Note: Descriptions are shown in the official language in which they were submitted.
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2~.309
SECURE C~MUNICA~ION SYST~M
3ack round of the Inven~ion
q
1. Field of the Invention
The present invention relates to the field of secure data
communication systems, and more particularly to such a system
employing frequency-hopping techniques and encrypted data.
2. Descri tion of the Prior Art
P
Various tyFes of communications technioues are employed in
order to ensure the security of the information ~eing tra~smitted
and to make such communications more resistant to intentional
jamming or noise. ~or example, in a spread-spectrum
communications system an initial narrcw-band information signal is
modulated with a pseudo-random code sequence having noise-like
properties which spreads the original narrcw-band signal over a
much larger bandwidth and at reduced amplitude. Assuming that
both a tra~smitter and a receiver utilize the same pseudo-random
sequen oe , detection and decoding of the information o~ the spread
signal is simply a matter of synchronizing the pseudo-random code
generators of both the transmitter and receiver to the same
initial bit position of the pseudo-random sequen oe . Such a
spread-spec~rum communication system offers good data security
sin oe a third party who wishes to~decode the transmission would
have to have in his possession the pseudo-random bit sequence and
would have to properly synchronize the sequence with that of the
transmitter. In addition,~ such a system is inherently resistant
to jamming, sin oe the ~nformation borne by the spread-spectrum
signal.is spread out over a relatively wide b~nd of frequencies.
Thus, an attempt to jam such a signal by utilizing a high pcwer
signal of relatively narrcw bandwidth would merely degrade
somewhat the reception of the broaaband information signal and
siightly increase the probabillty o~ errors in decoding the
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386
signal. Alternatively, broadband jamming can ~e used to
completely swamp the relatively low amplitude spre~d-s~ectrum
signal. However, this is at the cost of having to e~ploy a large
number of ~owerful transmitters in order to broadc~st enough
energy sver a broad enough bandwidth to ensure adequate jamming.
In addition, such broadband jamming techniques can be relatively
easily overcome simply by moving the fundamental carrier frequency
of the transmitted information signal to an area not covered by
the broadband jamming system. Also, such broadband syste~s tend
to be relatively easy to detect and locate, thus subjecting them
to conventional attack or electronic countermeasures.
~ second type of secure data communication system utilizes a
so-called "frequency-hopping" technique. In a frequency-hopping
system, information is transmitted and received on frequencies
which are frequently changed or ~hopped". The frequency-hopping
pattern is normally predetermined and built into the transmitter
and receiver. The pattern can be generated in various ways, such
as through the use of pseudo random number generators in both the
transmitter and receiver which, once synchr~nized to a particular
initial frequencv, will thereafter change freauencies n tandem
with one another.
Data encryption is frequently employed in fre~uency-hopping
systems in order to prevent unwanted reception of the data being
transmitted. Further, it is known to transmit the ~essages in
short bursts followed by a c~ange in frequency to make it more
difficult to jam the transmitted signal. This is because ~he
burst transmitted signal will have a much hisher ef~ective
signal-to-noise ratio, for a given amount of transmitted power,
than a continuous tone-jamming signal would have of com~arable
pcwer.
One drawback to current requency-hopping systems is that
although the hopping pattern is intended to be rzndcm, it is
actually a deterministic pattern wnich can be derived by someone
who observes the pattern of transmissions cver a sufficiently long
time period. The ~attern can also be easily determined if a
transmitter or receiver is captured and the frequency-hopping
~L~4~6
pattern generator is subjected to study.
In addition, the transmitter and receiver in sucn a system must
be initially synchronized to the beginning of the frequency-hopping
pattern before each series of transnissions. If such synchronization
does not occur (due, for example, to intentional jamming or noise),
then the transmission of information from the transmitter to the
receiver cannot take place.
Summary of the Invention
It is a general object of the invention to provide an improved
method and apparatus for secure communications.
