Radio-Wave Propagation in the Earth's Crust

Home , Radial (radio)

JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 6SD. No.2, March- April 1961 Radio-Wave Propagation in the Earth1s Crusf'2 Harold A. Wheeler

(F ebruary 26, 1960)

There is a reasonable basis for postulating t he existence of a useful waveguide deep in the earth's crust, of t he order of 2 t o 20 km below the surface. Its di electric is basem ent I'Oc], of very low conductivity. Its upper boundary is formed by the conductive layers ncar the surface. Its lower boundary is formed by a high-temperature conductive layer far below the surface, termed the "thermal ionosphere" by analogy to the well-known "radiation ionosphere" far above the surface. The electrical conductivity of the basem.ent rock has not been explored. An example based on reasonable estimates indicates that transmission at 1.5 kc/s might be possible for a distance of the order of 1500 km. This waveguide is located under land and sea over t he entire surface of the earth. It may be useful for radio transmission from t he shore to a submarine on the fl oor of t he ocean. The sending antenna might be a long conductor in a drill hole deep in the basement rock ; the receivi ng antenna might be a vertical loop in t he water .

In Lhe earth's crust, there appears to be a deep S waveguicleLhat has not yet been explored. This waveguide extends under all the surface area, so --::-~_:.-:J?C:__ ~ it suggests the possibility of wave propagation under I --_ R OCEAN the ocean floor. This might enable communication --- - _ 0 from land to a submarine lo cated on or neal' the -- -.;;;------ocean floor. If below a certain depLh , j t happens ------WAVEGUIDE that the excess radio noise from electric storms would become weaker than thermal noise, and no otber THERMAL IONOSPHERE so urce of appreciable radio noise is recognized. FIGURE 1. Communicati on tlwough the deep waveguide under This waveguide comprises basemen t rock as a the ocean. dielectrie between upper and lower conductive boundaries. The upper boundary is formed of the guide into the ocean ju t above, and is sampled by well-known geological straLa lo cated between the an antenna at the receiver. surface and the basement rock, with conduetivity Figure 2 shows an arrangement for the sender provided by electrolytic solutions and semiconductive antenna. It is a long conductor (pipe) sunk into a minerals. The lower boundary is provided by high drill hole flIl ed with oil insulaLion. The example temperature conductivity in the basement rock. shown has conducting material down to a depth of In concept, Lhe lower boundary is similar to Lhe about 1 km. Thl'ough this layer of earth, there is usual ionosphere, bein g formed by gradually increas an outer pipe which forms the outcr conductor of a ing conductivity. In the usual ionosphere [Wait, coaxial tran mission line. This pipe is in contact 1957], caused by extratenestl'ial radiation, tbe con with a conductin g surface slleh as water or radial ductivity increases with height. In the present wires in the ground. Below this layer of earth, the case, however, the conduetivity increases with deptb inner conductor extends further about 2 km into and is caused by the increasing temperature in the the basement-rock dielectric. This extension radiates dielectric material. Therefore it may be designated into the waveguide in the usual manner. as the "inverted ionosphere" or "thermal iono sphere." Figure 1 shows how this waveguide may be used S for eommunication from a shore sending station (8) WATER to an underwater receiving station (R). The latter may be a submarine on the bottom of the ocean. The sender launches a vertically polarized tl'ansverse electro-magnetic (TEM) wave by means of a vertical ~"~"'~ wire projecting into the basement rock. The wave WAV EGUIDE : ~ ( T EM MODE ) : is propagated in the deep waveguide between the :I 1 I I 2 km surface conductor and the thermal ionosphere. I I Some power from the wave leaks out of the wave- I, I I I Contribution from '~'hee l e r Laboratories, Great Neck, N.Y., and Develop I L_ -' mental Eng in eerin~ Co rp ., Leesburg, Va. , Paper presented at Conference on tbe Propagation of ELF Radio Waves, Boulder, Colo ., January 27, 1960. FIGU RE 2. Sender antenna in the dee p waveguide. 189 (J", CON DUCTIVITY (mhos/m) The locatioll ot each boundary depends on the log 10 IT frequency, t he conductivit)T, and the rate of change - 12 -6 o of conductivi ty with depth. In the example to be o o outlined, these boundaries occur at temperatures in the range of 300 to 600 °C. UPPER BOUNDARY As an example of the behavior that might be

