Method and apparatus for rotating an electronically-scanned radar beam employing a variable-dwell scanning process

A method and apparatus for controlling the scan rate of an electronically-scanned (E-scan) antenna (24) of a secondary radar system to provide a variable-dwell time for the E-beam (62) of the E-scan antenna (24) at selected scan angles. The secondary radar system operates in conjunction with a primary radar antenna (20) that scans at a constant angular rate throughout a scan cycle. The E-scan antenna (24) scans through any given azimuth sector at an approximately constant data rate, whereby the beam (62) remains in the azimuth sectors that have high-target densities (52) for extended periods of time and moves rapidly through azimuth sectors that have low-target densities (54).

FIELD OF THE INVENTION 
The present invention relates, in general, to Air Traffic Control (ATC) 
radar systems. More particularly, it relates to a method for controlling 
the scan of an electronically-scanned, secondary radar antenna to provide 
a variable-dwell time as a function of the target density. 
BACKGROUND OF THE INVENTION 
The air traffic control system currently in international use is the Air 
Traffic Control Radar Beacon System (ATCRBS). ATCRBS combines the target 
responses produced by a primary surveillance radar with the target 
responses produced by a secondary surveillance radar. It provides the air 
traffic controller with a more complete picture of the traffic status 
within a controlled area than is available from a primary radar alone. The 
primary radar depends upon aircraft reflections of radar transmissions to 
indicate aircraft ranges and bearings. The secondary radar relies upon 
responses to interrogation signals by transponder (beacon) equipped 
aircraft to indicate range, bearing, altitude, and identity of such 
aircraft. 
In the majority of ATCRBS installations, the primary radar antenna and the 
secondary radar antenna are mounted on the same pedestal for scan rotation 
by a common drive mechanism. In such installations, the scans of the 
primary radar beam and the secondary radar beam are synchronized, and the 
dwell times for both the primary and secondary radars are constant 
throughout a scan cycle. An improved form of ATCRBS is provided by 
replacing the mechanically-scanned secondary radar antenna with an 
electronically-scanned, circular-phased array antenna as described, for 
example, in U.S. Pat. No. 4,639,732, issued Jan. 27, 1987. 
The electronically-scanned (E-scan) antenna is capable of shifting the beam 
pointing direction rapidly to any desired azimuth angle. The agility of 
the E-scan antenna has been used to provide more frequent updates of 
tracking information on priority targets, such as fast-moving or close 
range aircraft, than is available from mechanically-scanned (M-scan) 
antennas. When an update of information on a priority target is due, the 
search operation of the E-scan antenna is interrupted and the antenna beam 
is directed to the azimuth of the priority target. After the update is 
obtained, the antenna beam returns to the position at the time of the 
interruption and resumes its search operation. 
This limited utilization of the dexterity of the E-scan antenna fails to 
exploit the full potential of the antenna. The benefits that an E-scan 
antenna can provide to primary and secondary radar systems are related to 
the data processing problems created by the characteristic, non-uniform 
distribution of targets in range and azimuth in the ATC environment. In 
addition, the Mode S data-link requirement intensifies the problems which 
target bunching and channel time already present to radar data processing. 
Target bunching is a problem for radar data processing since it places peak 
data load demands upon the data processing resources. For a fixed-scan 
antenna, these peak data loads are directly proportional to target 
density. System design considerations for a fixed-scan system must take 
into account the appropriate balance between processor power, buffer size, 
and target report delay so as to ensure that all target bunching scenarios 
can be handled when the system is at full target capacity. In order to 
process peak data loads without excessive delay, a well designed ATCRBS 
includes excess data buffering and processing capacities. 
Channel time is the amount of time available for RF data communications 
between the ground system and those aircraft within the sensor's active 
beam. In a fixed-scan system, this time is fixed and is a function of the 
scan rate and beamwidth. It has the same numerical value as antenna dwell 
time, but usually refers to the data link aspect rather than the 
surveillance aspect of the radar problem. Improved secondary surveillance 
radar systems perform both surveillance and data link transactions. 
Channel time defines the communication bandwidth within the active beam 
and determines the type and number of data link messages which may be sent 
or received within a given beamwidth. In a fixed-scan system, 
communication transactions which exceed the available channel time are 
either lost or delayed until the next scan. Message prioritization is used 
to ensure that the most critical messages are sent first. Channel time is 
independent of processing power in the sense that unlimited processing 
capability cannot compensate for insufficient communications bandwidth. 
