Voiced alerting system

A ground-to-air, voiced alerting system whereby a ground controller can communicate traffic warning messages to a pilot in an automatic fashion via a voice radio band and a voiced message generator. The system generally comprising means for transmitting a frequency shift key (FSK) encoded message to the pilot via the voice band and receiver means for decoding the same message. The decoding means, in turn, generally comprising means for identifying specific aircraft and additional means for decoding the message and producing the voiced message over the pilot's head set.

BACKGROUND OF THE INVENTION 
The present invention relates to voiced communication systems and in 
particular to a ground-to-air communication system, whereby FSK encoded 
messages can be transmitted over a voiced radio band and from which 
transmissions the encoded messages can be decoded to produce voiced 
messages. 
A major problem in most air traffic control centers is that over the years, 
as the amount of air traffic has increased, it has become more and more 
difficult for a ground controller to monitor each and every aircraft and 
keep the individual aircraft advised of their relative positions with 
respect to other aircraft within an area of space. While the individual 
aircraft are monitored by systems such as the Sperry Univac.RTM. Automatic 
VFR Advisory Service (AVAS) which employs the Automated Radar Tracking 
System (ARTS III) it becomes very difficult, due to the limited time 
bandwidth of the controller's radio channel, to communicate 
collision-avoidance messages to all pilots. This problem is especially 
critical during periods of high traffic density and during which periods 
traffic-warning messages are occasionally lost or delayed due to radio 
channel saturation. From the standpoint of air traffic safety, it is 
therefore highly desirable if the AVAS system were also able to 
automatically advise each pilot independent of controller action as well 
as to be able to transmit more messages in any given period of time. Such 
an improved system would also reduce the ground controllers workload, 
conserve radio channel use and increase the maneuvering time available to 
aircraft. 
It is therefore a primary object of the present invention to transmit 
digital messages, via FSK encoded analog signals over a ground 
controller's voice radio band, to an identified aircraft and to decode the 
message and produce a voiced traffic warning to a pilot over his head set. 
It is also an object of the present invention to accomplish the former 
objective using readily available circuitry but configuring the same in a 
system that has a vocabulary sufficient to accommodate the majority of the 
traffic-warning messages which might typically be transmitted from a 
ground controller. 
It is a further object of the present invention to produce a system which 
is compatible with the Discrete Address Beacon System/Army Tactical 
Airspace Regulation System (DABS/ATARS) or with the Air Traffic Control 
Radar Beacon System (ATCRBS). Thus permitting the transmissions of alerts, 
warnings and resolutions in automatic or semi-automatic fashion. 
These objects and others will become more apparent upon a reading of the 
hereinafter described appartus. 
SUMMARY OF THE INVENTION 
A ground-to-air voiced alerting system that enables the transmission of air 
traffic warning messages to identified aircraft over a voice radio band. 
The encoded messages, upon receipt, being digitally decoded and processed 
to produce the corresponding voiced message that is received by the pilot 
over his head set. 
The system generally comprising an air traffic control system for 
monitoring air traffic and producing digital data indicative of the 
relative positional differences between aircraft. The system also 
comprising means responsively coupled to the digital data for producing an 
FSK encoded message and means for transmitting the FSK encoded signal as a 
series of audio tones over a controller's voice radio band. The system 
further comprising means for receiving the FSK encoded message and 
decoding the message via a voice generator that produces a voiced message 
to the pilot. 
The voice generator, in turn, comprising analog-to-digital decoding means 
for receiving the audio signal and identifying particular aircraft. The 
voice generator further comprising means for processing the message by 
selecting adaptive differential pulse code modulated (ADPCM) encoded words 
from a vocabulary memory and converting the words into an analog format so 
that they may be understood by the pilot as a voiced message.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a generalized block diagram is shown of the major 
elements contained within the present voiced alerting system. As 
indicated, the present voiced alerting system is essentially a subsystem 
adaptable to presently existing air traffic control systems, such as the 
ARTS III System that is available from the Sperry Univac Division of 
Sperry Corporation. The operation of the ARTS III System, however, is 
generally well known to those skilled in the art, and since information 
relative thereto is readily available upon contacting Sperry Univac, its 
operation will not be described in detail. 
