Operation Overview
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The complete radiation package is an integration of five primary components: (1) a broadband shortwave pyranometer (PSP), (2) a broadband infrared radiometer (PIR), (3) the fast-rotating shadowband radiometer (FRSR), (4) an attitude sensor that measures platform pitch, roll, and azimuth, (5) a small, low-power controllerlogger that performs the sweep-by-sweep processing algorithms, and (5) an uninterruptable power supply and backup battery module that eliminates power surges, minimizes power supply noise, and supports wind or solar power operation for remote operation. The fast-rotating shadowband radiometer (FRSR) (pdf 2MB jtech paper) accurately decomposes downward shortwave (solar) irradiance into direct beam and diffuse components from a moving platform such as a ship on the ocean. The FRSR has seven channels, one broad-band silicon detector and six ten-nm-wide channels at 415, 500, 610, 680, 870, and 940 nm. The shadowband technique produces estimates of the directbeam normal irradiance, the diffuse irradiance (sky component) and the total irradiance. The direct-beam normal irradiances produce time-series of aerosol optical thickness. |
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All measurements are synchronized to the rotation of the shadowband.
Broadband measurements and housekeeping measurements are
taken when the arm is at each horizon. During the sweep, when the shadowband crosses
the upper hemisphere, 250 measurements are made for each channel. The first and last
ten samples of each sweep are averaged and we refer to these measurements as the global
measurements, vG1 and vG2. Global measurements are computed for each filter channel.
During the lower half of a revolution, the heater is operated as necessary to maintain a
head temperature of 35±0.5°C.
At the end of each sweep of the shadowband, the channel-1 voltage measurements are examined and a decision made whether a viable shadow was present. The shadow ratio is a sensitive measure of the intensity of the direct solar beam and is computed by the equation |
| shadowratio = ( Vav - Vmin ) / Vsigma |
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where Vav is the mean signal voltage for the entire sweep,
Vmin is the minimum signal
voltage, and Vsigma is the standard deviation for the sweep. (Measurements±0.3 sec from
the minimum value are excluded from the computation of mean and standard deviation.)
It has been found that a criterion of shadowration >2.3 captures almost all true shadow cases and seldom permits a false positive. On sunny days the shadow ratio can exceed a value of 100.
Any sweep where shadowratio >= 2.3 is block averaged and stored in a compressed binary packet. Block averaging of the sweep retains all of its significant characteristics but significantly reduces data storage requirements. Block averaging begins at the minimum index value, imin, and moves left and right through the sweep array with increasing block sizes. Twenty-three contiguous block averages, b(i,j), where i is the channel number 1-7, and j is the bin number, are computed according to Table 1 below. The shadow index, imin, depends on the solar azimuth and zenith angles, the ship heading, and the pitch and roll, and thus can occur anywhere in the 250-point sweep array. In the block averaging process some bins fall outside the sweep and are given a "missing" value. |
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Binary Data Packet
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The compressed binary packet with global and sweep data for all detectors is transmitted
as an EIA422, 28800 bps, serial stream to the base computer. An EIA422 balancedline
transmission is highly immune to electronic noise and radio interference and can be
transmitted over long transmission lines. The binary packet has start and end character
strings and a Cyclic Redundancy Checksum (CRC) for error-free transmission. Once the
packets are transmitted, the shadowband cycle begins again.
The baud rate of 28800 bps is an odd choice and was selected as a compromise between the data acquisition software and the CDU. After the last measurement (P2 in the figure above) and the heater is checked, the blolck averaging for each channel is done and the packet is created. At that point the packet must be transmitted with enough time remaining that the CDU can detect the shadowpand nadir point. It is essential to detect the nadir point as this synchronizes the system for the next measurement cycle. A binary packet has a format of |
<<nnn,Hbbbbbbb...bbbbbbbbbbb,cc>>
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where << and >> mark the start and end of the binary packet,
"nnn," is the number of binary characters after the mode character,
"H" is operation mode, high (daytime) or low (night),
"bbb...bbb" are the nnn binary characters in the packet, and
"cc" is the CRC packet chacksum.
