fig1 illustrates the total of all possible answering codes which is designated by the symbol m . the areas kl through kn represent valid answering codes during a time δt . fig2 is a time diagram with δt , time being broken up into increments δtl - δtn + 1 . associated with each δtl - δtn + 1 is a code element kl - kn + 1 as illustrated . certain code elements as , for example , code elements k2 and k3 may overlap in time . the code elements kl - kn are chosen so that they have sufficient capacity such that even when a great many interrogations have occurred that at the end of the time interval δt there is a high probability that answering codes will be contained in the respective code element for answering an interrogation . thus , for example , with reference to fig1 the element kn includes a large number of usable answering codes for use in the time δtn . for example , the portion of the code element kn which is shaded with slating lines indicates the maximum possible code values fixed by the particular system conditions that can be interrogated . the cross - shaded area in the code element kn illustrates the part of the usable code elements which have actually been used during the time δt and which have been blocked due to their use in interrogation . the shaded portion which is determined by cross - shaded lines can at the maximum , be as great as the slant - shaded portion . the answering codes selected for a code element k depends on the length of the time intervals δt during which they are valid , on the maximum answer sequence ( repetition frequency ) of the answering apparatus fa , and on the desired minimum answer probability of the answering apparatus pa . this may be expressed by the equation the total amount k of the disposable code should be very great as compared to the code elements kl - kn so as to prevent an enemy from determining by randomly guessing codes that provide suitable answers . since k can be very great , especially where interrogations rapidly follow one another , under some circumstances it will be necessary to use codes with high decision content m where m = 2logm . the relationship between k and m and the relationship among the individual sub elements kl - kn should be such that there is no discernible pattern that would allow the codes to be discerned or calculated by the enemy . this can be accomplished by utilizing suitable encoding processes . which code element kn will be used during a particular time segment as , for example , in the time element δtn can be established by utilizing a key known by all of the interrogating and responding stations and in which the key is valid for only a limited time as , for example , one day . synchronous master timers at the interrogating and responding stations assures that the particular code elements kl - kn are valid during their associated time intervals δt . the systems may also repeat the particular code elements over different time intervals as long as the time intervals are large enough that the use of the same code would be undetectable by the enemy . during a transition from a time interval tn to the time interval δtn + 1 special measures have to be provided to bridge over synchronization gaps either by overlapping or by the suppression of answers or interrogations during the transition time so as to avoid confusion . since during each time interval a different code element kn is used , the storing means which contains the already used answering codes can be erased at the end of each time segment 66 tn . thus , the storage capacity required in the answering apparatus must correspond at most to the maximum possible number of answers occurring during the time δt . possible distribution of answering codes are explained below with the use of a numerical example . the accuracy of the synchronous timing system should be chosen such that time intervals of δt = 0 . 1 sec . are possible . if it be assumed that the maximum answering sequency ( repetition frequency ) be fa = 10 khz and the answer probability should in the most unfavorable case be at least equal to pa = 90 % for each interrogation and responding occurrence , then the storage requirement in the responding equipment is 1000 codes ( δt . fa = 1000 ). fig3 allows the determination of the code element k . where on the abscissa is plotted the number i of the transmitted inquiries during a time interval against minimum answer probability of the answering apparatus pa . for the answer probability pa , the equation this curve illustrated in fig3 is valid if it be assumed that the number of interrogations occuring within the time t = 10 , 000 ( k = 10 , 000 ). for high values of pa the following equation approximately holds : ## equ1 ## in the above assumed example the range of interest lies to the left of the line g in fig3 . if the decision content of the interrogation code of m = 36 bits ( m ≈ 6 . 87 . 10 10 ), then the ratio m / k in the above example where k = 10 , 000 become m / k = 6 . 87 . 10 6 . thus , the probability that the enemy can guess the code becomes very small . if the maximum interrogation sequency ( repetition frequency ) is equal , for example , to the responding sequence frequency , fa = 10 khz ( that is , every 100 μsec an interrogation is possible ), the enemy will receive , with steady statistical interrogating for locating purposes on the average only one answer every 11 . 4 minutes . these numerical examples show that in the system of the present invention there are no unrealistic requirements and further illustrate that the system is practical . thus , a secure system which is difficult to jam and also makes locating of the interrogating and responding station by the enemy difficult is provided by this invention . the essential conditions for obtaining the advantages of this invention are the synchronous time standards at both the interrogating and responding stations and the storage means in the answering apparatus or interrogating equipment and these may be easily accomplished in the present state of the art . the time standard for the control of the sequence ( repetition ) of the interrogation code or the code elements of the interrogation codes can be dispensed with if in the responding apparatus there is a precisely calibrated time standard . for example , aircraft containing collision avoidance systems operating on a time basis carry a precise standard and this may be utilized for the control of the interrogation codes or the code elements of the present invention . also , modern techniques make it possible to provide precise time standards constructed , for example , by integrated circuit means which are relatively low in cost and are small . fig4 illustrates an interrogating station 10 and a responding station 19 . the interrogating station includes a transmitting antenna 