Abstract:
Frequency resources of a DME/TACAN band specified for the aircraft band are effectively utilized, and a GPS reinforcing data is overlapped onto distance information to be broadcast. A header is added to a DGPS reinforcing data, and the transmitting pulse level of a conventional ground DME system is modulated, and the data is broadcast to an airborne system. In the airborne system, a threshold value of “1” and “0” is generated by a level detecting device ( 5 ), and the start point of the data is detected by a header detecting device ( 6 ), and the reinforcing data is supplied to the airborne system. Consequently, the function of broadcasting a data by overlapping the data onto the distance information of the DME is achieved. Since the DME uses the L band, the radio interference with the existing ILS, VOR, aircraft radio transmission, and broadcasting station can be avoided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a DME (Distance Measuring Equipment) system, and specifically, it relates to a GBAS (Ground Based Augmentation System) which broadcasts a reinforcing data of the GPS in the DGPS from a ground unit. 
     2. Description of the Prior Art 
     Previously, an amplitude of the pulse transmitted from a DME ground unit has automatically been controlled so as to become a constant value by a method shown in Japanese Patent Publication No. 2-47710. Accordingly, there is no data broadcasting function in pulses other than that of the Morse code which is transmitted once for every 30 seconds. 
     On the other hand, in case of constructing a landing guidance system using the GPS, it is necessary to transmit the data to reinforce the accuracy and integrity of the GPS. Therefore, at present, a data broadcasting system using a VHF which is used by an ILS (Instrument Landing System) and a VOR (VHS Omnidirectional Radiorange) is being developed. 
     However, this frequency band has already been assigned to a lot of facilities, and the interference with the ILS, the VOR and the aeronautical radio is inevitable. For obtaining a sufficient number of stations, there is such a problem that it is necessary to wait for the removal or service stop of the ILS or the VOR. 
     A conventional DME system gives attention to the pulse width, and works so as to keep the sliced pulse width constant, and as a result, the amplitude is also kept constant, and therefore, it is impossible to perform the data broadcasting by the amplitude modulation method using this DME unit. 
     Furthermore, as mentioned above, the data broadcasting system using the VHF has such a disadvantage that the interference with the ILS or the VOR is inevitable in this country where the assignment of frequencies has already been saturated. In order to solve this problem, there is a method of improving the spectrum, but it has a disadvantage of requiring a large-scale unit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide means by effectively utilizing the frequency resource of the DME/TACAN band specified for the aircraft band, and which overlaps the GPS augmentation data onto the distance information and broadcasts that. 
     It is another object of the present invention to achieve the GBAS function without mounting any new unit on the aircraft side. 
     A DME system of the present invention is characterized by modulating the amplitude of the transmitting pulse of a DME ground station within an allowable range so as to transmit information other than the distance information and the station identification code (ID). 
     In the present invention, in order to clearly express the start point of the data and to make the bit synchronization easy, a training sequence and a unique word which include “1” and “0” and have previously been determined are added just before the data by header adding means. In this header, the training sequence is used as the reference value of level “1” and level “0” showing the data, and the unique word is used for preventing a wrong detection of a signal because of wave form degradation caused by noise or propagation. 
     The present invention comprises: a ground unit having level modulating means which modulates the output of a conventional DME transmitter in a way of level modulation for each paired pulse stimulation according to a signal including “1” and “0” with a header; a detecting means which detects level “1” and level “0” of the video output of a conventional airborne DME receiver; and an air borne DME unit having a decoding means which judges “1” or “0” from this level signal and the receiving pulse timing. 
     The header adding means adds a header including a previously determined code string of “1” and “0” just before the inputted data. This header includes a training sequence and a subsequent unique word. The training sequence includes “1” and “0” which are alternately repeated, and clearly expresses the level average of “1” and “0.” Furthermore, the unique word shows the start of information. 
     The level modulating means uses a method of changing the peak level of each paired pulse stimulation according to the information of “1” and “0” for each paired pulse stimulation by an attenuator or a pulse modulating means, and it changes the level of the output of a conventional ground DME transmitter according to this information of “1” and “0.” At that moment, the transmitter separates a pulse showing “1” and a pulse showing “0,” and keeps the pulse width constant, and it maintains the spectrum within the prescribed value. 
     The level detecting means provided in the airborne DME unit determines the average value of the receiving pulse level from the training sequence pulse received for a certain time, and it makes this average level (mean level) the threshold value to judge that a pulse higher than that value is “1” and a pulse lower than that value is “0.” 
