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APR-2 Dual-Frequency Airborne Radar Observations, Wakasa Bay

Summary

In January and February 2003, the Airborne Second Generation Precipitation Radar (APR-2) collected data in the Wakasa Bay AMSR-E validation campaign over the sea of Japan on board a NASA P-3 aircraft. Data were collected on all P-3 flights that encountered precipitation. APR-2 collected data at 13.405 and 35.605 GHz in a downward-looking, cross-track scanning geometry. The experiment was designed to validate the AMSR-E shallow rain and snow retrievals, and to extend the precipitation database needed to implement a physical validation strategy.

The Level 1 data product consists of the calibrated reflectivity at both 13.405 and 35.605 GHz, as well as the Doppler and linear depolarization ratio (LDR) at 13.405 GHz. Data are available via FTP in Hierarchical Data Format (HDF) and JPEG browse images.

The Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E) is a mission instrument launched aboard NASA's Aqua Satellite on 4 May 2002. AMSR-E validation studies linked to rainfall experiments are designed to evaluate the accuracy of AMSR-E precipitation data.

Citing These Data

Im, E. 2004. APR-2 Dual-frequency Airborne Radar Observations, Wakasa Bay. [indicate subset used]. Boulder, Colorado USA: NASA DAAC at the National Snow and Ice Data Center.

Overview Table

Category Description
Data format Hierarchical Data Format (HDF) and browse images in JPEG format
Spatial coverage 30°N to 45°N, 130°E to 150°E
Temporal coverage 14 January to 03 February 2003
Tools for accessing data HDF is a multi-object file format developed at the National Center for Supercomputing Applications (NCSA) at the University of Illinois. HDF software and libraries may be accessed from NCSA. For Matlab users, a sample Matlab routine ("PR2_HDFv2read.m") is available with the data for reading APR-2 HDF data.
File naming convention Binary and browse image files share file names but have different extensions. The convention is:
APR2.YYMMDD.UTC_time.version#
The binary file extension is .HDF and the browse image extension is .jpg.
For example, the following files contain data for the same flight pass:
APR2.030114.084057.2.HDF
APR2.030114.084057.2.HDF.jpg
File size Binary files range from 307 KB to 68 MB. Browse image files range from 112 KB to 250 KB.
Parameter(s) Reflectivity at 13.405 and 35.605 GHz and Doppler and LDR at 13.405 GHz.
Procedures for obtaining data Data are available via FTP.

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigator(s) Name and Title

Im, Eastwood
Principal Investigator
Jet Propulsion Laboratory (JPL)
Pasadena, California, USA

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Acknowledgements

Dr. Steve Durden, JPL.
Jet Propulsion Laboratory (JPL)
Pasadena, California, USA

2. Detailed Data Description

Format

Data are provided in Hierarchical Data Format (HDF) and browse images in JPEG format.

The table below describes objects within the APR-2 HDF data files, which are described in more detail after the table. Three-dimensional objects such as "look_vector" and "zhh14" are interleaved by line.

Name Format Size Notes
fileheader int32 18  
scantime int32 nscan x nray Beginning of scan in seconds since 1 January 1970
scantimus int32 nscan x nray Beginning of scan; microseconds past scantime
lat float nscan x nray From P-3 navigation files
lon float nscan x nray From P-3 navigation files
roll float nscan x nray From P-3 navigation files
pitch float nscan x nray From P-3 navigation files
drift float nscan x nray From P-3 navigation files
alt_nav float nscan x nray From P-3 navigation files (meters)
alt_radar float nscan x nray From APR-2 surface echo (meters)
look_vector double nscan x nray x 3 From P-3 navigation files
look_vector_radar double nscan x nray x 3 From APR-2 surface echo
range0 float nscan x nray Distance of the first radar range bin from aircraft
isurf int32 nscan x nray Index of radar range bin intersecting surface (starting from 0).
sequence int32 nscan x nray Ray number within the file
v_surfdc8 float nscan x nray Apparent surface Doppler velocity as estimated from P-3 navigation
v_surf float nscan x nray APR-2 measured surface Doppler velocity
beamnum float nscan x nray Ray number within a scan
surface_index float nscan x nray Preliminary surface classification index
zhh14 int16 nscan x nray x nbin Radar Reflectivity at Ku band (scaled dBZ) (scaling factor is given in file header)
zhh35 int16 nscan x nray x nbin Radar Reflectivity at Ka band (scaled dBZ) (scaling factor is given in fileheader)
ldr14 int16 nscan x nray x nbin Linear Depolarization Ratio at Ku band (scaled dB) (scaling factor is given in fileheader)
vel14 int16 nscan x nray x nbin Doppler Velocity at Ku band (scaled m/s) (scaling factor is given in fileheader)

