The Polarimetric Scanning Radiometer (PSR) is a versatile airborne microwave imaging radiometer developed by the Georgia Institute of Technology and the NOAA Environmental Technology Laboratory (now NOAA/ESRL PSD) for the purpose of obtaining polarimetric microwave emission imagery of the Earth's oceans, land, ice, clouds, and precipitation. The PSR is the first airborne scanned polarimetric imaging radiometer suitable for post-launch satellite calibration and validation of a variety of future spaceborne passive microwave sensors. The capabilities of the PSR for airborne simulation are continuously being expanded through the development of new mission-specific scanheads to provide airborne post-launch simulation of a variety of existing and future U.S. sensors, including CMIS, ATMS, AMSU, SSMIS, WindSat, TMI, RAMEX, and GEM.

The basic concept of the PSR is a set of polarimetrc radiometers housed within a gimbal-mounted scanhead drum. The scanhead drum is rotatable by the gimbal positioner so that the radiometers (Figure 2.) can view any angle within ~70° elevation of nadir at any azimuthal angle (a total of 1.32 pi sr solid angle), as well as external hot and ambient calibration targets. The configuration thus supports conical, cross-track, along-track, fixed-angle stare, and spotlight scan modes. The PSR was designed to provide several specific and unique observational capabilities from various aircraft platforms. The original design was based upon several observational objectives:

1. To provide fully polarimetric (four Stokes' parameters: Tv, Th, TU, and TV) imagery of upwelling thermal emissions at several of the most important microwave sensing frequencies (10.7, 18.7, 37.0, and 89.0 GHz), thus providing measurements from X to W band;
2. To provide the above measurements with absolute accuracy for all four Stokes' parameters of better than 1 K for Tv and Th, and 0.1 K for TU and TV;
3. To provide radiometric imaging with both fore and aft look capability (rather than single swath observations);
4. To provide conical, cross-track, along-track, and spotlight mode scanning capabilities; and
5. To provide imaging resolutions appropriate for high resolution studies of precipitating and non-precipitating clouds, mesoscale ocean surface features, and satellite calibration/validation at Nyquist spatial sampling.

The original system has been extended - as discussed below - to greatly exceed the original design objectives by providing additional radiometric channels and expanded platform capabilities.

The PSR scanhead was designed for in-flight operation without the need for a radome (i.e., in direct contact with the aircraft slipstream), thus allowing precise calibration and imaging with no superimposed radome emission signatures. Moreover, the conical scan mode allows the entire modified Stokes' vector to be observed without polarization mixing.

The nucleation-mode aerosol size spectrometer (NMASS) measures the concentration of particles as a function of diameter from approximately 4 to 60 nm. A sample flow is continuously extracted from the free stream using a decelerating inlet and is transported to the NMASS. Within the instrument, the sample flow is carried to 5 parallel condensation nucleus counters (CNCs) as shown in Fig. 1. Each CNC is tuned to measure the cumulative concentration of particles larger than certain diameter. The minimum detectable diameters for the 5 CNCs are 4.0, 7.5, 15, 30 and 55 nm, respectively. An inversion algorithm is applied to recover a continuous size distribution in the 4 to 60 nm diameter range.

The NMASS has been proven particularly useful in measurements of nucleation-mode size distribution in environments where concentrations are relatively high and fast instrumental response is required. The instrument has made valuable measurements vicinity of cirrus clouds in the upper troposphere and lower stratosphere (WAM), in the near-field exhaust of flying aircraft (SULFUR 6), in newly created rocket plumes (ACCENT), and in the plumes of coal-fired power plants (SOS ’99). The instrument has flown on 3 different aircraft and operated effectively at altitudes from 50 m to 19 km and ambient temperatures from 35 to -80ºC.

Accuracy. The instrument is calibrated using condensationally generated particles that are singly charged and classified by differential electrical mobility. Absolute counting efficiencies are determined by comparison with an electrometer. Monte carlo simulations of the propagation of uncertainties through the numerical inversion algorithm and comparison with established laboratory techniques are used to establish accuracies for particular size distributions, and may vary for different particle size distributions. A study of uncertainties in aircraft plume measurements demonstrated a combined uncertainty (accuracy and precision) of 38%, 36% and 38% for number, surface and volume, respectively.

