Vol.13 No.4 JOURNAL OF ELECTRONICS Oct. 1996

C E N T R A L I Z E D M U L T I R A D A R I N T E G R A T E D T R A C K I N G

He You Wang Guohong Tang Jinsong

(Nava/Aeronautical Academy, Yantai 264001)

A b s t r a c t This paper deals with the integrated tracking mechanics of centralized multiradar

system,such as trae~in E system files management, operation mechanics, etc. Simulation is made

for a certain ship tracking 8 t~rgets under various tactical m~neuvering situations.

Key w o r ~ Multiradar trac.k~, Data fusion; Operation mechanics

I. In troduc t ion

Multkadar tracking is one of the important subjects in multisensor data fusion, whereas

the operation of multiradax tracking system is the key problem in multiradar integrated

tracking[ 1-4]. If the early returns are processed prior to the late returns in multiradar

system, there exist some special problems such as 'time inversion' (i. e., the late received

data are used to update the track prior to the early detected data) when the handover

between two radars or among more than three radars takes place. Although there were

reports about the operation mechanics of two radars, the detailed reports about that of

more than two radars have not been found. This paper deals with a practical integrated

tracking mechanics for centralized multixadar system, presents a method of tracking system

file management by using stack to establish track-number files, and lays emphasis on the

operation mechanics of tracking system. Finally, system simulation is made for a multiradar

system of a certain ship.

II. Tracking System Storage Files and Management

In the tracking system, there axe four types of files: track-number file, sector file, track

file and plots file. The plots file is used to record the detected data from each radar [~).

Track-number file, sector file and track file are only introduced as follows:

1. Track-number file

Track-number file is primarily used to assign numbers to the established tracks. Conse-

quently, the manipulation of track-number file is referred to as automatic batching. Track-

number file contains firm-track-number file, tentative-track-number file (or track-number file

in short ) and fixed-track-number (or clutter-number in short ) file. The present methods

involve timing-batch and linkage file. Since the two methods are not practical, it is reason-

able to use stack to build track-number (clutter-number) file. The detailed procedures of it

are as follows:

After the system is started, the stack is initiated first, and the track-number is placed

304 JOURNAL OF ELECTRONICS Vol.13

into the stack in sequence of the larger number prior to the smaller one. If the stack-top-

pointer is not zero, it is indicated that there are some track-numbers available in the stack.

And in this case, when a new track-number is requested, a track-number is pushed out so

as to be assigned to the new track, and in the meantime, 1 is subtracted from the stack-top-

pointer. If the stack-top-pointer is equal to zero, it is shown that there are no track-numbers

in the stack. Thus, no track-number will be assigned to the new track in this case. When

a track is eliminated, its track-number is pushed back to the stack, and 1 is added to the

stack-top-pointer. From the above discussion, we can see that it is convenient to use stack

to build track-number files.

As to the clutter-number file, the manipulation is identical to the track-number file.

Therefore, it will not be described here.

2. Sector flies |s'S]

In order to reduce the number of correlation, the azimuth-range plane is separated into

32 equal azimuth sectors, and the radar data are processed in azimuth sectors. Therefore,

the sector files are established to record tracks in each sector. After a track is initiated or

updated, the predicted position of the track is checked to see which sector it occupies, and

the track is assigned to tha t sector. If the track is dropped or moves to a new sector, the

track-number is dropped out of the sector in which it was previously located.

Sector files include track sector files and clutter sector files. The structure of the file-data

and the management method of both files are identical.

Since the insertion and elimination of the sector files is frequent, it is suitable to use

linlmge form for data organizat ion.

3. T r a c k files In radar data processing, there are firm track files, tentative track files and fixed track

files. Firm track files and tentative track files are both named track files, while fixed track

files are referred to as clutter files. Taking a track file for example, the contents associated

with the track (clutter) file are: predicted velocity, smoothed velocity, last time when each

radar updated the track, next time when each radar updates the track, track quality scores,

and the smoothed accuracy of the track, etc.

