Method for the improved utilization of response signals in a _secondary radar system and a secondary radar system for implementing the method

The time interval between the pulses of an interrogation signals transmitted to aircraft in the vicinity of an airport is varied at will in accordance with air traffic conditions. The time interval under heavy traffic conditions may be varied from its nominal value as a function of the rate of response signals, the garbling rate thereof, or the decoding rate. The time interval may be controlled manually as a function of air traffic conditions displayed on a screen associated with the interrogator and receiver. By varying the time interval the percentage of responses to interrogation may be varied to optimize overall reception conditions and utilization of potentially available information.

The invention relates to a method for the improved utilization of response 
signals transmitted from aircraft and received at a ground-based secondary 
radar station, and a radar system implementing the method. 
Most radar equipped airports are provided with both a radar system for 
locating aircraft in the vicinity of the airport, known as primary radar, 
and a secondary radar which scans the sky and interrogates the aircraft. 
The reception of interrogation signals aboard the aircraft triggers an 
airborne responder which transmits a response radar signal to each 
interrogation signal which is then decoded upon reception at the airport 
and furnishes requisite information. 
Customarily, the ground-based secondary radar station is designed to 
operate according to several modes as a function of which the responder 
provides information of a particular type, most often aircraft 
identification number or a flight pattern number in an interrogation mode 
called mode A, and the altitude in an interrogation mode called mode C, 
etc. 
Such radar intercommunications between an airport or airfield secondary 
radar station and approaching aircraft has given satisfactory results as 
long as there are not too many aircraft in the vicinity of the airport. 
But as air traffic has increased, the number of aircraft has increased 
greatly, especially at certain times of the day, with attendant 
difficulties to process the information emanating from each of the 
aircraft being interrogated. 
Airborne responders are all tuned at the same wavelength and despite the 
shortness of the response signals from the various aircraft it is not 
unusual that upon reception at the airfield it is not possible to separate 
response signals emanating from different aircraft. 
This difficulty is so much greater as: 
(1) the main lobe corresponding to the interrogating radar transmission, 
although defining a very small solid angle, may at any given time be 
directed at several different aircraft; 
(2) several aircraft may be at substantially the same distance from the 
airfield at any given moment; and 
(3) it is desired to be able to make available at the airfield receiver a 
multiplicity of messages from an aircraft to derive the sought-after 
information. 
In fact, an overlapping of in-coming signals simultaneously received from 
two or more aircraft may occur upon reception at the airfield, and this 
phenomenon, which is commonly known as garbling, requires the installation 
of so-called "degarbling" means for the airfield receiver in an attempt to 
particularize the signals received and unequivocally correlate them with 
the aircraft from which they were transmitted. 
Sometimes, in addition to the garbling there is a phenomenon known as 
"fruit" which originates in the reception at a particular airfield of 
signals transmitted by airborne responders in response to interrogations 
from another airfield interrogator, as the airborne responders transmit 
omnidirectionally. 
For some time, in spite of the operation of "degarbling" means for airfield 
receivers and also "defruiting" means, the identification of aircraft at 
certain times of the day becomes difficult and long in resolving, with the 
potential risk which may not be ignored. 
To attempt to mitigate against these difficulties it has already been 
proposed to de-automate the responses from the various airborne responders 
by providing each one with a random opening electronic gate which permits 
or inhibits the transmission of a response to an interrogation in 
accordance with a random function. The operation of such a gate reduces 
the number of signals received on the ground practically in half. 
Moreover, even if in the case of heavy traffic such a system is able to 
facilitate the identification of aircraft or utilization of other 
information, it does not permit full advantage to be taken of the 
technical capacity of the receiver on the ground, especially when traffic 
is light. Besides, adopting such an arrangement would require the 
conversion of all aircraft responders which would involve enormous 
expenditures. 
The invention is based on the observation that the responders equipping 
aircraft furnish responses not only when the receivers with which they are 
associated receive interrogations whose two characteristic pulses are 
separated by a nominal time interval corresponding to an interrogation 
mode, but also when they receive pulse pairs separated by time intervals 
deviating from the nominal value. When the deviation from the nominal 
value increases, rates of response less than 100% result. The rates of 
response diminish until the deviation reach limit values to either side of 
the nominal value, beyond which no responses are furnished. 