While such a system is required for the present invention to operate, its 
primary function is to supply the relative positional information of the 
various aircraft within any area of space to a ground controller, and who 
in turn transmits an appropriate traffic warning message. As mentioned and 
to date, such messages have been individually transmitted via the ground 
controller's spoken speech on a dedicated ground controller's radio 
channel. Such transmissions have thus been dependent upon the controller's 
time availability and the density of the traffic at any given point in 
time. The present system differs, however, in that while the particular 
position information is still available, it now is coupled in an 
appropriate parallel format to the present automatic voiced alerting 
system. Thus, the digital data from the ARTS III System is now supplied 
directly from the ARTS III System 10 via the single, six bit wide, 
parallel I/O channel 12 to the parallel interface 20, where data is 
received and retransmitted via the six bit data channel 14 to the latch 
22, where the data is temporarily latched. The parallel interface 20 
typically is of a Naval Tactical Data System (NTDS) type that conforms to 
Mil. Std. 1750A, such as the corresponding Sperry Univac NTDS type 
interfaces described in Sperry Univac Specifications DS-4772. The latch 22 
is also typically comprised of six flip-flops that temporarily hold the 
data prior to its being written into the buffer memory 32. 
The particular data that is received and the particular message length, 
however, depend upon the relative positions of the aircraft, and thus the 
message length varies depending upon the circumstances at any point in 
time. In any event though, the data is sequentially shifted into the latch 
22 and out of the latch 22 via the six bit data channels 14 and 26 to the 
buffer memory 32. The buffer memory 32 is typically configured as a 
first-in, first-out (FIFO) type memory and is sized to store at least the 
maximum length message. The buffer memory 32, in turn, transmits the 
parallel data to the digital code generator 42 via the six bit data 
channel 36, where the data is encoded into an FSK message. 
During this encoding process, the digital code generator 42 reads the 
individual lines of parallel data at a 1200 bit per second rate and 
generates the FSK encoded messages. It is to be recognized, however, that 
the actual technique for generating the FSK encoded message is well known 
to those of skill in the art, but which technique will again be described 
below with respect to FIGS. 2a and 2b for the present invention. The FSK 
encoded messages are thus generated and transmitted serially to the ground 
transmitter 44, as well as to the voice generator 46, that is coupled to 
the controller's head set. 
Referring to FIG. 2a, each FSK encoded message is generally comprised of a 
100 millisecond carrier tone header followed by three synchronization 
codes, one start of message code, an aircraft indentification code, the 
coded message itself, an end of message code and a checksum character 
code, which essentially acts in the fashion of a parity check. The 
function served by each specific code will, however, be described in more 
detail hereinafter with respect to the operation of the voice generator 
50. In general though, the length of each message is less than 0.3 seconds 
in duration. Each message also has a single level approximately 6 DB below 
the average speech level of the controller's voice radio band. And, the 
typical message is comprised of thirty-one or less codes; where each code 
consists of six binary bits. 
Referring to FIG. 2b, each message is thus comprised of a series of codes, 
where each six bit code is comprised of six corresponding frequency 
modulated pulses. Each binary bit of each code thus corresponds to either 
a 1200 or 2200 Hz pulse for a respective binary "0" or "1". Each message, 
in turn, consists of the series of pulses and which messages, due to the 
0.3 second time duration, are merely heard as a beep to any listener on 
the controller's voice band. It is to be noted though that the FSK coded 
messages are transmitted sequentially, so that it remains for the voice 
generator to properly decode the messages. 
Returning now to the present description, it is to be noted that the block 
diagram of FIG. 1 contains two types of voice generators (i.e. voice 
generators 46 and 50). Each type of voice generator is essentially 
identical to the other, with the exception that voice generator 46 is 
ground based while the voice generators 50 are air based within the 
individual aircraft. The voice generator 46, however, also differs from 
the others in that it is programmed to respond to all aircraft 
identification codes, and therefore the voice generator 46 is able to 
monitor all of the messages that are produced by the ARTS III System. The 
controller is thus able to monitor and control all the FSK messages that 
are transmitted. The voice generator 46 consequently responds to all FSK 
encoded messages and converts the audio signals into voiced messages that 
are comprehendible by the controller. It is to be noted, though, that 
while the above description typically contemplates a controller's 
monitoring each audio message, the controller may choose not to, just as 
easily as choosing to adjust the timing and sequence of the messages. It 
is to be noted also that the controller may initiate messages to aircraft 
before previous messages have been completed, so that different messages 
will be heard by pilots of different aircraft, nearly simultaneously. In 
this case voice generator 46 would convert only the first message in the 
sequence of time-overlapped messages into voice audible to the controller. 
The ultimate objective of any control scheme, however, is to place the 
control of all advisory messages under the controller's auspices, unless 
he or she chooses to select an automatic mode, whereby the messages will 
be transmitted against some priority criteria. 