Within the table:

The fileheader data are stored as Vdata. The remaining items are Scientific Data Sets (SDSs). Missing data are replaced by -9999.

Altitude and look vector (the 3 components of the antenna relative to a global coordinate system where x is the aircraft ground track and z is vertical) are provided in two estimates. The alt_nav and look_vector items are calculated based on the aircraft navigation information, but alt_radar and look_vector_radar are calculated based on the observed surface return in APR-2 data. The latter pair is reliable only when flying over the ocean, as it provides a more accurate geolocation than the navigation-based pair.

The predicted and observed surface Doppler velocities are also provided. The Doppler velocities were corrected for occasional aliasing and were used to correct the Doppler measurements of precipitation for the bias introduced by the aircraft motion.

The surface_index is estimated by analyzing APR-2 surface return (roughness, angle dependence of the surface normalized radar cross section, apparent surface inclination, and LDR at nadir). It assumes one of six values (this classification is preliminary, see Error Sources for known issues):

The fileheader contains information about the APR-2 data. These are parameters that are constant over the entire file. The next table describes the file header structure. All file header items are expressed as 4 byte integers.

1 PRF Pulse repetition frequency in Hz
2 Pulse LengthRadar pulse length in 1 us units
3 Antenna LeftAntenna scan left-limit in degrees
4 Antenna RightAntenna scan right-limit in degrees
5 Scan DurationScan time for antenna in second * 100
6 Return DurationAntenna retrace time in second * 100
7 Ncycle Number of pulse averaged by Wildstar board
8 AZ Average Number of blocks averaged in a beam or ray
9 Range averageNumber of 30 m range cells averaged in a bin
10 Scan averageNumber of scans averaged
11 Number of BinsNumber of range bins in the ray
12 Number of BeamsNumber of rays in each scan
13 Range Bin SizeThe vertical resolution of range bin
14 Z scale factorFactor multiplying reflectivity
15 V scale factor Factor multiplying Doppler
16 Valid Ka scan beginScan number where the valid Ka data begin
17 Valid Ka scan endScan number where the valid Ka data end
18 CalVersion Version number of the calibration table

In the course of processing, browse images were also created. These are saved in JPEG format. They show the beam #12 (pointing downwards in the aircraft reference) of APR-2 data, versus along-track time (in minutes), providing a downward-looking slice of the 3-D data set.

File and Directory Structure

There are tar files containing data for each date and a "browse" directory for the browse images.

File Naming Convention

Binary and browse image files share file names but have different extensions. The convention is:

APR2.YYMMDD.Time(UTC).version#

The binary file extension is .HDF and the browse image extension is .jpg.

For example, the following files contain data for the same flight pass:

APR2.030114.084057.2.HDF

APR2.030114.084057.2.HDF.jpg

File Size

Binary files range from 307 KB to 68 MB. Browse image files range from 112 KB to 250 KB.

Spatial Coverage

Southernmost Latitude: 30° N
Northernmost Latitude: 45° N
Westernmost Longitude: 130° E
Easternmost Longitude: 150° E

Temporal Coverage

Data were collected from 14 January to 03 February 2003.

Parameter or Variable

Parameters are reflectivity at 13.405 and 35.605 GHz and Doppler and LDR at 13.405 GHz.

Sample Data Record

The example browse image below is from the file "APR2.030119.043716.2.HDF.jpg." The browse images show the along-track section provided by ray #12. Vertical axis is altitude (km), and horizontal axis is time (minutes). Reflectivity values are expressed in dBZ, Doppler velocity in m/s, Linear Depolarization Ratio in dB. The value of the surface index is represented by colored dots at 0 m altitude (0 = black, 1=blue, 2=cyan, 3 = green, 4=yellow, 5=red). The four plots at the bottom visualize the navigation data. They should always be checked before interpreting the radar data.

