Identification, friend or foe IFF installation

In an installation for identification friend or foe, coded interrogations and replies of an interrogation installation of the interrogating station and of a transponder of the interrogated station are transmitted in utilizing a radiotelephony installation. The transmission of the reply (identification) proceeds according to the frequency jumping procedure via the radiotelephony installation. The interrogation of the interrogating station consists of a known encoded code word and an encoded variable data part which contains (or comprises) a frequency address. The making available (or offering) of the transmit frequencies of the transponder proceeds by means of a frequency synthesizer controlled (or operated) by the frequency address. The reply is, in certain circumstances, repeated on several, successively transmitted frequencies.

The invention relates to an installation for identification friend or foe 
utilizing coded interrogations and replies in conjunction with an 
interrogation installation of the interrogating stations and a transponder 
of the interrogated stations, and with a radiotelephony installation for 
the encoded (1) communication (or message transmission) between the 
interrogating and the interrogated station. 
In order to obtain usable information from interrogated stations in 
IFF-systems, in certain instances a large number of measures is necessary 
in order to protect the interrogation and the response by a transponder, 
above all, from targeted (or aimed) interference (or jamming) maneuvers of 
an enemy and to make them illegible (or unintelligible) for unauthorized 
[personnel]. One of these measures can e.g. consist in encoding the 
interrogation in order, in this fashion, to make it impossible for enemy 
interrogations to trigger any IFF-reply. On the other hand, the encoding 
of the IFF-reply is very important in order that an enemy weapons system 
cannot transmit any positive friend-identification. It is equally 
important that the friend-identification can be received in an 
interference (or jamming) free fashion. Since the IFF-reply up to the 
present time is being transmitted in a relatively broad-band fashion on a 
reserved frequency, an interference (or jamming) of the reply 
transmission, and hence a reduction in the effect of rapidly reacting 
detection systems is possible with low enemy outlay. 
The object underlying the invention consists, in the case of an 
installation for identification friend or foe of the type initially cited, 
in improving the interference (or jamming) resistance (or protection) with 
a relatively low outlay. 
In accordance with the invention, this object is achieved in that, for the 
purpose of interference (or jamming)-resistant IFF-transmission, pursuant 
to co-utilization of the installations, or parts of the installations, of 
an interference (or jamming)-resistant-designed radiotelephony 
installation, the transmission of the reply (identification) of the 
interrogated station proceeds in accordance with the frequency jumping 
procedure via the radiotelephony installation in such a fashion that the 
interrogating station transmits, as the interrogation, a fixed encoded 
code word, known at the interrogation location, an encoded variable data 
section containing a frequency address which simultaneously adjusts the 
reply receiver of the radio installation of the interrogating station to 
the transmit frequency of the expected reply of the interrogated station, 
and that the making available (or offering) of the transmit frequencies of 
the interrogated station proceeds by means of a frequency synthesizer 
controlled by the frequency address, and that, following a first reply of 
minimal redundancy, if necessary, a numbered second reply having great 
redundancy is successively transmitted on several different frequencies. 
The advantage of the inventive installation lies in that the interrogation 
is very interference (or jamming)-resistant and that a minimal additional 
outlay becomes necessary for generating (or producing) the frequency 
addresses and the code texts for the purpose of encoding the 
identification, since the reply address is implicitly communicated. 
Since virtually all weapons systems henceforth will carry interference (or 
jamming)-resistant radiotelephony installations, it is advantageous to 
jointly utilize these installations, or parts thereof, for an interference 
(or jamming)-resistant identification transmission. The IFF-response here 
has absolute preeminence. The interference (or jamming) effect on a 
currently existing radiotelephony connection (or communication) can be 
intercepted by the redundant speech transmission, assuming that the 
IFF-reply is brief (approximately 10 ms) and that it is to be relatively 
seldom broken off (or interrupted) (two interrogations per second). 
In order to increase the interference (or jamming) resistance of the 
radiotelephony, or to first reach it at all, respectively, the frequency 
jumping, which is particularly suitable for 
ground-to-ground-communication, is employed. This signifies that, in every 
radiotelephony apparatus, a series of frequencies ([or] better, binary 
adjustment values for the synthesizer) are stored. Moreover, for the 
radiotelephony-connection setup signalling, methods of correlation 
reception are employed. The call signals consist of a clear time signal 
(or character), selected according to good correlation properties, and an 
encoded call word marking the receiver. 