This and other obiects are attained, in accordance with one
aspect of the invention by a method of secure communications
utilizing frequency-hopping and having at least a tran~mitter and a
receiver, said metnod preventing intentional jamming an~ comprising
the steps of: (a) generating at the transmitter an information
signal including at least a portion representative of the frequency
of transmission of the next informational signal; (b) transmitting
the information signal at a transmission frequency determined by the
frequercy representative portion of an lmmediately preceding
transmitted information signal; and (c) receiving the info~nation
signal at the receiver at a freq~er~y determined by the frequency
representative portion of the immediately preceding received
information signal.
Another aspect includes a secure communications apparatus of the
freq~ency-hopping type comprising: a transmitter including means for
generatir~ an information signal including at least a portion
representative of the frequency of transmission of the next
information signal, ar~ means for transrnitting the information
signal at a transmission frequency determined by the fre~uency
representative portion of an Lmmediately precedir~ transmitted
information signal; and a receiver for receiving the information
signal, incIudin3 means for tuning the receiver to a frequency
determlned by the frequency representative portion of the
immediatel~ preced mg recelved information signal.
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Brief Description of the Drawing Figures
These and other features and advant~ges of the present invention
5will be clear from the following detailed description of the
preferred ert~oditrRnt, when taken in conjunction with the drawing
figures wherein:
Figure 1 is a block diagram. of a secure data comnunication
system arranged in accordance with the present invention; an~
10Figure 2 shows diagramatically a typical informa~ion signal as
used in the present invention;
Figure 3 shows diagramatically the transmission of a series of
information signaLs, such as shown in Figure 2, in accordance witn
the preser~ invention; and
15Figure 4 is a flow chart illustrati~g the operation of the
secure data co~nunication system of the present invention.
etailed Descr ption of the Preferred Embodiment
With reference to Figure 1, there is shown a block diagram of a
secure communication system which utilizes the techniques of the
present inventïon. The communication system comprises a pair of
transceivers 101 and 201, each including a transmi~ter section 103
30and 203, respec~ively, and a receiver section 105 and 205,
respectively.
.
~4~3~36
Both transceivers 101 and 201 include ccntrollers 107 and
207, respectively. Each controller 107 or 207 ~ay include or
c~nprise a microprocessor 2nd associated memory and input/output
(I/O) circuits.
Controller 107 is in com~unication with a data encryptor 109
while ccn ~ oller 207 is in communication with a data decryptor
209. The function of data encryptor 109 and data decryptor 2C9
are explained belcw.
In one exemplary embodiment, the invention is designed to be
used to enable communication between transceivers 101 and 201 to
take pla oe in a secure fashion, including a high resistance to
jamming or noise which may occur o~er a transmissicn li~k 300.
Transmission link 300 may, for example, be a radio link, fiber
optic link, telephone link, etc.
For example, transceiver 101 may be set up as a ground based
control system for ccntrolling a remote vehicle (e.g. a pilotless
surYeillan oe aircraft) which includes trans oe iver 201. The
transmission link 300 is a radio link. Transmitter section 103 is
designed to transmit control signals via ~an up-link portion of
transmission link 300 to receiver section 205 of the remote
vehicle. The pilotless vehicle includes various sensors and/or
optical systems for observing a predetermined area. These sensors
and/or imaging devioe s (not shown) feed telemetry and/or video
signals into the transmitter section 203 for broadcast back to
transoe iver lOl at the ground station via a down~link portion o~
transmission link 300. Receiver section 105 of transceiver 101
receives these down-link signals and further processes them or
displays them. The up-link and down-link transmission frequencies
may be either the same or different from each other.
T~e transmitter section 103 of transceiver lOl and receiver
section 205 of transceiver 201 whicn communicate over the up-li~k
portion of transmission link 300 utilize a burst-mode
frequen~y-hopping technique in order to reduce susceptability to
portions of the broadcast spectrum which may be particularly noisy
or ~here intentional ja~ning signals are present. To this end,
ccntrol signals and other data or m formaticn are transmitted as
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digital signals cver the up-link by transmitter section 103 a~d
are received by receiver section 205. As shown in Figure 2, a
typical information signal 301 includes an mitial preamble 303
of, for example, 128 bits, with the bit pattern chos2n to enha~oe
the ability of the receiver to establish symbol synchronizaticn.