APR-2 browse image

Error Sources

This section lists all known problems with the APR-2 version 2 data. Some of these problems are caused by problems in the raw data, while others are processing problems.

Quality Assessment

APR-2 data quality has been assessed by examining a number of engineering parameters related to the radar's stability and calibration. The observed minimum detectable reflectivity (Z) for APR-2 at both frequencies was derived from clear-air observations of the radar return signal and of the receiver noise floor. (In the Wakasa Bay experiment, no pulse was transmitted in ray #1 of each scan to measure receiver noise.) The values for both Ku-band and Ka-band are below 5 dBZ at 10 km range from the radar. Due to system non-linearities, the effective minumum detectable reflectivity was approximately 5 dBZ at 6 km range. The surface return, along with pulse compression sidelobes, can be seen at approximately 6 km range. The pulse compression sidelobes, rather than thermal noise, limit performance near the surface. Achieving such low pulse compression sidelobes required careful design of the transmit waveform and control of gain and phase errors.

Radar calibration can be verified using observations of the ocean surface. This technique has been used previously, since the ocean backscatter near nadir is well known, especially near 10 degrees incidence, where sensitivity to wind speed is a minimum. Ocean backscatter at Ka-band is not as well characterized, although models show similar behavior to the Ku-band. At Ka-band, the reflectivity in very light rain should be nearly identical to that at Ku-band, since Rayleigh scattering should apply at both frequencies.

Observations of the ocean surface with APR-2 show a cross section near 7 dB, which is close to previous measurements. Ocean backscatter comparisons with surface reflectivities calculated with Geophysical Model Function (GMF) or from TRMM/PR measurements indicate a bias of less ~0.5 dBZ, but strong winds and clouds undetected by APR-2 are possible contributors for this bias at Ku band. In-depth analysis is required to further refine calibration. The Ka-band data have reflectivities within about 1 dB of the Ku-band reflectivities in light rain. Surface Doppler measurements can be compared with Doppler calculated from the P-3 navigation parameters and the APR-2 antenna pointing. Such a comparison indicates the bias between the observed and calculated Doppler is very small.

3. Data Access and Tools

Data Access

Data are available via FTP.

Volume

Total volume is 3.5 GB, but the tar files are between 7.7 and 195 MG. If the tar files are too large for you to download, please contact NSIDC's User Services to provide the data on CD- or DVD-ROM or on some other media.

NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org

Software and Tools

HDF is a multi-object file format developed at the National Center for Supercomputing Applications (NCSA) at the University of Illinois. HDF software and libraries may be accessed from NCSA.

For Matlab users, a sample Matlab routine ("PR2_HDFv2read.m") is available with the data for reading APR-2 HDF data.

Related Data Collections

4. Data Acquisition and Processing

Theory of Measurements

APR-2 operated on eleven out of the twelve atmospheric science flights of the NASA P-3 aircraft during the Wakasa Bay Experiment (WBE). It did not operate on flight number 6, a clear-air flight, or on the sea-ice flights into Russian air space. Parameters under operator control were set to the same values throughout the experiment, with the exception of the receive window attenuation, which varies with surface brightness. The pulse length was always set to 10 microseconds and the PRF to 5000 Hz. The number of pulses averaged in real time was 250, equivalent to about 60 independent pulses. The elevation angle of the antenna (along-track angle) was set during flight to maintain a near-zero Doppler from the surface, minimizing platform motion contamination to the measured Doppler from precipitation. The platform motion was estimated from the surface and subtracted during ground processing. The azimuth scan limits were about +/- 25 degrees.

For the flights on January 14th and 30th and February 1st, different radar bit-processor configurations were tested, so only a limited amount of the data from these flights have been processed for this release of the data.

The following table is a brief summary of the data collected during the WBE. A more extensive description of the observations, including cooperation with other aircraft and satellite overpasses, is listed on the Colorado State University Wakasa Bay site.