     The decoding means generates a signal string of “1” and “0” from this judgment value and the timing of the pulse receiving, and in the meantime, it detects a unique word from the signal string by correlation processing, and separates and outputs the data. 
     According to the present invention, for example, a header is added to the DGPS (Differential GPS) augmentation reinforcing data, and the transmitting pulse level of a conventional ground DME system is modulated, and the data is broadcast to an airborne system. In the airborne system, a threshold value of “1” and “0” is generated by the level detecting means, and the start point of the data is detected by the header detecting means, and the DGPS augmentation data is supplied to the airborne system. Consequently, it is possible to achieve the function of broadcasting a data by overlapping the data onto the distance information of the DME. Furthermore, since the DME uses the L band, the radio interference with the existing ILS, VOR, aircraft radio transmission, and broadcasting station can be avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This above-mentioned and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram showing an embodiment of the present invention; 
     FIG. 2 is a timing chart for describing the operation of the present invention; 
     FIG. 3 is a block diagram showing a concrete example of the embodiment of the present invention; 
     FIG. 4 is a block diagram showing a concrete example of the embodiment of the present invention; 
     FIG. 5 is a block diagram showing a concrete example of the embodiment of the present invention; and 
     FIGS.  6 ( a ) and  6 ( b ) are figures of the wave forms showing another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram showing an embodiment of the present invention. As shown in FIG. 1, the embodiment of the present invention comprises broadcasting data adding means  1  which is provided in a ground DME unit  10  and has header adding means  2  and level modulating means  3 , and broadcasting data detecting means  4  which is provided in an airborne DME unit  20  and has level detecting means  5  and header detecting means  6 . 
     FIG. 2 is a timing chart showing the operation of the present invention. A transmit data  102  is inputted into the ground DME unit  10  from a data generating means  101 . The header adding means  2  adds a header in which “1” and “0” are alternately arranged, before the transmit data  102 . This header includes a training sequence which transmits the reference value of level “1” and level “0,” and a unique word for clearly expressing the start point of the data. The transmit data  102  is sent out as a data with a previously determined specific length continuously after this unique word. 
     The relative frequency of “0” and “1” in the training sequence is precisely 50%, and the value determined by averaging peak levels of these pulses is most suitable for the threshold value of level “0” and level “1.” 
     Furthermore, the unique word is a previously determined random string of “0” and “1.” For example, in the case where the unique word is a word of 16 bits, letting the relative frequency of “1” and “0” be 50%, the probability Pu that all the data string of 16 bits is in accord is as follows: 
       Pu= 0.5 16 =1.52×10 −5   
     An example of the unique word is shown by Su as follows: 
     Su=000 010 011 110 000 001 101 110 001 100 011 111 101 111 100 010 
     The transmit data  102  having a header added is delivered to a level modulating means  3 . The level modulating means  3  generates a transmitting trigger  16  of the header part at a previously determined interval T using a transmitting trigger  15  as a trigger, and inputs that into a conventional ground DME transmitter  14 , and in the meantime, it outputs the output from the ground DME transmitter  14  in a way of damping or not damping the output by an attenautor or the like according to “0” or “1” of the data. This high frequency signal passes through a circulator  13 , and through an antenna  12 , it enters an airborne antenna  17 , and is inputted into a conventional airborne DME receiver  18 . 
     In the conventional airborne DME receiver  18 , a normal processing of measuring distance is performed, and in the meantime, a video signal  19  of the receiving pulse is outputted to the outside. This signal is inputted into the broadcasting data detecting means  4 , and is separated into a group with a large amplitude and a group with a small amplitude. 
     That is, in the level detecting means  5 , the level of paired pulse stimulation of the video signal  19  is measured, and the average of the previously determined number of times (n times) is found by the sliding window to determine the threshold. Then, the result obtained by judging that the inputted video signal is “1” or “0” on the basis of this threshold is sent out to the header detecting means  6 . 
     The header detecting means  6  detects a unique word by performing the correlation processing of the result obtained by judging that the signal is “1” or “0” in the level detecting means  5 , and it outputs the signal following this unique word as a receive data  106 . 
     Next, an example of the present invention will be described in detail by referring to drawings. FIG.  3  and FIG. 4 are respectively a block diagram showing an example of the header adding means  2  and a block diagram showing an example of the level modulating means of the ground unit  10 , and FIG. 5 is a block diagram showing an example of level detecting means  5  and an example of the header detecting means  6  of the airborne unit  20 . 
