NCAR/TN-325+EDD Design of the ELDORA Airborne Doppler Radar.

148 Pages·2003·8.11 MB·English

NCAR/TN-325+EDD
NCAR TECHNICAL NOTE
-
December 1988
Design of the ELDORA
Airborne Doppler Radar
Peter H. Hildebrand
Chuck Frush
Craig Walther
ATMOSPHERIC TECHNOLOGY DIVISION
NATIONAL CENTER FOR ATMOSPHERIC RESEARCH
BOULDER, COLORADO
TABLE OF CONTENTS
TABLE OF CONTENTS ................... 0 00 i
LIST OF FIGURES ................... . . . v
LIST OF TABLES ......... ... ...... . . vii
ABSTRACT a ) a · · a & I 0 . . ix
1.0 Evaluation of Scientific Needs for Airborne
Doppler Radar .. . . . . . ... 1
1.1 Introduction . . . . . . . . . . 0 0 1
1.1.1 Historical Perspective ......... 9 0 0 1
1.1.2 Assessment of Needs ........... 0 0 0 & 4
1.2 Scientist Questionnaire ........ 0 0 0 4
1.3 Presentation of Results .... 9 0 0 0 4
1.3.1 Areas of Research Interest . .. 0 0 & 0 5
1.3.2 Radar Characteristics ......... 0 6
1.3.2.1 Radar data density ........... 0 0 0 6
1.3.2.2 Sampling domain size .......... 0 9
00* 00
1.3.2.3 Radar sensitivity ............ 11
1.3.2.4 Measurement accuracy ...... 00 0 11
1.3.2.5 Aircraft capabilities ..... 14
1.3.3 Interrelations Between Measurement Needs 0 0 0 16
1.4 Review at the 21st Radar Meteorology Conference . 18
1.5 Comments and Summary .... .* * * 18
1.6 Questionnaire ......... 0. .019. .
1.7 Sources for Literature Determination of
Measurement Needs ........... 23
0 0 0
2.0 Radar Design Considerations ... 00* 0 25
2.1 Introduction .. . ......... 0 0 0 0 25
2.2 Selection of Radar Frequency ...... 0 0 25
2.2.1 Beam-filling of Space and Radar Frequency 0 25
2.2.2 Sensitivity Calculations ... 0 26
2.2.3 Gaseous Atmospheric Attenuation .. 0 0 0 0 34
2.2.4 Attenuation by Hydrometers ... 0 0 0 9 35
2.2.5 Range and Velocity Ambiguity .. 38
2.2.6 Spectral Measurement Considerations . . . 39
2.2.7 Recommended Wavelength ......... 41
2.3 Radar Sampling Considerations ...... 42
2.3.1 Spatial and Temporal Sampling Goals . . . 42
00
2.3.2 Scan Method ............... 43
2.3.3 Radar Sampling Rate . ......... 46
2.3.4 Transmitted Wave Form . . .. . . . . 49
2.3.4.1 A Comb of Frequencies .......... 50
2.3.4.2 Phase-Modulated Pulse Compression . . 0 0 a 0 50
2.3.4.3 Pulse Compression using an FM Chirp . 52
· * *
2.3.4.4 Summary and Complex Wave Form Discussion 52
2.3.5 Measurement Accuracy .......... 52
2.4 Hardware Design Approach ........ 54
2.4.1 Design Goals . ..... . 54
i
TABLE OF CONTENTS CONT'D
2.4.2 Division of the Radar System into Modules . . . .54
2.4.3 Communication Between System Modules ...... 54
2.4.4 System Maintenance and Documentation .... 55
2.4.5 System Control ................. 55
2.4.6 Safety Considerations .......... . . . . 55
2.4.7 Component Reliability ...... . . . . . . . 56
3.0 Design of the ELDORA Receiver/Transmitter Unit . 57
3.1 Introduction . . . . . . . . . . . . . . 57
3.2 The Proposed Phase I Wave Form ......... 57
3.3 Synthesis of the Transmitted Signal and the
Doppler Reference . . . . . . . . . . . . . . . 61
3.3.1 Multiple-Frequency Generation ......... 62
3.4 Transmit Signal Switch ............ . 63
3.5 Intermediate Power Amplifier (IPA) .. .... 63
3.6 The Output High Power Amplifier ......... 63
3.6.1 Output Tube Modulation ............. 64
3.6.2 Output Tube Protection ...... 64
3.6.3 Output Tube Cathode and Filament Considerations . 65
3.6.4 Output Isolator and Harmonic Filter ..... . 66