Thus, it is to be noted that while numerous messages are generated by the 
ARTS III System and the present ground control apparatus, the messages are 
typically reviewed prior to transmission. It is also to be noted that 
since each message is only approximately 0.3 seconds in duration, 
significantly more messages than before can be transmitted within a given 
time period to all the aircraft within the controller's scope of 
responsibility. It is to be noted, too, that while each message is 
transmitted by the airport radio transmitter 44 to all the aircraft within 
the controller's area of responsibility, the complete message is only 
decoded by those aircraft having a corresponding identification code. Each 
identical aircraft thus receives the individual messages via their 
receivers 48, decodes the messages via their voice generators 50 and 
produces the voiced message for the pilot. 
Referring now to FIG. 3, the stored vocabulary that is used by the present 
voiced alerting system is shown in detail. Specifically, thirty-three 
encoded words are provided and which words are accessed via their 
respective six bit vocabulary unit codes (shown in decimal form). Thus, 
each six bit byte or code corresponds to one voiced word, and each message 
of up to thirty-one vocabulary unit codes and when organized according to 
the message format of FIG. 4 comprises an intelligent message. 
Thus, referring to FIG. 4, the alternative voiced messages are shown that 
may be transmitted via the apparatus of the present preferred embodiment. 
Each message generally states that traffic exists at some o'clock position 
relative to the identified aircraft, at some range from the identified 
aircraft, at some relative altitude and with some suggested advisory 
action. The messages are therefore relatively short and succinct and 
provide only the pertinent information that the pilot needs to react to 
any given traffic situation. It is to be recalled, too, that depending 
upon the cirumstances, the controller can interrupt at any time and 
redirect the pilot or queue the message to accommodate the situation. It 
is also to be noted that the voiced message provides specific information 
relative to altitude or headings and which information is voiced as three 
decimal digits. 
Referrng now to FIG. 5, particular attention will be paid to the voice 
generators 50 that are contained within each of the aircraft. Each voice 
generator 50, as mentioned, is coupled to a receiver 48 within each 
aircraft and each receives all the traffic warning messages that are 
transmitted by the transmitter 44. Each voice generator 50, however, only 
decodes those messages that contain the aircraft's corrsponding aircraft 
identification code. In particular, each voice generator 50 is comprised 
of a parallel interface 52, a digital decoder 54, a microprocessor 56, a 
program memory 58, a vocabulary memory 60, as well as a digital-to-analog 
converter 62, low pass filter 64, mixer 66, and an amplifier 68. And, each 
voice generator 50 generally acts, upon receipt of an FSK message, to 
supply the message to the digital decoder 54, where the FSK signal is 
decoded and converted into a parallel byte of binary information at 
transistor transistor logic (TTL) levels. Before decoding the entire 
message, however, each message is sampled by each aircraft to see if the 
message contains a corresponding aircraft identification code. If a match 
doesn't occur, the message is ignored. If a match occurs though, the rest 
of the message is received and stored in a vocabulary index buffer of the 
microprocessor 56 and from which buffer the message is read and 
interpreted by the microprocessor 56. Prior to describing the operation of 
the voice generator 50 though, attention should be directed to the digital 
decoder 54. 
Referring to FIG. 6, the digital decoder 54 is shown in detail and is 
generally comprised of a receiver filter 70, a threshold detector 72, a 
limiter 74, a 1 megahertz crystal oscillator 76, and a demodulator 78. 
While the principle of demodulating FSK signals is not new and can be 
performed by various modems, such as a Motorola Part No. MC6860 or a Bell 
Part No. 202, the present decoder 54 differs in that it receives its 
signals from an aircraft radio receiver, instead of a telephone line. It 
is therefore necessary that the decoder 54 filter the outside carrier 
bands from the FSK signals. The receiver filter 70 performs this function, 
and essentially filters out all frequencies outside the range of 1200 
Hertz to 2200 Hertz. This bandpass filtering thus improves the 
detectability of the FSK message by increasing the signal-to-noise ratio. 
The filtered signal is then impressed upon the threshold detector 72 and 
which acts to detect when a received signal is of a sufficient level so 
that the demodulator 78 can act upon it. The limiter 74, at the same time, 
samples each of the filtered signals to determine whether or not the 
filtered signal is symmetric and within the dynamic range expected by the 
demodulator 78. If the filtered signal passes both the limiter 74 and the 
threshold detector 72, the demodulator 78 is then able to convert the work 
space frequency charges of the filtered signals into the parallel, TTL 
level binary signals. In this regard, attention is again directed to FIG. 
