Scanning L-Band Active Passive (SLAP)

The Scanning L-band Active Passive (SLAP) is a new airborne simulator for NASA’s smap soil moisture satellite that launched in early 2015. 

SLAP has both passive (radiometer) and active (radar) microwave L-band imaging capabilities, just as SMAP does.   The radiometer observes at 1.4 GHz using duplicate front end hardware from the SMAP satellite radiometer. It also includes a duplicate of the digital backend development unit for SMAP, thus the novel Radio Frequency Interference (RFI) detection and mitigation features and algorithms for SMAP are duplicated with very high fidelity in SLAP. The digital backend provides 4-Stokes polarization capability. The real-aperture radar operates in the 1215-1300 MHz band with quad-pol capability. Radar and radiometer share one antenna via diplexers that are spare units from the Aquarius satellite instrument.

SLAP’s initial flights were conducted in Dec 2013 over the eastern shore of Maryland and successfully demonstrated radiometer imaging over 2 full SMAP 36x36 km grid cells at 1.3 km resolution within 3 hrs. A second flight on the same day also demonstrated SLAP’s quick-turn abilities and high-resolution/wide-swath capabilities with 200m resolution across a 1500m swath. 

Additional flights were conducted as part of the GPM iPHEX campaign in May, 2014. SLAP is currently preparing for the SLAPex series of campaigns to various locations in North America during 2015.

SLAP Data Imagery

 Figure Caption: Passive microwave image from SLAP. A 50x80 km region was mapped at 1.3 km resolution in <3 hrs, including two SMAP 36 X 36 km grid cells shown by the black squares. The yellows & oranges indicate slight variations in temperature and moisture across this portion of eastern Maryland and Delaware. Dark blue on the far right (Delaware Bay) corresponds to the low brightness temperatures typical of open water. Light blue is seen along land-water boundaries, including some rivers. 

ComRAD Instrument System

Hydrological Sciences currently supports a ground-based microwave instrument system called ComRAD for Combined Radar/Radiometer which is mounted on a 19-m hydraulic boom truck. This instrument package is an outgrowth of a network analyzer-based L, C, and X band polarimetric radar system developed jointly by NASA/GSFC and George Washington University which has provided reliable calibrated radar data in soil moisture field campaigns across the United States since the early 1990s. The system has been upgraded with the addition of a dual-polarized 1.4 GHz total power radiometer. A novel broadband stacked-patch dual polarized feed resonates at both the 1.4 GHz radiometer and 1.25 GHz radar frequencies, enabling the L band radar and L band radiometer to share the same 1.22-m parabolic dish antenna. Having the radar and radiometer utilize the same antenna with the same 12° field of view greatly simplifies the development of active/passive microwave retrieval algorithms. External calibration of the radiometer is achieved using cold sky and ambient microwave absorber targets during each measurement run, while radar calibration is achieved using known microwave reflectors (flat disks and dihedral) at the beginning and end of each extended measurement series. In addition to supporting basic science on microwave remote sensing of hydrological parameters like soil moisture, ComRAD can also function as a ground-based instrument simulator for L band space missions such as SMOS, Aquarius, and SMAP.