MULTI MODE LADAR/RADAR ACTIVE TRANSPONDER SYSTEMS
FOR TRACKING VERY LONG OPERATION RANGES COOPERATIVE TARGETS
J. Gavan, M.Haridim
Holon Academic Institute of Technology
POB 305, Holon 58102, Israel
Fax : 972-3-502-6685 – http://www.cteh.ac.il
ABSTRACT
Tracking of cooperative airborne targets is very important for numerous applications
especially for space missions. Previous researches have shown that tracking Laser
RADAR (LADAR) offers very high precision and reliability but their operation range
is limited to 40 km for good visibility and much less for adverse propagation
conditions. Higher operation ranges under all weather conditions require a high power
and costly RADAR and /or a light compact tactical low cost RADAR system using
active transponders (on the cooperative target) at different frequency bands.
In this paper a multimode LADAR/RADAR/Transponder system is investigated.
Providing optimum performances and maximum operation range of hundreds of kms up
to Low Earth Orbit (LEO) Satellites or space shuttle orbits. These performances are
obtained without reducing significantly the tracking accuracy.
1. Introduction
A multimode LADAR/RADAR Transponder system for tracking remote cooperative
airborned targets is investigated for optimum performances. Beginnning with most
remote targets, a L band RADAR is operated for all weather long distance ranges up
to Low Earth Orbit (LEO) satellites or space shuttle orbits. In addition, an active
transponder in the L frequency band should be operated on the tracked target When the
distance is reduced to less than 450 km, for instance, the L band Mode active
transponder is disconnected and is switched to a X band mode and/or Mm wave mode
RADAR with or without active transponders. For operation distances less than 60 km,
for instance, an accurate LASER mode is switched instead of the RADAR modes for
the final tracking steps first, an optical transponder may be used and in the final
docking stages the active transponder is disconnected to obtain maximal precision. The
system analysis and parameter computations will be presented in the following sections.
2. LADAR System Operation Mode
LADAR systems have been proposed for detecting and tracking cooperative targets
with a very high precision [1,2]. A Nd YAG LADAR was designed to provide an
effective range of 10 km for a detection probability Pd of 0.99 and a False Alarm Rate
5
(FAR) of 10 as shown in table 1, using the probability concepts developed in
previous papers [3,4].
2
The data of table 1 can be computed from [5] using the equation.
 2 K1.0 D(4r )  (1.0 ret )
P
S c  retro 
Pclutter 42 . K 2 . R(1.0 dif ) .  . R 2.10 6
.
.
.
(1)
where Pretro is the desired signal power from the cooperative target with a retro-reflector,
Pclutter is the signal from the cluttering interference, R is the cooperative target range in km,
K1.0 is the target factor due to atmospheric turbulence, and is taken as K (1.0)=
0.65
where
e
e = 2.71 is the natural logarithmic base.
The cooperative aperture diameter is D(r)0.038m [1] and the attenuation factor of the
cooperative target is (1.0 ret)0.9. The retro factor for circular cross sections is K 2 3.7. The
clutter or scattering causes an equivalent reflection factor R(1.0 dif) for average reflections
from various back-scattering sources such as clouds or ground.
A medium value of R(1.0 dif)=0.6 is taken, R(1.0 dif)= 0.8 is a realistic worst case value applied
for the computation of minimum operation range [4].
To achieve accurate detection and tracking for small ranges the following LADAR has
been proposed [6].

Nd-YAG pulsed Laser operating at a wavelength of  = 1.06m.

Pulse Peak Power Pp= 2 MW.

Single pulse width = 10 n sec and a period T = 300 m sec.

Full angle Tx beamwidth  = 1.2 m rad.

