IEEE GEOSCIENCE AND REMOTE SENSING LETTERS, VOL. 13, NO. 11, NOVEMBER 2016 1621 Television Frequency Interference in AMSR2 K-Band Measurements Over Reflective Surfaces Xiaoxu Tian and Xiaolei Zou Abstract—The Advanced Microwave Scanning Radiometer 2 (AMSR2) measurements at K-band channels are used for snow retrieval. However, television (TV) signals transmitted from DirecTV satellites at the K-band, if reflected by snow surfaces, could enter the antenna of AMSR2 to introduce errors in AMSR2 snow products. This letter investigates TV frequency interference (TFI) in AMSR2 K-band measurements over areas of land covered with snow. Since a necessary condition for TFI over land to occur is for AMSR2 measurements to have small glint angles, a principal component analysis algorithm constrained by TFI glint angles for TFI detection is thus developed in this letter. Using AMSR2 observations in January 2014, it is shown that TFI signals exist along the two 55 ◦ incident angle curves of DirecTV-11 and DirecTV-12, which are located around 44 ◦ N over the North American continent. Index Terms—Advanced Microwave Scanning Radiometer 2 (AMSR2), K-band channels, television frequency interference (TFI) over snow surface. I. I NTRODUCTION T HE Advanced Microwave Scanning Radiometer 2 (AMSR2) is the only remote sensing instrument onboard the Global Change Observation Mission—Water 1 (GCOM-W1) satellite, which was successfully launched onto a sun- synchronous orbit at an altitude of 700 km on May 18, 2012. It crosses the equator at 1:30 P. M. of local time [1]–[4]. AMSR2 is the successor of the AMSR-E onboard the Aqua satellite, which stopped its operation in October 2011. AMSR2 inherited all 12 channels of AMSR-E and added two new C-band channels at 7.3 GHz. It is well known that a number of channels in microwave imagers, such as 6.925 (C-band) and 10.65 GHz (X-band) of AMSR2, suffer from radio-frequency interfer- ence (RFI) from active sensors over densely populated cities [5]–[10]. The RFI sources primarily consist of radars, radio communication devices, traffic control monitors, etc. Over oceans, the reflected television (TV) signals from geostation- ary satellites cause TV interference (TFI) in microwave radi- ance observations [11]–[13]. In Europe, the TFI are found in X-band channels because the TV signals in the region are Manuscript received April 6, 2016; revised June 5, 2016 and July 11, 2016; accepted July 22, 2016. Date of publication August 25, 2016; date of current version October 12, 2016. This work was supported in part by the National Oceanic and Atmospheric Administration (NOAA) through the Hurri- cane Forecast Improvement Program (HFIP) under Project NA15NWS4680002 and the NOAA Proving Ground and Risk Reduction Program under Project NA11OAR4320199. (Corresponding author: Xiaolei Zou.) The authors are with the Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740-3823 USA (e-mail: xzou1@ umd.edu). Digital Object Identifier 10.1109/LGRS.2016.2598058 transmitted at X-band frequencies. Over coastal ocean areas of North America, TFI mainly exists in imager observations at K-band channels since the TV signal sources transmitted by DirecTV-11 and DirecTV-12 are around a frequency of 18.7 GHz. McKague et al. [14] investigated the possible interference by TV signals to observations of WindSat, AMSR-E, and SSMI. The accumulated maximum spectral differences between ob- servations of channels at 18.7 GHz and those at 23.8 GHz showed that K-band channel observations over land are likely to be interfered by TV signals reflected from snow surfaces. Zou et al. [12] pointed out that, the TFI glint angle, i.e., the angle between the line-of-sight vector and the reflected TV signal vector, is a necessary condition for the interference to occur. This letter develops a TFI detection algorithm based on principal component analysis (PCA) with TFI glint angles as the constraint. Since TFI is caused by reflected TV signals, it is not correlated with natural emission from a snow-covered land surface. Based on this characteristic, the PCA can isolate the TFI from the observations even when obscured by snow. Numerical results are made with AMSR2 L1B observation data. II. AMSR2 CHARACTERISTICS AMSR2 is a conical scanning microwave imager with a constant local incident angle of 55 ◦ . Compared with its prede- cessor AMSR-E, the diameter of the antenna is increased from 1.6 to 2 m for AMSR2 to make measurements at higher spatial resolutions than AMSR-E. AMSR2 has in total 14 channels, with their center frequencies located at the following seven different frequencies: 6.925, 7.3, 10.65, 18.7, 23.8, 36.5, and 89 GHz. Each frequency has two channels measuring radiation at horizontally and vertically polarized states. Except for two channels at 7.3 GHz, the other 12 channels are inherited from AMSR-E. The purpose of adding two new channels is for RFI mitigation of observations at C-band. The swath width of AMSR2 is 1450 km. Each scan cycle has 486 Earth-scene field of views (FOVs) in 89-GHz channels and 243 FOVs in lower frequency channels. GCOM-W1 orbits the Earth about 14 times a day. This allows AMSR2 onboard GCOM-W1 to scan a majority part of the Earth twice daily. The GCOM-W1 satellite has a 16-day repeating cycle. The spatial resolution of instantaneous FOV (IFOV) at different channels, the sampling interval, and the beamwidths can be found in Table I. III. TFI DETECTION ALGORITHM The U.S. is fully covered with TV signals at K-band fre- quencies from both DirecTV-11 at 99.2 ◦ W and DirecTV-12 1545-598X © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.