When a differential bi-phase code is employed, for instan oe , an
all "zero" pattern might be used. The purpose of the preamhle
bits i~ to enable a receiver to synchronize and lock on to the
signal. Following the preamble is a short series of bits 305
(e.g. 24 bits) for fra~R synchronization purposes. The fra~e
synchronization bits may be one of many different patterns sucn as
those specified by the Interange Instrumentation Group (IRIG).
The fra~e synch ~its are utilized by a receiver to indicate the
beginning of a message or a series of digit 1 bytes or words.
Follcwing the frame synch pattern are a series of bits 307
defining the actual message. The message bits may be of any
predetermined number (e.g. 256 bits or thirty-two eight-bit
bytes). The ~essage data can include such things as control
signals for steering the remote vehicle, si~nals for activating or
turning off certain surveillan oe sensors, etc.
An important feature of the present invention is that at
least a portion of the transmitted message 307 includes a word
which is representative of tne frequency on wnich the next
information signal will be broadcast. This information is used by
the receiver in the remote vehicle, as more fùlly explained below,
to enable it to retune the receiver in order to receive the next
messase transmission at the new frequency.
The information signal may further include a series of parity
check bits 309 (e.g. 16 bits) wnich, as explained more fully
below, enable the receiver in the remote vehicle to determine
whether it has correctly received a particular information
signal. It is to be undefstood that the above arrangement of
information signal 301 and the pa~ticular bit lengths and patterns
employed are merely exemplary and may be suitably changed to other
values or arrangements in accordan oe with a user's particular
needs.
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A second important feature of the invention is that at least
the word representative of the next tran~nission frequency is
encrypted, and preferably the entire s~quence of message bits 307
is encrypted. Data encryption is performed by data encryptor lO9
wnich can use any one of a number of well-known techniques for
encrypting the message portion of ~he information signal. For
example, the National Bureau of St~dard's Data Encryption
Standard ~DES), as descri~ed in Federal Information Processing
Standard Publication ~o. 46 Jan~ary 1977, may be used. By
employing the DES, the security of the message being transmitted
by transmitter section 103 to receiver section ~05 is relatively
high, especially in view of the fact that only a few seconds will
go by before the next transmission, at a different frequency,
takes place.
Sin oe ~he word representative of the next frequency of
transmission is also encrypted it would be highly unlikely that
someone would be tuned in to the proper frequency to receive the
up-link transmission, and then also be able to decode it qulckly
enough in order to know which frequency the next data transmission
would take place. Thus, someone who wished to eavesdrop on the
up-link transmission or intentionally jam such transmission would
have a very difficult time in doing so. Further, sin oe the
frequency-hopping pattern is determined only at the ground station
~transceiver lOl) there is no loss in .security if the remote
vehicle should fall into the possession of an unauthorize~
Ferson. This is because the frequency-hopping pattern generated
by controller 107 can be changed periodically and in a random
fashion.
In addition, transceiver 205, wnich is situated Ln the remote
vehicle, may include sensors for detecting the spectrum of any
jamming signals which may be applied to either the up-link or
down-linX portions of transmission link 300. This spectral
information for the jamming signals ~ay be sent back to receiver
section ~105 of the ground statlon by transmitter section 203 of
the remote vehicle over the down-link. This information can then
be assimilated by controller 101 and used to change the
L4~3~6
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frequency-hopping pattern employed by transmitter section 103 in
order to minimize the effects of such jamming signals on its
transmission. The down-link informaticn tra~smitted by
transmitter section 203 of transceiver 201 to reoe iver section 105
of transceiver 101 may employ the same encryption and message
format as employed in the up-link signal.
Io further enhance the security of transmission of the
information signals, each signal 301, such as shown in Figure 2,
is sent as a short burst with a relatively long delay between each
such burst, as shown in Figure 3. For example, if each
information signal is 424 bits long and transmitted at
approximately 20 kilobits per second, the duration Tl of
information signal 301 will be approximately 21 milliseconds. As
shcwn in Figure 3, the ti~e T2 between each such information
signal transmission is greater than the length of the information
signal transmission itself, and preferably is much greater than
the duration Tl of a single information signal transmission.