For the IFF-reply, these radiotelephony-typical call signals; in 
particular, the corresponding signal transmitters and signal receivers, 
can be jointly employed. A suitable IFF-specific masking of the call word 
ensures the separability of IFF- and radiotelephony-signals. 
In order to emit (or transmit) an IFF-reply, the following signals must be 
transmitted (or emitted) to the radiotelephony apparatus: 
frequency address or adjustment information for the frequency synthesizer 
a masked (or veiled) text for the purpose of IFF-individual masking of the 
radiotelephony call word 
transmit command 
The processing (or editing) of the signals takes place either in an 
adaptation (or matching) apparatus between the IFF-transponder and the 
radiotelephony apparatus, or it is a fixed constituent (or component) part 
of the transponder. 
In addition, it is advantageous to design the IFF-response adaptively; 
i.e., a first interrogation is to trigger only a brief reply. The 
interference (or jamming) effect of this reply on the radiotelephony is 
then only minimal. Only in 10% of all interrogations is a so-called second 
interrogation to become necessary and trigger a frequent IFF-reply to be 
transmitted on many frequencies, so that the overall recognition 
protection of an IFF-reply becomes very great. 
The encoding and decoding of an IFF-interrogation signal assumes 
synchronism of the corresponding coding apparatus (or cipher machines). 
This synchronism can be forced (or constrained) by more or less precise 
time normals (clocks). A simple method of correctly decoding, in spite of 
synchronization errors, consists in that, by means of a 
start-stop-operation of the coder, a new code text is generated and stored 
per time increment (milliseconds to seconds). For decoding three code 
texts are simultaneously employed. 
1. Code text from a preceding time increment 
2. Code text from the currently valid time increment 
3. Code text from the next-following time increment. 
It is readily apparent that the allowed synchronization error can be 
permitted to be .+-.1 time increment. One of the three code texts can then 
decode the message. However, the beginning, or end, respectively, must be 
recognizable in the received message; moreover, an encoded code word, 
known at the transmitting- and receiving location (or station), and 
contained in the message, must determine the respective code text. 
Depending upon the quality of the clocks employed, various methods of 
IFF-interrogation and -response are possible. The clock precision is of 
importance insofar as very brief time increments can be made with precise, 
but expensive clocks (error 10.sup.-8); i.e., very frequently new code 
texts become effective (or operative). Imprecise clocks lead to long time 
increments, thus also [to] long validity periods (or durations) of a code 
text. This connection shall be explained in the following on the basis of 
two examples. 
1. Clock precision .+-.10.sup.-6. The "clock" is corrected every 8 days. 
Eight days identical=691 200 s. 691 200 2 10.sup.-6 -1.38 s. The clock 
error after 8 days can amount to .+-.1.38 s. A time increment follows 
therefrom of approximately 1.5 s; i.e., every 1.5 s a new code text is 
employed and all interrogations within 1.5 s have the same encoding. 
2. Clock precision .+-.10.sup.-8. The "clock" is corrected every 8 days. 
691 200 2 10.sup.-8 =13.8 ms. 
Thus, a new code text is possible every 15 ms. Virtually every 
interrogation has a different code text as a basis. 
As previously mentioned, in the case of imprecise clocks, several 
interrogations can fall into the validity (or current, or significant) 
period of a code text and thus bring about identical interrogation 
signals, or reply signals, respectively. This could be exploited by the 
enemy for interference (or jamming) purposes or utilized for a 
friend-simulation respectively. For this reason it is expedient to 
incorporate, in every interrogation, a variable data part, generated per 
interrogation, respectively. This variable data part, together with the 
code text, which is valid for a longer [period of] time, can serve as 
basis-information for generating frequency addresses and for masking 
reply-identifications. 
The interrogation in this instance contains (or comprises) not only a fixed 
or encoded code word, but additionally a data part varying per 
interrogation. Whereas, in the fixed code word, errors are permitted, in 
the variable data part of the interrogation, transmission errors must at 
least be recognized through data protection measures. Methods of redundant 
coding can find application here. 