12 connected to a transmitter 11 . the transmitter periodically transmits one of a plurality of time dependent interrogation codes 14 - 18 identified as i - code 1 through i - code n . a clock ( time standard ) 20 controls the selection of the particular code to be transmitted . for the realization of this selection device those skilled in the art are familiar with a number of possibilities . for instance , the interrogation codes 14 to 18 can be stored in a shift register which is controlled by the clock 20 with a certain sequence . at the output of the shift register the interrogation codes 14 , 15 , etc . to 18 sequentially occur and can correspondingly be subsequently modulated in the transmitter 11 onto the carrier and be transmitted via the antenna 12 . each interrogation code 14 to 18 is thus only effective for the predetermined prescribed period , as controlled by the clock 20 . it is also possible to store the interrogation codes 14 to 18 in a storer or memory and to read them therefrom in a sequential manner . in this case the clock 20 controls the reading address to send the first code 14 then the code 15 , etc . on to code 18 which are read out in sequence . the responding station 19 is remotely located from the interrogating station 10 and might be mounted on a friendly aircraft , for example . the responding station 19 includes a receiving antenna 13 connected to a receiver 21 which feeds a decoder 22 . the decoder 22 feeds time dependent answer - codes 23 through 27 which are successively blocked on a time dependent basis as used . for example , codes a - code 1 through a - code 3 are blocked in fig4 and the responding station 19 responds with a - code 4 at the particular time shown . a clock ( time standard ) 28 controls the selection of the particular answer codes to be utilized . a transmitter 31 receives the selected answer code and radiates it from an antenna 32 . the blocking of the codes which have already been used may be accomplished in different ways . it is possible , for instance as shown in fig5 a , to accommodate the codes 23 to 27 in a shift register . the clock 28 shifts this shift register . for instance , the code 23 is read , then blocked , and the code 24 is made ready for the next readout as shown in fig5 b . since in this case the code 23 is blocked , it is assured that it cannot be emitted again for a long time . ( blocking by cancelling ). it is , however , also possible to read the answer codes without cancelling them . this may be done similar as described with the interrogation station . the codes 23 to 27 may be contained in a storer as shown in fig6 a and fig6 b , which has a reading address that is controlled by the clock 28 . the first reading address is such that the code 23 is read ( left side of the storer shown in fig6 a ) and emitted as an answer signal . after a certain time the reading address is changed so that the code 24 is read ( fig6 b ) and so forth . by this progressive control of the read address from a - code 1 - a code n , it is assured that reading of only one code at a time will occur and that codes which have already been used are blocked at least for a long period before being reused . reading without cancellation according to fig6 a and 6b has the advantage that the codes can be used anew after the full sequence has been used . this means in the present example that after the a - code n has been used that the next code will be the a - code 1 . it is to be noted , of course , that between the emitting of a - code 1 and a - code n that a long period will have lapsed and an enemy will not easily recognize the repetition . for example , codes might not be repeated in less than one hour . a receiver 34 at the interrogating station is connected to an antenna 33 for receiving the signal from transmitter 31 and furnishes it to a decoder 36 which determines if the received code is a valid code . in the receiver of the interrogation station which is also controlled by the clock 20 , a device for the storage of a - code 1 ( 23a ) to a - code n ( 27a ) is provided which is constructed similar to the device in the responder 19 . this device will therefore not be described in detail since its structure and operation is the same as the device 23 - 27 and clock 28 in the responder . for the illustrated operational condition in fig4 an interrogation with the i - code 4 is answered by the a - code 4 signal . if the decoder 36 is connected with the transmitter 10 a - code 4 , herein denoted with 26a and if an answer is received from a responder 19 which correctly answers with the a - code 4 , there will be coincidence between the decoded answer code from the decoder 36 and the a - code 4 from 26a . the observer will be informed by an indicator 37 that the responder has responded with the correct answer code . the indicator 37 might indicate in this case -- &# 34 ; code ok &# 34 ;. if , however , an answer is received which has been sent by an enemy it cannot be the a - code 4 but will be a former used code . if for instance such an imitated answer arrives with the a - code 2 ( 24a ) the decoder 36 will supply the a - code 2 whereas at this moment the a - code 4 is supplied by device 26a , and the recording device 37 shows noncoincidence and indicates &# 34 ; code not ok &# 34 ;. thus , it is immediately known by the operating personnel that an enemy is attempting to confuse them . if the interrogation station is switched to the i - code 5 by the clock 20 and simultaneously at the responder 19 the a - code 5 is enabled and the a - code 5 will be emitted at interrogation station 10 . the clocks 20 and 28 operate synchronously due to synchronizing signals on line 41 at station 10 and signal on line 42 at responder 19 . the synchroneity signal is not necessary , if the clocks 20 and 28 run exactly enough . it is assured that this switching takes place simultaneously . thus , it is assured that only answers of the correct answering code are decoded whereas an answer with the previously used a - code 4 would be recognized as an enemy interference . thus , this invention provides increased security , anti - jamming and eliminates the possibility of an enemy locating a transmitting station by providing means for storing codes used during a responding period and for inhibiting their use after they have once been used for a particular time interval . interrogating stations include means for storing used answering codes and for detecting when such used codes are used while they are in storage so as to detect transmission by the enemy and indicate this fact to the operator . although the invention has been described with respect to preferred embodiments it is not to be so limited in that changes and modifications may be made which are within the full intent and scope as defined by the appended claims .