3.7 Power Transmission ........ 67
3.7.1 The Circulator ........ 67
3.7.2 Directional Couplers . . . . . . . . .... 67
3.7.3 Rotary Joint . 67
3.7.4 Waveguide Pressurization ...... 68
3.7.5 Antenna ..................... 68
3.7.6 Radome ..................... 68
3.8 The Receiver .................. 69
3.8.1 The REF Preselector ............. 70
3.8.2 The 1st Mixer . ............. 70
3.8.3 IF Section .............. 71
3.8.4 Conversion to Baseband and Digitizing ..... 71
3.8.4.1 Linear Amplifier and Quadrature Detection .... 72
3.8.5 The Test Pulse and Receiver Calibration ..... 73
3.9 Radar Power Requirements ....... 74
4.0 Data System for Airborne Doppler Radar . 75
44..11 IInnttrroodduuccttiioonn . . . . . . . . . . . ...... 7755
4.2 Data System Overview .......... 75
4.2.1 Design Philosophy ........ ....... 75
4.2.2 Mechanical Design Approach ..... 76
4.3 Radar Processor .... ............ 78
4.3.1 Radar Processor Task ....... . 78
4.3.2 Radar Processor Design Considerations ..... 78
4.3.3 Radar Processor Design ........ ..... 80
4.3.4 Programming the Processor ........... 81
4.3.5 Processor Signal Flow and Bussing .... . 81
4.3.6 Processor Mechanical Specifications ....... 82
4.3.7 Radar Processor Output ............. 83
4.3.8 Intermediate Processing of the Radar Data . . 83
4.4 Housekeeping Processor . . ..... 85
4.4.1 Purpose ............. 85
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TABLE OF CONTENTS CONT'D
4.4.2 Control Bus ... 85
4.4.3 Radar Calibration .87
4.4.4 INU Interface .................. 87
4.4.5 ADS Interface ... 87
4.4.6 Data Stream Manipulation .. . ....... 88
4.5 Data Reduction Processor . . . . . ..... 88
4.6 Control Processor ............ 88
4.6.1 Purpose . . . . . . . . . . . 88
4.6.2 Design .................... 90
4.6.3 Hardware Configuration ............. 90
4.6.4 User Interface . . . . ............ 90
4.7 Timer Module .................. 91
4.8 Display Processor ......... ...... 92
4.8.1 Overview ................. . 92
4.8.2 User Control of Displays ............ 93
4.8.3 Radar Data Displays ............... 96
4.8.4 Display Data Rates ............... 96
4.8.5 Future Display Options ............. 97
4.9 Data Recording .......... 97
4.9.1 Data Rates ................... 97
4.9.2 Recording Devices .............. 98
4.10 Data System Design and Repair Philosophy . . . 101
5.0 Aircraft Modification and Rotodome
Configuration ................ 103
5.1 Aircraft Selection .... .. 103
5.2 Aircraft Modification .... .. 103
5.3 The Rotodome Concept ............. 104
5.4 Drive Mechanism ...... .. 107
5.5 Construction ...... .. 109
5.6 Lightning Protection . ........ .. . 109
5.7 Electrical Power ............. 110
5.8 Component Location, Aircraft Weight and
Balance . .... ............ . 110
5.9 Electra Weight and Balance .......... 112
5.9.1 Electra Payload Capabilities ......... 112
5.9.2 Weight and Balance Calculations ........ 113
6.0 Antenna Rotational Control and Position
Measurement ........ 115
6.1 Introduction ...... .. 115
6.2 Antenna Mounting and Structure ........ 115
6.3 Rotodome/Aircraft Frictional Coupling ..... 116
6.3.1 Rotary Joints . 116
6.3.2 Slip Ring ................... 116
6.3.3 Rotodome Imbalance .... .. 116
6.3.4 Rotodome Windage ............... 116
6.3.5 Spindle Bearings ............... 117
6.3.6 Acceleration ..... . ........ . 118
6.3.7 Total Torque Acting Against Rotodome Moment . . 118