2b, wherein the FSK modulating-demodulating criteria are shown, except 
that now the FSK signal is demodulated into its corresponding, coded TTL 
level binary signal. It should be noted that the demodulator 78 takes 
approximately 100 milliseconds to "turn on", but that the remaining 200 
milliseconds are sufficient to demodulate the FSK message. It is to be 
noted too, that the demodulator 78 operates under the microprocessor 56's 
control via the control signals that are transmitted over the duplexed bus 
80. 
Upon demodulating each coded message, the demodulated message is 
transmitted in the parallel binary format at the TTL levels via bus 80 to 
the microprocessor 56. The microprocessor 56, which in the preferred 
embodiment is capable of responding to eight bit bytes of data and which 
typically is a Motorola 6802, Intel 8085 or an equivalent eight bit 
microprocessor, then controls the subsequent conversion of the received 
message into analog speech via the stored program of FIG. 7. Recalling 
though that each FSK message contains three sync codes and an aircraft 
identification code, it is first necessary for each voice generator 50 to 
synchronize and identify itself. This, however, assumes that the voice 
generator 50 has already initialized itself. Thus during the "Idle Loop" 
of the program, the voice generator 50 via the microprocessor 56 
synchronizes itself to the sync codes and compares its preset aircraft 
identification code to the transmitted aircraft identification code. 
Assuming a match exists, the rest of the message is then received and 
stored in the vocabulary index buffer before being converted to a voiced 
message during the "Speak Message Loop." 
Next, during the Speak Message Loop of the stored program, the 
microprocessor 56 coordinates a table lookup operation from the vocabulary 
memory 60 of each of the vocabulary units that correspond to the 
vocabulary unit codes that are contained in the demodulated message in the 
vocabulary index buffer. Upon looking up each vocabulary unit, the 
microprocessor 56 then converts each unit from its four bit ADPCM format 
into an eight bit pulse code modulated (PCM) format. The algorithm for 
performing this function being well known and described in an article by 
P. Cummisky et al entitled "Adaptive Quantization in Diffential PCM Coding 
of Speech" in the Bell System Technical Journal, Vol. 52, No. 7, 
September, 1972. The sequential, PCM formatted message is then transmitted 
over the bus 80 at a 6 Kolohertz rate to the digital-to-analog converter 
62 where it is converted and subsequently transmitted as analog speech to 
the pilot. This conversion, however, will be described in greater detail 
hereinafter. 
Referring first though to the flow chart of FIG. 7, the program stored in 
the program memory 58 will now be described. Essentially, as mentioned, 
the program acts to direct the conversion of the received signal, and 
which it performs pursuant to FIG. 7a via the various modes of FIG. 7b and 
which modes are also shown in Table 1 below. 
TABLE 1 
______________________________________ 
MICROPROCESSOR MODES 
Mode Description 
______________________________________ 
0 Waiting for sync code 
1 Expecting start of message code 
2 Expecting a/c identification code 
3 Expecting vocabulary I.D. code 
4 Expecting checksum 
______________________________________ 
Prior to receiving messages though, it is necessary to "power up" and 
"initialize" each voice generator 50, and which initialization process 
also typically includes running a test message to ensure that the 
apparatus is operating properly. Specifically, during initialization the 
parallel interface 52 and the digital decoder 70 are enabled and the test 
message stored in the microprocessor 56's internal read only memory (ROM) 
is inserted into the vocabulary index buffer. The program then enters its 
"Speak Message Loop" and samples the successive index values stored in its 
ROM and thus performs a table lookup of the contents of each vocabulary 
unit code or address in the vocabulary memory 60. It does this by locating 
each unit code or address in vocabulary memory 60 and each corresponding 
vocabulary unit, and which vocabulary unit information is then transferred 
to the digital-to-analog converter 62 where each ADPCM sample of that test 
message is converted to the PCM format, until the end of the test message 
is reached. The microprocessor 56 thus queries the vocabulary index buffer 
after accessing each vocabulary unit to determine if any additional 
vocabulary units are stored therein. If additional units are present, the 
index value (I) of vocabulary index buffer is incremented by one, and the 
next vocabulary unit contained within the buffer is selected. The process 
is then repeated until the end of the vocabulary index buffer is reached 
and the answer to the query is "yes" that the message is complete. It is 
to be recognized, however, that the test message, as just described, is 
replayed only during initialization. Upon completing the test message and 
thereafter, the voice generator 50 is then ready to receive and respond to 
the various messages that are received by the aircraft's receiver 48. 