Generally, sufficient time is allotted between each burst
transmission to enable a receiver to check for errors in the
transmission of the information signal and to perform the
oecryption process on at least the word representative of the next
frequency of transmission. In the example given, this period of
time is on the order of 3-4 seconds. While such a time period is
adeq~ate to enable error detection and decryption of the fre~uency
representative word to take p.la oe , is much too short a time for
another par y, who does not have the decry,otion key, to decode the
encrypted word representative of the next transmission frequency.
Therefore, ~uch an unauthorized party would not be able to decrypt
the fre~uency representative word quickly enough to kncw what the
next transmission frequency wiLI be in order to retune his
receiver to receive the next message.
In addition, the burst-mode of transmission makes it more
difficult for an unauthorized third party to utilize
radio-location techniques to home in on and locate and/or destroy
either the ground station or remote vehicle. A further advantage
of the burst-mode of r~ssage transmission is that each message is
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~.~4~L88~i
effectively transmitted at a higher peak power due to the short
duration of each transmission. This enables relatively low pcwer
components '.o ~e utilized and makes the transmitted signal more
difficult 'o jam using broadband jamming techniques, sin oe the
jammer will need to employ a much higher average power across a
much larger bandwidth in order to be assured of blanking out the
burst-mode transmitted message.
Referring back to Figure 1, the operation of transceivers 101
and 201 wiIl nc~ be explained Ln more detail. Initially,
instructions or other ~essages or information in a format similar
to that shown in Figure 2 are generated by controller 107.
Controller 107 is, for example, a microprocessor including a
oertain amount of random access memory (R~) and read only me~ory
(RCM), such as the 8085 processor sold by the In.el Corporation.
The message generated by controller 107, either under program
control or by input from an operator (not shcwn), is applied to
data encryptor unit 109. Data encryptor 109 may implement any one
of a number of well-known data encryption algorithms. For
example, the Data Encryption Standard promulgated by the National
Bureau of Standards is one such high-security encryption
algorit~m. One advantage to using the Data Encryption Standard is
that there are available standard Lntegrated circuit chips which
perform the data encryption and data decryption functions. One
such chip is the Intel 8294 data encryption unit.
After the message has been encrypted, it is returned to
controller 107 for further processing and then output to
transmitter section 103 of transceiver 101.
Transmitter 103 includes a parallel-to-serial and code
converter unit 111 whicn takes the output of controller 107 (which
is in parallel form) and converts it int~ a serial bit-stream for
application b~ transmitter unit 113. The code converter portion
of unit 111 changes the standard binary outp~t of controller 107
; m to a differential bi-phase code for application to transmitter
113. A differential bi-phase code is one in which, for example,
a "1" is represented by a change in phase from the previously
transmitted signal while a ~l0" is represented by no change in
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phase. Although other types of encoding schemes can be used,
differential bi-phase coding has the advantage of being self-
clocking and therefore does not re~uire a separate synchronization
signal.
The data signal applied to transmitter 113 is then used to
modulate a carrier signal at a predetermined frequency using
standard frequency shift keying (FaK) techniques. The particular
frequency of transmission is determined by frequency synthesizer
115 whose setting, in turn, is determined by controller 107.
Fre~uency synthesizer 115 preferably is capable of being
programmed to select from a large number of transmission
fre~uencies to enable the transmitted messages to be
frequency-hopped over a wide enough range so as to avoid most
types of narrcw-band jamming.
As mentioned earlier, each information signal 301 is sent as
an individual unit in a short burst transmission. For ex2mple, if
the data rate output by unit 111 is 20 kilobits per second, and
assuming an inform tion signal co~taining approxim~tely 424 bits
of data, then the length of transmisston of an individual
information signal will ~e approximately 21 milliseconds. The
exact timing of when tne infor~ation signal is transmitted is
under control of controller 107, 2s is the period of silen oe T2
(no transmission) between each information signal. These pauses
in data transmission are designed to oe just long enough for
2S receiver section 205 of transceiver 201 to detect the transmitted
signal, check it for errors, and to decrypt the message contained
therein.
The encrypted FSK signal 301 is then sent over transmission
link 300 via the up-link section as shown in Figure 1. m is
up-link may be a radio communications channel, a fiber optics
coTmunicaticn link, telephone line, or other such co~munications
channel.