     In FIG. 3, by the input of a data transmitting trigger  103 , a parallel data  102  is read out from a data generating means  101 . The data transmitting trigger  103  passes through an AND gate  201 , and it is inputted into a flip flop  202 . The output of the flip flop  202  is set to “1” to set a parallel serial converter  204  in the operable state. The output of the flip flop  202  is used as a signal to show transmitting (transmitting status  108 ), and while making the AND gate  201  in the prohibited state, it turns a switch  209  in the ON state. 
     Furthermore, the output of the flip flop  202  makes a counter  203  in the operable state, and the counter  203  is counted up by a TX trigger  105 . The output thereof is used as an address of a ROM  205 . In the ROM  205 , a training sequence and a unique word are recorded, and they are outputted to the level modulating means  3  through a switch  206 . By the series of operations, a header is outputted to the level modulating means  3  just after the input of the data transmitting trigger. 
     After that, when the counter  203  reaches the previously determined value, a decoder  207  shifts the switch  206  to connect the output of the parallel serial converter  204  to the level modulating means  3 . Furthermore, the output of the decoder  207  shifts a switch  208 . Consequently, the TX trigger  105  is supplied to the parallel serial converter  204 , and the information to be broadcast is inputted into the level modulating means  3  in a mode of a serial data. 
     In FIG. 4, the TX trigger  105  inputted into the level modulating means  3  passes through multi-vibrators  301 ,  302 , and it is shaped to be a double pulse rectangular wave. Furthermore, the data recorded in a ROM  303  is converted into an analog signal by a D/A converter  304 . This analog signal is made a previously distorted wave form for reducing the distortion of a solid state power amplifier (SSPA)  403 . This signal and the reshaped double pulse rectangular wave are added by an operational amplifier  305  as analog values, and are amplified by a current amplifier  306 , and are supplied to a modulator  402 . 
     The CW output of an SG  401  is subjected to the pulse modulation by the modulator  402 , and is amplified in the SSPA  403 , and passing through a directional coupler  404  and a circulator  13 , it is radiated via an antenna  12 . A part of the signal sampled in the directional coupler  404  is inputted into a switch  314 . The switch  314  is shifted by a bit stream  104  inputted from the header adding means  2 , and the input signal is outputted to a peak holder  312  in the case where the bit stream  104  is “1,” and it is outputted to a peak holder  313  in the case of “0.” 
     The output of the peak holder  312  is compared with the “1” level reference voltage by the operational amplifier  310 , and the difference thereof is inputted into a “1” level filter  308 . The “1” level filter  308  operates as a loop filter of a feedback circuit to stabilize the “1” level pulse output. Similarly, the peak holder  313 , an operational amplifier  311 , and a “0” level filter  309  stabilizes “0” level pulse output. 
     On the other hand, the bit stream  104  shifts an analog switch  307  to connect the output of the “1” level filter  308  to the D/A converter  304  in the case of “1,” and to connect the output of the “0” level filter  309  to the D/A converter  304  in the case of “0.” Since the “0” level reference voltage is set to be lower than the “1” level reference voltage by 1 dB, the peak electric power of the pulse radiated via the antenna in the case of “0” is also lower than that in the case of “1” by 1 dB. 
     In FIG. 5, in the airborne unit  20 , the signal inputted from a normal antenna  17  is received by a conventional airborne DME receiver  18 . The video signal  19  outputted from the receiver  18  is inputted into a decoder  501  of the level detecting means  5 . In the decoder  501 , in the case where the input pulse signal has a regular pulse interval, the signal is judged to be a DME signal to be inputted into a ring counter  502 . The output of the ring counter  502  sets the sample holders  505 - 1  to  505 -m to be operable in turn. On the other hand, the peak level of the received video signal  19  inputted into a delay element  503  is detected by a peak holder  504 . 
     Accordingly, by the decoder  501 , only the peak levels of the pulses judged to be DME signals are going to be held in turn from the peak holder  504  to the sample holder  505 - 1  to  505 -m. When m pieces of samples have been held, returning to the sample holder  1  again, the sample is held. This output is added by the operational amplifier  506  as an analog value. 
     According to this operation, if m pieces of bit streams of the training sequence have been set, the output of the operational amplifier  506  becomes a value proportional to the average value of the peak levels of the training sequences. This voltage is amplified in an amplifier  507 , and passes through a filter  508 , and is averaged, and it is supplied to the input on one side of a comparator  509  as the reference value. To the input on the other side of the comparator  509 , the output of the peak holder  504  is connected. According to this configuration, the comparator  509  outputs “1” in the case where the peak level of the inputted video signal  19  is larger than the average value of the levels of the training sequences, and it outputs “0” in the case where the peak level is smaller than the average value. 
