Specifications

Detailed Description

The ISS subsystems are integrated physically and digitally. A Sunworkstation is the heart of the digital integration. The Sun is thecenter of the ISS computer network and serves to collect, display, andarchive data from each of the subsystems. The Sun is connected topersonal computers in both the radiosonde sounding system and theprofiler/RASS sounding system via SAMBA. Data from the surfaceobserving station is routed serially via RS-232 directly into the Sunworkstation. The Sun can also format data and control data flow fortransmission to sites well removed from the ISS site.

Datatransmission from an ISS site can be via modem and phone line or viasatellite. In the TOGA COARE project, ISS data from island ISS sites inthe Pacific were transmitted to the GOES WEST satellite and ultimatelyrelayed to Boulder, Colorado and other project locations. WMO formatGTS messages, both sounding and surface, were transmitted in real timefrom the TOGA COARE ISS sites. These messages were transmitted via theGOES WEST satellite to forecast centers in real time and incorporatedinto the forecast models. (Those messages and others were alsotransmitted to Boulder and then relayed back to Townsville, Australia.)

The balloon borne radiosonde navaid (GPS) soundingsystem is the standard NCAR "GAUS" sounding system. This soundingsystem typically uses the Vaisala RS-80 GH which uses GPS satellitenavigation signals for wind finding. Launching configurations can varydepending on the installation. An ISS site may have a "bag" launcher,or possibly no launcher at all. The balloon and sonde are secured tostructures available before release in the latter case.

Theenhanced surface observing station consists of two instrumented towersand a rain gauge. A ten-meter tower is instrumented with wind velocitysensors as well as pressure, temperature, and humidity sensors. Aseparate one-meter tower is typically instrumented with radiometers.The data are formatted and processed by a Campbell CR10x datalogger.The datalogger is programmable. It is typically configured to generateone-minute average data which are sent via RS-232 to the Sunworkstation. Shipboard surface observing station installations requiresome type of shipboard navaid to correct for ship velocity in the windvelocity measurement.

The 915 MHz wind profiler deployedduring TOGA COARE was a three-beam Doppler beam swinging system. Thesystem is now being modified to a five-beam system. The profiler is along-wavelength Doppler radar which detects the backscattered signalsfrom turbulence-induced refractive index variations. The profiler thustracks the motion of the turbulent eddies which drift with the meanflow, providing a measurement of the mean wind velocity.

Radialwind velocities are obtained over a variable number of range gates(e.g. 25 to 40) using spectral moment and consensus averagingtechniques. Consensus averages can be calculated over varying periodsfor two vertical modes: one for lower altitude sampling with a highvertical resolution and one for higher altitude sampling with reducedvertical resolution. The raw data are recorded on site in real time toallow for alternative post-processing.

Any shipboard ISSprofiler installation requires special attention and adaptation. Themicrostrip profiler antenna needs to be mounted on a gyro stabilizedplatform. A three axis accelerometer may be required to measureoscillatory ship motions (backscattered signals are doppler shifted bythe motion of the ship) and a GPS receiver is used to determine thespeed and heading of the ship. Details of shipboard testing of thissystem can be found in Carter, et al. (1992).

The radioacoustic sounding system (RASS) utilizes the vertical beam of theprofiler to track a broad band acoustic frequency wave front producedby four speakers. (The radar actually tracks refractive indexperturbations induced by the acoustic wave traveling at the local speedof sound.) The broad band frequency is used to assure that the Braggcriterion is met. A vertical virtual temperature profile is obtainedfrom the tracking of this wave front (acoustic shell) from therelationship between air density and the propagation velocity (speed ofsound) of that wave front.

Vertical virtual temperatureprofiles with 100 meter resolution can be obtained periodically by thetracking of the generated acoustic signals. A correction can be appliedin the real time processing to remove the vertical wind motion from themeasured speed of the acoustic shell. Note that the RASS temperatureprofiles obtained in real time will be drastically affected byhydrometeors. The return from the hydrometeors will invalidate themeasurement of vertical wind and the resultant correction will giveerroneous profiles. In some cases where there are hydrometeors present,it may be possible to obtain a reliable shell propagation speed inpost-processing by using rainfall rate measurements to estimate andremove the hydrometeor fall speeds which would otherwise contaminatethe vertical wind measurement.