u expected in this waveguide, the following numerical E 0 ~ ~ -'" values arc suggested. DIELECTRIC CONDUCTOR k'6 Frequency 1.5 kc/s Dielectric constant 6 10 200 Wavelength (in dielectric) 80 km Efl'ective boundaries of E fi eld (depths) 1- 18 km Effective boundaries of M fi eld (depths) 1- 27 km (TEM WAVE) I "- S ki n depth for E fi eld (lower boundary) 1.5km f- ::;;: "- w Skin depth for JIll field (lower boundary) 4km W f- L ength of radiator (ill waveguide) 2km 0 R eactance of radiator 1600 ELECTRI C ohms Effective length of radiator 1 km 20 400 Radiation resistance (in waveg uide) 0.4 ohms Ot her r esista ncc 20 ohms x x Radiation efficiencv 0.02 0 0 A verage power fadm' of E and JIll fields, about 0.1 0:: 0:: "- "- Napier distance (for wave attenuation) 130 kill "- "-

30 600 If these values are to be experienced, communica tion ranges of the order of 1500 km will be possible FIGURE 3. 11ariation of conductirity with depth to form the de ep waveguide. under the surface of the earth. The assumptions for this example are based on preliminary estim ates of the best conditions that are Figure 3 shows how tll e temperature and the at all likely to be realized. The high-temperature resulting conductivity may vary with depth, espe conductivity needed for the lower boundary is typical cially in the basement rock at depths exceeding a of quartz and other similar minerals. The extremely few kilometers. This diagram will be used to explain low conductivity at lower temperatures is unlikel.Y, the expected behavior of the deep waveguide. At but need not be quite so low to provide a dielectric depths of about 2 to 20 km, the basement rock is that could give the indicated performance. indicated to have such low conductivity that it is a dielectric suitable for wave propagation. Above As for the properties of the basement rock, it is and below this dielectric, the conductivity is high very doubtful how low its conductivity may be. enough to serve the function of boundaries for the Its seismic properties are explored but not its waveguide. electrical conductivity. Its principal chemical com The upper boundary is fairly weU defined, in a ponents are known, but apparently not its small depth of the order of 1 km (perhaps down to several content of " impurities" that may determine the kilometers). Its conductivity, in the most common conductivity. It seems that core samples have been materials, ranges from a maximum of 4 mhos/m in made to only a sm.all depth (less than 1 km) in the sea water down to about 10- 4 in rather dry nonco n- basement rock, presumably because there has been ductive minerals. . little prospect of valuable mineral products at a Tbe dielectric layer, shown between depths of reasonable cost. It is notable that some tests show about 2 and 20 km, may have very low conductivity, a trend toward lower conductivity (below 10 - 6) in of the order of 10- 6 to 10- 11 mho/m. The lowest the transition from the surface layers in to the base conductivity is observed in fu sed quartz, but prob men t rock. A con tinuation of this trend may enable ably is not found in nature. The present plan is such performance as is indicated in the example. useful if the conductivity is around 10- 8 or lower. Returning to the waveguide properties, the TEM The dielectric constant is about 6. mode (with vertical polarization) is t he one that has The lower boundary has some unusual properties. the greatest probabilit.v of enabling long-range (These are also characteristic of the ionosphere at communication . It is the only propagating mode frequencies below VLF.) The gradual increase of at frequencies below about 2 kc/s (i ncluding the conductivity [Van Hippel, 1954] provides an effective above example) . boundary Jor each kind of field, that for the electric This preliminary study has indicated that the field being closer than that for the magnetic field. deep waveguide is probably a definite physical In each case, there is a sort of skin depth in the phenomenon. The properties of its dielectric and boundary [Wl1eeler, 1952]. Both of these boundaries boundaries are not known quantitatively, so it is make comparable contributions to tbe total dissipa uncertain to what distances this waveguide may be tion factor of the waveguide, which determines the useful for communication or related purposes. Some exponential attenuation rate. rather optimistic assumptions as to these properties 190 - I