High traffic densities can also lead to garbling of the replies made by 
aircraft. Since all aircraft within the beam of an ATCRBS secondary radar 
respond to an interrogation signal by the ground system, replies by 
aircraft are often overlapped and undecipherable. An improvement to 
ATCRBS, known as Mode S, is now being implemented. Mode S provides 
selective addressing to target aircraft within the secondary radar beam, 
as well as data link communications with those aircraft. The selective 
addressing feature of Mode S eliminates unwanted replies, reducing the 
probability of message garbling and reduces channel time limitations on 
the traffic handling capacity of the system. However, when data link 
communications are conducted, reply messages are of greater length, and 
channel time limitations again influence the traffic handling capacity of 
the system. 
Potential performance advantages of an E-scanned antenna over an M-scanned 
antenna are generally recognized. However, additional performance 
advantages can be realized for primary and secondary surveillance radar by 
implementing the variable-dwell scanning method disclosed in this patent. 
These advantages are evident in both general radar applications as well as 
in a Mode S addressable communication environment. In addition, a 
particular embodiment of the disclosed invention is adaptable to existing 
mechanically-based systems, and allows those systems to realize the 
aforementioned benefits with minimal impact to the existing hardware and 
software design. 
SUMMARY OF THE INVENTION 
The invention provides a method for controlling an electronically-scanned 
radar beam ("E-beam") that is capable of scanning a range of azimuth 
sectors so that the beam scans through any given azimuth sector at an 
approximately constant data rate, whereby the beam remains in the azimuth 
sectors that have high-target densities for extended periods of time and 
moves rapidly through azimuth sectors that have low-target densities. That 
is, the E-beam scans at a rate that provides constant data to the system 
rather than scanning at a constant angular rate. 
The amount by which the dwell time of the E-beam is lengthened or shortened 
over the constant dwell time of the primary radar beam is dependent upon 
the time required to complete surveillance/communication transactions 
within a particular sector of the beam scan. The amount by which the dwell 
time is increased in high traffic density sectors is limited to a maximum 
value, and the amount by which the dwell time is shortened in low traffic 
density sectors is limited to a minimum value so that the average scan 
rate of the E-beam is made equal to the scan rate of the primary radar 
beam. Preferably, the azimuths of the E-beam and the primary radar beam 
coincide at the beginning and end of each scan cycle. 
In an ATCRBS having a primary radar antenna and a secondary radar antenna 
that scan in synchronism at a constant angular rate, target reports are 
received by the primary radar system and by the secondary radar system at 
substantially the same times and azimuths. In this mode, the target 
reports from both radars are easily correlated using standard known 
procedures. In the present invention, however, procedures must be 
implemented for correlating the target reports from the constant rate 
primary radar and the variable-dwell secondary radar. 
In the present invention, as a result of the lack of synchronization 
between the primary radar beam and the E-beam, a target report on a 
particular aircraft received by the primary and secondary radar will 
usually differ in the time of reception. In order to compensate, the range 
and azimuth of the target report from the primary radar is appropriately 
adjusted to allow the use of existing procedures for correlating the 
target reports from the primary and secondary radars. 
Variable-dwell operation adapts the scan rate to the type of non-uniform 
target environment which is typical for air traffic control operations. 
Unlike the M-scanned primary and secondary radar beams, an E-scanned beam 
with variable-dwell provides a dramatic increase in processing efficiency 
by spending the appropriate amount of time, as determined by the 
surveillance/communication requirements, in each azimuth sector. 
It is therefore an object of the invention to provide a method of 
controlling the scan of an E-scan antenna for a secondary radar of an 
ATCRBS by which method the dwell time of the antenna beam is varied during 
a scan cycle. 
It is a further object of the present invention to provide a variable-dwell 
E-beam that conforms to the non-uniform target environment that is typical 
for ATC operation. 
It is a further object of the invention to provide a method of controlling 
the scan of an E-scan antenna for a secondary radar of an ATCRBS that will 
reduce peak data loading of the system data processor. 