The ISS sites are housedin a standard 20-foot sea container modified to serve as an equipmentshelter and laboratory for project scientists and engineers. Themodified sea container houses the Sun workstation, the profiler/RASScomputer, the balloon borne sounding system computer, as well asstorage for expendables, disks, and tapes. The wind profiler and RASSspeakers are typically placed on the ground outside of the container.The surface meteorological instrumentation and Campbell datalogger areoutside away from the container.

ISS Surface Measurement Instrumentation Description

TheISS surface meteorological instrument installation includes severalsensors mounted on two separate towers as well as a rain gauge mountedindependently. An anemometer is mounted on the top of a ten-metertower. Temperature and humidity sensors are mounted on the end of aone-meter boom attached to the ten-meter tower at two meters above thesurface. The temperature and humidity sensors are aspirated andprotected with a radiation shield. The pressure sensor is housed in thebox containing the Campbell CR 10x datalogger. That box is mounted onthe ten-meter tower at one meter above the surface and a "pressureport" is connected and mounted at 2 meters. The "pressure port" reducesnoise in the pressure sensor do to the venturi effect of from the wind.

The radiation sensors are mounted on a one meter boom onthe top of a separate one-meter tower. The standard ISS radiationsensors include an up-looking Eppley PSP solar radiation sensor, EppleyPIR sensor and a net radiation sensor. In situations which require morecomplete radiation measurements, additional sensors can be added.

Theoutput from all the sensors is directed to the Campbell datalogger forprocessing. The Campbell datalogger, which is independentlyprogrammable, typically generates one-minute average data which aresent via RS-232 to the ISS Sun workstation. The data input to theCampbell datalogger are five-second sample data.

ISS Surface Measurement Instrumentation Specifications

Pressure Measurement

Thesurface pressure sensor used in the ISS installation is either aVaisala PTA427 or PTA427A pressure sensor. The PTA427 sensor pressurerange is 800 to 1060mb while the PTA427A sensor pressure range is 600to 1060mb. These sensors have an accuracy of +/- 0.5mb and +/- 0.8mbrespectively. They are both silicon capacitive pressure sensorspatented by Vaisala. Both are temperature compensated and produce alinear voltage output over the full operating range. In order tointerface with the Campbell datalogger, a 2:1 voltage divider isincorporated into the cable from the pressure sensor.

Temperature and Humidity Measurement

Thetemperature and humidity sensors are contained in a Vaisala 50Yhumitter which has been carefully calibrated with a curve fit. Theactual sensors are a PRT and a Vaisala "humicap" capacitive relativehumidity sensor. The temperature sensor accuracy is +/- 0.4 degrees Cover the range -33 to +48 degrees C. The accuracy of the humiditysensor against field references is approximately +/- 2% with a longterm stability of better than 1% RH per year. These specifications foraccuracy are achieved by internal calibration at EOL and data curvefitting in real time. The 50Y humitter sensor probe is protected andventilated by an RM Young aspirated radiation shield model number43-408 and external high flow aspiration fan.

Wind Measurement

Windspeed and direction are measured with an R.M. Young 05103 Wind Monitor.The monitor is a propeller wind vane with a 0.9 m/s threshold for windspeed and a 60 m/s maximum. Wind direction is measured using a 360degree mechanical precision conductive potentiometer. Directionmeasurements have a threshold of 1.0 m/s at a 10 degree displacementand 1.5 m/s at a 5 degree displacement. The potentiometer is 10 K-ohm,with a life expectancy of 50 million revolutions and has a 0.25%linearity through the entire range.

Radiation Measurements

Thestandard ISS radiation measurements are incoming solar radiation andnet radiation. The incoming solar radiation measurement is made with aEppley PSP precision pyranometer. The infrared measurements are madewith an up looking Eppley PIR precision PIR pyrgeometer. The netradiation sensor is a Radiation and Energy Balance Systems, Inc.Fritschen net radiometer. The net radiation sensor is a single sensorwith up-looking and down-looking hemispheric sensors mountedback-to-back at the end of a 0.5 meter boom.

The EppleyPrecision Spectral Pyranometer (PSP) comprises a circularmulti-junction wire-wound Eppley thermopile which has the ability towithstand severe mechanical vibration and shock. Its receiver is coatedwith Parson's black lacquer (non-wavelength selective absorption). Thisinstrument is supplied with a pair of removable precision ground andpolished hemispheres of Schott optical glass. Both hemispheres are madeof clear WG295 glass which is uniformly transparent to energy between0.285 to 2.8µm. Please refer to http://www.eppleylab.com/ forspecifications.