The invention is based precisely on this observation which reflects the 
inevitable limitations of a technological nature and gave rise to a method 
considerably improving the operation of secondary radar transmission 
systems between airborne responders and ground receivers. 
The invention contemplates no modifications in airborne responders, but 
rather, varying at will upon transmission of the radar interrogation the 
time interval between two consecutive pulses characteristic of the 
interrogating mode, in order to, where appropriate, cause the responders 
to give responses only to a proportion of interrogations corresponding to 
the best possible utilization, at each moment, of responses from the 
various aircraft reaching the airfield receiver. 
As long as garbling and fruit are not troublesome the interval between two 
consecutive pulses remains at the nominal value for which the responder 
furnishes a response to each interrogation received. From the signals 
received by the airfield receiver are then derived the requisite 
information at maximum speed. 
As soon as garbling and/or fruit become troublesome the time interval 
between two consecutive pulses of a pair transmitted by the secondary 
radar is altered, the quantitative change in the time interval being 
varied in accordance with the amount of garbling or fruit. 
The amount of signals received at the airfield from each or some of the 
airborne responders in a predetermined period is then smaller than if the 
interrogation pulses where at their nominal spacing, thereby reducing the 
overlapping of signals received and facilitating their utilization. 
Secondarily, advantage is taken here of the peculiarities of the diverse 
airborne responders which cause the rate of signals emitted to vary 
differently depending on the time interval between the pulses of a pulse 
pair. 
If, after a decrease in the number of aircraft in a predetermined azimuthal 
zone, or changes of position of the aircraft relative to one another, 
garbling becomes less troublesome, the secondary radar transmitted is once 
again adjusted so that the interval between two consecutive pulses which 
are characteristics of the interrogating mode is closer or equal to the 
nominal value; the system thus constantly operates at its maximum 
operating efficiency. Such an adjustment may either be continuous or 
intermittent at predetermined intervals. It may be manual or automatic. 
The invention provides a variation in the time interval between two 
consecutive pulses of a pulse pair, characteristic of an operating mode, 
in correlation with the degree of air congestion of an azimuthal zone.

The graphic representation of FIG. 1 shows the response curve of an 
airborne responder complying with present-day regulations for mode A 
operation to the transmission of interrogation signals from a ground-based 
secondary radar station at an airport or airfield, the interrogation 
signals consisting of repeated pulse pairs the two pulses of a pair being 
separated by an 8 .mu.s time interval. 
The horizontal line segment ab along the 100% marking on the ordinate axis 
indicates the fact that the responder furnishes 100% of the responses in 
case the interrogations received from the airfield radar consist of a pair 
of pulses not only those spaced from 8 .mu.s, which is the nominal value, 
but also those the time spacing of which deviates from this value by, at 
most approximately 0.3 .mu.s below the nominal value and at most 
approximately 0.5 .mu.s above the nominal value. Such a responder complies 
with the regulations as it provides a 100% response rate to interrogations 
the pulses of which are separated by a time interval T comprised between 
7.8 .mu.s and 8.2 .mu.s. 
Line segment ac indicates the fact that the responder furnishes a 
decreasing proportion of responses if the interval T between the pulses of 
a pulse air decreases below 7.7 .mu.s, up to not furnishing responses for 
interval values T between consecutive pulses of a pulse pair interval less 
than approximately 7.4 .mu.s. 
Likewise the responder considered furnishes increasingly fewer responses if 
the interval T between two pulses of an interrogation pulse pair exceeds 
approximately 8.5 .mu.s; it provides no responses for interval values 
greater than approximately 8.7 .mu.s, as shown by segment bd of FIG. 1. 
Other responders have different characteristics. Curve B of FIG. 2 is a 
graphic representation relating to a responder providing 100% of responses 
for interrogation pulse pairs spaced an interval T comprised between 7.7 
.mu.s and 8.4 .mu.s and which provides increasingly fewer responses for 
lower and higher values, and no responses for interval values less than 
about 7.4 .mu.s and greater than about 8.7 .mu.s. 