It is another object of the invention to provide a method of controlling 
the scan of an E-scan antenna for a secondary radar of an ATCRBS that will 
increase the channel time for carrying out data transactions in sectors of 
the antenna scan having high air traffic densities. 
It is still a further object to provide a variable-dwell E-beam that will 
provide the maximum degree of compatibility with existing systems. 
One of the benefits of the present invention is that a variable-dwell 
E-scanned beam provides a dramatic reduction in peak data loading combined 
with a reduction in target report delay, independent of any target 
density. In terms of a new design, this benefit translates to reduced 
processing power requirements and/or a simplification in the software 
implementation of the required data processing. In terms of existing 
M-scan designs, this benefit would enable systems with insufficient 
processing power to meet full requirements and, alternatively, provide an 
added margin of safety to those systems which meet current requirements, 
thus allowing the accommodation of future increases in system demand. 
A further benefit of the present invention is that variable-dwell E-scanned 
beams provide for the dynamic adjustment of channel time to match 
communication requirements on a real-time basis. This effectively 
increases the available communication bandwidth within the active beam to 
accommodate the desired number and kinds of messages between the aircraft 
and ground system. 
It is still a further benefit of the present invention that a 
variable-dwell E-scanned beam is compatible with the Mode S data 
communication and preserves the investment in the hardware and software of 
that system.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the primary radar antenna 20 is mounted on a pedestal 
22 containing a mechanical drive for scanning the antenna 20 in azimuth. 
The secondary radar antenna 24 comprises a circular phased array 
positioned below the primary radar antenna 20 with the vertical axis of 
the phased array aligned with the scan axis of antenna 20. Antennas 20 and 
24 are mounted atop a tower 26 to reduce the effects of ground reflections 
on the system performance. Antenna 20 may be scanned at a constant rate of 
12.5 r.p.m. for terminal installations or at a lower rate for en-route 
installations. The width of the beam of antenna 20 is typically 
2.4.degree. between 3 dB points. Assuming a scan rate of 12.5 r.p.m., the 
time required for antenna 20 to scan through one beam width is 32 ms. One 
beam width may be considered to be scanned in four steps of 0.6.degree. 
each, with a constant dwell time of 8 ms. per step. 
The scanning of antenna 24 is electronically controlled by the settings of 
digital phase shifters (not shown), as is known in the art. Suitably, the 
phase shifters of antenna 24 are controlled to scan antenna 24 through 
360.degree. in 600 steps of 0.60.degree. each. The width of the beam of 
antenna 24 is assumed to be 2.4.degree., but other beam widths may also be 
used. If the scan of antenna 24 was synchronized with the scan of antenna 
20 throughout a scan cycle, the beam of antenna 24 would be advanced in 
azimuth one 0.6.degree. step at uniform intervals of 8 ms. However, to 
provide the variable dwell times required by the invention, the beam of 
antenna 24 may be advanced in steps of 0.6.degree. at non-uniform 
intervals. 
FIG. 2 is a simplified block diagram of an ATCRBS-Mode S secondary radar 
system. Block 28 labelled "E-scan Antenna Subsystem" represents antenna 24 
and its associated r.f. components and phase shifters. A transmitter 30 of 
R/T unit 32 furnishes antenna 24 with modulated r.f. signals for 
transmission as interrogation signals. The modulating signals for 
transmitter 30 that determine the nature of interrogating signals, i.e., 
ATCRBS mode A, mode C, or Mode-S all-call, roll-call and data, are 
supplied to transmitter 30 by a channel management section 34 of data 
processing unit 36. Other functions of channel management section 34 
include providing steering commands to antenna 24; determining the times 
at which steering commands are issued to establish scan dwell times; and 
preparing, scheduling and updating communications transactions. 
Replies to interrogation signals made by airborne transponders, frequently 
referred to as "beacons", are detected by a receiver 38 and decoded either 
by ATCRBS reply processing 40 or Mode S reply processing 42, as is 
appropriate. Processed replies from unit 40 are supplied to ATCRBS 
surveillance processing 44 for establishing a separate surveillance file 
for each ATCRBS target detected. Similarly, processed replies from unit 42 
are fed to Mode S surveillance processing 46 for establishing Mode S 
surveillance files. The surveillance files contain information on target 
identity, azimuth, range, and a prediction of the target position on the 
next scan. 