6.4 Antenna Drive Motor . 119
6.5 Shaft Encoder................ 120
6.6 Drive Motor Controller ............ 120
i1i i1
TABLE OF CONTENTS CONT'D
6.7 Inertial Navigational Equipment .. 120
6.8 Housekeeping Processor .. 121
7.0 Eldora Project Planning ....... .... 123
7.1 Introduction ..... 123
7.2 Initial Testing of the Airborne Doppler Concept
and Development of the Plans for ELDORA . . . 123
7.3 Development of the ELDORA Design and the
ELDORA Design Review Workshop ....... 125
7.4 ELDORA Development During FY 1986 . .. ..125
7.5 ELDORA Development During FY 1987 . . . . . . 126
7.6 ELDORA Development During FY 1988 ..... . 126
7.7 FY 1989 Development Plans ......... .. 127
7.8 FY 1990 Development Plans .. .... 127
7.9 FY 1991 Development Plans ........... 128
7.10 FY 1992 Development Plans ..... 128
ACKNOWLEDGMENTS ..................... 131
REFERENCES ......... .. 133
APPENDIX ....................... 137
1
LIST OF FIGURES
Figure 1.1 Observations of a thunderstorm using ground 3
based (top) and airborne (bottom) Doppler
radars.
Figure 1.2 Requirements for horizontal and vertical radar 7
data density (top) and steady state time
(bottom) for radar observations of atmospheric
phenomena.
Figure 1.3 Requirements for horizontal and vertical domain 10
size (top) and range (bottom) for airborne
Doppler radar.
Figure 1.4 Minimum sensitivity requirements for airborne 12
Doppler radar.
Figure 1.5 Measurement accuracy requirements for airborne 13
Doppler radar.
Figure 1.6 Aircraft capability requirements including 15
aircraft range and flight duration (top) and
maximum flight altitude (bottom) for airborne
Doppler radar.
Figure 1.7 Relationships between resolution and domain 17
size (top) and domain size and steady state
time (bottom) for airborne Doppler radar.
Figure 2.1 Correlation between adjacent radar beams as a 27
function of beam separation for a 2° Gaussian
radar beam.
Figure 2.2 Illustration of the relation between desired 28
sampling interval, radar beamwidth, and range.
Figure 2.3 Minimum detectable reflectivity values as a 30
function of range for the X-, Ku- and Ka-band
radars described in Table 2.1.
Figure 2.4 Minimum detectable reflectivity values through 37
0.25 g/m3 rain as a function of range for the
X-, Ku- and Ka-band radars described in Table
2.1.
Figure 2.5 Scan geometry for the current NOAA P-3 airborne 44
Doppler radar system.
Figure 2.6 Scan geometry for the NCAR ELODRA system. 45
v
LIST OF FIGURES CONT'D
Figure 2.7 Mean velocity measurement accuracy for a 48
X-band radar as a function of signal to noise
ratio and number of independent samples.
Figure 2.8 Illustration of candidate waveforms for 51
multiple frequency transmission. Possibilities
are shown for 1 through 4 transmitted frequen-
cies.
Figure 3.1 Schematic diagram of the R/T Unit showing a 4 58
frequency unit and 2 radars.
Figure 3.2 R/T Unit detail. 60
Figure 4.1 Data system layout. 77
Figure 4.2 RP7 radar processor input module. 79
Figure 4.3 Schematic of intermediate data processor. 84
Figure 4.4 Housekeeping processor schematic. 86
Figure 4.5 Control processor schematic. 89
Figure 4.6 User console. 94
Figure 4.7 Zoom and roam schematic. 95
Figure 5.1 Exploded view of the ELDORA rotodome. 105
Figure 5.2 Electra aircraft tail configuration for 106
ELDORA showing the antenna location and
the fore and aft beam positions.