The variable data part of the interrogation, together with the currently 
valid code text (parts thereof), can, in a simple fashion, serve as start 
information of a linear, fed-back shift register (scrambler). After the 
recognition of an interrogation, the decoding of the interrogation signal, 
and the setting of this register, by means of defined clock-pulsing of 
this register the frequency address and masked text for the masking of the 
reply identification can be obtained and retransmitted to the radio 
apparatus for the purpose of response. In this manner also the so-called 
second reply; namely, many identifications on different frequencies, can 
be generated. In this instance, the register is repeatedly (or multiply) 
charged (or fed, or loaded) with clock pulses in a compulsory (or forced) 
sequence, and thus different frequency addresses and different masked 
texts are produced (or generated) per operating (or work) step. Of course, 
a similar procedure must be carried out on the interrogation side for the 
purpose of decoding the reply. Delay times, brought about by the 
processing time of the interrogation signal at the transponder, must be 
taken into account on the reply receiving side. Likewise, the maximally 
occurring distance (or link path) transit (or propagation) time (2) can be 
taken into account by means of a "receiving window" for the respective 
identification. The degree of safety (or protection) of this method, in 
relation to enemy interference (or jamming) capability, or simulation 
capability, respectively, is, indeed, restricted, but is nevertheless 
sufficiently high. This method can, of course, also be applied in the 
[case of] utilization of precise clocks. The degree of safety (or 
protection) increases as the validity (or current, or significant) period 
of the code text becomes shorter. 
In the utilization of precise clocks, or in the case of a brief validity 
period of the code text (several milliseconds), it is possible to dispense 
with the transmission of a variable data part in the interrogation, since 
every interrogation, with [a] high [degree of] probability, is encoded 
with another code text. The evaluation (or analysis) of the interrogation 
is then only still restricted to the decoding of the fixed encoded code 
word for the purpose of synchronization of the transponder coder to the 
interrogation coder and decoding: 1./2. Interrogation, frequency address 
and masked text for masking the identification can be taken (or removed) 
from the coder at the transponder; i.e., in addition to the generation of 
the code text for the purpose of decoding the interrogation (3) texts, 
respectively), additional code-bits for frequency addresses and 
identification encoding are generated and, with the recognition of an 
interrogation, are retransmitted to the radio apparatus. In a second 
interrogation, the coder is clock-pulsed in a compulsory (or forced) 
sequence (or train) and the resulting code texts are retransmitted to the 
radio apparatus. This occurs at the interrogator as well as at the 
transponder (taking into account the signal processing time).

The interrogation signal, demodulated via an optical, or electronic, 
respectively, receiving system 1 (FIG. 1), is input into a decoding 
circuit 2. In the decoding circuit, at three [combinational] logic 
elements M1, M2 and M3, the interrogation signal is decoded simultaneously 
with three code texts stored in registers SR1, SR2 and SR3. The three 
decoded interrogation signals pass through one comparison register VR1, 
VR2 and VR3 each. The contents of these registers are checked (or 
examined) as to identify with a fixedly adjusted code word at comparison 
circuits V1, V2 and V3. The results of the identity check are supplied to 
threshold value circuits SW1, SW2 and SW3. In the threshold [value] 
circuits the number of correspondencies of [the] received [code word] with 
the fixedly adjusted code word is ascertained (or determined), and, upon 
exceeding a predetermined threshold (number of correspondencies), a signal 
is emitted (or transmitted) to a decision logic EL. The decision logic 
then activates switches S1 and S1' in such a fashion that only that 
particular code text remains effective (or operative) which was able to 
correctly decode the code word. 
The variable data part following the code word and the respective check (or 
test) part of the interrogation signal is now decoded with the correct 
code text and can be further processed. 
As previously stated, the received interrogation signal, in particular, the 
code word-portion, is simultaneously decoded with three different code 
texts. This is necessary in order that the precision demand on the control 
of the code apparatus S remains in realizable boundaries. A clock control 
of the code apparatus 5 ensures, at the location of the interrogator as 
well as of the transponder, the generation of the code texts; i.e., per 
time increment (e.g. sec), a new code text is generated. Between two new 
code texts, however, several interrogations can take place; therefore, in 
the case of every interrogation, an individual part (variable data part) 
is incorporated in the interrogation signal. Due to the permitted clock 
error--as already described above at the location of the transponder the 
code text valid for the interrogation must be searched out (or selected). 
