The ARMAR data set resulted from atmospheric measurements collected during the intensive observation period of the Tropical Ocean Global Atmosphere-Coupled Ocean Atmosphere Response Experiment (TOGA-COARE). The ARMAR instrument was operational aboard the NASA DC-8 aircraft under the direction of Principal Investigator Fuk Li of NASA/Jet Propulsion Laboratory.
2.1 Instrument Mission and Objectives. ARMAR has been developed by NASA/JPL for the purpose of supporting future spaceborne rain radar systems, including the radar for the Tropical Rain Measuring Mission (TRMM) to be flown in the late 1990s. It flies on the NASA Ames DC-8 aircraft and is operated by JPL. Its primary goal in TOGA COARE was measurement of the three-dimensional reflectivity of rainfall.
2.2 Instrument Geometry. ARMAR operates with the TRMM frequency and geometry, measuring reflectivity at 13.8 GHz in a cross-track scan +/-20 degrees from nadir along the flight track of the aircraft. Nadir-looking, non-scanning measurements can also be acquired.
2.3 Principles of Operation. ARMAR is a pulse compression radar. The digital controller causes the chirp generator to produce chirps with the selected length, spacing, and start frequency. In normal operation, the waveform is a linear, frequency modulated upsweep chirp with 4 MHz bandwidth and amplitude weighting for range sidelobe suppression. The chirps are upconverted to 13.8 GHz and amplified by a high power traveling wave tube amplifier (TWTA). The amplified chirp is then sent to the antenna system. A small amount of power is sent directly to the receiver through a calibration loop with 84.5 dB attenuation. The TWTA is operated in the non-saturated mode to maintain the desired chirp amplitude characteristics. The antenna system consists of a dual, linearly polarized scalar feed horn which illuminates a precision offset parabolic reflector. The signal is focused by the parabolic reflector and reflected by a flat, mechanically scanned elliptical reflector which scans the beam +/-10 degrees in the crosstrack direction. The reflector can also be pointed or scanned up to +/-10 degrees in the along track direction, if desired. Both transmit and receive polarizations can be varied on a pulse to pulse basis, allowing a combination of like- and/or cross-polarization data to be collected. The signal reflected from the rain is collected by the antenna and then amplified by a low noise amplifier (LNA). Following the LNA, the received signal is downconverted to the 70 MHz intermediate frequency (IF). Here, the signal is split into radar and radiometer signals. The radar signal is passed through a programmable attenuator before IF ampliiers and filters. In the final stage, the signal is downconverted to baseband (offset video) where it is digitized by a 12-bit, analog/digital converter (ADC) at a rate of 10 MHz and recorded. The radiometer signal is acquired during a short time within each interpulse period after return from the transmitted pulse has reached zero. This signal is integrated in analog circuitry, sampled every 10 ms, averaged, and recorded.
3.1 General Characteristics. The ARMAR data has an approximate volume of 4 GB and a typical file size of up to 10 MB. The ARMAR file naming convent align=centerat "ddhhmm.ARM."
The ARMAR data set is level 1B (calibrated and earth located).
3.2 Data Format. The ARMAR files are being archived in their native binary format. Software is provided to convert the ARMAR data to NCAR's DORADE format. ARMAR files in DORADE format will contain single polarization, doppler data.