Precision. The precision is controlled by particle counting statistics for each channel. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are recorded with 10 Hz resolution, and the instrument has demonstrated response times of this speed in airborne sampling. However the effective response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements with high concentrations of nucleation-mode particles may be processed at 10 Hz.

Specifications: Weight is approximately 96 lbs, including an external pump. External dimensions are approximately 15”x16”x32”. Power consumption is 350 W at 28 VDC, including the pump.

The NAST-M currently consists of two radiometers covering the 50-57 GHz band and a set of spectral emission measurements within 4 GHz of the 118.75 GHz oxygen line with eight single sideband and 9 double sideband channels, respectively. To be added prior to CRYSTAL-FACE are five double side band channels within 4 GHz of the 183 GHz water vapor line and a single band channel at 425 GHz. For clear air, the temperature and water vapor information provided by the 50-57 GHz, 118 GHz, and 183 GHz channels is largely redundant; but, for cloudy sky conditions the three bands provide information on the effects of precipitating clouds on the temperature and water vapor profile retrievals and enables sounding through the non-precipitating portion of the cloud, a feature particularly important for CRYSTAL-FACE.

QCM/SAW was developed to perform in-situ real time measurements of aerosols and chemical vapors in the stratosphere. The instrument is integrated into a fuselage centerline pod. The instrument is controlled by an embedded micro controller. The preset sampling sequence is triggered by a single command issued by the pilot. Using an air pump, samples are collected and decelerated in two stages to match the velocity requirement at the cascade impactor. Once the sample enters the cascade impactor, aerosols contained in the sample are separated into 8 size bands. The separated particles are collected on the surface of the piezoelectric microbalance crystals. The samples are analyzed post-flight.

The MASTER is similar to the MAS, with the thermal bands modified to more closely match the NASA EOS ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) satellite instrument, which was launched in 1998. It is intended primarily to study geologic and other Earth surface properties. Flying on both high and low altitude aircraft, the MASTER has been operational since early 1998.

Instrument Type: Multispectral Imager
Measurements: VNIR/SWIR/MWIR/LWIR Imagery

The MODIS Airborne Simulator (MAS) is a multispectral scanner configured to approximate the Moderate-Resolution Imaging Spectrometer (MODIS), an instrument to be orbited on the NASA EOS-AM1 platform. MODIS is designed to measure terrestrial and atmospheric processes. The MAS was a joint project of Daedalus Enterprises, Berkeley Camera Engineering, and Ames Research Center. The MODIS Airborne Simulator records fifty spectral bands.

The MAMS is a modified Daedalus Scanner flown aboard the ER-2 aircraft. It is designed to study weather related phenomena including storm system structure, cloud-top temperatures, and upper atmospheric water vapor. The scanner retains the eight silicon-detector channels in the visible/near-infrared region found on the Daedalus Thematic Mapper Simulator, with the addition of four channels in the infrared relating to specific atmospheric features.

The scanner views a 37 kilometer wide scene of the Earth from the ER2 altitude of about 20 kilometers. Each MAMS footprint (individual field of view) has a horizontal resolution of 100 meters at nadir. Since the ER2 travels at about 208 meters per second, a swath of MAMS data 37 by 740 kilometers is collected every hour. The nominal duration of an ER2 flight is 6 hours (maximum of about 7 hours).

The Meteorological Measurement System (MMS) is a state-of-the-art instrument for measuring accurate, high resolution in situ airborne state parameters (pressure, temperature, turbulence index, and the 3-dimensional wind vector). These key measurements enable our understanding of atmospheric dynamics, chemistry and microphysical processes. The MMS is used to investigate atmospheric mesoscale (gravity and mountain lee waves) and microscale (turbulence) phenomena. An accurate characterization of the turbulence phenomenon is important for the understanding of dynamic processes in the atmosphere, such as the behavior of buoyant plumes within cirrus clouds, diffusions of chemical species within wake vortices generated by jet aircraft, and microphysical processes in breaking gravity waves. Accurate temperature and pressure data are needed to evaluate chemical reaction rates as well as to determine accurate mixing ratios. Accurate wind field data establish a detailed relationship with the various constituents and the measured wind also verifies numerical models used to evaluate air mass origin. Since the MMS provides quality information on atmospheric state variables, MMS data have been extensively used by many investigators to process and interpret the in situ experiments aboard the same aircraft.