Upon completing the test message, the voice generator 50 assumes its zero 
mode by setting the mode register contained within the microprocessor 56 
[i.e. a portion of its internal random access memory (RAM)] to a value of 
zero and clears the value in the running checksum register, also contained 
in the microprocessor's RAM, to its initial value. The voice generator 50 
next reads the aircraft identification code that is established by the 
parallel interface 52, and which, in turn, is preset by the aircraft 
identification thumb wheel switches contained in the parallel interface 
52, and stores the aircraft identification code value in another register 
contained within the microprocessor's RAM. It is to be noted also that 
each microprocessor 56 in each aircraft reads its own aircraft 
identification code for each and every message that is transmitted, since 
a controller may from time to time require an aircraft to change its 
identification code. 
Upon storing the aircraft identification code, the microprocessor 56 then 
queries the demodulator 78 by the control portion of the bus 80 to 
determine if a valid character (i.e. code) has been received and is ready 
to be processed. If the digital decoder 54 does not indicate that it is 
ready, the microprocessor 56 again reads the aircraft identification code 
and again checks for a control signal from the digital decoder 54. This 
process thus continues until both of the conditions are met, and in which 
event, the microprocessor 56 accepts one character from the digital 
decoder 54. This character is then compared to a sync character that has 
been stored in an additional register of the microprocessor's RAM to 
determine if a match exists. If a match does not occur the microprocessor 
56 queries its mode register to see if it is in the fourth mode. If it is 
not in the fourth mode, it adds the value of the character that has been 
received and checked to the initial value contained in the running 
checksum register. The microprocessor 56 then branches to the mode 
contained within its mode register. Since in the present case, the 
microprocessor 56 is in its zero mode, it again branches to the zero mode 
and re-enters the program at the "first entry point" (i.e. indicated by a 
circle enclosing a 1) and continues through the idle loop until a sync 
character is detected. 
Assuming the next character is a sync code and it matches the sync code 
contained within the sync code register, the microprocessor 56 increments 
the mode register by "one" so that the program is placed in its first mode 
and wherein it next expects the start of a message code. First, however, 
the microprocessor 56 sequentially calls the next two characters from the 
digital decoder 54 to see if they too match the sync code; and each time 
the programs loops, looking for the start of the next message code. 
Assuming that the next two codes match the sync code, the program 
continues in mode 1 until the start of message code is detected. Once the 
the fourth character or start of message code is received and compared to 
the stored sync code, the codes won't match and the microprocessor 56 will 
check to see if the mode register is in the fourth mode. If the mode 
register is not in the fourth mode, the start of message code is added to 
the value of the running checksum and the program then branches to the 
first mode. 
Referring now to the first mode branch in FIG. 7b, the microprocessor 56 
compares the fourth code or presumably start a message code to the 
character contained within the start of message register of the 
microprocessor 56. If a match occurs the mode register is incremented by 
"one" to its second mode. If a match does not occur, a false start is 
indicated and the program returns to the "third entry point" and the mode 
register and running checksum registers are cleared to their zero values. 
Assuming, however, that a start of message code is detected, the mode 
register is incremented by "one" and thus switches to its second mode and 
the program re-enters itself at the "first entry point." The program then 
looks for the fifth character or aircraft identification code that 
normally follows the start of message code. Prior to describing the second 
mode, though, it should be recalled, that the aircraft identification code 
contains seven bits of data, as opposed to the six bit bytes that are 
received for each of the other codes. The microprocessor 56, however, is 
capable of handling eight bits so no problems are encountered with this 
anomaly and again the microprocessor 56 performs each of the idle loop 
steps and adds the fifth code to the accumulated value in the running 
checksum register and branches to the second mode that is now contained 
within the mode register. 
Referring to the second mode branch of FIG. 7b, the aircraft identification 
code or fifth character is next read from the digital decoder 54 and 
stored in the vocabulary index buffer. The microprocessor 56 then checks 
to see if the portion of the vocabulary index buffer that contains the 
received aircraft identification code is full (i.e. all seven digits have 
been received), and if not, it re-enters the program at the first entry 
point and which activity permits additional time for the aircraft 
identification code to be written into the vocabulary index buffer. 
