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Information provided here are extracted from JPL/ARMAR user's guideData Set OverviewThe ARMAR measurements were collected during the KWAJEX (Kwajalein) field experiment, which took place during the months of August and September, 1999 on Kwajalein Atoll, Republic of the Marshall Islands. The ARMAR instrument was flown aboard the NASA DC-8 aircraft under the direction of principal investigator Dr. Steve Durden of NASA/Jet Propulsion Laboratory. SponsorThe distribution of these data sets is funded by NASA's Earth Science Enterprise. The data are not copyrighted; however, we request that when you publish data or results using these data, please acknowledge as follows:The authors wish to thank Dr. Steve Durden, NASA/JPL, for the production of these data and the Data and Information Services Center(Code 610.2) at the Goddard Space Flight Center, Greenbelt, MD, 20771, which archives and distributes them under sponsorship of NASA's Earth Science Enterprise. The DataCharacteristicsThe ARMAR data set provided here is calibrated and earth
located. since the data status is preliminary, before using
this data in your publication, please contact the Principal
Investigator Dr. Steve Durden (sdurden@jpl.nasa.gov).
Known ProblemsIn using the data in some of our analyses we have found that the calibration for the last three files on day 218 (August 6, 1999) is in large error. For files 2180446, 2180502, and 2180505, the reflectivities are approximately 12 dB too low. 12 dB should be added to these reflectivities. The source of the problem is not clear. However, these data were acquired following a crash of the ARMAR control computer. It would appear that the receive attenuation setting was not recorded properly. The calibration signal power is unchanged from files prior to these times, suggesting that ARMAR was transmitting the same power before and after the computer problem. However, the apparent system noise dropped, suggesting a different receiver attenuation. Specifically, if the data were processed using a smaller receiver calibration than was actually set in the instrument would cause such a problem. At this point, the only solution is to add 12 dB to the reflectivities in these three files. This brings the ocean backscatter from about -5 dB to +7 dB, as would be expected. The FilesThe ARMAR data files are archived in their native binary format. The typical data file size is ~10 MB. Programs in c are provided to read the ARMAR data and to convert it to NCAR's DORADE format.File Naming ConventionThe ARMAR calibrated data files are archived only. These files are named as dddhhmm.arm , where ddd is the Julian day, hh is the hour, and mm is the minute. The file names reflect the UTC time of collection. The times of the file names are approximate; data times should always be taken from the #A header.Typical file sizes are around 10 megabytes, which corresponds roughly to 5 minutes of data. FormatsQuick-look/Browse ProductQuick-look (browse) ARMAR images have been produced for a few selected flight lines during KWAJEX. These gif files were produced during the experiment and provide annotation. Their names correspond to the year, month, day, and time (GMT) at the start of the image.calibrated dataThe format of the calibrated data is a sequence of headers. Each header begins with "#" followed by a capital letter unique to the particular type of header.#V header: The format is #V, followed by 156 bytes, providing the processing software version numbers. Hence each file begins with first two bytes as #V. #A header: a binary header (with first two bytes being #A) storing information for a single radar radial or ray,including antenna pointing, brightness temperature, radar data #C header :containing DC-8 DADS navigational and aircraft data . The C-I headers are saved in ASCII just as they are received from the DC-8 DADS system. Each line is terminated by a carriage return and line feed. Because we do not modify the format of the DADS data, we refer the reader to the DC-8 DADS group at NASA DFRC for information on the format of these headers. A full set of DADS headers are recorded about once per second, and each header can appear at random intervals with respect to the radar data (A) headers. An A-header begins with an 80 byte structure which describes characteristics of the data. This is followed by the radar data itself. The data contains several possible parameters: reflectivity in dBZ, velocity and spectrum width in m/s, HH-VV phase difference in degrees, and HH-VV correlation coefficient (unitless). These quantities were computed as floating point numbers and have been converted to 2 byte integers to save space. The floating point quantities were multiplied by 100 before converting to integers and can be recovered by converting the 2 byte integers to floating point and then dividing by 100. The exception is the HH-VV phase difference, which was mutliplied by 10 rather than 100. The data have been created on a Sun SPARCstation, which uses Big Endian ordering and IEEE format. The order in which the radar parameters are stored is as follows: all range bins of the first radar parameter (usually reflectivity), followed by all range bins of the second radar parameter, etc. A maximum of 400 range bins are possible. In Table 1 we list the contents