lead o ne to s pe(; uhtc 0 11 di sL nllccs of Lite order of of workill g with this group is acknowledged wi Lh 1500 kill . 'vVhile Lh c d eep wavcguide exLend s under apprecia tion. the cntil'c s urfacc o f la nd and sea, it is most n eed ed for md io Lr ,U1 SJlli ssion to a subm arille on t ilC ocean References floor, b ccfw se t hi s locatioll is s bi elded from the usual radio \Y ,lyeS above th e surfacC' . Von ] I ippel, A. It., Dielectri c materials and ap p licatiolls. F used q ua rtz at high temperatures, p . 403 (John Wiley & Sons, New York, J'\ .Y ., 1954). ' Vait, J . R ., The mode t heo ry of VLF ionosphere propagation for fini te groulld co nducL iviLy, Pl'oc. IRE 45, 760- 767 This concep t occurred to tbe wri ter r ecen tly (June 1957). d uring disc Ll ssions with L ester H . Carr a nd his Wh eeler, E . A., Uni ve rsal skin-cffect chart for condu cting associates in D evelopmental Engineering Corpora materials, E lectroni cs 25, No. 11 , 152- 4 (?\fov. 1952). tion, Il ot" bly L . E. R a wls, G . F . L eidorf, and t heir (Including land a nd sea.) geologic'Ll consultant, P . P arker. The opportunity (P ap er 65D2- 119)

191 JOURNA L OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 65D, No.2, March- April 1961

Publications of the National Bureau of Standards*

Selected Abstracts

Use of the incoherent scatte r technique to obtain ionosph eric M i c ro ~econ d synchronization is obtained by use of nhe temperatures, T. E. VanZandt and K. L. Bowles, J . Geophys. Loran-C navigation system as the link betwee n a m aster Research 65, No.9, 2627-2628 (S ept. 1960). clock at Boulder, Colorado, a nd any slaved clock any where If t he ion-electron gas is in di ff usive equilibrium on t he top in t he Loran-C service area. side of t he F layer, t hen the electron density decreases T he timing system also includes a un ique method for distribu exponentially wit h height, and its logarithmic dec rement is t ion of several t ime code formats 0 11 a single UHF channel. proportional to the neutral gas t emperature. From an electron density p rofil e obta ined by the scatter radar tech Comment on m odels of the ionosphere a bove h m ax F" J . W. niq ue, it is shown that t llis interpretation is co nsistent. Wright, J . Geophys. R esearch 65, No.9, 2595- 2596 (Sept. Moreover , t he ded uction of ionospheric temperatures in t his 1960). way from scatter radar electron density profiles has several Evidence for a gradient of scale height in t he F region is advantages over other methods. shown, a nd di scussed in relation to a simple Chapman model of t he P region a bove hm ax F,. It is suggest ed t hat a simi lar Correlation of a n auroral arc a nd a subvisible monochromatic model, but a ll owin g for a scale-height gradient, may give 6300 A Arc with outer-zone radi ation on Nove mber 28, 1959, somewhat better agreement 1\"ith recent obser vations. B. J . O'Bri en, J. A. VanAll en, F . E. R oach, and C. W. GarU ei n, J . Geo phys. R esearch 65, No. 9, 2759- 2766 (S epl. I mprove ments in radio propagatio n prediction service, 'vV. B. 1960) . C hadwick, Elec. Eng. 79, 721- 724 (Sept. 1960). During a severe geomagnetic storm on November 28, 1959, Data from world-wide ionospheric a nd solar stations permit two Geiger t ubes on satelli te Explorer VII (1959 iota) fou nd close observation of t he changin g state of t he iOll osphere so anomalies in the oute1: radi ation zone at an altitude of about t hat t he maximum usable frequen cy for radio communications 1000 km whi ch appear to be correlatcd in space and timc between a ny two points in t he world can be accurately wit h optical emi ssions from t he atmosphere beneath. Very predicted 3 months in advance. intense narrow zo nes of radiation were detected over a visible aurora during o ll e p ass. The radi'l.tion in t lu-ee such zo nes was h arder t oward low latit udes. On t hree subsequent p asses Other NBS Publications t he radiation zo ne was deduced to be over a subvisible 6300 A arc, whose brightness diminished as the radi ation zones Journal of Research, Vol 65A, No.1, J a nuary- February 1961. became less in tense. The correlation is discussed. 70 cents.