The primary radar, often referred to herein as the "radar", generates 
target reports on detected targets. The radar target reports contain 
information on the ranges and azimuths, but not identities, of the 
targets. In accordance with the invention, the azimuth of the secondary 
radar beam may lag behind the azimuth of the radar beam because of the 
variable dwell of the secondary radar beam. Such lag causes the target 
reports generated by the primary radar and the target reports generated by 
the secondary radar to differ in the times of generation, and possibly in 
azimuth due to target movement in the interval between the respective 
reports. The radar target reports and surveillance reports from unit 44 or 
unit 46, as appropriate, are applied to radar-beacon azimuth compensation 
unit 48 for adjustment of the azimuth contained in the surveillance 
reports in compensation for secondary radar beam lag. The radar reports 
and the azimuth compensated surveillance reports are then correlated in 
radar-beacon correlation unit 50 where the range and azimuth data of the 
radar target reports are compared with similar data of the surveillance 
reports, as adjusted by compensation unit 48. Target data from correlation 
unit 50 is furnished to channel management unit 34 for the purpose of 
establishing and updating the target track files maintained by unit 34. 
A complete description of the ATCRBS-Mode S system, as implemented with 
M-scanned, constant dwell-time, primary and secondary radar antennas, is 
given in Federal Aviation Agency Specification No. FAA-E-2716. Copies are 
available to the public through the National Technical Information 
Service, Springfield, Va. 22161. Specification No. FAA-E-2716 includes a 
more complete description of the functions of channel management unit 34 
and a full description of the algorithms used by unit 50 for correlation 
of radar-beacon target reports. 
FIG. 3 shows the operation of the variable-dwell E-scan antenna 24 through 
a portion of a scan cycle and FIG. 4 shows the corresponding operation of 
the constant dwell radar antenna 20. Initially, the E-beam 62 of Fig.3 and 
the radar beam 64 of Fig.4 coincide at 0.degree. azimuth. Both beams scan 
in synchronism until a sector 52 is encountered containing a heavy 
concentration of aircraft. At that time, the dwell time of the E-beam 62 
is lengthened, for example, to T=300 ms. to provide adequate channel time 
to conduct all required data transactions. After the E-beam 62 passes 
through sector 52 into areas of light traffic concentrations, such as 
sector 54, the dwell time of the E-beam 62 is shortened to a minimum 
value, suitably T=10 ms. This minimum dwell time provides sufficient 
channel time for ATCRBS operations while allowing the E-beam 62 to 
approach synchronism with the radar beam 64 which maintains a constant 
dwell time of 32 ms. 
While the variable-dwell concept can be implemented within a scheme where 
the position of the E-beam depends upon the azimuth for the next target 
update, the preferred embodiment of the invention provides the maximum 
degree of compatibility with existing radar systems, and in particular, 
Mode S applications. This approach is to implement variable-dwell 
operation such that it simulates a hypothetical, variable-speed, 
mechanically rotating antenna. The amount of dwell time within a given 
azimuth sector is a function of the surveillance/communications activity 
within that sector. Therefore, the angular rate at which the beam is 
scanned will be adjusted as required. Two considerations in the 
implementation of a variable-dwell E-beam for a Mode S application are the 
potential variations in target update from scan-to-scan and the 
asynchronous nature of the secondary E-scanned beam with respect to the 
primary radar beam. 
The first consideration results from the fact that although the target 
environment is unlikely to change substantially within one scan, 
communication requirements may change dramatically from one scan to the 
next, causing the update rate to change for some targets. In a 
mechanically-scanned system, the surveillance processing software will 
update the track file on a given target within some time delay following 
the generation of the target report for that target. The position of the 
target is predicted for the next scan assuming a fixed scan period and the 
predicted positions are entered into the track file. The track files are 
then arranged in earliest expected azimuth is order within the 
surveillance file. 