APR-2 Operation on Wakasa Bay P-3 Flights:

Flt. # Date Comments
1 1/14/03 APR-2 in Engineering Operations (testing and optimization of configuration) - one file of science data available in this release
2 1/15/03 Mainly snow over land
3 1/19/03 Rainfall over ocean - APR-2 turned off during low altitude lags
4 1/21/03 Mainly rainfall over ocean, some snow
5 1/23/03 Widespread rain and squall line over ocean
6 1/26/03 Clear air flight for PSR calibration, but APR-2 not turned on
7 1/27/03 Widespread rain over land
8 1/28/03 Scattered snow showers over land and ocean
9 1/29/03 Widespread snow over land and ocean
10 1/30/03 Snow showers over ocean - APR-2 in Engineering Operations after 0500 UTC
11 2/1/03 No precipitation - APR-2 mainly in Engineering Operations, no files of science data available in this release
12 2/3/03 Rainfall with varying freezing level

Sensor or Instrument Description

The Precipitation Radar (PR) aboard the Tropical Rainfall Measuring Mission (TRMM) satellite launched in 1997 was the first-ever spaceborne radar dedicated to three-dimensional, global precipitation measurements over the tropics and the subtropics.

A second generation, dual-frequency precipitation radar (PR-2) was designed to include digital, real-time pulse compression, extremely compact radio frequency (RF) electronics, and a large deployable dual-frequency cylindrical parabolic antenna subsystem. The antenna is fed by a linear active array for electronic beam scanning.

To demonstrate many of the key PR-2 technologies and designs, an airborne version called APR-2 was developed. The cylindrical reflector antenna and linear feed array for the spaceborne PR-2 have been replaced by traveling wave tube amplifiers (TWTAs), front-end electronics, and an offset parabolic reflector antenna with mechanical scanning. The APR-2 operational geometry is shown in the figure below; it looks downward and scans its beam across-track, with each scan beginning at 25 degrees to the left of nadir and ending at 25 degrees to the right.

Diagram of aircraft and sensor operation

APR-2 consists of a 0.4 m offset reflector antenna with a mechanically scanned flat plate. For PR-2 the 13.8-GHz antenna feed has been replaced by a dual-frequency feed (13.4 and 35.6 GHz) and the aperture at 35.6 GHz is under-illuminated to provide matched beams at the two frequencies. This choice results in poor Doppler accuracy at Ka-band, but is needed for rain retrieval.

The RF circuitry can be divided into two categories: circuits operating at frequencies of less than 1.5 GHz and circuits operating at frequencies above 1.5 GHz. The lower-frequency circuitry is contained in a single unit, the local oscillator/intermediate frequency (LO/IF) module. This unit converts transmitted chirp signals from 15 MHz up to 1405 MHz and down-converts received IF signals from 1405 MHz to 5 MHz. The unit contains both upconversion channels and all four receive channels and fits into the equivalent of a double-wide 6U-VME card.

The RF front-end electronics are unique to the APR-2 design and consist of five units: one local oscillator/up converter (LO/U) unit, two TWTAs and two waveguide front end (WGFE) units. In the NASA P-3 installation, the two TWTAs are stacked vertically in a standard rack with the LO/U in between. The two WGFEs are mounted on top of the antenna pressure box, near the antenna feed. A calibration loop is included for each channel. This feeds some of the transmit power to the receiver, allowing in-flight variations of the transmit power and receiver gain to be monitored and removed from the data.

The digital electronics consist of a control and timing unit (CTU), an arbitrary waveform generator (AWG), and a data processor. The CTU generates the pulse timing and all other timing signals. It also provides control signals to RF. The AWG is loaded with a digital version of the linear FM chirp to be transmitted. The data processor is based on Field-Programmable Gate Array (FPGA) technology. It performs pulse compression and averaging in real time. The pulse compression scheme in APR-2 is based on real-time filtering in the time domain. The 4 MHz bandwidth received signals are sampled at 20 MHz, then digitally downconverted to complex samples, resulting in I and Q samples at 5 MHz rate. The data processor also includes pulse-pair Doppler processing. The output of the processor is the lag-0 (power) and lag-1 (complex Doppler data) for the co- and cross-polarized channels at each frequency. A Virtual Machine Environment (VME) workstation runs the radar, including ingesting and saving the processed data. Following calibration on the ground, the APR-2 data are stored in a binary HDF format similar to that for the TRMM PR.