III. O p e r a t i o n M e c h a n i c s

In this section, we discuss the operation mechanics of the centralized multiradar sys-

tem,which mainly involves: initiation of the system,overflow judgement of the system, clutter

map processing, handover processing, and track processing.

1. Sys tem initiation

System initiation involves arrangement of radars, initiation of the start positions in

radars ' azimuth-scan, and initiation of track-number files, input data and processing sectors.

Arrangement of radars is to order radars in sequence of their rotation-rates. Initiation of the

positions for azimuth-scan is to preset the s tar t position of radars ' azimuth-scan, through

which the nonuniform phenomenon of the instantaneous data in each sector is avoided to a

No.4 CENTRALIZED MULTIRADAR INTEGRATED TRACKING 305

certain extent so as to reduce the handover-burdon. For example, for a 4-radar system with

the scan periods being Is, 2s, 6s and 10s respectively, if their initial azimuth positions (at

board) are preset at 0 ~ 180 ~ 90 ~ and 270 ~ respectively, the simulation results show that

there will never be the case of three radars being in one sector. Initiation of track-number

files was discussed previously. Initiation of input data is to make the pointer, size and time

of each data block be zero. Initiation of processing sectors is to let the processing sectar of

each radar be the start sectors in which they are now located.

2. Sys t em overflow j u d g e m e n t

In the trackin~ process, system overflow should be prevented. The method is to take the

radar with highest rotation-rate as criterion, and let its current processing sector lag a few

sectors behind the radar's current sector. The lagged value should be more than (the longest

scan period/the shortest scan period )+1 so as to prevent the radar from being in waiting

states[i,2]. If the processing sector of the radar which possesses the highest rotation rate lags

behind the radar's current sector, the other radars are bound to satisfy the lag requirement.

If the lagged value is more than the preset value, then the computer is in burdon state, i.

e., there are too many plots for the computer to process. If the phenomenon takes place,

overflow processing should be required.

3. C lu t t e r map

Since the radars work in different f~equency-bands, the detected clutter-plot data are

quite different. Therefore, multi-tuple clutter maps should be adopted. That is to say, each

radar should have the clutter map of its own. The manipulation sector of it is prior 2 sectors

to the processing sector. The plots correlated with clutter maps will be deleted in plots files.

The update of clutter map is based on the idea of the nearest neighbour, and is executed by

using an adaptive filter. If the velocity of the fixed track is more than a certain value, the

fixed track will be converted into a firm track.

4. Handover processing

Handover processing is important in system executing. The method is to make data-

rearrangement and let it be prior 1 sector to the processing sector. The sector for' data-

rearrangement is called handover-processing sector. The basic idea of data-rearrangement is

to arrange the data of the sector in the manner of azimuth-time, i. e., to arrange the data in

the same azimuth according to the time. After the data-rearrangement, the early detected

data will be used to update the track in advance of the late detected data. Assuming that

there are N radars in the multiradar system and TMRK(j, i) denotes the time of the i-th

radar through the j - th sector. The procedures of data-rearrangement are as follows:

(1) Deciding if handover arises in the handover-sector,what time the handover takes

place and which radars participate the handover.

Denot~ j as the sequence-number of the handover processing sector. Deciding if han-

dover takes place contains 2 steps: (a) Vi E {1 , . - . ,N}, TMRK(j , i ) are ordered in the

sequence from the smaller to the larger such that the se,quence-number vector, IORDC, of

radars through the handover sector is obtained. Where, IORDC(i) = k denotes that the

306 JOURNAL OF ELECTRONICS Vol.13

s ,t.l' t.~

Fig.1 Handover of three _r,~d,~s

sequence-number of the/-th radar through the sector is k. If two radars go through the

border of the sector at the same time, then the radar with the higher rotation rate is put

ahead. If the radar IR1 comes across the sector at the earliest, then IORDC(IR1)--1. If

IR1-- N, it means that the radar IR1 rotates at the lowest rate, and hence there is no

handover taking place. (b) Vi E {1,.-., N}, TMRK(j O 1, i) are arranged such that the

sequence-number vector, [ORDL, of the radars through last sector is obtained. Where, @

means the modulo subtraction with the modulos of 32; IORDL(i) = k denotes that the

sequence of radar { through last sector is k. If IORDL(IR1)=I, it means that radar IR1

enters the sector at the earliest, and hence there was no handover taking place; otherwise,

the handover was consided to have taken place. Fig.1 illustrates the handover process of

three radars, where, tci is the time of radar { entering the sector, t~ is the time of departure,

tH, ij is the handover time between the radars i and j .