The PIR comprises a circularmulti-junction wire-wound Eppley thermopile which has the ability towithstand severe mechanical vibration and shock. Its receiver is coatedwith Parson's black
lacquer (non-wavelength selectiveabsorption). Isolation of long-wave radiation from solar short-waveradiation in daytime is accomplished by using a silicone dome. Theinner surface of this hemisphere has a vacuum-deposited interferencefilter with a transmission range of approximately 3.5 to 50 µm. Referto http://www.eppleylab.com/ for specifications.

TheRadiation and Energy Balance Systems, Inc. Fritschen net radiometer,measures the sum of all incoming radiation (direct solar, diffusesolar, longwave skylight) minus the sum of all outgoing radiation(reflected radiation and terrestrial longwave radiation). A single datastream, the difference of incoming and outgoing radiation, is recordedfrom the Radiation and Energy Balance Systems, Inc. Fritschen netradiometer. The wavelength range of this instrument covers both theshortwave and longwave bands.

When more preciseradiation measurements are required, three separate radiationmeasurements can be made in the surface meteorological installation,shortwave, longwave, and net radiation. Both the shortwave and longwaveradiation measurements are then made with pairs of Eppley radiometers,one up-looking and one down-looking radiometer in each pair. The netradiation measurement is again made with the Radiation and EnergyBalance Systems, Inc. Fritschen net radiometer.

Theshortwave radiation measurements are made with Eppley PSP pyranometers.The wavelength range of these instruments is 0.285 to 2.8 um. Separatedata streams are recorded from both the up-looking and down-lookingpyranometers. The longwave radiation measurements are made with Eppleypyrgeometers. The wavelength range of these instruments is 3.5 to 50.0um. As with the pyranometers, separate data streams are recorded fromboth the up-looking and down-looking pyrgeometers.

Rain Measurement

ATexas Electronics TE525 tipping bucket rain gauge is used at all landbased ISS sites for measurement of rainfall. The rain gauge resolutionis 0.254 mm. The gauge is typically positioned 1.5 meters above theground about 7 or 8 meters from the ten-meter tower. Shipboard rainmeasurements can be made with a Scientific Technology Inc. optical raingauge, STI ORG-100. These STI rain gauges are currently not availableas standard ISS furnished equipment. At present they would have to bepurchased by the project. They may become available as standardequipment at a later time as funding permits.

ISS Balloon borne Sounding System Description

TheISS balloon borne sounding system is the standard NCAR "GAUS" soundingsystem (See Chapter addressing the GAUS system for completeinformation). The integration of the surface meteorological data intothe sounding data is somewhat different in the ISS. In the NCAR ISSconfiguration, that surface data are made available to the "GAUS"sounding system through the ISS computer network via SAMBA. (Thesurface data are first transferred from the Campbell datalogger viaRS-232 to the ISS Sun workstation.)

Radiosonde Deployment

Thereare differing launch configurations possible at any ISS site. There canbe a "bag" launcher used where a heavy vinyl tarp contains and protectsthe balloon prior to launch. A "bag" launcher is typically used onshipboard ISS installations. In addition, a site can function without alauncher, in which case, the balloon and sonde are tied outside the ISScontainer and then released. All releases are environmentally aspiratedand shaded from direct sunlight.

ISS Wind Profiler and RASS Specifications

Upto four sets of radar parameters can be defined for the ISS windprofiler and RASS system. These are set up interactively by theoperator. The operator enters values for the inter-pulse period (IPP),the pulse width (PW), the number of heights (NHTS), the time from thefirst transmitted pulse to the first sampled height (DELAY), and thespacing between the sampled heights (SPACE). The four pulse widthsavailable define the vertical resolution of the wind profiler and RASS.Those pulse widths are as follows:

400 nS pulse - 60 meter resolution;
700 nS pulse - 100 meter resolution;
1700 nS pulse - 250 meter resolution;
3300 nS pulse - 500 meter resolution.

Thepulse width also defines the amount of energy transmitted by the radarand in most cases is reflected in the maximum altitude coverage.Another key factor in altitude coverage is the availability ofscatterers to provide refractive index irregularities within the samplevolume. Most variations in refractive index depend on the variation oftemperature and humidity caused by turbulent eddies that are in a sizerange on the order of one-half the radar wavelength.

Thefollowing tables summarize the wind profiler and RASS systemcharacteristics. Note that on the wind profiler height resolutionentries, resolutions are given for two modes, a low altitude mode (highresolution) and a high altitude mode (low resolution). The height towhich the profiler can measure wind velocities is highly dependent onthe atmospheric characteristics.