The horizontal resolution for the two channels depends on both the particular Antenna Beamwidth and the aircraft altitude, with the latter changing frequently. The horizontal resolution D can be calculated as follows:

D=2[h·tan-13dB/2]

where h is the aircraft altitude and θ3dB is the 3dB antenna beamwidth.

The following table describes the APR-2 parameters:

Frequency 13.4 GHz 35.6 GHz
Polarization HH, HV HH, HV
Antenna diameter 0.4 m 0.14 m
Beamwidth 3.8 deg 4.8 deg
Antenna gain 34 dBi 33 dBi
Antenna sidelobe -30 dB -30 dB
Polarization isolation -25 dB -25 dB
Peak power 200 W 100 W
Bandwidth 4 MHz 4 MHz
Pulse width 10-40 ms 10-40 ms
PRF 5 kHz 5 kHz
6 dB Pulse Width 60 m 60 m
Range Bin spacing 30 m 30 m
Horizontal Resolution at 6 km Altitude 400 m 500 m
Ground Swath at 6 km Altitude 4.5 km 4.5 km
Noise-equivalent Ze (10 km range) 5 dBZ 5 dBZ
Doppler precision 0.4 m/s >1 m/s

Processing Steps

The raw APR-2 data are saved in a unique APR-2 format. These data are run through a processor that calibrates the data to reflectivity Z, LDR, and velocity. A second processor uses these data and the aircraft navigation data to create a geolocated Level 1B product. This product is saved in a Hierarchical Data Format (HDF) format similar to the TRMM Precipitation Radar.

5. References and Related Publications

Next Generation Precipitation Radar

Kummerow, C., W. Barnes, T. Kozu, J. Shiue, and J. Shiue. 1998. "The Tropical Rainfall Measuring Mission (TRMM) sensor package." J. Atmos. Oceanic Technol. 15:3 809-817.

Im, E. Durden, S.L. Haddad, Z.S. Sadowy, G. Berkun, A. Huang, J. Lou, M. Lopez, B.C. Rahmat-Samii, Y. Rengarajan, S. 2000. "Second-Generation Spaceborne Precipitation Radar," Geoscience and Remote Sensing Symposium, 2000. Proceedings. IGARSS 2000. IEEE 2000 International. vol. 3. pp. 1361-1363.

Durden, S. L., E. Im, F. K. Li, W. Ricketts, A. Tanner, and W. Wilson. 1994. "ARMAR: An airborne rain mapping radar," J. Atmos. Oceanic Technol. 11:3 727-737.

Sadowy, G. A., A. C. Berkun, W. Chun, E. Im, and S. L. Durden. 2003. "Development of an advanced airborne precipitation radar," Microwave J. vol. 46, no. 1, pp. 84-98.

6. Document Information

Acronyms and Abbreviations

The following acronyms and abbreviations are used in this document.

APR2 Airborne 2nd Generation Precipitation Radar
ARMAR Airborne Rain Mapping Radar
AWG Arbitrary Waveform Generator
CTU Control and Timing Unit
FPGA Field-Programmable Gate Array
FTP File Transfer Protocol
GMF Geophysical Model Function
HDF Hierarchical Data Format
LDR Linear Depolarization Ratio
LO/IF Local Oscillator/Intermediate Frequency
LO/U

Local Oscillator/Up Converter

PR Precipitation Radar
PR-2 Dual-Frequency Precipitation Radar
PRF Pulse Repetition Frequency
SDS Scientific Data Set
TRMM Tropical Rainfall Measuring Mission
TWTA Traveling Wave Tube Amplifier
UTC Universal Time, Coordinated
VME Virtual Machine Environment
WBE Wakasa Bay Experiment
WGFE Waveguide Front End

Document Creation Date

February 2004

Document URL

http://nsidc.org/data/docs/daac/nsidc0195_rainfall_wakasa_apr2.gd.html