Since variable-dwell allows the update rate to change, a scheme must be 
provided to compensate for predicted positions based on a fixed overall 
scan period. FIG. 5 illustrates the operational considerations of a 
variable-dwell E-scanned beam with respect to scan-to-scan target 
predictions. Assume a steady state condition in which 700 targets are 
bunched within a 252.degree. sector 56 with maximum data link activity. In 
this situation, the E-scanned beam moves from 0.degree. to 252.degree. in 
about 3815 milliseconds and from 252.degree. to 360.degree. in about 985 
milliseconds. Therefore, target 58 would be updated every 4.8 seconds. If 
however, at point 60, the air traffic control cancels all data link 
activity for the 700 targets, except for surveillance, the total number of 
data link transactions between 0.degree. to 252.degree. drops from about 
2100 transactions to 700 transactions. If each non-surveillance data link 
transaction required 0.5 milliseconds, then the E-scanned beam would 
update target 58, 700 milliseconds earlier than the previous scan. 
Therefore, the predicted target positions for a given azimuth step must be 
modified prior to initiating data link activity within that step. 
There are two aspects to implement the variable-dwell approach to solve 
potential variations in target updates from scan-to-scan. The first aspect 
is beneficial for solving both of the considerations mentioned above and 
involves operation of the rotating E-beam at the same nominal scan period 
as the radar antenna. For a steady state surveillance/communications 
environment, this will provide a constant update rate for all targets, in 
spite of the fact that the instantaneous scan rate varies according to the 
spatial distribution of the target densities within that nominal scan 
period. The algorithm which governs dwell time must consider not only 
surveillance/communication loading requirements, but also the relative 
position of the E-scanned beam with respect to the mechanically-rotating 
radar antenna. 
FIGS. 6 and 7 illustrate the operational considerations of a fixed-dwell 
primary radar antenna correlating with a variable-dwell secondary E-scan 
radar under steady-state conditions. If the scan rate of the E-beam was 
based only on surveillance/data link loading alone, synchronization 
between the primary and secondary scan rates would be virtually 
impossible. FIG. 6 illustrates that the E-beam 62 would lead the radar 
beam 64 in a lightly-loaded traffic environment 66. FIG. 7 illustrates 
that the E-beam 62 would lag behind the radar beam 64 in a heavily-loaded 
traffic environment 68. 
The second aspect is to provide correction for the predicted target 
positions when scan-to-scan update variations about the nominal update 
period are introduced. This software module must use the actual update 
time to correct the predicted target positions within the surveillance 
file prior to interrogating targets for a given beam position. 
Channel management unit 34 (FIG. 2) controls the dwell time of E-beam 64 
according to the process summarized in the flow chart of FIG. 8. After 
initializing variables 70 at the beginning of a scan, the beam is advanced 
one step 72. Construction begins of a table 74 that records the azimuth of 
the radar beam 64 of antenna 20 at the time of each advance of the E-beam. 
The data of the table is used, as described hereinafter, for compensating 
the azimuth data contained in radar target reports. Substantially 
simultaneously with the scan advance of the E-beam, an all-call 
interrogation is initiated 76 and the time of the transmission of the 
all-call (T.sub.last) is recorded 78. 
The all-call interrogation triggers a response of either altitude or 
identity from all ATCRBS beacons in the beam and a response only from 
those Mode S beacons that have not responded to an all-call interrogation 
during a previous scan. After the first all-call response, the Mode S 
beacons respond only to selectively addressed interrogations. The active 
target list 80 is updated and, on the basis of the updated target list 80, 
the incremental time, T.sub.mode S, required to complete Mode S data 
transactions at the current E-beam azimuth is determined 82. The number of 
entries in the active target list is, of course, proportional to the 
traffic density at the current azimuth. 
At 84, T.sub.mode S is compared with T.sub.nom, the nominal delay of the 
radar beam. If T.sub.mode S is greater than T.sub.nom, the time T.sub.adv 
at which the E-beam is to be advanced to the next scan step is calculated 
from the relationship 86: T.sub.adv =T.sub.last +T.sub.allcall +T.sub.mode 
S, where T.sub.allcall is the T.sub.mode S incremental time required to 
transmit an ATCRBS interrogation and listen for replies. If T.sub.mode S 
is less than T.sub.nom, providing an opportunity for the E-beam to move 
more rapidly, the scan angle .theta..sub.E of the E-beam is compared in 88 
with the scan angle .theta..sub.R of the radar beam. If .theta..sub.E is 
less than .theta..sub.R, indicating that the E-beam is lagging the radar 
beam, T.sub.adv is calculated from the relationship 90: T.sub.adv 
=T.sub.Last +T.sub.allcall +T.sub.min, where T.sub.min is the minimum 
incremental dwell time for the E-beam. If .theta..sub.E is greater than 
.theta..sub.R, then the time for the next scan step is calculated from 
relationship 86. 
