JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 66D, No. 3, Ma y-June 1962 Observations of Radio Wave Phase Characteristics on a High-Frequency Auroral Path 1. W. Koch and W. M. Beery Contribution from Central Radio Propagation Laboratory, National Bureau of Standards, Boulder, Colo. (Received June 22, 1961; revised November 7, 1961) Experimental observations of phase perturbations to continuous wave and pulse signals, over short time periods, have been carried out on a high-frequency a uroral path. Statistics on phase perturbations occurring in time intervals of one t o t went y milliseconds were obtained for t he continuous wave signals. P ulse-t o-pulse phase st ability measurements were made, using one-millisecond pulses with a pulse repetition rate of 250 pulses per second. The phase of corresponding parts of successive pulses were compared continuously, and then t he integrated values of phase differences during one-millisecond pulse periods were deter­ mined. Comparisons of statistics of phase perturbations for continuous wave signals a nd one-milliseco nd pulse signals ind ic at e no significant differences for approximately four­ millisecond sampli ng intervals and comparable fading speeds on t his auroral path. 1. Introduction fairly long auroral path. Fast "flutter " type fading has been observed for many years on radio paths A high-frequency radio ignal, propagated by the crossing the auroral zone; phase per turbations of ionosphere over long distances, usually arrives at radio waves may be expected to be quite severe the receiver via several different propagation paths under such conditions. and with different time delays for the various paths. Interferen ce between the multipath signal com­ 2. Experimental Facilities ponents, along with movements of ionospheric layers Observations wero carried out over a path from and irregularities, results in phase and amplitude Barrow, Alaska to Boulder, Colorado during D e­ per turbations of the received signal. E ven when cember 1959, and the first half of 1960 at frequencies one component is much stronger than the others, of 9.9475 M c/s, 14.688 M c/s, and 19.247 M c/ . The rapid movement of the ionosphere may still cause great circle distance is 4,470 km. Figure 1 shows phase per turbations of the received signal as the phase path-length changes. - APPROXIMATE ZONE OF MAXIMUM AURORAL ACTIVITY Bramley [1951] and Voelcker [1960] have consid­ ered the theoretical phase per turbations to be expected over short time periods with R ayleigh III fading signals. Observations of changes in phase path lengths for high-frequency radio waves have been obtained by several workers, notably Findlay [1947], [1951 ], and Price and Green [1957]; however, most of the observing equipment was limited to time resolutions of the order of a second or greater. Lutz, Losee, and Ladd [1959] have suggested that the amount of multipath falling within a received signal element, and hence the phase per turbations, may be reduced by a technique of transmitting short pulses separated in time by intervals greater than the longest multipath delay times ; they sug­ gested I-msec pulses at a repetition rate of 250 pulses per second. A phase-keyed communication system, using this separated pulse technique, would then compare the phase continuously between corresponding parts of successive pulses, the inte­ grated value of the phase comparisons determining the signalling state. 140' 130' The purpose of the observations reported in this 12 0' 11 0' paper is to provide some statistical information on phase perturbations occurring within a few milli­ FIGURE 1. Geographic locations of transmitter and receiver seconds for continuous wave and pulse signals on a stations. 291 the geographic location of the path relative to the is much stronger than others, the pulse-to-pulse undistmbed amoral Zone. A discription of th e anel con tinuous wave phase stabili ties would be the transmitting and receiving equipment is given in same; likewise, if flll of the important modes con­ appendix A. The phase stability recording and tribute energy elming a received pulse period, the analysis equipment is discussed in appendix B. pulse-to-pulse phase stability is not likely to be much better than continuous wave phase stability. 