Ate~ remote vehicle receiver section 205 of trans oe iver 201
receives the encrypted FSR signal it is processed for utilization
by controller 207.
~ 2~ 36
The incoming information signal 301 is first ap~lied to
receiver unit 211 ~ihose reoeption frequency is controlled by
frequency synthesizer 213. Frequency synthesizer 213 is in turn
controlled by controller 207. Receiver unit 211 demodulates the
5 received information sisnal and a~plies the demodulated signal to
syrnbol detector and synchronizer unit 2i5. Symbol detector and
synchronizer unit 215 operates in a well-knatJn manner to detect
preamble 303 of the inormation signal 301 (see Figure 2) and to
synchronize a voltage-controlled oscillator (~CO) of a
10 p~ase-locked loop (PLL), which are part of unit 215, to the symbol
rate of the bits of the information signal. Cnoe the received
bits are detected and the phase-locked loop has been locked to the
symbol rate of the bits of the information signal, the serial
bit-stream is applied ~o word synchronizer and serial-to-parallel
converter unit 217. Unit 217 operates in a well-knc~n fashion o
detect frame synchronization bits 305 tsee Figure 2) of each
information signal to produce a signal alerting controller 207
that ~he start of the message portion 307 of the information
signal has been located. The serial data stream is then converted
20 into` parallel blocks (e.g. 8 bits wide) for application to
controller 207.
Controller 207 is similar in structure to controller 107 and
includes a microprocessor and both R~S and RCM menories.
Controller 207 may further have stored in its n~e~ory one of a
25 number of well-kno~n error detectial and/or error correction
algorithms for determining whether the message portion of the
information signal has been properly received. For example,
simple error detection can be provlded by inspecting the parity
bits 309 ap~ended to the end of each information signal and
30 comparing them with the detected pattern of data bits in the
encrypted message portion 30? of the informati~ signal. Error
correction can be provided by initially inserting a pattern of
error correcting bits into the encrypted message portlon 307 prior
to transmission by transmitter section 103. E`or exanple, a code
of the BCH (30, 25) type or Reed-Solomon encoding may be used.
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If controller 207 detects an error in the received data
message portion, or if the message is not received at all during a
predetermined time period, an error signal is generated by
controller 207 and applied to transmitter section 203 of tne
remote vehicle. It should be noted that error detection takes
place prior to data decryption. This is because data decryption
is a relatively slcw process, whereas error detecticn can be
performed within as little as a few microseconds after reception
of the information signal using suitable hardware. Ihis enables
controller 207 to determine whether it h2s properly received
message portion 307 of the information signal and to request
retransmission immediately withalt bothering with data decry2tion.
If the received message is error free, controller 207 applies
encrypted message portion 307 of the information signal 301 to
data decryptor 209. Data decryptor 209 perfo~ms data decryption
in accordance with the particular data encryption algorithm
employed. Once decrypted, the informatiGn signal is returned to
controller 207 for further processing. It should be noted that
one porticn of the decrypted message contains a word
representative of the frequency of transmission of the next
information signal to be transmitted from the gro~nd station.
Controller 207 then uses this fre~uency representative word to
ccntrol the frequency selected by frequency synthesizer 213 which,
in turn, is used to retune receiver 211 to the frequency the next
information signal is to be transmitted on.
Thus it can be seen that infor~ation signal 301 transmitted
by transmitter section 103 to receiver section 205 contains all
the data necessary for receiver section 205 to know what the next
transmlssicn frequency will ~e. Unlike prior-art
frequency-hopping systems, this means that r~ceiver section 205 of
the remote vehicle does not have tD know ahead of time what the
frequency-hopping pattern will be. This makes the design of the
remote vehicle receiver ~section 205 simpler and enhances the
security of the overall communic~tion system sin oe , if by some
misfortune the remote vehicle were to fall into unwanted hands,
there is nothing contained in its circuitry which would enable
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someone to know what the frequency~hoppi~g pattern is. ~nis is
because the frequency hopping ~attern is generated ~olely by
controller 107 at the ground station.