Receiver (Rx) sensitivity Psen = -35 dBm.
The optimal computed operation ranges under normal atmospheric conditions without
rainfall are presented in Table 1 and figure 1. Simulations and practical field measurements
have shown similar results [1].
Table 1
3
Minimum and medium detection ranges “Rmin” and “Rmed” computation results as
a function of Pf, Pd and the number of integrated pulses n for the proposed LADAR system
Number of
Integrating
Signal Pulses
Pd=0,9
n =1
n =2
n =8
n=16
Pd=0,99
n=1
n=2
n=8
n=16
1.
2.
3.
4.
5.
Pf=10-3
Rmin
Rmed
km
km
Rmin
km
Pf=10-5
Rmed
km
13,70
18,50
32,50
41,00
15,80
21,40
37,60
47,40
11,30
15,20
26,70
33,50
13,00
17,50
29,80
38,70
11,30
15,30
26,90
33,80
13,10
17,70
31,10
39,10
9,60
13,00
22,80
28,70
11,10
15,00
26,40
33,20
The main conclusions for the LADAR mode operation are as follows :
LADAR system maximal operation range using only one n=1 detection integrating
signal pulse per period, is less than 12 km for a probability of detection Pd = 0.99, a
probability of False Alarm Pf = 10-5 and an acceptable signal to clutter ratio of Sc= 14
dB as shown in the graphs of figure 1and the data of table 1.
Improvements of the LADAR system by increasing the number n up to 16 and
increasing the corner recto-reflector cross section can enhance the LADAR maximum
detection and tracking range up to 40 km.
Unfavorable atmospheric conditions, including rainfall, increase signal attenuation
and scattering effects will significantly reduce detection and tracking operations ranges
[7].
Novel active LASER Transponder on the cooperative target can enhance the
operation range up to 80 km.
The extreme tracking resolution and accuracy of LADAR cannot be achieved for
long range operation and/or bad atmospheric conditions where the RADAR mode must
be used [8].
3. RADAR System operation Modes
In ground to air tracking RADAR systems the operation range is limited by the radio Line
Of Sight (LOS) dL distance.For normal atmospheric propagation conditions in most
locations with temperate climate dL can be evaluated using the equation [9]:


d L  17 h1  h2 km
(2)
where h1 and h2 represent the height of the system airborne coooperative target and the
ground RADAR antenna, respectively, in m. For high altitude cooperation tracking h the
influence of h2 be neglected, and then
d LOS  130 h1
(3)
Here dL and h1 are both expressed in km.
When d< dLOS the propagation conditions as well as the received signal-to-noise and
interference are favorable and subject to Gaussian or Rician statistic distributions [10].
When d dLOS, non favorable
Rayleigh statistic distributions prevail decreasing
detection probability and significantly increasing False Alarm Rate (FAR) [2]. Thus the
dLOS distance is the limit maximum the practical RADAR system operation range. Table 2
4
provides the medium operation range dL for RADAR systems as a function of the target
elevation h1 above the tracking ground station.
In the RADAR operation modes, the system detection and tracking resolution and
precision significantly deteriorate with decreasing frequency and the antennas dimensions
and weight increase, keeping on similar gain and directivity values [3]. However, on the
other hand at lower frequencies the RADAR equipment, Tx power output and efficiency and
the Rx noise figure and gain are improved when the frequency decreases as well as the
atmospheric losses and scattering [2]. Thus, the operation range is improved when
frequency decreases as shown in Table 3. Table 3 presents the main system characteristics
of the RADAR modes and the operation ranges for three typical frequency bands in the:
35GHz mm waves (Ka band), military 9.375 GHz (X band) and long range operation 1.33
GHz (L band) without and with active transponders. The L band RADARs operate as an all
weather system whose range is not significantly affected even by heavy rainfall in
comparison to the higher frequency (ka) band and LADAR systems which are significantly
affected by atmospheric conditions [3].
3. RADAR Modes Operation Ranges
The medium operation range d for the described cooperative target tracking RADAR is
computed using the following equation [11]:
1

4
 
 Pp . T .gT . Ar . a . .n. E i (n) 
 

d
2
 4  k T F B f ( S / N ) L 
i s
B a eq 1 p



(4)
where
Pp '
,
T
gT , Aa , , n, Ei (n), k B , T0 , Feq , B, 1 , f p , ( S / N ), Ls
represent respectively the : Tx peak power output in W, duty cycle, transmitting antenna
gain, receiver antenna effective area, antenna efficiency, retroreflector cross section area,
number of integrating pulses, incoherent integration efficiency, Boltzmann constant,
k B  1.4 .10 23 J o ,
k
ambient temperature, equivalent noise figure of the receiver,
receiver bandwidth, pulse width, Pulse Repetition Frequency (PRF), Signal-toNoise+Distortion Ratio at the RADAR receiver input and system margin losses.
Eq. 4 shows that the desired returned signal intensity and the signal-to-noise and distortion
ratio at the RADAR Rx input decrease as a function of d4.
The RADAR characteristics and operation ranges as a function of frequency bands are
presented in Table 2. The range computations were carried out using equations (3,4). As
shown in Table 3, the operation height for maximum LOS range is about 1 km at the 35
GHz mm band, and about h 40 km at the L band all weather operation RADAR.
Table 2
RADAR characteristics and operation ranges as a
function of frequency band for a S/N of 14 dB
5
Ka (35)
Frequency band f 0
parameters in GHz
X
(9.375)
L
(1.33)
A
5
B*
1
25
40
0.17
5
5
5
5
5
Dish Antenna Diameter, D(m)
Tx Antenna Gain GT (dBi)
Tx Antenna Efficiency 
Rx Antenna Cross Section, m2
or (Gain) dB
Number of Integrating Signal
Pulses, n
Rx Equivalent Noise Figure, Feq(dB)
Rx IF bandwidth, B(KHz)
Clear Air attenuation (dB/km) [13]
16 mm h Severe Rainfall -
0.6
44.5
0.5
0.2
0.6
44.5
0.5
(10dB)
1.0
38.5
0.65
0.8
2.00
27.7
0.75
3.2
0.6
17
0.7
(10dB)
16
1
16
16
1
6
500
0.15
6
500
0.15
6
250
0.01
3
333
-
4
333
-
attenuation,  r dB km [8]
4.0
4.0
0.26
0.01
0.01
Medium Operation Range d (km)
Operation Range under
16 mm h Rain, dT (km)
195
28
350
60
310
110
450
445
850
840
Tx Average Power, Pa
 KW
Tx Duty Cycle, (%)