The data acquisition program (PRPRX.EXE) takes all data from the PRP as ASCII
data strings until it sees the << key. It then switches to a binary mode
and collects all
characters in memory until it detects the >> end key. The checksum is computed and
compared to the transmitted checksum.
If the checksums agree, the packet is processed as a legitimate data packet.
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| Variable | Bytes | Range | Resolution |
|---|---|---|---|
| Time | 4 | 1970 to present | 1 sec |
| Head Temperature 1 | 2 | -20 to 100 °C | 0.01 °C |
| Pitch 1 | 2 | -90 to 90 ° | 0.01 ° |
| Roll 1 | 2 | -90 to 90 ° | 0.01 ° |
| Azimuth 1 | 2 | 0 to 360 ° | 0.01 ° |
| Vpsp 1 | 3 | -500 to 500 mV | 0.01 mV |
| Vpir 1 | 3 | -500 to 500 mV | 0.01 mV |
| Vcase 1 | 3 | -500 to 500 mV | 0.01 mV |
| Vdome 1 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan4 1 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan5 1 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan6 1 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan7 1 | 3 | -500 to 500 mV | 0.01 mV |
| Pitch 2 | 2 | -90 to 90 ° | 0.01 ° |
| Roll 2 | 2 | -90 to 90 ° | 0.01 ° |
| Azimuth 2 | 2 | 0 to 360 ° | 0.01 ° |
| Vpsp 2 | 3 | -500 to 500 mV | 0.01 mV |
| Vpir 2 | 3 | -500 to 500 mV | 0.01 mV |
| Vcase 2 | 3 | -500 to 500 mV | 0.01 mV |
| Vdome 2 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan4 2 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan5 2 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan6 2 | 3 | -500 to 500 mV | 0.01 mV |
| Vchan7 2 | 3 | -500 to 500 mV | 0.01 mV |
| HIGH MODE ONLY | |||
| Shadow ratio | 2 | 0 to 6553 | 0.01 |
| Shadow ratio limit | 2 | n.a. | 0.01 |
| Global 1, Horizon 1, chan 0-6 | 14 | 0 to 4095 mV | 1 mV |
| Global 2, Horizon 2, chan 0-6 | 14 | 0 to 4095 mV | 1 mV |
| IF SHADOW ELSE SET TO MISSING (0) | |||
| Bins 0-22, chan 0, 2Bytes/bin | 322 | 0 to 4095 mV | 1 mV |
| Bins 0-22, chan 1, 2Bytes/bin | 322 | 0 to 4095 mV | 1 mV |
| Bins 0-22, chan 2, 2Bytes/bin | 322 | 0 to 4095 mV | 1 mV |
| Bins 0-22, chan 3, 2Bytes/bin | 322 | 0 to 4095 mV | 1 mV |
| Bins 0-22, chan 4, 2Bytes/bin | 322 | 0 to 4095 mV | 1 mV |
| Bins 0-22, chan 5, 2Bytes/bin | 322 | 0 to 4095 mV | 1 mV |
| Bins 0-22, chan 6, 2Bytes/bin | 322 | 0 to 4095 mV | 1 mV |
PRPRX and the Raw Data Files
The data acquisition program is called PRPRX.EXE and it is well described in the PRP Operations
manual (see link at the top of this page). PRPRX monitors the serial data stream
from the PRP and performs the following functions:
A set of eight raw data files are written to the hard drive each rotation of the shadowband. The raw data files are raw data as collected by the PRP without any calibration data applied. The radiation and case/dome temperatures are in millivilts, directly from the 12-bit or 18-bit ADC circuits. The file names are as follows
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The DA0 Raw Global Data File
| The raw data file (.DA0) contains all the data that was taken at the two shadowband horizons (see the figure above). It has no header. | 
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The different fields in each record (line) are as follows:
N is the record count beginning at record 1 at the start of the day.
SH is the shadow ratio,
SHL is the shadow ratio limit for a usable shadow. This is usually set to 2.3 but is settable.
YYYY MM DD HH MM SS is the time that the packet was received. This is the time of the data acquisition PC. This is the primary time base. Thus, it is essential that the data acquisition PC is
set properly to GMT. If the PC is not synchronized to a www time server then the operator must manually set the PC clock so it is accurate to GMT within a few seconds.
