The results of a conceptual design study and the performance of key components of the Bread Board Model (BBM) of the Tropical Rainfall Measuring Mission (TRMM) radar are presented. The radar, which operates at 13.8 GHz and is designed to meet TRMM mission objectives, has a minimum measurable rain rate of 0.5 mm/h with a range resolution of 250 m, a horizontal resolution of about 4 km, and a swath width of 220 km. A 128-element active phased array system is adopted to achieve contiguous scanning within the swath. The basic characteristics of BBM were confirmed by experiments. The development of EM started with the cooperation of NASDA and CRL

Okamoto, Kenichi

English

NASA Technical Reports Server

TRMM Radar
Ken'ichi OKAMOTO
Communications Research Laboratory, 2-1, Nukui-kitamachi 4-chome, /9.,
Koganei-shi, Tokyo, 184, Japan
N94-15894
Abstract
Results of conceptual design study and performances of
developed key devices of BBM of the TRMM radar are presented.
The radar , operating at 13.8 GHz and designed to meet TRMM
mission objectives, has the minimum measurable rain rate of 0.5
mm/h with a range resolution of 250 m, a horizontal resolution of
about 4 kin, and a swath width of 220 km. A 128-element active
phased array system is adopted to achieve a contiguous scanning
within the swath. The basic characteristics of BBM were confirmed
by experiments. The development of EM has started with the
cooperation of NASDA and CRL.
1. Introduction
TRMM(Tropical Rainfall Measuring Mission) is a joint space program of the
USA and Japan(Ref. 1) to measure rainfall of tropics where about 60% of the global
rainfall is concentrated. The feasibility study(Phase A) of TRMM was performed
successfully under the joint efforts of the USA and Japan from January 1987 to March
1988 and TRMM project was evaluated to be feasible. Phase B activities of TRMM were
completed both in USA and Japan, and both countries are doing Phase C/D activities
now. TRMM will adopt a circular orbit with the altitude of 350 km and the inclination
of 35 ° . TRMM will be launched in August 1997 by the Japanese H-II rocket.
The goals of TRMM are, (1)to advance the understanding of the global energy and
water cycle by means of providing distributions of tropical rainfall, (2)to understand
the mechanism through which the tropical rainfall affects the global circulation, and
to improve the global circulation model, and (3)to evaluate satellite system for rainfall
measurement. The primary mission product of TRMM is the monthly averaged
rainfall over 5 ° x 5 ° grid boxes between the latitude of 37 ° N and 37 ° S with an
accuracy of about 10% over three years. TRMM is required to understand tropical rain
processes that play a key role in climate changes, particularly El Nino and Southern
Oscillation. TRMM will contribute to international global change programs such as
GEWEX.
2. Conceptual Design Study of TRMM Precipitation Radar
TRMM is the first space mission dedicated to measurements of tropical rainfall
with the first precipitation radar in space. Communications Research Laboratory
(CRL)performed a conceptual design study of TRMM precipitation radar in the
feasibility study of TRMM in cooperation with NASA/GSFC (Ref. 2,3). The mission
requirements for the TRMM precipitation radar basically specified by NASA are
PRECEDIN6 PAGE BLANK NOT R,LMED
119 ../
https://ntrs.nasa.gov/search.jsp?R=19940011421 2020-06-16T19:27:19+00:00Z
summarized in Table 1. These requirements were used as the guideline for the design
of TRMM precipitation radar.
In the precipitation model, the rain is assumed to be uniform and extends to a
height of 5 km. The thickness of the bright band or the melting layer is 0.5 km and the
attenuation coefficient in the melting layer is assumed to be twice as large as that of
rain below the melting layer. The following relations between the effective Z-factor
Ze(mm6/m3), attenuation coefficient A(dB/km) and rain rate R(mm/h) for 13.8 GHz
are assumed: Ze = 372.4 R 1.54 (Ref. 4) and A = 0.032R 1-124 (Ref. 5).
In the TRMM precipitation radar, radar antenna beam is required to scan a swath
width of 220 km in the cross track direction every 0.6 seconds in order to observe a
raining area without any gaps between scanning lines which are perpendicular to the
moving direction of the satellite. Therefore, high speed electric scanning becomes
essential.
Figure 1 shows the block diagram of the TRMM radar(Ref. 6). Active array radar
is selected as a reliable candidate for the TRMM precipitation radar after trade off
studies in the feasibility study. The system is reliable because it is still operational even
if some parts of transmitters or receivers are damaged. The total system _loss becomes