After the dwell time for the current E-beam azimuth is calculated, either 
by the relationship 86 or 90, the azimuth data contained in each of the 
target reports of the active target list is corrected, at 92, to the 
target azimuth predicted for the next beam position. 
When the correction of the azimuth data of the target reports is complete, 
preparation of the data transactions to be conducted during the next 
E-beam step is enabled at 94 and, in decision block 96, the active target 
list is checked to determine whether any Mode S transactions are scheduled 
for the current E-beam azimuth. If T-mode S=zero, the program waits, at 
98, until the time arrives to advance the E-beam and then returns to 72 to 
repeat the program. 
If T.sub.mode S does not equal zero, the program determines, at 100, 
whether the all-call listening period has expired. At the expiration of 
the all-call listening period, transmission of interrogation signals to 
selected Mode S targets commences, box 102. Decision block 104 determines 
whether T.sub.mode S, the time allotted to the conduct of Mode S 
transactions, has expired. At the expiration of T.sub.mode S the program 
waits, at 106, until T.sub.adv arrives and then returns to 72 repeat the 
program. 
The second consideration to implement a variable-dwell E-beam in a Mode S 
application is the effect of the variable-dwell on primary and secondary 
radar data correlation. Under conditions where the E-scanned beam 62 lags 
the radar beam 64, the position of a target as reported by the radar and 
E-beam may differ. As a result, it is necessary to identify which radar 
report corresponds to a given E-beam report prior to attempting E-beam to 
radar data correlation. 
Referring to FIGS. 3 and 4 as an illustration, the radar beam 64 of antenna 
20 scans at a constant rate, regardless of the traffic concentration in 
any sector. Thus, at the time the E-beam 62 has just passed through sector 
52, the radar beam 64 has scanned well beyond sector 52. Generally, when 
the scan angle E.sub..theta. the E-beam lags the scan angle R.sub..theta. 
of the radar beam in this manner, a beacon report from a particular 
aircraft in sector 52 will be received at a later time than the time at 
which the radar report for the same target was received. In order to 
correlate the radar report with the beacon report using the correlation 
algorithms of the above-referenced specification FAA-E-2716, the azimuth 
of the radar report is adjusted to the azimuth predicted for that aircraft 
at the time of reception of the beacon report. 
Specifically, this method is illustrated in FIGS. 9 and 10 for two separate 
occasions as described. During each scan cycle, function 74 (FIG. 8), 
constructs a Beacon/Radar Correlation Table of the following form: 
______________________________________ 
TIME E-BEAM AZIMUTH OF AZIMUTH OF 
ENTERED STEP E-BEAM (E.sub..theta.) 
RADAR BEAM 
STEP (T.sub.E) AT T.sub.E (R.sub..theta.) AT T.sub.E 
______________________________________ 
0 -- -- -- 
1 -- -- -- 
2 -- -- -- 
. -- -- -- 
. -- -- -- 
599 -- -- -- 
______________________________________ 
With data from the above table, the azimuths of the radar reports are 
adjusted by radar-beacon compensation unit 48 (FIG. 2) to azimuth values 
predicted for the targets at the time the beacon beam scans through the 
sector in which such targets are located. 
It should be noted that the azimuth compensation provided by unit 48, FIG. 
2, is separate from and in addition to the correction of the predicted 
azimuths of target reports performed by function 92, FIG. 8. The 
compensation performed by unit 48 is for the purpose of facilitating the 
correlation of radar and beacon reports, described below. The correction 
of azimuths performed by function 92 is for the purpose of updating the 
active target list of channel management unit 34 to account for targets 
that may be entering or leaving the beam at the next beam scan position. 
FIG. 9 illustrates the method of correlating track-correlated beacon 
reports and beacon tracks without beacon reports to primary radar reports. 
A track-correlated beacon report is a beacon report whose measured target 
position correlates with the predicted position for that target in a 
corresponding track file. A beacon track without beacon reports is a track 
file which has been maintained on a target but, for which, no beacon 
report has been received for the current scan. 