Assuming, however, that the aircraft identification portion of the 
vocabulary buffer is full, the microprocessor 56 then compares the 
previously stored aircraft identification code, which corresponds to the 
thumb wheel settings and which is stored in the microprocessor's RAM, to 
the value in the vocabulary index buffer to see if a match exists. If a 
match does not occur, the microprocessor 56 interprets this to mean that 
the message is not for this aircraft, and therefore it ignores the 
remainder of the message. If, however, a match occurs, the mode register 
is incremented to its third mode and the microprocessor 56 re-enters its 
program at the first entry point. 
Upon receipt of the sixth character or the first vocabulary unit code of 
the message, the program again runs through its idle loop and assuming 
that all the conditions have been met and that the running checksum 
register has been updated, the program then branches to its third mode. 
Referring to the third mode of FIG. 7b, the microprocessor 56 first 
compares the character stored in its end of message code register to the 
received character. Assuming a match does not occur, the sixth character 
is stored within the vocabulary index buffer at the zero index position 
(i.e. I=0). The microprocessor 56 then checks the vocabulary 
identification buffer to determine if an overflow condition has occurred. 
If an overflow occurs, it interprets this as an error and the program 
re-enters itself at the third entry point and begins anew. If, however, an 
overflow does not occur, the program re-enters itself at the first entry 
point. Subsequently and sequentially, each of the additional vocabulary 
unit codes are similarly received and stored within the vocabulary index 
buffer at the next successive index values. It is to be recalled though 
that only a maximum of thirty-one entries can be received before the end 
of message code so that this process may continue for up to thirty-one 
cycles. 
Upon receipt of all the vocabulary unit codes of the message and recalling 
that all the received character codes have been added to the initial value 
of the running checksum, the receipt of the next character code or end of 
message code should cause a match to occur upon comparison with the end of 
message code stored in the microprocessor's end of message code register. 
Upon the occurrence of this match, the mode register is incremented by 
"one" to its fourth mode and the microprocessor 56 again re-enters its 
program at the first entry point. 
Thus, upon receipt of the next and last character code or checksum 
character, the program should produce a match when it queries the mode 
register to see if it is in its fourth mode. With this match, the program 
then branches to the fourth mode of FIG. 7b and compares the last 
character to the current sum contained within the running checksum 
register. If a match does not occur, the microprocessor 56 re-enters the 
program at the third entry point and resets the mode register to zero and 
begins anew. If, however, as would be expected, a match does occur, the 
microprocessor 56 re-enters the program at the second entry point. The 
microprocessor 56 then, as with the test program, reads out the various 
successive vocabulary unit codes stored within the vocabulary index buffer 
for the various index values. And, each vocabulary unit code is thus used 
to form an address into the vocabulary memory 60 which is read in a table 
lookup fashion to determine each of the ADPCM coded units of speech that 
are stored therein. 
The ADPCM codes are next converted from their four bit ADPCM format to the 
eight bit PCM format, and which PCM signals are, in turn, transmitted to 
the digital-to-analog converter 62 where they are converted at a frequency 
of 6,000 samples per second. The analog output produced by the 
digital-to-analog converter 62 is then low-pass filtered by the low-pass 
filter 64 to remove any quantization noise that may exist above the speech 
band. 
The filtered, analog signal is next mixed via the mixer 66 with the signal 
that is currently being received from the receiver 48 via line 67. It is 
to be noted that, in general, voice communications will not be occurring 
during this time so that the voiced traffic warning will not be 
superimposed over any voiced communications on line 67. It is possible, 
however, for the above effect to occur, and in which event a "cocktail 
party" effect would occur. However, a listener could adjust to this effect 
and mentally filter out one or the other of the conversations, but in 
general, this cocktail party effect is not desired and the listener would 
thus typically hear only the voiced traffic warning message. The mixed 
analog audio from the mixer 66 is next amplified via the two input, 
operational amplifier 68 and the amplified audio is then heard by the 
pilot via his head set. 
A ground controller is thus able to selectively transmit a voiced traffic 
warning message, merely by pushing a button--typically the controller 
would do this after positioning his cursor over the aircraft on his/her 
screen so that the proper aircaft identification codes would be entered 
into the message--and thereby free himself to address other conflict 
situations and similarly transmit messages to each of these aircraft. 
Furthermore, each message only takes approximately 0.3 seconds and does 
not disrupt other voiced communications. The time savings clearly being 
significant over voiced instructions from the controller. 
While the present invention has been described with reference to its 
preferred embodiment, it is to be noted that variations or alternative 
embodiments may suggest themselves to those of skill in the art, upon a 
reading hereof. Therefore, the following claims should be interpreted 
broadly to include any such equivalents.