Another responder has characteristics shown by graphic representation D. It 
provides responses to 100% of the interrogations for an interval T between 
two pulses of a pulse pair whose value is between about 7.7 .mu.s and 
about 8.2 .mu.s. It provides no responses for interval values less than 
about 7.2 .mu.s or greater than about 8.7 .mu.s. 
In the illustrated graphs the variations between the rates of response 
between 100% and 0% are linear. For other responders the variations of the 
rates of response are not linear. 
The system comprises a receiving and transmitting antenna 11 (FIG. 3) 
rotating at uniform angular velocity around a vertical axis, of the 
conventional type used for secondary radar airfield antennas. 
Signals picked up by antenna 11 are carried via a duplexing means 12 to a 
receiver 13 connected by a circuit 14 to degarbling means 15 which is 
connected by a circuit 16 to a defruiting means 17. The information at the 
output 18 of the defruiting means is applied to a decoding means 19 whose 
outlet is connected by a circuit 21 to display means 22. 
A second receiver output is connected by a circuit 23 to a rate of response 
determining means 24, and likewise a circuit 25 connects the degarbling 
device 15 to a rate of garbling determining means 26. 
The decoding means 19 has a second output connected by a circuit 27 to a 
rate of decoding determining means 28. 
The rate of response, the rate of garbling and the rate of decoding as 
determined respectively by the determining means 24, 26 and 28 are applied 
by circuits 29, 31 and 32 respectively to the inputs of a date processing 
means 34. 
A device 35 for controlling the rate of repetition of the interrogations is 
also connected to the data processing means 34 by a circuit 36. A circuit 
42 connects the data processing means 34 to means for introducing an 
interrogating mode, or mode selector 43. 
The output 44 of the data processor 34 is connected to modulation control 
means 40, associated with a mode modulator 45, the outlet 46 of the mode 
modulator modulates the transmitter or interrogator 47, connected to the 
antenna 11 via circuit 48 and the duplexing means 12, for varying the time 
interval between the pulses of a pulse pair constituting an interrogation. 
This variation is therefore a function of the output of the data 
processing means 34. 
To operate the system according to a particular mode, e.g. mode A, the mode 
selector 43 is set for this code. 
In the event of light air traffic in the vicinity of the airfield, the 
transmitted 47 transmits interrogation signals with the time interval at 
the nominal value of the A mode, i.e. pairs of pulses time spaced 8 .mu.s 
between each other. The airborne responder or responders receive the 
interrogation signals and transmit a response to each pair of 
interrogation pulses received. If, taking into account the overall 
receiving characteristics, for example, owing to the fact of the increase 
in air traffic, or even the presence of several aircraft in the same 
azimuthal zone, the rate of responses becomes too high and/or the 
degarbling is no longer accomplished satisfactorily, bearing in mind the 
rate of repetition of the interrogations (and/or if the decoding rate 
falls too low) then the data applied to the data processing means 34 by 
circuits 29, 31 and 32 provides at the output 44 of the data processing 
means 34 a signal which by means of the modulation control means 40 causes 
the value of the interval T between the consecutive pulses of 
interrogation signals to decrease or increase relative to the nominal 
value of 8 .mu.s. 
If, for example, the value of the interval T becomes 7.6 .mu.s an aircraft 
responder which has characteristic response curve B provides a response 
rate of 60% whereas an aircraft responder having a characteristic response 
curve D provides a response rate of 80%. 
The number of responses received at the airfield during a predetermined 
time period decreases and the overlapping of responses is diminished, the 
identification numbers and/or flight pattern of each aircraft is thus 
determined quicker and more reliably. 
Data input to the data processing means 34 is, advantageously, controlled 
so that the system is constantly operating under optimum conditions of 
reception with regard to the identification of aircraft or the recognition 
of their flight patterns. 
The system may also operate according to another mode, for example mode C, 
by adjusting the mode selector 43, or successively to mode A and mode C, 
for example, with a view to determining at any time both the identity of 
various aircraft and their altitude. 
According to another embodiment, the selection of the length of time T 
separating two consecutive pulses of a pulse pair constituting an 
interrogation signal is produced by means of a manual control means 51 
provided on a console 52 and connected to the data processor 34. 
The operator then makes the adjustment of identification and garbling 
conditions as a function of traffic displayed on the radar screen.