3.2.1 Description of ARMAR native format:
| Table 1 | |||
|---|---|---|---|
| Parameters held in the first 80 bytes following an "#A" | |||
| param | format | units | description |
| prf | 2-byte-int | Hz | radar pulse repetition frequency |
| dat_type | - | radar data type- see Table 2 | |
| spare0 | - | not used (was radar "gate" mode) | |
| no_av | - | no. radar pulses read from original data | |
| nbin | - | range sample interval (x15 = meters per pixel) | |
| dt | ęs x10 | range sample interval (x15 = meters per pixel) | |
| no_av1 | - | no. pulses actually averaged in pulse 1 power | |
| no_av2 | - | no. pulse 2 power (2nd polarization) | |
| spare1 | - | not used (was pulse "id") | |
| spare2 | - | not used (was "nskip") | |
| no_sumc1 | - | no. pulses actually averaged in doppler 1 | |
| no_sumc2 | - | no. doppler 2 (2nd polarization doppler) | |
| no_sumr | - | no. pulses accumulated in lag-2 correlation | |
| v_offset | m/s x 100 | offset measured & applied to doppler | |
| v_predict | offset otherwise predicted from INS | ||
| n_miss | - | no. radar pulses lost to data system errors | |
| az1 | 4-byte-float | deg | antenna azimuth at start of accumulation |
| az2 | antenna azimuth at end of accumulation | ||
| el | antenna elevation (aft is positive) | ||
| tb | K | radiometer brightness temperature | |
| time | 8-byte-double | s | UT seconds |
| r0 | 2-byte-int | m | range to first pixel below aircraft |
| npulse | - | no. pulses to end of scan (1 is last pulse) | |
| x | x 10000 | along track cartesian antenna vector | |
| y | cross-track component | ||
| z | zenith component | ||
| pol1 | - | polarization 1: 1=HH, 2=VV, 3=HV, 4=VH | |
| pol2 | - | polarization of pulse 2: " | |
| day | - | Julian flight day (1=Jan 1, 1994) | |
| rcm | - | radiometer calibration mode | |
| scanmode | - | ant. scan: 0=bowtie, 3=retrace, <3=fixed | |
| spare3 | - | not used | |
| spare4 | - | not used | |
| Table 2 | |
|---|---|
| Interpretation of "ahd.dat_type" from Table 1 | |
| dat_type | meaning |
| 1 | Single polarization, no doppler: read nbin samples of dBZ into array z1 |
| 2 | Dual polarization, no doppler: read nbin samples nbin samples into z2.into z1, then |
| 3 | Single polarization doppler: read nbin dBZ values into z1, nbin m/s values into velocity array v1, then nbin m/s samples into doppler width array w1. |
| 4 or 5 | Dual pol. doppler: read nbin samples each into z1, v1, w1, z2,v2, and w2. Polarizations are given in "ahd.p1" and "ahd.p2". Type 5 normally implies cross-pol data for z2. Type 4 normally implies HH/VV polarizations. |
| 8 | Single pol. noise floor: read nbin samples of mean noise into n1array, and nbin samples of noise variance into nv1. |
| 9 | Dual pol noise floor: read nbin samples each into n1, nv1, n2,. These pol's should be paired with subsequent type 2, 4, or 5 dual polarized radar data. |
| note: z1, z2, v1, v2, w1, w2, n1, n2, nv1, nv2 should all be allocated as 400 integer arrays. | |
3.3 Documentation. Two documents are available to assist users in reading and understanding ARMAR data. The "Quick Reference Guide," included with this distribution (ascii file, "quickref.txt"), and the more comprehensive "User's Guide", available in this directory in file "user.ps" (see Section 8.1). The Quick Reference Guide describes the JPL internal format, how to read it, and describes software that is available to assist the user. The User's Guide provides a description of the hardware and its operating modes; data collection, processing, and calibration; and data quality assessment.
3.4 Browse Products. Color Post-Script images of selected flight segments reside in directory armar/armar_images. Images show nadir reflectivity as a function of height along with brightness temperature as a function of cross-track position. Each image represents about 10 minutes of data. The file names follow the dddhhmm.ps convention indicating the day (ddd) and approximate start time (hhmm) of the plot. Images can be viewed using any post-script viewer (i.e.pageview,ghostview,etc.). No doppler or cross-polarized data are plotted.