THEAD is the internal temperature of the FRSR head.
PITCH, ROLL, AZ are the pitch, roll, and azimuth measured by the PNI flux-gate compass.
The subscripts 1 and 2 designate measurements made at the first and second shadowband horizons.
All eight channels of the 18-bit analog-to-digital (ADC) Adam converter are given for each shadowband horizon. The output of the ADC is in millivolts over the range -500 to 500 mV. PSP, PIRU, TCASE, TDOME are the measurements of the signal from the Eppley PSP, the thermopile of the PIR, and the case and dome temperature thermistors. As before the numbers 1 and 2 designate measurements made on the first and second shadowband horizons. (Since a full rotation of the shadowband takes approximately six seconds, the radiometer measurements are spaced about three seconds.) VREF is the reference voltage for the 12-bit ADC that reads the FRSR head channels. (Vref12 = V6 * ). VBATT is a measure of the input battery voltage (Vbatt = V7 * .0308 volts). Finally, TT8 TIME is the date and time from the CDU's TT8 processor. The time here is based on the seconds since the initialization of the CDU program and begins with Jan 1, 2000. This clock can be set using the PRP menu but it is best to leave it alone. It is useful to cross check against the time kept by the data acquisition PC. |
The DAx Raw Sweep Data File
| The raw sweep data files, one per channel, are written with lines of the following type: | 
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Above is an example of six lines from of data from a good PRP operating at midday
in a sunny situation. The time of the records advances by about 6 sec per sweep.
The appearance of this file should be about the same for all of the FRSR sweep files (da1, da2, da3, da4, da5, da6, and da7)
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Post-processing -- Data file organization
| Before beginning the data processing software, the following data files and folders must be set up. (The forward slash designates a folder as opposed to a file.) |
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The folder cruiseID/ is the main data folder for the cruise. The cruise ID is a short arbitraty unique designator for the particular data set. It is used often in the processing so should be as short as possible. An example might be "RB0604" signifying the R/V Ron Brown cruise in April of 2006. The file INFO_cruiseID.txt describes the cruise and the configuration of instruments that were used in this cruise segment. The file prprx_cruiseID_calID.txt is produced as part of the calibration process. The file INFO_cruiseID.txt is highly essential to the data processing as it holds all the configuration information related to the cruise. An example of an INFO file is given below. The folder level0/ holds all of the original raw data files. The level0 folder can be quite large and thus can be located in a different location from the rest of the cruise folders. It is iincluded here for completeness. The folder nav/ holds the navigation data. The name is arbitrary. The folder prp/ holds the raw PRP text files described below (DA0, DA1,...DA7. The folder level1/ holds all of the level 1 data products. Level 1 data products are derived from level 0 raw data after calibration and other corrections are made to convert the quantities to physical units. The file named nav.tx0 is discussed in detail here. The folder level2/ holds final data products and all derived data products such as AOD, daily insolation, etc. Finally, the folder sw/ contains all of the software used for processing the data. The PERL and MATLAB scripts here are drivers to the general programs in the main software folder. |
Hardware and Software Requirements
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Computer
PERL MATLAB HTML BROWSER |
The Setup File
| Example Setup File |
Post processing: Program Sequence
| a01_CheckNav.pl | Described below. Checks navigation file, as described below. | level1/nav.txt |
| a02_MakeInfo.pl | creates the metadata file as described. It is important to use a program to create the INFO file because programs to follow use data here and look for key strings. | INFO_cruise.txt |
| a03_da0_avg.pl | Reads the raw PRP DA0 files and makes 2-min averages as described below. Integrates nav.txt clear-sky estimates. | level1/da0_av120.dat |
| a04_sweep_avg.pl | For each filter channel 1-7 compute the 2-min mean for each of the sweep bins. In a separate file compute the Edge, Shadow, and Global irradiance then use these to compute the normal and diffuse irradiance. | daX_sweep_av120.dat daX_computed_av120.dat where X = 1,2,3,4,5,6,7. |
| a05_ComputeAOD.pl | Computes the Aerosol Optical Depth (AOD). Channel 1 is the broadband channel and we do not compute an AOD for that channel. | level2/aodX.txt where X = 2-7. |