     In the header detecting means  6 , the output of the decoder  501  is delayed by a delay circuit  601 , and it is supplied to a shift register  602  as the clock. The output of the comparator  509  is inputted into the shift register  602 , and it is latched after a certain delay each time there is a decode pulse. The shift register  602  has the same number of steps as the number of bits of the unique word. 
     Now, letting 
     Su=000 010 011 110 000 001 101 110 001 100 011 111 101 111 100 010, n=48 is given. 
     In a ROM  603 , the reversed value of this value of Su is recorded in advance. Each bit of the ROM  603  and each bit of the shift register  602  are subjected to exclusive-OR by an exclusive-OR circuit  604 . Then, the outputs of the exclusive-OR circuit from all bits are added by an adder  605 . Accordingly, in the case where the inputted signal is set in the shift register  602  and each bit is equal to that of the unique word, the output of the adder  605  is  48 . 
     When 1 bit has not yet been matched after the input started in the case where all contents of the shift register  602  were 0, the contents Ss of the shift register are as follows: 
     Ss=00 010 011 110 000 001 101 110 001 100 011 111 101 111 100 010 0 At this moment, the output of the adder  605  is a voltage corresponding to 32. 
     A comparator  606  has a voltage corresponding to  40  as the reference value for detecting this difference. The output of the comparator  606  is inputted into a flip flop  607 . The flip flop  607  performs the work to show the receiving start of the broadcast data  102 , and stops the ring counter  502  of the level detecting means. Furthermore, it makes a counter  609  operable, and in the meantime, it turns on a switch  608 . Consequently, the bit stream to be inputted is stored in a RAM  611 . 
     Furthermore, the output of the delay circuit  601  is counted by the counter  609 , and when a previously determined number of pieces of signals are counted, a notice is given to an END detecting circuit  610  to stop a RAM  611 . The END detecting circuit  610  resets the flip flop  607 , and in the meantime, it gives the notice of a receiving end signal  107  to the outside. According to the series of processes, the broadcast data are accumulated in a RAM  611 . 
     FIGS.  6 ( a ) and  6 ( b ) are figures of the waveforms showing another embodiment of the present invention. In the above first embodiment, the modulation is performed by the paired pulse stimulation by the data, but in the embodiment as shown in FIG.  6 ( a ), one pulse of the paired pulse stimulation is made the reference pulse, and the other pulse is modulated by the data. That is, the reference level is put in a first pulse and the data is put in a second pulse. In this case, if the level of the first pulse is always level “1,” it can easily be performed to judge the level of the second pulse which is the data. 
     Furthermore, as shown in FIG.  6 ( b ), by inserting the paired pulse stimulation in which the level of the first pulse is “0” before the data transmitting, it is also possible to transmit the data start without using a header pulse string. If such a waveform is used, it is possible to simplify the processing circuit. Furthermore, since the reference pulse and the data pulse are close in terms of time, there is such an effect that it is difficult to be affected by the fluctuation of the level because of the movement of an aircraft or the like. 
     Of course, it is also obviously possible that the reference value of the first pulse is “0” and one in which the level of the first pulse is “1” is the header. 
     In addition to this, it is also considered to process the data to be transmitted. For example, it is considered to use an error correcting code, to overlap a bit scramble so that the relative frequency of “0” and “1” may be equal, to use an interleave method to cope with a burst error, to insert “0” or “1” according to a host protocol, or the like. 
     The data processing is considered to be a host protocol of the present invention, and the modulating procedure of the present invention is not affected by the presence or absence thereof. 
     According to the DME system of the present invention, it is possible to broadcast the data without affecting any conventional DME function. 
     Furthermore, since the VHF band in which the frequency assignment has already been saturated is not used but the DME/TACAN band in which the channel is divided for every 1 MHz is used, the frequency assignment is easy. Accordingly, the interference with existing landing systems such as the VOR or the ILS can completely be avoided. 
     Therefore, the use is possible without waiting for the removal of the VOR and ILS. For example, if the frequency of the terminal DME provided together with the ILS is used, the labor for changing the frequency in an aircraft is reduced. 
     Furthermore, since the same circuit as a conventional DME can be used at a high frequency, an aerial receiver can be shared in an aircraft, and an antenna and a transmitter can be shared on the ground.