Although not essential to the basic operation of the
invention, Figure l shows some additional elements for
transmitting information from transmitter section 203 of the
remote vehicle via the down-link porticn of transmission lirAk 300
to receiver section 105 of the ground station~ Preferably, this
down-link information includes signals indicative of the non-
receipt or the erroneous receipt of a signal transmitted over theup-link portion, or an acknowled~ment signal indicating that the
signal transmitted over the up-link has been procerly received.
m e error and acknowledgment signals are generated by controller
207 in response to the proper receipt or tne non-receipt/Lmproper
receipt of the up-link information signal.
The acknowled~ment or error signal is applied to telemetry
formatter unit 219 which may also receive other telemetry data
from sensors in the remote vehicle (not shown). If the re~ote
vehicle is, for example, a pilotless observation aircraft, the
teleme~ry data can be indicative of altitude, speed, camera angle,
etc. ~ne telemetry data can also include spectral information
con oe rning the distribution of noise and/or jamming signals over
the transmission link 300.
The acknowledge/error signals and other telemetry signals ~re
then applied to a telemetry subcarrier modulator/oscillator 221.
In addition, if visual observations are ~eing made by a video
camera (not shown) the video signal can be applied to a low-pass
filter unit 223 ~nose output, along with the output of the
telemetry subcarrier modulator/oscillator 221, is summed Ln
summing device 225 and then applied to transmitter unit 227. The
frequency of transmission of the down-link signal output by
transmitter 227 is selected by frequency s~nthesizer 229 wnich, in
turn, is controlled by ccntroller 207. The down-link transmission
frequency may be periodically changed, in a fashion similar to
that employed with respect to the up-link frequency, in order to
enhan oe the security of the do~n-link signal. Although no data
12~ 6
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encryption is shown for use with the down-link signal, transmitter
section 203 of the remote vehicle obviously can be m~di~ied to
include data encryption of the tel.emetry and/or video signals,
along with message-by~message frequency-hopping as is utilized
with the up-link transmitter sectiGn 103.
The telemetry and/or video signals are sent over the
down-link portion of transmissicn link 300 and are received and
processed by receiver section 105 of the ground station. Receiver
section 105 includes a receiver unit 117 which receives the
down-link signal at a frequency determined by frequency
synthesizer 119. Frequency synthesizer 113 tunes receiver 117 to
` a frequency determined by controller 107.
Receiver 117 demodulates the down-link signal and passes it
to a low-pass filter 121 and a band-pass filter 123. The output
of low-pass filter 121 contains video data information which ~y
then be supplied to a display devioe , such as a TV monitor (not
shown). The output of band-pass filter 123 contains telemetry
information which is then applied to telemetry decommutator unit
125. The ou~put of telemetry decommutator unit 125 is applied to
controller 107 and may also be applied to instru~ents or other
visual dislays (not shown) for observation or other action by an
operator. The telemetry information applied tD controller 107 may
be used to modify a subsequent info~mation signal which will be
transmitted over the up-link portion of transmission link 300.
It will be appreciated that ccntrollers 107 and 207 may
contain preprogr a d instructions relating to tne operation and
movement of the remote vehicle. F~rther, both controllers 107 and
207 are preprogrammed to try transmission and reception on an
initial predetermined frequency to initiate communication~ If
communication cannot be established within a preselected number of
attempts, then both controllers 107 and 207 are preprogrammed to
begin trying a predetermined serles of one or more fall-back
frequencies until communication is established, as expla med more
fully below.
Figure 4 is a flowchart illustrating the various steps
performed by the transceivers and controllers located both in the
~4~
ground base control system and the remote vehicle.
Prior to launching the remote vehicle, the initial
transmission frequency of the first message is programmed into the
memory contained in controller 207 of the remote vehicle. In
S addition, tne data decryption key for decoding the encrypted data
messages is stored in the memory of controller 207. The pattern
of predetermined fall-~ack frequencies is also stored in ~he
memories of controllers 107 and 207.
Controller 107 then selects the next up-link transmission
frequency and generates an information signal (such as that shcwn
in Fig. 2) includlng a word representative of the next frequency
of transmission. ~he message portion 307 of m formation signal
301, including the word representative of the nex~ f~equency of
transmission, is then encrypted and transmitted as a single bur~t
during a predetermined time period, as determined by ccntroller
107. The transmitted signal travels over the up-link portion of
transmission link 300.