The procedure is to identify the candidate radar report for correlation 
with a given beacon report scanned at position 108. That is, at position 
108, E-beam 62 is receiving a report from a target that was previously 
scanned by radar beam 64 at position 112. Because of a high-traffic area, 
E-beam 62 lags radar beam 64, which is currently located at position 110. 
Therefore, the beacon report does not correlate with the radar report. 
Using this procedure, the beacon report at position 108 is defined as the 
baseline data, and a corresponding radar report must be correlated with 
the baseline. The method consists of first viewing the 360.degree. 
coverage area as n, n.div.1000 azimuth steps. Preferably, n&gt;360, and even 
more preferably, the 360.degree. coverage area is divided into 600, 0.6 
azimuth steps. Upon each E-beam scan advance, channel management control 
34 enters the current time the E-beam entered the step and the current 
radar azimuth at the appropriate step location (0-599) in the Beacon/Radar 
Correlation Table, above. In order to identify the candidate radar report 
(or reports, if multiple targets existed at position 112), the target at 
position 108 is coasted in a counter-clockwise direction (for this 
example, it is assumed that the target is traveling in a clockwise 
direction, or positive velocity; the procedure works equally well for a 
target having a negative velocity)from position 108 at its assumed 
constant speed, and the radar beam 64 is coasted in a counter-clockwise 
direction from position 110 at its scan rate until the target and radar 
beam 64 intersect. The azimuth position at intersection point 112, defined 
as .theta..sub.I, identifies the radar report(s) whose range and azimuth 
qualify them as candidates for correlation with the beacon report at 
position 108. The candidate radar report(s) are then updated by coasting 
the range, altitude and azimuth positions to position 108 using the target 
velocity values. The beacon report at position 108 is then correlated with 
the updated radar reports using standard Beacon/Radar correlation 
procedures. 
FIG. 10 illustrates the method of correlating a track-correlated radar 
report with an uncorrelated beacon report. A track-correlated radar report 
is a radar report whose measured position correlates with the predicted 
position in a corresponding radar track file. An uncorrelated beacon 
report is a beacon report whose measured position does not correlate with 
the predicted position of any corresponding beacon track files. 
Generally, this procedure first determines those uncorrelated beacon 
reports which are candidates for correlation with a given radar report. 
The radar report is then modified with coasted azimuth, range and altitude 
values. Standard Beacon/Radar correlation procedures may then be employed 
to attempt correlation of the position-compensated radar report with one 
of the uncorrelated beacon report candidates. 
Using this procedure, summarized in the flow chart of FIG. 10, the radar 
report is defined as the baseline data to determine a candidate set of 
uncorrelated beacon reports. The objective of the first step is to 
determine the azimuth step corresponding to the position of the target at 
the time it was interrogated by E-beam 62. By referring to the 
Beacon/Radar table, the azimuth of the target in the radar report can be 
used to determine the step (ESTEP) which corresponds to the time and 
position of the E-beam at the time of the radar report in block 114. The 
position of the radar target when the E-beam finally interrogates it will 
depend upon the initial position and velocity of the target (data 
contained in the radar report) and the time intervals over which the 
target must be coasted (determined from the table). After determining 
ESTEP, the next step is to initialize the target to its position indicated 
by its radar report in block 116 followed by coasting the target in the 
appropriate direction (depending on whether the target has a negative or a 
positive velocity) for the time interval of ESTEP in block 118(the time 
interval is a function of the amount of targets within ESTEP); calculate 
the azimuth of the coasted target at the end of the time interval in block 
120; and compare the calculated azimuth with the position of the E-beam at 
ESTEP+1 in block 122. If the azimuths correspond, then the uncorrelated 
beacon reports found within this azimuth step are candidates for 
beacon-radar correlation at 124. If the azimuths do not match, then 
increment ESTEP one step and repeat step 118. Following identification of 
the candidate uncorrelated beacon reports, the original radar report is 
updated with its final coasted position. Standard beacon-radar correlation 
procedures can then be used to attempt to correlate the modified radar 
report with one of the uncorrelated beacon reports from the candidate set. 
It will be understood that the particular embodiments described above are 
only illustrative of the principles of the present invention, and that 
various modifications could be made by those skilled in the art without 
departing from the scope and spirit of the present invention, which is 
limited only by the claims that follow.