A Plarimetric Survey f Radi Frequency Interference in C- and X-Bands in the Cntinental United States using WindSat Radimetry Steven W. Ellingsn Octber, Cntents Intrductin WindSat Methdlgy Analysis f RFI Surce Cunts 7 Bradley Dept. f Electrical & Cmputer Engineering, Whittemre Hall, Virginia Plytechnic Institute & State University, Blacksburg VA 6 USA. E-mail: ellingsn@vt.edu
Intrductin This reprt presents a preliminary analysis f radi frequency interference (RFI) bserved in 6 mnths f bservatins in the C- and X-Band channels f WindSat, an Earth-rbiting plarimetric radimeter. In this reprt, we limit the analysis t bservatins f the cntinental United States. This analysis fllws the same apprach as in previus reprts by this authr ([] and []). Cmparable studies using AMSR-E are reprted in []. WindSat A detailed descriptin f the WindSat micrwave plarimetric radimeter is available in []. Here, the details relevant t the analysis in this reprt are summarized. WindSat is the primary paylad n the U.S. Air Frce s Crilis satellite, which was launched in January and is nw in an 8 km circular sun-synchrnus rbit. The 6.8 GHz radimeter has a bandwidth f 5 MHz and integratin time f 5 ms, yielding an abslute accuracy f abut.75 K. WindSat measures brightness temperature in a ftprint apprximately.5 (in latitude and lngitude) in diameter as travels ver the Earth. Fr this reasn, the duty cycle f bservatin fr any given pint n the Earth is very lw; n the rder f twice per day. After grund-based pst-prcessing, radimetry in fully-calibrated vertical ( V ) and hrizntal ( H ) plarizatins are prvided. ( H refers t the plarizatin parallel t the grund, whereas V refers t the plarizatin perpendicular t bth H and the directin f incidence.) Brightness temperatures bserved n the surface f the Earth in bth C- and X-Band typically fall in the range 7 K. Fr the purpses f this reprt, temperatures greater than K are cnsidered RFI, which als crrespnds the highest brightness temperature the WindSat radimeter can accurately measure. The basis f this reprt was six mnths f WindSat bservatins crrespnding t the mnths f September thrugh February. Methdlgy Analysis is mst cnvenient when the bservatins are spatially requantized t a grid which is rectilinear with respect t latitude and lngitude. Each pint in the grid defines a pixel fr which the brightness temperature statistics are cmputed. The grid used in this reprt has spacing equal t fur times the mean spacing f the swath data btained frm WindSat. This grid spacing is apprximately ne-half f a beamwidth, s there is significant spatial verlap in bth the WindSat data as well as the bservatins presented here. This is desireable in that smaller spacings tend t prduce results which are smewhat redundant (being finer than the reslutin f the instrument) whereas larger spacings tends t lead t large variatins in the number f bervatins per grid pint, which culd ptentially bias the statistics. Figure shws the actual prbability distributin functin (PDF) f the number f bservatins per grid pint. An bservatin is defined as fllws: Fr each measurement frm WindSat, the grid pint nearest the center f the beam is determined, and then this grid pint (and n ther) is declared t have been bserved. Nte that each grid pint is bserved at least 5 times ver the six-mnth perid; i.e., times/day n average. The Gaussian shape f the distributin suggests that the sampling f the Earth s surface was nt significantly biased, and s the results fr all grid pints bserved shuld have rughly the same statistical significance. Figure shws the mean and max hld C-Band brightness temperatures, displayed as a flatshaded cntur plt. Max hld means the maximum brightness temperature ever bserved at a certain lcatin. Nte that sme cautin is required in interpreting these results since the graphing rutine is essentially interplating between the grid pints t btain the temperatures at all ther pints. All plts shw clear evidence f nn-gephysical brightness temperture distributins in the frm f lcalized ht spts, which are nt expected t ccur naturally. Furthermre, many f these ht spts exhibit unnaturally-bright tempertures (in excess f K) making it almst certain that they are due t man-made RFI. There als seems t be sme crrelatin between ht spts and
