prevents any random pulses that may be present from being processed as the third pulse. This safeguard can be further improved by checking not only the time position in relation to the DME pulse pair, but also the center frequency of the carrier wave. If the carrier waves of the DME pulse pair and the third pulse have the same frequency, the check is made to determine whether at the instant the third pulse is present at the output of the second receiver 36, a distorted pulse is present at the output of the first receiver 6. Because of the narrow bandwidth of the first receiver 6, this distorted pulse is provided only if the pulse to be examined has the correct carrier frequency. The check can be performed by an AND gate (not shown). 
     If the carrier frequency of the DME pulse pair is different from that of the third pulse, the second receiver 36 contains a narrow bandpass filter for the center frequency of the carrier wave of the third pulse. Instead of the output signal of the first receiver 6, the output signal of this bandpass filter is then used for the comparison. 
     While we have described above the principles of our invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.