Figure 5.3 ELDORA rotodome drive mechanism. 108
Figure 5.4 ELDORA equipment layout on the Electra 111
aircraft.
Figure 7.1 ELDORA Development Plan Timetable 124
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LIST OF TABLES
Table 1.1 Areas of atmospheric sciences research indi- 5
cated as being suitable for airborne Doppler
radar.
Table 1.2 A comparison of data density requirements 8
according to results of the questionnaire
(top) and a review of the literature (bottom).
Table 1.3 Requirements for horizontal and vertical 9
sampling domain size as described by the
results of the airborne Doppler
questionnaire.
Table 1.4 Requirements for minimum radar sensitivity. 11
Table 1.5 A comparison of measurement accuracy require- 14
ments according to results of the questionnaire
(top) and review of the literature (bottom).
Table 1.6 Requirements for Aircraft Capabilities. 16
Table 2.1 Sensitivity calculations for X-, Ku-, and Ka- 32
band radars.
Table 2.2 Combined attenuation due to oxygen and water 35
vapor for X-, Ku-, and Ka-band radars
(Rosenblum, 1961).
Table 2.3 Attenuation by hydrometers. 36
Table 2.4 Unambiguous range and velocity calculations 38
for X-, Ku-, and Ka-band radars.
Table 2.5 Contribution to Doppler spectral width 40
caused by an aircraft flight velocity of
120 m/s.
Table 2.6 Time to Independence based on relations given 47
in Atlas (1964), p. 404.
Table 4.1. Design parameters used to specify data system. 97
Table 4.2 Data rate vs range gate and data density 98
requirements for total and reduced data
rates.
Table 4.3 Selected recording devices. 100
Table 5.1 Effects of ELDORA on the NCAR Electra. 114
Table 6.1 Torque budget for the ELDORA rotodome. 118
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viii
ABSTRACT
During the past decade the need for airborne Doppler radars for
meteorological research has become apparent. Airborne Doppler
weather radars can provide quick observation of large scale storm
systems and high resolution observations of small-scale weather
phenomena. First serious calls for airborne Doppler weather radars
were heard in the 1970's, and by 1982, a prototype airborne Doppler
radar was operating aboard the NOAA P-3 aircraft. Tests of that
system, jointly conducted by NCAR and NOAA proved that airborne
Doppler radar was indeed a valuable research tool, an observational
tool which could open new horizons for atmospheric scientists.
This Technical Note describes NCAR's plans for building a next
generation airborne Doppler radar system. This system builds upon
the excellent beginning of the NOAA P-3 radar. Major areas of
improvement will include radar sensitivity and measurement
accuracy, scanning technique, and data handling and display.
This dual-beam, X-band, rapid-scanning radar will be mounted on
the NCAR Electra aircraft, hence the name ELDORA for Electra
Doppler Radar. The radar will be a dual Doppler radar system
having two helically scanning antennas, one scanning 20° ahead of
normal to the flight track and the other scanning 20° aft of normal
to the flight track. This scanning technique will provide dual
Doppler air motion measurements, while flying in straight lines
past weather events of interest. In order to provide the needed
spatial sampling density for reflectivities and velocities, the
radar will be a rapid-scanning radar, which will scan at up to 144°
per second, about 5-10 times the scan rate of typical ground
weather radars. A complex transmitted waveform, plus over-sampling
in range will be used to obtain the necessary measurement
accuracies.
This Technical Note begins by describing the scientific needs for
ELDORA, including the polling of the community as to the need for
the radar and the necessary measurement capabilities for the radar.
The radar sampling design criteria are then discussed, including
frequency selection, radar sensitivity, scanning technique,
measurement accuracy and transmitted waveform. Separate sections
describe details of transmitter/receiver design, data system design
including the signal processor and aircraft modification design.
The development schedule for radar development is presented in the
last section of the report. Current plans for development of the
radar aim for completion of the radar during FY 1992, provided that
adequate funding levels can be maintained.
ix