of the A-header. Table 2 shows the radar parameters that are found in the A-header for each possible value of the data type parameter. As an example, suppose one has read the "#A" characters and then the 80 byte structure. Then from that structure one gets a data type of 3, and number of range bins of 310. From Table2, type 3 indicates that single polarization doppler data follows. So one must read, first, 310 words of dBZ values into an array, then 310 doppler velocity values, and then 310 values of doppler width. In all, this header occupied exactly (3x2x310+80+2=) 1942 bytes of mass storage (including the "#A" identifier). The A-headers are loosely organized by the cross-track scans of the antenna positioner. Preceding each antenna scan is always an A-header of data type 8 or 9 which contains the noise floor mean and standard deviation. This noise floor information is applicable to all radar pulses of the subsequent antenna scan. In general, radar data within 5 dBZ of the noise floor standard deviation should be considered unreliable. There are typically about 20 radar rays (or A-headers) across a single scan of the antenna. Each radar ray is typically an average of about 250 raw pulses (the actual value can be found in the A-header structure, as shown in Table 1). Each radial is numbered across the scan, with the last radial in the scan being 1 (see Table 1). The range from the aircraft to the first pixel of radar data is given in the A-header in meters. The range to subsequent pixels is computed as:
r_i = i * Delta t * 15 + r_0
where Delta t is the time between range bins as stored in the A-header in 100 nanosecond units. This variable normally has a value of 4, giving a pixel spacing of 60 m. The specific antenna pointing angle of a given pulse is contained in the 80 byte structure, as indicated in Table 1. The starting and ending azimuth angles and elevation angle are with respect to the nadir oriented antenna positioner. Negative azimuth is to the left, and positive elevation is aft of the aircraft. The two azimuths define the cross-track swath, or window, over which the 250 (or so) raw radar pulses were averaged. Note that these parameters do not account for aircraft roll, pitch, yaw, or positioner mounting offsets. As an alternative to the antenna angle, the Cartesian unit vector given in the A-header give the antenna vector as corrected for mount offsets and aircraft attitude. This vector is multiplied by 10000 to fit the integer format of the data storage. This vector places the antenna beam with respect to the aircraft ground track. This vector together with the heading, drift angle, latitude, longitude, and altitude as taken from the DADS headers, will locate a given pixel in Earth coordinates. The Following tables describe the 80-byte header #A .
SoftwareThe DISC distributes read and conversion software that was provided by the data producers at JPL:
In addition to read software, a number of other JPL documents are also available to assist users in reading and understanding ARMAR data. The JPL documents and text version of this DISC produced html document "README_tfb_armar.html" is also included in the distribution package. The DISC document is based on the JPL produced more comprehensive user guide "armar_users_guide.ps" and the document "quick.txt". These documents provide a description of the instrument, its operating modes; data collection, processing, and calibration; data quality assessment; and data format (special JPL internal format) and are available online at ftp://disc2.nascom.nasa.gov/data/KWAJ/aircraft/nasa_dc8/armar/doc/. Data Access and ContactsData AccessFTP SiteThe ARMAR data resides on DISC anonymous FTP. They are grouped by flight in directories labeled with the Julian days on which the flight occurred.Browse images and quick looks are available for most flights on the KWAJEX site at Ames Research Center. You may access the files from this document,
Points of ContactTechnical Inquiries about this Data should be addressed to the Principal Investigator for ARMAR,
For Information about KWAJEX data at the
Goddard DISC, contact
Hydrology Data Support Team
The ScienceARMAR has been developed by NASA/JPL for the purpose of supporting spaceborne rain radar systems, including the radar for the Tropical Rain Measuring Mission (TRMM). It flies on the NASA DFRC DC-8 aircraft and is operated by JPL. ARMAR was completed in late 1991, and the first airborne testing was performed in May of 1992. Additional tests were completed in December 1992, and the system was deployed during TOGA COARE in the western Pacific in early 1993. The ARMAR data described here were collected during KWAJEX in August-September 1999. Instrument CharacteristicsARMAR operates with the TRMM frequency and geometry, measuring reflectivity at 13.8 GHz in a cross-track scan. Nadir-looking, non-scanning measurements can also be acquired. Additional capabilities include dual-polarization, frequency diversity for increased independent samples, and Doppler (when frequency diversity is not used). While operating as a radar, a small fraction of time is spent measuring the 13.8 GHz brightness temperature in a radiometer mode at the same viewing geometry as the radar mode. ARMAR characteristics are shown in Table 3. More detailed descriptions are in Durden et al. (1994) and Tanner et al.(1994).