B*
A
B* with Active Transponder on the cooperative target.
Table 3
Optimal operation ranges of RADAR systems as function of airborne cooperative height h1,
Line of Sight distances dLOS and frequency range modes without and with active
transponders.
h1
dLOS
d(35GHz)*
d(35 GHz)**
d(9.375GHz)*
d(1.33GHz)**
d(1.33GHz)*
*
(km)
0.3
1.0
10
40
700
(km)
70
130
410
825
3450
(km)
165
350*3
-
(km)
25
200*3
-
(km)
310
-
(km)
445
3450*3
(km)
440
3420*3
* Clear weather conditions with optimal visibility
** Operation ranges under 16mm/h rain
*3 Cooperative Target Active Transponders operation Modes
The addition of an active transponder on the cooperative target enhances significantly the
detection and tracking operation range and also enables a reduction of the RADAR modes
weight, Tx power levels and dimensions of the antennas as shown in table 2.
6
The passive transponders are not useful due to their limited operation ranges of only
hundreds of meters [12 ].
The most appropriate active transponder design includes a superheterodyne Rx tuned to the
RADAR f0 frequency which excites a coded signal at a chosen non-harmonic related
frequency fr radiated toward the RADAR Rx. The active transponder Tx operates only
when the input signal from the tracking RADAR exceeds a threshold power level at the Rx
input circuits, thus decreasing the FAR probability and enhancing the operation time of the
cooperative target battery. A simpler method would utilize a time division half duplex
communication technique where the transponder transmits on the same frequency fT2 that it
receive fT2=fr2. Part of the tracking period time, the RADAR transmits bursts and the rest of
the period it operates as a receiver.
The common Friiss (LOS) equation can be used instead of the RADAR equation [2,5] for
operation distances less than the free space line of sight limit . In this cases, we obtain :
 PT1 g T 1g r2
 
Ls
 Pr2
0.5

 (5)

where f T is the ground RADAR Tx frequency in GHz and d T is the up-link (forward)
3  10 3
dT 
40  f T
path tracking distance in km from the ground tracking Tx antenna to the cooperative target
transponder Rx.
The dish antenna gain g T1 , using a given Diameter DT is
as
g T1 
1000 2
 D T  f T2 T (6)
9
where f T in GHz and DT in m.
Thus for the downlink distance d r from the transponder Tx to the Rx one can use the
following equation:
3
3  10
dr 
40  f r
0.5
 PT2 gT2  g r2 


 (7)
Ls 2 
 Pr1
Using these equations, the parameters of a multimode RADAR in the L and Ka bands as
shown in column B of figure 2 can be computed. The required active transponder PT =
10W and a simple printed antenna array of 4 elements at most can provide gain of up to
10dBi.
7
The transponder and RADAR receiver threshold sensitivity power are calculated using the
common equation
(8)
Psen  F.k.T0 .B
where F is the transponder or RADAR Rx equivalent noise figure, k is the Boltzmann
coefficient, To is the ambient temperature in 0K and B is the Intermediate Frequency Rx
bandwith in Hz. At the conventional ambient temperature of 3000K, KT0 4.1. 10-21 Joule
[9].
In fact, for high altitude aircrafts or LEO satellites the T o is significantly lower than 3000 K,
thus the real Psen may be significantly less than that computed here.
The preceeding equations are used for the chosen 3 modes frequency ranges of fT: 1.33;
9.375 and 35 GHz presented in table 2.
For L band, fT= 1.33 GHz the operation range can be up to 10.000 km. Therefore, the
required operation ranges shown in Table 3 can be obtained using significantly lower P T
and DT.
The system designed parameters for the uplink from the tracking RADAR to the
transponder are as follows:
Fr 2  6dB(2.5); KT0  4.1.10 21 Joule and B  333KHz
15
From equation (8) we obtain
Psen  5.310 W or (-113dBm). Thus, high detection
11
probability will be obtained for S/(N+D) exceeding 20 dB. For Pr 2  10 W or (-80
dBm), for instance, the computed signal-to-noise and distortion is around 33 dB which
enables a reliable tracking of cooperative targets.
We use Pp  2kW ,