Three code texts are here available. Relative to a clock U of the 
transponder and to the arrival of an interrogation, these are the code 
texts from a preceding, a currently valid, and a future time increment. 
Via the code word, information known at the transmitting- and 
receiving-location, the suitable code text is selected and with this code 
text the further processing of the interrogation signal is carried out. 
The transponder-code apparatus can be controlled (or operated) in the 
following manner: 
A clock U, which consists of a quartz-controlled clock-pulse distributor 
chain, ensures the control (or operation) of the coder 5. Per time 
increment, it is now possible to generate a code text which can be 
modified (or influenced) via a day code. 
If the value "0" is added to the clock time, the currently valid time 
increment (at) the coder S is effective (or operative). The past or 
future, respectively, time increment can be made effective through 
addition of -1 or +1, respectively, to the currently valid clock time. 
With the recognition of an interrogation in the transponder, the coder S is 
activated three times with a (excessive) clock pulse speed and the value 
-1.0 and +1 is connected to a summer SU. The resulting code texts are 
stored in the separate registers SR1, SR2, SR3 and the registers SIR1, 
SIR2, SIR3 of a scrambler 4. 
Another possibility of control (or operation) consists in that the clock U 
at the transponder generally advances (or gains) a time increment, and 
that a new code text is generated in the clock pulse of the clock per time 
increment. The storage of the three code texts then proceeds serially by 
means of series-connected registers. With the input of a new code text, 
the contents of the registers are automatically displaced (or shifted), so 
that always also the two preceded code texts are available. During the 
interrogation-decoding, the three registers must be (separated-up) and 
each individually must be closed in the circuit in order that the contents 
can be available to additional interrogations. 
The further processing of the previously decoded variable data part 
proceeds in the following manner. The variable data part is read 
simultaneously into two installations, into an error-recognition 
installation 3 and into a scrambler 4. In the error-recognition 
installation 3, by means of a division register DR, the variable data part 
and the respective check (or test) part is divided by the generator 
polynomial. If, at the end of the division operation, a remainder remains 
in the register DR; i.e., if not all positions of the division register 
are "0", then transmission errors have been recognized. A zero test 
installation NT responds (or is actuated) as soon as the division register 
is filled with zeroes; i.e., [as soon as], with [a] high [degree of] 
probability, no transmission errors have occurred. In the case of a 
successful zero test the further processing of the variable data part is 
continued. 
As previously stated, the variable data part is retransmitted (or 
forwarded) to the scrambler installation 4 and there entered into the 
scrambler-register SA. Simultaneously, from a code (or key)-register SIR1, 
SIR2, SIR3, selected via the switch S1, a code information is accepted (or 
transferred) into a scrambler-register SB. Both information parts, the 
variable data part and the code (or key) part, serve as the initial value 
for generating a bit-sequence in accordance with the characteristics of 
linear, fed-back shift registers. 
Basically, the code text-portion for formation of the start address could 
be dispensed with here. However, it increases the simulation protection 
(or resistance) of the identification and renders possible long register 
periods. 
The last bit of the variable data part represents a so-called 
interrogation-"number" in order that it is possible to distinguish between 
a first interrogation and a second interrogation. This last bit is 
deposited in an additional memory AN. 
If the zero test in the error-recognition installation 3 was successful, 
then the switches S2 and S2' of the scrambler 4 are brought into position 
b. The register SA, SB, loaded with a start information, now operates as a 
linear fed-back shift register (scrambler) and generates a bit-sequence 
(or train) as soon as a shift clock pulse is (applied). The period of this 
bit sequence (or train) amounts to 2.sup.n -1 bit; n is here the step 
number of the register. 
With a first interrogation this register is now subjected to few clock 
pulses. The bit-sequence (or train) thus generated represents a frequency 
address (a-bits) and a masked information for masking of the 
identification. Together with a transmit command this information is 
retransmitted to a radiotelephony apparatus SF1. The respective frequency 
is adjusted there, a masked identification signal generated and the 
transmitter switched-on for the duration of the identification 
[transmission] via a sequence control 6. 