A basic requirement of data rearrangement is to ensure that the early detected plots

should be processed ahead of the lated detected plots. Consequently, this paper presents

the method of complete-sector-rearrangement. In this method, the number of the complete-

sector-scan is the same as the number of the radars which take part in the handover. Taking

the scenarios in Fig.1 for example, three complete-sector-scans are obtained in Fig.2, where

three radars participate the handover, and the numbers over the axis indicate the radar-

numbers.

t. s 3t., 3to~ 3 t ~ 2 "I'Aa2 t'~. ! tp, . l ~ t,~ .t,s

2 2 3 l 2 2 2

1 l 3 3 3 3

Fig.2 Results of data ~ e n t for three radars

A~811m'[nf that that NHR is the number of radars (including radar IR1) which take part

in handover, then

N.R = IIORDC(ml) - IORDL(ml)I + 1 (i)

No.4 CENTRALIZED MULTIRADAR INTEGRATED TRACKING 307

Let

S~ = {il IORDC(i) < IORDL(IRI)} (2)

i. e., Se is the radar-number set which contains the radars entering sector j . Then the

radar-number set which contains the radars participating handover is

SH = Se + {IRI} (3)

(2) Handover time computation and data rearrangement

The condition under which the radars il and i2 carry out handover is

T M R K ( j O I , i l ) > T M R K ( j e l , i 2 ) , if J1 < i2 (4)

Assuming that the rotation rates of the radars il and i2 are 0)1 and w2 respectively and that

tH is the handover time, then I2]

t H = (wl" TMRK(j G 1, il) - w2" TMRK(j ~3 1, i 2 ) ) / ( w l - wg.) (5)

Tab.1 Tab le o f data rearrangement

Time-segment Orders of radars entering

1 4

tea re2 3

re2 tel 3

tel tH,23 3

tH,23 tH,13 2

tH,13 tH,12 2

tH,12 t~l 1

tpl tp2 tI,2

2 3

2

2 1

3 1

1 3

2 3

2 3

3

If tn > TMP~( j ,Q) , it is indicated that the handover between the radars il and i2

does not take place. After the handover is completed, the data are rearranged. First,

u E {1,--. ,N}, ordering is made to TMRK(j @ 1,i) which is the time of entering sector

j , handover time and the time through sector j . Then, these times are segmentalized in

the form of Fig.2, and the sequences of radars entering the time-segments are obtained and

recorded in the form of Tah.1. Thus, the data rearrangement is completed.

To get the sequences of the radars in each time-segment, one of the following two

methods may be used: (1) after the detected plots are directly scanned and the detected

bearings at the node in each time-segment are compared, the radar-number with larger

bearing value is ordered ahead; (2) the radar-number with larger bearing value is also placed

in advance, whereas the bearings are computed by using the formula in Ref.[2].

5. Tracks initiation, correlation and updat ion

Track initiation lags one sector behind correlation. The criterion of track initiation is

referred to Ref.[7]. Assnming that the track update is finished, the free plots with which

neither the clutter-maps nor the tracks correlate in sector j @ 1 are treated as tentative

tracks, and new tracks are initiated. Track-numbers are assigned to the new tracks and the

308 JOURNAL OF ELECTRONICS Vol.13

sectors in which the new tracks locate are determined. Then, the new tracks are placed at

the end of the sector-files.

