Instruments
P-Band Radar
The AirMOSS radar is a modification of the L-band radar in NASA's Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) program.
The NASA/JPL UAVSAR is a pod-based repeat-pass polarimetric SAR that currently operates in L-band (1.2 Ghz). The AirMOSS radar replaces the L-band front-end electronics and antenna with components that operate at P-band. The major components of the P-band front-end electronics are based on AIRSAR and GeoSAR heritage, and the P-band antenna utilizes the GeoSAR P-band antenna design.
The modular design of the pod electronics bay enables the AirMOSS team to adapt the radar to operate in different frequencies with ease.
AirMOSS utilizes a passive dual-polarized array antenna based on the GeoSAR P-band antenna design to take advantage of the good polarization isolation over the bandwidth of 280 to 440 MHz.
The radar utilizes the L-band RF electronics and an arbitrary waveform generator (AWG) from UAVSAR. The L-band transmit pulse is down-converted to P-band by mixing it with a tone from a direct digital synthesizer (DDS). The P-band transmit pulse is then amplified and sent to either the horizontal (H) or the vertical (V) polarization channel of the passive antenna. The transmit bandwidth is determined by the AWG and the selection of one of four filters (6, 20, 40, or 80 MHz) in a filter bank.
The radar pod is mounted underneath a Gulfstream-III (G-III) aircraft (operated by Johnson Space Center).
The radar has two identical receive chains. Both the H and the V radar returns are up-converted to L-band by mixing them with the same tone from the DDS as was used on transmit. The radar return is filtered using an identical filter bank as on transmit and then enters the UAVSAR receive electronics.
Example of AirMOSS pulse-to-pulse frequency selectability. This is not a nominal AirMOSS operating mode.
The DDS tone frequency, the filter choice within the filter bank, and the polarization of the antenna during transmit can be commanded on a pulse-to-pulse basis for a predefined sequence of up to 16 pulses, which is repeated over and over throughout a data collection. For example, it is possible to collect simultaneously data to form fully polarimetric SAR images for a low frequency 20 MHz sub-band and a high frequency 20 MHz sub-band by defining a 4 pulse sequence as shown in the figure. Note that this is not currently planned for AirMOSS operations.
Key Radar Instrument Parameters
Data Calibration
The calibration of the P-band SAR data is similar to that for UAVSAR. However, because AirMOSS operates at a lower frequency (longer wavelength), larger corner reflectors are needed for geometric and radiometric calibration. In addition to the 2.4 m corner reflectors that are used for UAVSAR calibration, AirMOSS will use newly designed 4.8 m corner reflectors to achieve its radiometric accuracy requirements.
Side view of 4.8m corner reflector, with 5 foot tall person provided for scale.
Ground Sensors
Each of the AirMOSS study areas included a FLUXNET meteorological tower, which also includes instrumentation to measure root-zone soil moisture (RZSM) within the tower footprint (~1 km).
Fluxnet tower in Harvard Forests, Maine
For most of the AirMOSS study areas, up to three additional ground sensor profiles were installed. These profiles measured spatial patterns of RZSM over the modeling grid cell, at several depths ranging from 2 cm down to approximately 100 cm. The measurements were used to validate soil moisture estimated from the P-band radar.
Sensors at each depth will include thermistors to measure soil temperature, heated ceramic needles to measure soil matric potential, and frequency domain reflectometers to measure soil moisture content.
Additionally, each sensor profile included a thermal IR sensor for measuring surface soil temperature and a tipping bucket rain gauge to measure precipitation.
In situ ground sensor installed
Data from ground sensors measurements are archived as L2-IGSM and L2-Precip products.
ALAR
The Airborne Laboratory for Atmospheric Research (ALAR) platform is a light twin engine aircraft, a Beechcraft Duchess, instrumented with a GPS/INS attitude system, and a turbulence probe (BAT probe) mounted on the nose.
Beechcraft Duchess in-flight
This system enables 50Hz 3D winds, enabling its use for airborne flux measurements. The wind system has been wind-tunnel calibrated, with a vertical wind uncertainty of +/-6 cm/s. The aircraft has an all-Teflon fast sampling system to enable eddy covariance flux measurements, and fast response CO2 instrumentation. It is often flown with a spectral radiometer for surface reflectance measurements. The aircraft also has a cloud water collector.
Wind measurement (BAT) probe, sampling inlets, and temperature probe on ALAR Beechcraft Duchess
For AirMOSS, the ALAR platform is being augmented with a Picarro Wavelength-Scanned Cavity Ring Down Spectrometer (WS-CRDS), a greenhouse gas analyzer that will measure spatial variations in atmospheric CO2, CH4 and H2O over selected study areas. This data will be used to validate the model simulations and the upscaling process of NEE from the flux-tower to the modeling grid cell.
Data from ALAR will be archived as the L2-CFlux product and will be used to validate...
- ED2 and data assimilation model predictions over 25km x 25km grid centered at flux tower sites
- the model simulations and the upscaling process of NEE from the flux-tower to the modeling grid cell