A second interrogation triggers a plurality of these identification 
transmissions. For this purpose, the register SA, SB, is repeatedly (or 
multiply) charged (or loaded) with clock pulses and the transmit command 
is repeatedly (or multiply) issued via the sequence control. Each one of 
these identifications takes place on a different frequency and contains a 
different masked information. The identification itself is the 
radio-specific call signal; it is decoded from the receiving signals at 
the location of interrogation by means of correlation methods. 
In comparison with the IFF-transponder, the interrogation-installation 
(FIG. 2) is substantially more simply constructed. It consists of the 
actual interrogator AF and the radiotelephony apparatus SF2. In 
particular, the code (or key) apparatus 10, together with peripheral 
register SR1, SR2, SR3; SIR1, SIR2, SIR3 must generate, per interrogation, 
only one code text and can thus be simplified. The identity check (or 
test) installations for the code words are eliminated. In addition, a 
noise [signal] generator RG is required in the interrogation installation. 
It generates, per interrogation, a new, variable data part which is 
independent of the preceding interrogation. The variable data part is 
converted (or revalued, or translated) in the data protection installation 
11 pursuant to addition of redundancy (switch SA2 in position a; switch 
SA3 in position a), so that an error-recognition is possible in the 
transponder. The last bit of the variable data part represents the 
interrogation number. For this purpose, switch SA2 is brought into 
position. The check (or test) bits resulting in the data protection 
installation 11 through polynomial division are appended to the variable 
data part in utilizing systematic codes. 
Switch SA3 is then disposed in position b. The interrogation signal, 
accordingly, has the following construction: code word; variable data 
part; check (or test) bits for error-recognition. 
In detail, the following control operations are necessary: 
Synchronously with the transmission of the interrogation signal, the code 
text is read out of the buffer register PR1 through the transmitter. On a 
[combinational] logic element V, the encoding of the interrogation signal 
takes place. The fixedly adjusted code word, stored in a code word 
register CR, is first transmitted (SA1 in position a). Subsequently the 
switch SA1 is brought into position b. The switches SA2, SA3 and SA4 are 
now in position a. 
The variable data part generated in the noise [signal] generator RG is once 
loaded into the scrambler-register SCR A, on the other [hand], in the data 
protection installation 11, the division by the generator polynomial is 
carried out and simultaneously the variable data are transmitted. The last 
bit of the variable data part is the interrogation number; for this 
purpose, the switch SA2 is brought into position b. Simultaneously with 
the reading-in of the variable data part into the scrambler-register SCR 
A, the scrambler register SCR B is loaded with the code (or key) 
information disposed in the buffer register PR2, so that, at the end of 
this operation, the same start condition for the generation of the 
scrambler sequence is entered as at the location of the transponder. 
Following transmission of the variable data part, the switch SA3 is brought 
into position b and the division-result--the check (or test) positions (or 
digits)--are read out of the data protection installation 11 and 
transmitted. Taking into account the signal processing time resulting at 
the transponder, the scrambler SCRA, SCRB is started via the switch SA4 
(position b). The resulting bit-sequence (or train), together with the 
receive command, is forwarded (or retransmitted) to the radiotelephony 
apparatus SF2. 
FIG. 3 
In the radiotelephony apparatus SF2, the transmission frequency of the 
identification is adjusted and the masked text for unmasking the 
identification is made available (or offered). An identification 
correlator KK in the radiotelephony apparatus now "seeks" the 
identification in the receive signal. If the identification is (detected), 
a corresponding signal is transmitted to a sequence (or job) control AST 
of the interrogator AF; i.e., a "friend"-communication has arrived. A 
second interrogation is then not necessary. 
If, by contrast, within a certain time window (.mu.s-ms), no identification 
is (detected), the second interrogation is started. The scrambler SCRA, 
SCRB is repeatedly (or multiply) driven by clock pulses for this purpose, 
so that, synchronously with the transponder-radio apparatus, the 
respective frequencies and mask texts become effective (or operative). The 
second interrogation is "successful" when at least one of the 
identifications was recognized. 
The control of the coder SAF for generating the code texts proceeds via a 
clock circuit. Per clock-time increment, a new code text can here be 
generated and deposited in the buffer memories 1 and 2. However, it is 
also possible to permit the clock time to act upon the coder only at the 
interrogation time; the code text can then be generated in step with the 
interrogation signal.