small because transmission lines between the antenna and transmitter/receiver are
short and loss of phase shifter can be compensated by the SSPA(Solid State Power
Amplifiers) and LNA(Low Noise Amplifier).
Main system parameters estimated in the feasibility study are shown in Table 2.
The attained horizontal resolution is 4.3 km at nadir and the range resolution is 250
m. The swath width is 220 km. The required number of independent samples of 64 is
achieved by the frequency agility technique which uses dual frequency separated 6
MHz. Dual frequency pulse will be transmitted mutually in the same pulse repetition
period keeping a pulse repetition period long enough to insure the sufficient time for
the data acquisition., The minimum detectable rain rate at the rain top is 0.5 mm/h
and the measurable rain rate at the rain bottom is between 0.5 mm/h and 52 mm/h,
with a signal to noise ratio of 0 dB for a single echo. If we use the attenuation data of
the sea surface echo by rain, we can observe the average rain rate as large as 80 mm/h
with a signal to noise ratio of 0 dB for a single echo.
3. Development of Key Devices of the Bread Board Model of the TRMM
Precipitation Radar
CRL has developed key devices of the Bread Board Model of the TRMM
precipitation radar since 1988(Ref. 7). These key devices are (1) slotted waveguide
antenna elements, (2) 5-bit PIN diode phase shifters, (3) SSPAs and (4) LNAs.
Integration of these components to form an 8-element Bread Board Model of the
TRMM precipitation radar was also made. We developed 8-element slotted waveguide
array antenna. Major required characteristics of the antenna are (1) the low sidelobe
level to suppress the surface clutter, (2) the wide band width for the frequency agility,
and (3)the high speed electric scanning with scan angle of +17 degrees. We adopted the
non-resonance type(wide band width)slotted wave guide antenna. The separation
between slots cut on the narrow wall of the rectangular waveguide is 13.65 ram. Slot
conductance is designed to attain the Taylor amplitude distribution(sidelobe = -35 dB,
parameter n = 6) along the waveguide axis. Figure 2 shows outside appearance of the
developed 8-element slotted waveguide array antenna part of the Bread Board Model
of TRMM precipitation radar. The antenna length is 2.2 m. Measured antenna
120
=
E|
Table 1. Mission Requirements
Frequency
Satellite altitude
Scan angle
Swath width
Range resolution
Horizontal resolution
Minimum measurable
rainfall rate
Observation range
Nadir
Scan angle: e
Number of independent
samples
Antenna sidelobe level
13.8 GHz
350 km
+17.0 degrees
220 km
250 m
around 4 km at nadir
0.5 mm/h at storm top
15 km + 5 km
15 km/cos0
64
around -30 dB
T
E
N
N
f
Bement 01
O
/
C
0
N
V
Element #128
ele.
X
16
_T/R Subsystem (Active array unit)
T/R Subsystem
,,/
[ F'eq'=nv"Iq Sig;eno.
I IFunit.
Signal Processing Subsyslem
"x
f-
System
control
unit
Data
processing
unit"
Thermal
Control
Subsystem
Structure
Subsystem
• EM is developed by CRL
Fig. 1 Block diagram of TRMM radar
Fig. 2 Outside appearance of the 8-element slotted wave guide antenna part of
the Bread Board Model of TRMM precipitation radar
121
radiation pattern in the plane of the wave guide axis of an 8-element slotted wave
guide array antenna at 13.8GHz is shown in Fig. 3 The half power beam width of 0.7°
is attained. The peak sidelobe level about -30 dB is attained except for the shoulder
level of -28 dB. The wide angle sidelobe level less than -35 dB is also attained. The
antenna beam tilts about 4° from the direction normal to the slotted waveguide wall.
We developed PIN diode 5 bit phase shifter at 13.8GHz. In the PIN diode 5 bit
phase shifter, 11.25°, 22.5°, 45° and 90° bits are of loaded line type and 180° bit is of
hybrid coupler type. We attained at the worst casebetween -10°C and +40°C, insertion
loss values of 4-5 dB for the variation of the phasevalues. Phaseerrors about - 5°- + 2°
and amplitude errors less than 0.9 dBp_p are attained. As the phase and amplitude
errors and variation of the insertion loss are directly related to the characteristics of
phased array antenna, more detailed examination and evaluation of the phase shifter
will be performed for the Engineering Model. Although values of the insertion loss
are larger than those of the ferrite phase shifter, these loss values can be compensated
by the SSPA and LNA. The massof the developed phase shifter is about 150 g.
In order to satisfy the peak transmitted power shown in Table 2, the output
power of 10 W around the central part of the array is required. By using multi-stage
GaAs FET amplifiers, output power of 10 W and an efficiency of more than 15 % is
developed. In order to reduce the power consumption, the efficiency of SSPA itself, of