Meanwhile, re~eiver section 205 of the remote vehicle is
tuned to the initial transmission frequency-(as this frequency has
been previously preprcgrammed into the memory of controller 207)
and awaits the transmissicn of the information signal. If an
information signal 301 is detected the ~essage portion 307 of the
signal is checked for errors by controller 207 and, if properly
received, the message portion 307 will then be decrypted by data
decryptor 209. The decrypted portion of the word representative
of the frequency of the next data word transmission is then
utilized by ccntroller 207 to retune receiver unit 2ll to this
next frequency.
If no signal is received within a predetermined time period
or if the received signal contains errors, control1er 207
generates an error signal which i5 then im~ediately transmitted by
transmltter section 203 back to the ground station via down-link
portion of transmission link 300. Both controllers 107 and 207
contain real-time clocks which can synchronized prior to launching
of the re~ote vehicle. By suitably programming the controllers
107 and 207, transmitter section 103 of the ground station can be
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caused to t~ansmit the information signals during preselected time
~eriods and receiver section 205 of the remote vehicle can be
caused to look for these signals during the predetermined time
period.
If the message transmitted over the up-link is properly
received by re oe iver section 205 then an acknowledgment signal is
generated by controller 207 and sent back by transmitter setion
203 to the ground staticn over the down-link. So long as the
receiver section 105 of the ground station receives the
acknowledgment signal from the remote vehicle over the down-link~
controller 107 will operate in a normal fashion causing the next
message to be transmitted at the frequency specified ~y the
frequency representative word portion of the previously
transmitted information signal. If no acknowlegement signal is
received, or if an error signal is received by receiver section
lOS, ccntroller 107 then will begin one of two different fall-kack
m~des. ~f no down-link signal is reoe ived fro~ the remote vehicle
at all, then contro}ler 1~7 will regenerate the m itial
information signal and attempt to retransmit it a predetermined
number of t~mes over the up-link to the remote vehicle until a
proper acknowledgment signal is transmltted by the re~ote vehicle
back over the down-link. ~ontroller 107 can also be programmed to
cause the information signal to be retransmitted, but over a
series of predetermined fall-back frequencies if communication
with the remote vehicle is not made within a predetermined time
period. Likewise, con ~oller 207 of the remote vehicle will begin
retuning receiver unit 211 to tnese predetermined fall-~ack
frequencies if a predetermined time period has ela?sed without
receiving any signal from the ground station cver the up-link.
Gnce a proper acknowledgment signal has been received by
receiver section lOS of the ground station, the up-link signal
will be transmitted on the next preprogrammed frequency as
described earlier. If, for some reason rec iver section 205 of
the remote vehicle receives the message but it contains errors or
is otherwise garbled, controller 207 will generate an error signal
w~ich will then be transmitted vla the down-link back to the
- 17 -
ground station. In the event this occurs, both controllers 107
and 207 are preprogrammed to begin utilizing a predetermined
series of fall-back frequencies until communication between the
ground station and the remote vehicle is reestablished. This
ensures that if for some reason the information signal is
improperly received by the re~ote vehicle aue to intentional
jamming or noise in a particular area of the transmissicn link, it
will still be possible to reestablish communication at sc~e other
frequency.
While the present invention has been described in
considerable detail, it is understo~d that various changes and
modifications will fall within the scope of the invention. Fbr
example, while message-by-message fre~uency-hopping has been
discussed, it will be understood that word-by-word or
symbol-by-symbol frequency-hopping may also be utilized. In ~he
case of word-by-word or symbol-by-symbol frequency hopping, tne
encrypted up-link command would co~tain the starting frequency of
a pre-prcgra~med hoFping sequen oe to be used until a new command
is received. In addition to data concernin~ the next frequency of
transmission, the transmitted information signal ~ay contain data
designating the tLme of the next transmissicn or data concerning
the predetermined pattern o~ fall-bac~ frequencies which are to be
utili~ed.
It should be understood that the foregoing detail~ed
description of the preferred embodiment of the invention is merely
illustrative, but not limitive, of the invention which is defined
by the appended claims.