5 x 5.5.5 PDF.5.5.5 5 5 5 56 58 6 6 6 Number f bservatins Figure : Distributin f the number f bservatins per grid pint fr the 6 mnth dataset analyzed in this reprt. ppulatin centers, althugh this assciatin is weak and many large cities shw n evidence f a crrespnding ht spt. An dd feature visible in Figure is the bright plume alng the mid-atlantic cast in the max hld images. This feature des nt appear in the mean data and is therefre mst likely assciated with sme shrt-term event. Future analyses f the data shuld attempt t determine reslve the cause. Figure shws the mean and max hld X-Band brightness temperatures fr the vertical and hrizntal plarizatins. N bvius RFI is visible in these plts; hwever the bright plume ff the mid-atlantic cast, bserved in the C-Band results, is als visible in the max hld results here. This apparent brad bandwidth casts sme dubt that the plume is due t man-made RFI, althugh the pssibility exists that the surce is a very strng man-made emitter which is capable f simultaneusly saturating the C- and X-Band receivers. Figure shws the mean and max hld X-Band brightness temperatures fr the crrelatin channels U and. The values f these crrelatins due t gephysical mechanisms may be either psitive r negative, but in either case is expected t be very small. Fr this reasn, statistics f the magnitudes f these channels were cmputed, s that RFI culd be mre easily identified as a large psitive value. Unnatural RFI ht spts, similar t thse seen in the C-Band results, are clearly apparent. Hwever, the lcatins d nt seem t be well crrelated with the C-Band ht spt lcatins. Furthermre, cmparisn f the max hld and mean results suggests that these emissins are intermittent; hwever anther pssibility is that they are cntinuus but appear t be scintillating because the emissin is directinal and s nly bserved fr certain rientatins f WindSat with respect t the surce. There are tw ther ntable features in Figure. First is the apparent high brightness cincident with castlines in the results. This is thught t be due t a slight misalignment f the beams leading t imperfect subtractin as the beams pass frm water t land r vice-versa. Secnd is the trail f ht spts running in a Nrth-Suth line in the Atlantic in the max hld result; this may be assciated with the bright plumes bserved in the Figures and.
5N 5N 5N 5N 5N 5N W 8W 96W 8W 7W W 5 (a) C-Band V, Max Hld 8W 96W 8W 7W 5 (b) C-Band H, Max Hld 5 N 5 N 5N 5N 5N 5N W 8W 96W 8W 7W W 5 (c) C-Band V, Mean 8W 96W 8W (d) C-Band H, Mean Figure : C-Band Brightness Tempertures, in K. 7W 5
5N 5N N N 5 N 5 N N N 5N 5N W 8W 96W 8W 7W W 8W 96W 8W 7W (a) X-Band V, Max Hld (b) X-Band H, Max Hld 5 N 5 N 5N 5N 5N 5N W 8W 96W 8W 7W (c) X-Band V, Mean W 8W 96W 8W 7W (d) X-Band H, Mean Figure : X-Band Brightness Tempertures (Linear Plarizatins), in K. 5
5N 5N N N 5 N 5 N N N 5N 5N W.5 8W.5 96W.5 8W.5 7W W.5.5 (a) X-Band U, Max Hld 8W.5 96W.5 8W.5 7W.5 (b) X-Band, Max Hld 5 N 5 N 5N 5N 5N 5N W.5 8W.5 96W.5 8W.5 7W W.5.5 (c) X-Band U, Mean 8W.5 96W.5 8W.5 (d) X-Band, Mean Figure : X-Band Brightness Tempertures (Crrelatin Channels), in K. 6 7W.5
.8.8.7.7.6.6.5.5 PDF [%]. PDF [%]....... 5 T b [K] 5 T b [K] (a) C-Band, Mean (b) X-Band, Mean.8.8.7.7.6.6.5.5 PDF [%]. PDF [%]....... 5 T [K] b 5 T [K] b (c) C-Band, Max Hld (d) X-Band, Max Hld Figure 5: PDF f T B. Blue/Slid: V, Red/Brken: H. Anther way t characterize the brightness tempertures illustrated in Figures thrugh is thrugh PDFs as shwn in Figure 5. There is n particular evidence f RFI visible in these plts except fr the dramatic spike at the high end f the C-Band max hld PDF. Analysis f RFI Surce Cunts In this sectin we attempt t characterize the tempral behavir f the RFI. Due t the large quantity f data in this 6 mnth dataset, this is best dne in a statistical manner. Furthermre, the nly unambiguus criterin fr the presense f RFI is brightness temperature in excess f K. We shall refer t a single bservatin f a brightness temperture in excess f K as an excess brightness event (EBE). In this sectin, we determine the fractin f bservatins at each grid pint which exhibit EBEs, and present the results in terms f a PDF. Since EBEs were bserved nly in the C-Band data, nly the C-Band data is analyzed in this manner. Furthermre, we nte that this analysis is sensitive nly t the strngest RFI; in truth, there may be plenty f RFI at levels adversely affecting radimetry but which is undetected in this analysis simply because the recrded brightness temperture is less than K. In fact, the prevelence f ht spts belw K in the maps presented in the previus sectin strngly imply that this is the case. 7