T
 5% which represent an average power of PT1=100 W
instead of 40 KW without transponder and a ground antenna dish with D = 0.6m,
significantly less than for the former L band RADAR. For the transponder antenna, a
simple and compact microstrip antenna array of 4 patch elements can provide a gain of
more than 10dBi at L band and Ls ~2.5dB [14].
From equation (5) we obtain
d Tmax  850km which is sufficient to provide an all
weather conditions detection and tracking for all high altitude aircraft as shown in Table 3.
LEO satellites can be tracked only if the operation distance is slightly increased which can
be obtained with a slight increase of the Tx power, for instance.
For the downlink , the active transponders parameters are:
PT 2  10W , gT 2  10. For the ground RADAR; D = 0.6m and gr260; Ls2=2.5.dB and
Fr1= 3dB (Table 2). We obtain from equation (7) P sen1-116dBm and we choose a
minimum desired Pr1-90 dBm and dr  1150km . Thus, the up link is critical, where the
maximum operation distance is 850 km. For Low Earth Orbit (LEO) satellite tracking this
operation distance has to be slightly increased. This can be achieved by a reduction in
transponder receiver sensitivity or a slight increase of the RADAR Tx power output.[5]
At L band even heavy rain conditions have not any significant effect on the operation range
as shown in tables 2 and 3.
At ka band the system parameters are as follows for the up link : fT= 35GHz DT= 0.6m,
T~0.5, gT~24.400, gr2 =10 and Ls= 8 as shown in Table 2. Fr2=6dB and Psen=1.23.10-14W(109dBm).
For similar detection probability and FAR performance as in the L band active transponder,
Pr1-75dBm, was chosen.
8
Therefore, using equation (5), we obtain dT400km. The operation range for the active
transponders is significantly less than that for the L band.
For the downlink operation distance dr the following parameters were chosen;
PT2=10W, gT2=10, Psen2-109dBm and Pr1-88 dBm, gr1= 24400 Ls1 = 8 dBm, gr1=24400;
Lst= 8 from the data of Table 2.
From equation (7) dr~350km. Thus, the downlink is critical at ka band.
CONCLUSIONS
A Multi-mode LADAR/RADAR system is presented which provides both a maximum range
all-weather operation and a good tracking accuracy.
The power and dimensions of the ground RADAR can be significantly reduced by using
airborne active radio and Infra Red wavelength transponders.
The Multimode LADAR/RADAR using active transponder systems for tracking very long
operation ranges cooperative targets can be illustrated and summarized by table 4.
TABLE 4
Tracking Operation Typical Distances using a Multimode LADAR/RADAR System
Mode /Operation Distance (km)
Good
Bad
Visibility Visibility
Nd YAG Laser Mode
25
10
Nd YAG Laser Using an Optical Transponder Mode
60
20
Mm Waves 35GHz RADAR Mode
190
30
35GHZ RADAR using an Active Transponder Mode
350
65
X Band 9.375 GHz RADAR Mode
310
110
L Band 1.33 GHz RADAR Mode
450
445
1.33 GHz RADAR using an Active Transponder Mode
850
840
For maximal operation ranges up to 850 km the non accurate 1.33 GHz L band tracking
RADAR using a cooperative active transponder should be used . When the operation range
decrease to less than 445 km the active transponder is disconnected. For distances less than
350km under good visibility conditions or 65 km under bad visibility the 9.375 GHz X band
RADAR or 35 GHz Ka band RADAR with a cooperative active transponder is activated.
For distances less than 190 km under good visibility or 30km under bad visibility the active
transponder is not required. For the last steps of tracking for distances less than 60 km under
good visibility or 20 km under bad visibility the RADAR mode is disconnected and the
most accurate Nd Yag LASER is activated.
It was shown recently that an optical transponder can enhance significantly the operation
range of the accurate LADAR this may be the subject of a further paper.
References
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9
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