3. Phase Stability Characteristics If pulses arc used which are short enough to isolate the vflrious multipath components, and then phase Regardless of the number of multipath components comparisons made for successive pulses of each present in samples of the received wave, if the of the most stable modes, an improvement over ionosphere is essen tially fixed in characteristics and continuous wave phase stability should result dm-ing in space over the sampling interval time, there will periods when fl number of multipath components be no phase pertmbation from one sample to the are present. next. However, movements of irregularities and of t he regular layers in the ionosphere, as well as 3.1. Theoretical Distribution of Phase Varitaions changes in ionization along the path, will cause changes in phase path lengths of the various received In the case of Rayleigh distributed faeling of a components and in the interference pattern between continuous wave carrier envelope, and with the the components. Since the ionosphere is constantly assumption of a normal-law power spectrum of the in motion, and especially so in the amoral regions, fading carrier, one may derive theoretical probability it is logical to assume that phase pertmbations will distributions of carrier phase pertm-bations for increase with the number of multipath components various length sampling in tervals and fading speeds present in the samples. Therefore, one might con­ from the work of Bramley [1951] and Voelcker clude that the phase stability would be better for [1960]. The expression short pulses than for continuous wave signals, with phase sampling intervals equivalent to the pulse P (IL1<P I 2:<p,) = l -! [ <P i+ R sin <P i COS-I( - R cos ¢;) ], repetition intervals. However, if one arriving mode 7r (1-R 2c os2 <P i)} obtained by integrating the expression given by Bramley [1951] for the probability density function of phase variations, is plotted on figure' 2; R, the envelope of the normalized autocorrelation function 100 ~-~-~--~-~----;,-----.---~ of the fading carrier, is given by the expression, <! <f) <f) u <f) (D <! <f) In the last expression N m is the number of times pel' 8 10r-+-~-------------~ w second the envelope voltage of the fading carrier <Jx crosses the median level with a negative slope, and T W z is ~he tin1e interval in milliseconds between sampling Q !;; pomts. Admittedly, the envelope distribution of (D a:: fading HF waves may depart considerably from a ::J I­ R ayleigh distribution [Koch and P etrie 1961]; much a:: w of the time the distributions approach' those for the "'­ w sum of a specular com ponen t and randomly varyin g <f) <! I components. In the latter case, the phase variations "'- should be less than those indicated by figure 2. ~ 0.1 r------~,__------"'-...._--_j I- L>- o 3.2. Observed Phase Perturbations of CW Carriers W <! '"I- Observations of phase perturbations of the re­ Z t! 0.0 1r----------------="---j ceived continuous wave carriers were carried out in n: NOTE : Nm IS THE AVERAGE NUMBER Of TIMES PER g: SECOND THE INSTANTANEOUS ENVELOPE VOLTAGE D ecember 1959 and in February and April 1960. CROSSES THE MEDIAN LEVEL WITH A NEGATIVE These observations have been analyzed to obtain SLOPE (DEFINED AS THE fADING RATE ). stati.stics on phase sta ~ility o! the propagating medIUm between samplmg pomts separated by 0.001 ~o ---:17"0 ----:-40=---7::60--80:-0---:10""0 -----":11""'0 -...,.J140 various length intervals. Sample distributions of phase perturbations of the DEGREES PHASE PERTURBATION received carrier is plotted in figure 3 for phase sampling intervals spaced from 1 to 20 msec. All FIGURE 2. Theoretical distributions of phase perturbations for fading CW signals for various length sampling inte1'vals distributions are for a 200-sec sample of the received in milliseconds. signal. Figure 4 shows distributions of phase 292 100.0 100 BO.O (I) 113 4 MST, DECE MBE R IB, 1959, FAD I NG RATE - 2c/ s <! ~>, (fJ ' ......... (fJ 40P ",,- u (fJ en 1\\'\ -""""' 0 <! 20.0 "- (fJ 0 ~\ \ "'-0 '''-.,. w 10 10.0 , w B.O u s. x \\ \ ~;o( w ~ ",'",' 4.0 " 0 z . ~ '2 \ 0,, 0,,~, w \\ ~Sf>C '"w "'~ ~ n !;;: u 2.0 x en \ 0 ° ~ w a: • "-0 "0,,- :::l z \ -,'" "'. 1'---0 1"-0 c- 1.0 , :!;.s", '- ...... 0 a: ", '" 0.8 s·c w I " , § \ 0 ,,~ Q. " ~ ,,~ ~S w " '" (j) ~ 0.4 ", r-.... w'" -............. <! '", ~ ~ I ", Q. " i"-- "-0,,- . w 0.2 "<?- 0 w ~ 0.1 ~c "'0 :;; ) ~'" ~ >= w 0.1 " "- ,. 0.08 lL ;: 0 ~ 1\ W '" 0.04 "<! W c- 0 \, , Z ..~ "'. w Z 0.02 u 0.01 W U "" a: \ w W , Q. ~'" 0.01 0.00 B EACH DISTRIBU T ION CU RVE IS FOR A 200 SECOND SA MPL E 0.00 4 0.002 0.001 ;----0 1;;;:0--::40:----;60::----:8:::-0 ---;1;;;00--:';11:-0 -~1 4 0 DEGREES PHASE PER T URBATION 0.00 I o 20 40 60 BO 100 120 140 DEGRE ES PHASE PERT URBATI ON F I G URE 4. T ypical phase stabiltty oj 19.247 A1c/s CW stgnals betwee n sttccess'ive sam pling lJoints at 4,5 msec i ntervals FlOURIJ 3. Distribt,tions oj CW cania phase pertUl' bation fo r various fading m tes . between various length sampling intervals. Sempling period 200 sec IF bandwidth 800 cis (-G db) 3.3. Observed Pulse-to-Pulse Phase Perturbations Media ll carric r·to·noise ratio 33 db A vcragc fading rate 7 cis 19.247 ;\le/s, April 25, 19GO; 1<156 M S'r Observations of pulse-Lo-pul e phase sLabiliLy were carried ouL from June 20, 1960 through July 2; 1960.