.5.5.5.5 lg ( PDF ).5 lg ( PDF ).5.5.5.5.5 5...6.8 Fractin f Observatins Exhibiting EBEs 5 Fractin f Observatins Exhibiting EBEs Figure 6: PDF f the fractin f C-Band bservatins exhibiting EBEs (bservatins f brightness temperture K) per grid pint. The right plt is the same as the left plt except the hrizntal axis is pltted in lg scale. : Vertical plarizatin, : Hrizntal plarizatin. Figure 6 shws the PDF f C-Band EBEs ver the entire 6 mnth dataset. A few interesting cnclusins can be reached frm this result. First, we nte that nly a tiny fractin f the grid pints ever exhibit EBEs, and that n grid pint shws EBEs n every bservatin. In fact, the highest rate f ccurrance f EBEs was abut 7%, which ccurred fr just ne grid pint in vertical plarizatin. Secnd, we nte that the EBE statistics are rughly the same fr bth linear plarizatins. Third, we bserve that the PDF seems t be exhibiting a pwer law relatinship ( 6 )x fr x > where x is the fractin f EBEs bserved per grid pint; this is mst easily seen when x is pltted in lg scale, in which case the PDF plt becmes nearly a straight line. The pwer law relatinship must break dwn fr sme value f x < since the bserved result fr x is nearly, which is simply anther way f saying that mst grid pints d nt exhibit EBEs. This pwer law relatinship may be a clue that can be used t deduce the surce f the EBEs. Figure 7 shws the rate f C-band EBEs per grid pint in map frm. We nte here that EBE activity seems t be limited t relatively few gegraphical regins near Ls Angeles, CA; Seattle, WA; Tucsn, AZ; Washingtn, PA; Winstn-Salem, NC; and Cnnecticut, t name a few. We further nte that areas f EBE activity d nt appear as discrete spikes with respect t the grid, but rather as a distributin ver a relatively large number f adjacent grid pints. In fact, the mre active EBE regins exhibit a tail f diminishing EBE activity with increasing distance frm sme central pint. This tail may be simply the cnvlutin f the beam shape with the actual regin f EBE activity, and may be the surce f the pwer law relatinship nted abve. We nte ne mre interesting feature f Figure 7, mst easily seen in the hrizntal plarizatin results. Nte that the regin brdering the Mississippi River frm the Suthern tip f Illinis t the Nrthern brder f Luisiana shws cncentrated EBE activity. A smewhat similar feature is seen running Nrth-Suth thrugh Utah and Idah, which very clsely fllws the path f Interstate 5. Other similar linear features can be seen in central Texas and Califrnia. Thus, the surce f the EBEs may be assciated with radi frequency systems in use alng these transprtatin crridrs. 8
5 N 5 N N N 5 N 5 N N N 5 N W 8 W 96 W 8 W 7 W 5 N W 8 W 96 W 8 W 7 W.5.5.5 (a) Vertical Plarizatin.5.5.5 (b) Hrizntal Plarizatin Figure 7: Fractin f C-Band bservatins exhibiting EBEs, per grid pint. The clr bar is labeled in lg scale; i.e., crrespnds t, and s n. 9
References [] S.W. Ellingsn, A Preliminary Survey f C-Band RFI in the SMEX Area f Operatins using WindSat Radimetry (Infrmal Reprt), Virginia Plytechnic Institute & State University, May 8,. [] S.W. Ellingsn, A Survey f C-Band RFI in the Chesapeake Bay Regin using WindSat Radimetry, Infrmal Reprt (Infrmal Reprt), Virginia Plytechnic Institute & State University, June 7,. [] P.W. Geiser et al., The WindSat Space Brner Plarimetric Micrwave Radimeter: Sensr Descriptin and Early Orbit Perfrmance, Submitted t IEEE Trans. Gescience & Remte Sensing, February,. [] L. Li et al., A Preliminary Survey f Radi-Frequency Interference ver the U.S. in Aqua AMSR-E Data, IEEE Trans. Gescience & Remte Sensing,, 8,.