the power supply and of the guard time margin for the pulsed operation must be
considered. The output power variation is 0.2 dBp-p and the phase variation of the
each SSPA is as large as 30° for the temperature variation between -10° C and +40° C.
However, variation of the phase difference of 2 SSPAswhose phase difference is set to
be equal at room temperature is less than 6° p_pfor the temperature variation between
-10° C and +40° C, because the temperature dependence of the phase variation of 2
SSPAs is almost equal. The efficiency of the SSPA itself will be improved as large as
25% in the development phase of the Engineering Model. The mass of the developed
SSPAis about 400g.
LNA is composed of the multi-stage MIC amplifiers using the HEMT(High
Electron Mobility Transistor). Good characteristics with gain of 30 dB and noise figure
of 1.8 dB are attained. Variation of the phase difference of 2 LNAs whose phase
difference is set to be equal at room temperature is less than 3°p_pfor the temperature
variation between -10° C and +40° C, becausethe temperature dependence of the phase
variation of 2 LNAs is almost equal. It is important to attain the temperature stability
of the phase of each SSPA and LNA, and also to keep uniform the temperature of the
whole radar system in order to attain the stable antenna radiation pattern.
Integration of PIN diode phase shifters, LNAs, phase adjusters and a
divider/combiner is made to compose the 8-element transmitter and receiver part of
the Bread Board Model of TRMM precipitation radar and the appearance is shown in
Fig.4. The basic electric antenna beam scanning function by the switching of the 5-bit
digital phase shifter was confirmed. The preliminary measurements of both the
transmitting and receiving pattern of the 8-element active array antenna were also
made to show good electric performances.
4. CONCLUDING REMARKS
Development of the Engineering Model of the radar has started with the
cooperation of National Space Development Agency of Japan(NASDA) and the CRL.
The Proto-Flight Model of the radar will be developed by NASDA, aiming at the
launch of TRMM in August 1997 through joint efforts of USA and Japan. TRMM is
122
I O'dB
7"
-lOdB1 =
-20dB
• L
8" 6" 4" 2 O" 2" 4" 6 ° 8"
30dB
Fig. 3 Radiation pattern of a slotted waveguide array antenna element
Fig. 4 Outside appearance of the 8-element transmitter and receiver part of the
Bread Board Model of TRMM
123
Table 2. Main System Parameters of TRMM
Precipitation Radar
(estimated in the feasibility study)
Frequency
Antenna
Type
Gain
Beam width
Aperture
Sidelobe level
Scan angle
Transmitter
Type
Peak power
Pulse width
PRF(#1)
Receiver
Noise figure
IF frequency
Band width
Characteristics
Dynamic range
Linearity
Stain
Others
Required S/N(#2)
Total system loss
Filter weighting loss
Number of independent
samples(#3)
Data rate
Power consumption
Mass(#4)
13.8 GHz
Slotted waveguide
array(128 elements)
47.7 dB
0.71 x 0.71 degrees
2.2 m x 2.2 m
around -30 dB
+17.0 degrees
SSPA(x 128)
577.8 W
1.67_s x 2ch
2778 Hz
2.3 dB
156 MHz, 162MHz
0.78 MHz x 2ch
logarithmic
more than 70 dB
less than + 0.5 dB
-112.8 dBm
0 dB for single echo
2.0 dB
1.5 dB
64(32 x 2)
85 kbps
224 W
347 kg
(#1)
(#2)
(#3)
(#4)
Fixed PRF; Frequency agility technique
(e.g. 13.796 GHz and 13.802 GHz) is applied.
0.5 mm/h at storm top.
Frequency agility technique is applied.
Without margin
m
E
i
124
expected to improve the understanding of global atmospheric mechanism and water
cycle, which is essential for human life.
References
1. Simpson, J., R.E.Adler and G.R.North, "A proposed tropical rainfall measuring
mission(TRMM) satellite", Bull.Amer.Meteor.Soc., 69, 278-295 (1988)
2. Okamoto, K. et al., "A feasibility study of rain radar for the Tropical Rainfall
Measuring Mission", J. Commun. Res. Lab. Vol.35, No.145, 109-183(1988)
3. Nakamura, K. et al., "Conceptual design of rain radar for the tropical rainfall
measuring mission", Int.J. of Satellite Communications.,VoI.8. 257-268(1990)
4. Ulbiich,C.W., Private communication, based on the empirical fits on the
experimental data by Joss. (1987)
5. Olsen, R.L., D.V.Rogers and D.B.Hodge, "The aR b relationship in the calculation of
rain attenuation", IEEE Trans. Antenna Propagat., AP-26, 318-329(1978)
6. Kozu, T. et al., "Development status of rain radar for Tropical Rainfall
Measuring Mission" Proc. of IGARSS'92, pp.1722-1724, 1992
7. Okamoto. K. et al. "Development of Bread Board Model of TRMM
Precipitation Radar", Proc. of the 18th International Symposium on Space
Technology and Science, pp. 1919-1924, 1992
125
iE
z
|
II
qE
i
IE|
2|
E
|
_L


TRMM radar

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server