We describe measurements of radio frequency interference (RFI) in the 1200-1800 MHz band as observed from NASA's P-3 research aircraft during a flight along the Mid-Atlantic coast of the U.S. at altitudes of 2,000 and 20,000 ft. Both power spectra and coherently-sampled waveform data were obtained. Our results indicate that the spectrum below 1400 MHz is typically dominated by pulses from ground based radars, which, while very strong, individually have transmit duty cycles of on the order of 0.1%. We detect but are unable to identify some relatively weak intermittent RFI inside the 20 MHz protected band centered at 1413 MHz. We detect no significant RFI above 1420 MHz, but the limited sensitivity of this particular ex- periment makes it impossible to rule out the presense of RFI at levels damaging to total power radiometry. Im- plications for radiometer design, including possible active countermeasures for RFI mitigation, are discussed. I. Introduction Radio frequency interference (RFI) can impair the op- eration of radiometers operating at L-Band, even for sys- tems that operate in the protected 26 MHz frequency band around 1413 MHz. While bandwidths on the order of 100 MHz at L-Band are desirable for improving sensitiv- ity in soil moisture and ocean salinity applications, the use of active RFI mitigation techniques is required to achieve these bandwidths. To be effective, such techniques must take into account the spectral and temporal properties of the RFI. This paper presents some findings from an initial sur- vey of RFI in the frequency range 1200-1800 MHz, as ob- served from an airborne platform. The data were collected using a portable instrument known as the L-Band Inter- ference Surveyor/Analyzer (LISA), which was developed at the Ohio State University ElectroScience Laboratory in 2002. LISA includes two complementary subsystems: an off-the-shelf computer-controlled spectrum analyzer and a custom wideband high-dynamic-range coherent-sampling receiver. The former is useful for understanding the dis- tribution of RFI over large frequency spans and long time periods, whereas the latter provides waveform capture ca- pability with high temporal resolution. Our analysis of data captured during an initial test flight above the mid- Atlantic coast of the U.S. reveals some properties of the RFI which are helpful to know in the design of new air- borne and space-based radiometers, and especially in the development of mitigation techniques.