| m01_nav2mat_mirai.m | Matlab routine that reads the file nav.txt and converts it to a Matlab binary file. | level1/nav.mat |
| m02_track_plot.m | Produces a Matlab track plot of the ship during the experiment. Note, this routine requires the Matlab package called "Crusty". | images/track_cruise.pdf images/track_cruise.png |
| m03_level1_da0_dat2mat.m | Matlab converts ASCII DA0 2-min average file to mat binary. | level1/da0_av120.mat |
| m04_RadSummary_level1.m | Produces a Matlab plot of 2-min and daily mean values of radiation and motion. | images/radsummary_level1_cruise.pdf images/radsummary_level1_cruise.png |
| m05_aod_dat2mat.m | Convert ASCII AOD files to Matlab binary. | aodX.mat where X = 2-7 |
| m06_aod_summary.m | Make a plot of AODs for the entire cruise. | |
| m07_DaySummary.m | cc | |
| a06_MakeWebPage_mirai.pl | cc |
Navigation 2-Min File (nav.tx0)
| The ship raw navigation data must be processed to produce a 2-min average ascii data file named "nav.tx0" in the level1 folder. A description of the 2-min navigation file and algorithms for its creation are given here. |
NAV.TXT FILE
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a01_CheckNav.pl
The program a01_CheckNav.pl converts the nav.tx0 file as described above and creates the nav.txt file shown above. Consecutive GPS position data are used to compute the velocity of the ship, course over ground, COG, and speed over ground, SOG. The SOG is compared to a speed threshold, in this case 40 m/s. In the even of a bad position the speed almost always exceeds the threshold. When this occurs the bad position line is discarded and consecutive navigation lines are read until the computed SOG falls below the threshold. This technique is very effective at removing bad position data.
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Program a01_CheckNav.pl
| Before beginning the PRP data processing we need to review the navigation file CheckNav.pl to sure it is well behaved. It is possible for GPS data appear good but to be in error. For instance, the MIRAI SOJ data records will have the GPS fields marked valid, but will have a value to latitude or longitude of 0.0000 degrees. Since zero is a valid value, the number will be accepted. |
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The figure to the left shows ship speed as computed from consecutive 2-min averaged GPS positions.
The equations for this computation is given by the equations below.
The figure indicates that the ship SOG is at times quite noisy. The typical steaming speed is around 15 kts which is reasonable. |
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During the cruise the ship made many manuevers often holding position then steaming between stations. Typically, the maximum speed of the ship is around 25 kts. However, there are many cases where computed SOG far exceeds that speed. It seems that random noise in the GPS data cause these jumps. Note, that all of the GPS data have quality indicators such as number of satellites, signal strength, and reasonableness. Nevertheless, random jumps sneak past the quality filters.
The program a01_CheckNav.pl reads the 2-minute data in the nav file record by record and computes the distance, in latitude degrees, between consecutive records. A ship travelling 24 kts (double the ships top speed) will cover 0.0133° in two minutes. We look for jumps the ship position that exceed this amount. When a jump is detected, the new record is skipped and consecutive records are checked until computed SOG between a record and the last good position are less than the threshold value. We use a threshold value of 20 kts.
Running the program-- |
Program a02_MakeInfo.pl
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The INFO file is necessary before any more processing can take place.
The INFO file contains all key information related to the cruise and the data processing.
The INFO file uses the "nav.txt" to define the geography related to the cruise, and so it is essential that the navigation data are all as complete as possible. Therefore, the program
a01_CheckNav.pl
is the first process and a02_MakeInfo.pl follows.
A typical INFO file is as follows:
INFO_mr00-4-1 And the program a02_MakeInfo.pl is used to ensure all INFO files are formated correctly and that all the needed information is available for the processing programs. |
Program a03_da0_avg.pl
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The program a03_da0_avg.pl produces calibrated two-minute averages
of the global hemispheric radiation as well as all of the engineering information related to the
PRP.
CALLING: CD to the folder with the program and enter
An example of a call to the program is The averaged data are written to a file located in .../cruiseID/level1/da0_av120.dat. An example of the header and a few data lines from an averaged file is shown below:
Program: a03_da0_avg.pl Version: 1.03 (mirai) EditDate: 2006-05-23 |