Sea sona l variations in the twiligh t enhanceme nt of [OT] 5577, Faint lines in the arc spec trum of iron (F e I). C. C. Kiess, L. ll. Megill , P. M. J amnick, a nd J . E. Cruz, J . Atmosphe1·ic V. C. Rubin, a nd C. E. Moore. and T en·est. Phys. 18, No. 4, 309- 314 (Aug. 1960). Infrared a bsorpt ion of spectra of so me l-aceta mido pyra noid Measurements of t he t wi li ght en han cement of [01] 5577 were deri vatives a nd reducin g, acetylated pyrano se~. R . Stuart obtained during t he p eriod September 1957 to December Tipson and H . S. Isbell . 1958 at Rapid City, S.D. All these measurements were Monolayers of linear saturated s uccinate p olyester and a ir normalized to t he intensity at sunset or sunrise at a height li q uid in terfaces. 'vV . M . Lee, J . Leon Shercshefsky, a nd of 100 km. The results obtained indicate t h at t here was a R. ll. Stromberg. seasonal dependen ce of t he twilight enhancement of [01] H eat of formation of beryllium chl oride. 'vY . H . Johnson 5577 emission . The enhancement occurred most frequently a nd A. A. Gilliland. in the a ut umn a nd winter m on t hs, t he maximum occurring H eat of decomposit ion of potass ium perchlorate. 'vV . H. about 1 November. The enha ncement alm ost never OCC UlTed Johnson and A. A. Gillil and. during t he sp ring a nd summer months. H eats of formation of lit hium perchlorate, a mmonium perchlorate, and sodium perchlorate. A. A . Gill iland and Some magnetoionic phenomena of the Arcti c E -region, W. H . Johnson. J . W. Wright, J . A.l11lospheric and 'l'en·est. Phys. 18, No.4, H eat of formation of N -dimethy la minodiborane. W. H. 276-289 (A ng. 1960). Johnson, I. J affe, and E. J . Prosen. Several unusual phenomena of E -region ionogram echoes Sep aration of hafnium from zirconium by a nion excha nge. obtained at Thule, Greenlaud (mag. dip 85.5°) are described. J . L. Hague a nd L . A. Machla11. They ar e exp lained as t he effects of electron collisions on the Reaction of sulfur, hy drogen sulfide, a nd accelerators with propagation of radio waves at high-magnetic latit udes. The propylene and butadiene. F. J . Linnig, ]I;. J . Parks, a nd third magnetoionic compone nt (Z-echo) is explained in t his L. A. Wall. way a nd several of its d istingui shi ng features are explained a nd illustrated. New phenomena demonstrate t he existence Journal of Research, Vol. 65C, No.1, January- M arch 1961. of a ll E-pause (vall ey a bove hm .. E), and permit t he measure 75 cents. ment of electron densit ies and collision frequencies therein. Electronic scanning microscope for a spectrographic plate Widely separated clocks with microsecond synchronization comparator. M. L. Kuder. and independe nt di stribution systems, T. L. D avis and R . H. Viscoelastometer for measurement of flow and elastic recovery. Doherty, IRE Wescon Conv. R ecord 4, pt. 5, 3-17 (1960). R . J . Overberg and H. 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T emperature dependence of Young's modulus of vitreous VLF attenuation for east-west and west-east daytime propa germania and silica, S. Spinner and G. W . Cleek. J. Appl. gation using atm~sp h e rics. ViT. L. Taylor, J. Geophys. Phys. 31, No.8, 1407 (1960). Rese'lrch 65, No. 7, 1933 (July 1960). 194 Mechani zed co nversion of colorim eLri e d,ILa to munsell R epl y 10, On the st ru cture of t rimethylamine-t rim ethylboron , l'enotations, W. Rheinboldt and J. P. Y enard. J . Opt. . D . H.. Li ciC' , Jr., J . Chem. Phys. 32, No.5, 1570 (May 19GO). Soc. Am . 50, N o. ,802 (Aug. 1960). Dimensio nal changcs occurring in dent ures dllring processing, The usc of geostationary satellites for the stud.v of ionospheri c J . B. Woelfcl, G. C. Pa ff enba rger, a nd "V. T. Sweeney, J. electron co ntent a nd ionospheri c radio-wave propagation , Am. D en tal Assoc. 