3.5 Software. The DAAC distributes read and conversion software that was provided by the data producers at JPL:
The following table relates ER-2 and DC-8 flight numbers to the dates for the 13 mission flights of the NASA/TOGA COARE campaign. An ER-2 flight on February 7 is also included because it yielded data, although it has not been designated as a mission flight. The objectives (Obj) column is included for the convenience of the user; the mission objective defaulted to radiation (Rad) unless convection (Con) was forecast in the target area. The tar file names are of the format ddhhmm_ddhhmm where the values to the left and right of the underscore are, respectively, the Julian dates of the first and last data files in the tar file.
| Date(UTC) | ER-2 Flight | DC-8 Flight | ARMAR tar files | Obj |
|---|---|---|---|---|
| Jan 11-12 | 93-053 | 93-01-06 | 112345_120252.tar | Rad |
| Jan 17-18 | 93-054 | 93-01-07 | 172338_180443.tar | Con |
| Jan 18-19 | 93-055 | 93-01-08 | 190157_190849.tar | Con |
| Jan 25-26 | 93-056 | 93-01-09 | 260010_260546.tar | Rad |
| jan 31-Feb 1 | 93-057 | 93-01-10 | 312211_320518.tar | Rad |
| Feb 4 | 93-060 | 93-01-11 | 51443_351752.tar | Con |
| Feb 6 | 93-01-12 | 371437_371710.tar | Con | |
| 371712_371904.tar | ||||
| 371911_372126.tar | ||||
| Feb 7 | 93-061 | |||
| Feb 8-9 | 93-062 | 93-01-13 | 391826_392019.tar | Con |
| 392020_400005.tar | ||||
| Feb 10-11 | 93-063 | 93-01-14 | 411905_412232.tar | Con |
| 412248_420223.tar | ||||
| Feb 17-18 | 93-01-15 | 481903_490150.tar | Con | |
| Feb 20-21 | 93-065 | 93-01-16 | 511927_520209.tar | Con |
| Feb 22-23 | 93-066 | 93-01-17 | 532016_540036.tar | Con |
| Feb 23-24 | 93-067 | 93-01-18 | 542055_552359.tar | Rad |
Data quality is still being checked by the data producers by looking at a number of parameters. These include the calibration pulse power to check the system stability, the system noise flow and the measured cross section of the ocean, which can be compared with models. For Doppler mode data the standard deviation of ocean surface velocity measurements is being computed. All of these parameters are being tracked through the entire TOGA COARE data set. The goal for calibration accuracy is +/-1 dBZ for reflectivity and +/-0.5 m/s for velocity.
The Principal Investigator for the ARMAR instrument was:
Fuk K. LiQuestions about the data may be directed to:
Steven L. DurdenThe ARMAR data first became available on DAAC public FTP in July, 1994. See other sub-directories of the FTP directory "toga_coare" for other TOGA COARE data sets.
8.1 Instrument/Data Processing Documentation
User's Guide to ARMAR TOGA COARE DATA, Version 4.0, June 13, 1994, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.
Quick Reference Guide for ARMAR TOGA COARE Data Set, May 26, 1994, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109.
NASA/TOGA COARE Science Data Workshop II, Proceedings of a workshop held in Albuquerque, New Mexico, March 15-17, 1994, July 1994, FIRE Project Office, NASA Langley Research Center, Mail Stop 483, Hampton, VA 23666.
Mission Summary Reports, TOGA COARE, November 1993, FIRE Project Office, NASA Langley Research Center, Mail Stop 483, Hampton, VA 23666
8.2 Journal Articles and Study Reports
Durden, S., A. Tanner, W. Wilson, F. Li, E. Im, W. Ricketts, "The NASA/JPL airborne rain mapping radar (ARMAR)," Proc. 11th International Conference on Clouds and Precipitation, Montreal, August 1992, pp. 1013-1016.
Tanner, A., S.L. Durden, R. Denning, E. Im, F. K. Li, W. Ricketts, W. Wilson, "Pulse compression with very low sidelobes in an airborne rain mapping radar", IEEE Trans. Geosci. Remote Sensing, in press.
Science Data Workshop II Proceedings, Albuquerque, New Mexico, March 15-17, 1994
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