When the targets are sparse, the correlation is carried out by correlating the tracks in

the processing sector with the plots in the processing sector and its left-side and right-side

sectors. In the dense targets scenarios, track correlation contains two steps. The first step

is coarse correlation, i. e., the determination of plots. The plots which are laid in the

correlation gate are consided to be correlated with the track, and the correlation matrix is

established. The second is fine correlation. In the second step, the plots are assigned to the

tracks according to the following criterion: plots are assigned to the nearest tracks (when

in the conflict situation); tracks are assigned to the nearest plots (when in the situation of

more than one plots in the correlation gate); when the distances are equal, the plots are

assigned to the track with the higher track-quality scores; each track only possesses one plot

and each plot can only be assigned to one track. After the plots are assigned to the tracks,

the distances are designated as zero such that there is at most a zero element in each row

and each column in the correlation matrix.

1~ack update is made only in the main sector. If no plot is assigned to the track, then

by using a maneuvering gate, the track is assigned to the plots, which have not been assigned

to any tracks, in the sectors j @ 1, j and j @ 1. Where, $ means modulo addition of modulos

32. If there are more than one plot in the maneuvering gate, the nearest plot is only assigned

to the track. If there is no plot in the maneuvering gate yet, we say that the track lost plot

one time. If the data rate of the system is very high, a maneuvering gate may not be used

since the movement caused by the target 's maneuver is very small.

In tracking state, the track quality is assigned according to the correlation state. If a

track changes its sector or is deleted, a manipulation of the sector files is required [1'2].

IV. S imulat ion R e s u l t s

To verify the performance of the method presented in this paper, simulation is made in

FORTRAN language in computer AST-486. The simulation involves the following four steps:

(1) creation of the targets tracks; (2) generation of the radars measurements (containing

errors, false alarm and missed detection); (3) generation of the data in multiradar system; (4)

simulation testing. Where, the targets ' tracks are generated in the Descartes coordinate, the

false numbers in each sector are assumed to be Poission distributed in generating false alarm,

the positions of the radar-sectors are set when generating the data of the multiradar system,

and the sector-pointers change step by step according to the relation of time-sequence among

the radars. Thus, the sectors for multiradars detection are generated so as to coordinate the

relation of the time-sequence among the radars.

Simulation testing is made with the multiradar system of a certain ship as the engi-

neering background. Eight targets are tracked under various tactical maneuvering scenarios

which simulate the usual tactical maneuvers of fighters. Through changing the initial value

of the random sequence, simulation is made 40 times. Since there are 8 targets in each sire-

No.4 C E N T R A L I Z E D M U L T I R A D A R I N T E G R A T E D T R A C K I N G 309

ulation, there are 320 tracks in all. The simulation results are: the total number of falsely

tracking being 31 and the total number of missed tracking being 27, and hence the correct

probability being 0.81.

V. Conclusion

The operation of multiradar tracking system is the key problem in centralized multiradar

integrated tracking. Since the detailed reports about the operation of more than two radars

have not been found, this paper discusses the mechanism of centralized multiradar integrated

tracking, with the emphasis on the tracking files management, operation mechanics and

other special issues. Simulation of tracking 8 targets under various tactical maneuvering

situations is made to the multiradar system of a certain ship and the results are satisfactory.

The simulation results show that the method presented in this paper is suitable for solving

the multiradar data merging and radar-handover, etc.

References

[1] He You, ~ Jinsong, et al., Study ou the algo~ithxn of multiradar integrated tracking. Technical

Report of Naval Aeronautical Engineering Academy, Chap.4, 1991.7, (in Chinese).

[2] Tang Jinsong, Multiradar integrated tracking, Master thesis, Naval Aeronautical Engineering Academy,

1991.3, (in Chinese).

[3] J. D. Wilson, B. H. Cantrell, Tracking system for asynchronously scanning radars with new correlation

techniques and an adaptive filter. AD A020540, 1976.

[4] J. D. Wilson, A multiple radar integrated tracking (MERIT) program. ADA055915, 1978.

[5] A. Farin_a, F. A. Stud~r, Radar data proceeJing. ~ Studies Press LTD., 1988, ~3.2.

[6] G. V. qYunk, Automatic detection, tracking, and sensor integration. AD-A198 347, NRL Report 9110,

1978, 24-28.

[7] He You, Tang Jinsong, Wang Guohong, Optimization of tracks' quality management in multiradar

tracking, Modern Rada.r, 17(1995)1, 14-19, (in Chinese).