61, No.4, 413 (Oct. 1960). O. K . GarriolL and C. G. Li ttle, J . Geop hys. R esearch 65, T eeth, a rtifi cial, G. C. Pa f"f enbarger a nd G. B. D enton, E ncy No. 7, 2025 (July 1960). clopcdif. Britannica 21, 878 (J a n. 1960). Our measurement system a nd nflt ional needs, A. V. Astin, A method of improvin g isolat ion in multi-cha nnel waveguide Sprrl" yscope 15, No.6, 16 (1960). systems, G. F . Engen, IR1£ T rans. lYI icrolyave Theory and Standards of heat capacity a nd thermal co nductivity, D. C. T ech. Letter MTT-8, 460 (July 1960). Ginnings, Book, Thermoelectri city, p. 320 (Inclu d in g Proc. Innuence of earth curvature a nd t he terrestri fl l magnetic fi eld Conf. Thermoelectri city, Sponsored by t he Na vfll R esea rch on VLF propagation, .I . R. Wait a nd 1(. Spies, J . Geophys. Lab., Sept. 1958) (1960). Research 65, 2325 (Aug. 1960). N eutron-insensitiv e proportional counter for ga mma-ra.v Charge transfer and electron production in H- + H co llisio ns, dosimetry, R . S. Caswell , R ev. Sci. Instr. 31, No.8, 869 D . G. Hummer, R. F . Stebbings, " '. L . Fite, a nd L . ~1. (Au g. 1960). Bransco mb, Phys. Rev. 2, 668 (July 19(0). A study of 2-"NIc/s iono spheri c absorption measurements at The characteri stic energy losses of electrons in carbon , L . B. hi gh a ltitudes, K . D avies, J. Geophys. R esearch 65, 2285 Leder a nd J . A. Suddeth, .I. Appl. Ph ys. 8, 1422 (Au g. (Aug. 1960). 1960). Cnrrier-f requ eney dependence of t he basic transmi ssion lo ss Note hi storique sur les p rem iercs a nnces de la microscopie in trop osp heri c forward scatter propagation, 1\:. A. No rto n, electronique, L. Ma rto n, Extrait 131 111. Acad. R oy. Bclg. J . Geophys. R esearch 65, 2029 (July 1960). (Classr drs Sciences) 5, 119 (Mar. ] 959). The ionization co nstants of 2-ch loro-4-ni trophenol a nd :2- An optical study of t he boundary layer t ra nsition p rocesses ni tro-4-chl orophenol, V. E. Bower a nd R . A. R obinso n, J. in a supersoni c a ir system, 'V. Spangenberg a nd 'V. R . Ph.vs. Chem. 64, 1078 (1960). ROII' la nd, Phys. of F luids 3, No. 5, 667 (Sept.- Oct. 1960). Closed cir cu it li q uid hydrogen ref ri geration ystem. D. n. Thermodyna mi c structure of t he outer solar a tmosphere. VI. Chelton, J. ' V. D ean, and B. ' V. Birmingham, R ev. Sci. Effect of dcpartul'cs from thc Saha equat ion on inferred Instr. 31,712 (.Jul y 1960). properties of loll' chromosphere, S. R . Pottasch a nd R . N. A qua nt itative formulation of Sylves ter's law of in ertia, A. i\l. Thomas, Ast rophys. J. 132, 195 (Jil ly ] 960). Ostrowski , NaLi . Acad . Sci. Proc. 45, No. 5, 740 (i\ ay ] 959). Poly mer deco m position: Therm odynamics, mecha nism., and Infra red transm ission of clouds, D . :\'r. Ga tes and C. C. Shaw, energetics, L. A. Wa ll , Soc. Plastic E ngrs. Pt. I, 810 (Aug. J . Opt. Soc. Am . 50, 876 (Sept. 1960). 1960); P t . II, 1031 (Sept . ] 960) . Comparative fi xation of calcium a nd stronti um by synthetic The role of surfa ce ten.. ion in delermining certain clay-water hydroxyapatite, R . C. Likins, rr. G. McCann , A. S. Posner, properties, ' V. C. Ormsby, Bull. Am. Ceramic Soc. 39, ~o . a nd D . B. Scott, J . Biolog. Chem. 235, No.7, 2152 (July 8, 408 (Aug. 1960). 1960). , A co mpa rison of atomic beam frequenc.v standards, R. E . Beehl e r, R. C. :vrock ler, a nd C. S. Sn id er, Nature Letter 187 681, (Aug. 20, 1960). VI. M icroscopic a nd macroscopic energy loss d ist ribul ions. 1. Theo retica l rcvie"'s: A summa r.v, U. Fa no, Natl. Acad. *Publi cations jor whi ch a price is indicated (exce pt j 01' Tech Sci., ~at l. R esearch Coun cil Publ. 752, Report 29, p. 24 nical Notes) are available only jmm the S uperi ntendent oj (Au g. 1960) . Documents, U.S. Government P1'inting O.fJice, lVashin(fton 25, The mecha ni cal properties of cera mi cs a nd their mcasul'e D. C. (Jo reign postage, one-fourth additi onal). T he Technical ment at eleva ted temperatu res, S. .I. Sch neid cr, Book, N ews Bulletin and Basic Radio Propagation P redictions are Thermoelect ri city, Chflpter 21, 342 (1960). available on a 1-, 2-, or 3-year subscription basis, although no A rating method for refri gera ter! t ra il er bodi es ha uling peri sh 1'eduction in rates can be made. Reprints from outside .iou1"1~ al s able foods, C. W. Phillips, W. F . Goddard, Jr., a nd P. H.. and the NBS J ournal oj Resew'ch may ojlen be obtained directl y Achen bach, ASTI RAE J. 2, No. 5, 45 ( ~1 ay ]960). j rom the auth01·s.