Principles of OperationARMAR is a pulse compression radar. The transmitted signal is a relatively long, frequency modulated signal, referred to as a chirp. The ARMAR digital controller causes the chirp generator to produce chirps with the selected length, spacing, and start frequency. In normal operation, the chirp is a linear frequency modulated upsweep with 4 MHz bandwidth and amplitude weighting for range sidelobe suppression. The chirp start frequency is 60 MHz. A sequence of up to seven different chirps can be transmitted, with each chirp possibly differing in polarization and other parameters. 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. 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 +/- 25deg in the crosstrack direction. The reflector can also be pointed or scanned up to +/- 10 deg in the along track direction, if desired. Scanning can be done either in a bow-tie mode in which the elevation angle is changed across the scan or in a retrace mode in which the antenna is scanned with constant elevation angle from left to right and then rapidly re-positioned to the left. 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 amplifiers and filters. In the final stage, the signal is downconverted to baseband (offset video, rather than I-Q) 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. At the end of each antenna scan, or every few seconds in non-scanning operation, the radar enters a calibration mode in which the radar signal passing through the calibration loop, as well as radiometer noise diode and reference load measurements, are recorded. The main control computer for ARMAR is a 486 Personal Computer (PC), which runs a Quick Basic program for overall system control. This computer serves as the operator interface, allowing the operator to start, stop, and re-configure the radar and display data. The radar configuration includes such parameters as transmit chirp length, chirp start frequencies, polarization(s), and antenna scanning parameters. A sequence of up to seven chirps with differing characteristics, such as IF and polarization, can be programmed. A VME 68000 computer is used for real-time control of the radar through a specially designed digital controller. The VME computer also controls the antenna scanner. The signal conditioner and formatter board in the PC receives the data from the 12-bit ADC and transmits it to a high speed Ampex DCRSi tape recorder. Auxiliary data, including radiometer voltage, system temperatures, radar configuration, timing information, and aircraft parameters from the DC-8 data system, are recorded along with the radar data. The digitized voltages are recorded in binary, while the auxiliary data are recorded in the form of ASCII header strings. Radar data and auxiliary data are recorded as received. The radar system is mounted in the cargo bay of the NASA DC-8 aircraft. The antenna beam is directed through an opening in the bottom of the DC-8 aircraft. A thin radome covers this observation port, and the entire antenna system is surrounded by a pressure box. The radar RF section is mounted on a plate which lays on top of the pressure box. This plate also includes IF, video, and ADC sections. A rack for the TWTA and other equipment is mounted in the cargo bay next to the pressure box. The system computer, tape recorder, and data processing system are mounted in a rack in the DC-8 cabin where the ARMAR operators sit. ReferencesDoviak, R. J. and D. S. Zrnic, Doppler Radar and Weather Observations. Academic Press, 103-107, 1984. Durden, S. L., E. Im, F. K. Li, W. Ricketts, A. Tanner, and W. Wilson " ARMAR: An airborne rain mapping radar," J. Atmos. Oceanic Tech., vol. 11, no. 3, pp. 727-737, 1994. Durden, S. L., A. Kitiyakara, E. Im, A. B. Tanner, Z. S. Haddad, F. K. Li, and W. J. Wilson," ARMAR observations of the melting layer during TOGA COARE," IEEE Trans. Geosci. Remote Sensing, vol.35,no.6, pp. 1453-1456, November 1997. 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, vol.32, no. 1, pp. 211-213, 1994. 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.
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ARMAR Calibrated Data