195 The National Bureau of Standards

announces a three-week course In Radio Propagation

July 31 to August 18, 1961

Central Radio Propagation Laboratory Boulder, Colorado

This course is designed to provide a discussion of the fundamentals of radio propagation, the latest advances in t he state of the art, and t he application of t his knowledge to the design and development of communication systems. Tropospheric Propagation and Ionspheri c Propagat ion will be considered in two separate sections which may be taken individually or in succession. Details of t his co urse may be obtained from t he Educational Director at t he address given below. The course will consider communication via the entire range of useable radio frequencies and will extend into the modes of propagation which are being explored for the future. In both sections the cont inuing emphasis will be on t hose elements of propagation which affect system design and frequency all ocation. In addition to t he subject matter mentioned above t he two sections will include discussion of the following :

Tropospheric Propagation (July 31-August 4, 1961) The effect of atmospheric t urbulence, and of both normal and unusual atmospheric strati5cation, upon the refraction and attenuation of radio waves . .. Climatology of the atmospheric radio refractive index and its measurement by refractometers or weather data ... Diffraction and refl ection from irreg ular terrain and a bsorption b y t rees and buildings .. . The phase stability of microwave signals and its effect upon systems of t racking, guidance, and geodetic measurement . .. Mechanisms of t ropospheric propagation . .. Variability of t ransmission loss and the theoretical basis for t ransmission loss predic tion ... Modulation studies and techniques . .. Methods for pred icting t he probability of satisfactor y point-to-point co mnlllnication, broadcast coverage, and communication via satelli tes.

Ionospheric Propagation (August 7-18, 1961) Theory of radio wave propagation via the ionosphere, from t he very lowest frequencies to micro waves .. The distorting effects of ionospheric irregularities and dispersion on broad-band radio signals . . A description of the ionosphere-its spatial and temporal variat ions and their predictabil ity . . Transmission loss and its varia bility as a function of frequency and other system parameters . . . Special problems of earth-space communication ... Statistical character and average power of atmos pheric, cosmic, and artifi cial ra dio noise . .. Characterization of t he propagation medium as a time variant communication channel ... Consideration of pert urbat ions of amplitude and phase, mult ipath propagation, and noise as factors affecting modulation techniques, and t he capacity and reliability of systems ... Prediction of performance of ionospheric radio systems fOT communication, detection and pos itioning, navigation and timing.

Prerequisites: Tuit ion: A bachelor's degree in E lectrical Engineering, Tropospheric Propagation- $100 Physics, or other suitable academi c or practical Ionospheric Propagation-$200 experie nce. Entire cOlll'se- $300

R egistrations will be limited and earl y application should be made to enSlll'e consideration. Further detail s of t he course and registration forms w ill be available March 1, 1961, from : Edmund H . Brown, Educa t ional Director , Boulder Laboratories, National Bureau of Standards, Boulder , Colorado.

196 Editorial Notice

Conference on Transmission Problems Related to High-Frequency Direction Finding

This issue includes a group of papers presented in June 1960 at a confer ence sponsored by the University of California at Los Angeles in cooperation with the Office of Naval Research. Thc purpose of the conference was to discuss the aspects of long-range high-frequency radio propagation that affect radio lo cation and direction finding, and the related problems of measurement and analysis. A comprehensive bibliography of published work on direction finding and related ionospheric propagation topics for the period 1955- 1959 has been pre pared by the Numerical Analysis Research Staff of the University of Ca lifor nia at Los Angeles. This bibliography wi ll be edited by the Radio Systems Divisioll of the NBS and published as a Technical Note of the NBS. THOMAS N. GAUTIER, Associate Editor.