tion on scintillation of the full complex signal, in the form of .... University of Alaska's Poker Flat Rocket Range, near the ...... The observed tilt of the ellipse arises.
Radio Science, Volume 13, Number 1, pages 167-187, January-February 1978
Earlyresults fromtheDNA Wideband satellite experiment--Complex-signal scintillation E. $. FremouwI, R. L. Leadabrand, R. C. Livingston,M.D. Cousins,C. L. Rino, B.C. Fair, and R. A. Long .
SRI International, Menlo Park, Cali[ornia 94025 (Received March 16, 1977.)
A multifrequency(ten spectrallines betweenVHF and S band) coherentradio beaconis presently transmittingcontinuouslyfrom a 1000-km, high-inclinationorbit for the purposeof characterizing the transionospheric communicationchannel.Its highphase-referencefrequency(2891MHz) permits direct observationof complex-signalscintillation,and its very stable, sun-synchronous orbit allows repeatedpre-midnightobservationsat low latitudesand near-midnightobservationsat aurorallatitudes. We present here early results of the observations;salient points include the following. First, most of the data are consistentwith phase-screenmodelingof the productionof ionosphericscintillation,
includingan f-2 frequencydependence for phasevariance.Second,propagation theoriesinvoking weak, single scatter seldom are adequate, because even moderate intensity scintillation usually is accompaniedby phasefluctuationscomparableto or greaterthan a radian.Third, underconditions producingGHz scintillation(nearthegeomagnetic equator),lowerfrequenciesshowmarkeddiffraction effects,includingbreakdownof the simplef-2 behaviorof phasevarianceand lossof signalcoherence across a band as narrow as 11.5 MHz
1.
at UHF.
INTRODUCTION
1.1. Objectives. On 22 May 1976, satellite P76-5 was launched into near-polar orbit for the solepurposeof carryinga multifrequency,coherent radio beacon (DNA-002) into a 1000-km orbit for transionosphericpropagation studies. The spacecraft is a modified Navy Navigation Satellite that was strippedof its navigation-datamodulatorsand augmentedwith a 1.5-m ground screen for earthcoverage radiation of 10 CW signals from the payload.The mutuallycoherentsignals,whichrange from VHF to $ band, are beingrecordedat ground stationsin Alaska, Peru, and the mid-Pacific; initial
auroral and equatorial latitudes. The nature of deleterioussystemseffects arising from such perturbations depends on the time structure of the resulting signal fluctuations. For example, if the scintillation rate is comparableto or greater than the time spent in transmitting one data bit, the modulation
will be distorted
and errors will result.
The severity of performance degradation in this fast-fluctuation regime depends critically on the type of modulation used. In many communication systems, the bit transmission rate is much greater than the scintillation rate. For such systems, phase scintillation is of little concern and amplitude fading is of primary observations were made in California. interest. For other types of systems(such as synThe experimental program, called the DNA thetic aperture radars and certain kinds of positionWideband satellite experiment was conceived, deing systems),phaseis very important. A key element signed, constructed, and fielded by Stanford Reof the Wideband experiment is to provide informasearch Institute for the Defense Nuclear Agency tion on scintillation of the full complex signal, in (DNA). The program objective is to extensively the form of both high-time-resolution records and characterizethe perturbations(mainly scintillations) reduced-datasynopses.Besidesintensity and phase imposed on passing radio waves by structured scintillation indexes at the various frequencies, plasmassuch as those found in the ionosphereat second-orderstatisticalmeasuresare being obtained in the temporal, spatial, and spectraldomains. 'Now with PhysicalDynamics, Inc., Bellevue, Washington The second-order temporal statistics are being 98005. characterizedin terms of power spectraof intensity CopyrightI• 1978by the American GeophysicalUnion. and phase,while spatialstatisticsare beingobtained
0048-6604/ 78/ 0102-0167503.00
167
168
FREMOUW, LEADABRAND, LIVINGSTON, COUSINS,RINO, FAIR, AND LONG
in the form of intensity and phase correlation coefficients between UHF receiving antennas spaced on baselines of a few hundred meters. Spectralcoherenceis being characterizedby means of analogouscorrelationcoefficientsbetween spectral lines in the seven-lineUHF spectrum. 1.2. TheDNA Widebandbeacon. The payload is a multifrequency beacon that radiates a total of ten phase-coherentRF spectral lines: one at VHF, seven at UHF, one at L band, and one at S band.
All transmitted frequencies are harmonics of 11.4729MHz, being derived from the same crystal oscillatoroperatingat 34.4187MHz, and the intended polarizationfor all frequenciesis right circular. The radiated frequencies are shown in Table 1, together with the maximum effective radiated
•
to other receivers •HF
%!tttttttt SlITTER AND PLOs•. m
REFERENCE I I'-¾- offset • .......... tl MEASUREMENT R•C•,V•R/ spectruml •1 RECEIVER I? .sl
W
/
/
final
,.ectru
IJ
•to vco _1 S.•CTRU• I I • GENERATOR
TO FILTERS*
*Selectable bandwidth
AND A/D
of 37.5, 75, or 150 Hz
powers.
The antennas are nadir-directed; however, in an attempt at approximately uniform earth coverage, all beams except that for VHF were tailored to radiate somewhat
Fig. 1. Simplifiedblockdiagramof coherentreceiveremployed, illustrating thereferencechannel andoneof the9 to 12(depending upon station configuration)measurementchannels.
less at beam center than toward
the earth's limbs. In the case of the four highest UHF spectral lines, the beam observed after launch has a much deeper central null than intended (approaching 20 dB). Observers who may want to receive one of the UHF signalsfrom DNA-002 are
2.
GROUND
INSTRUMENTATION
AND DATA
HANDLING
2.1. Coherentreceivers. The key to the experiment is to retain
the coherence
inherent
in the
transmittedsignals,throughoutthe receivingoperaThe satellite is gravity-gradient stabilized, and tion on the ground.This is accomplishedby means of the receiver, illustratedschematicallyin Figure the orbit is sun-synchronousand highly stable. At 1. The heart of the receiveris a frequencysyntheepoch 1 January 1977, equator crossingswere at 1122:43(northward) and 2322:43 (southward) local sizerthat is essentiallya replicaof the satellite-borne time and were drifting later at the rate of 1.6 sec transmitter.The synthesizerproducesa spectrum per day. This orbit was chosenas a compromise identical to that transmitted, which is then offset signalsplusothersignals betweendesiresfor pre-midnightobservationsnear to providelocal-oscillator the geomagnetic equator and post-midnight ob- employedin severalfrequencytranslations. servations in Alaska. The synthesizeris phase-lockedby meansof a loopoperatingon the outputof the S-bandreference receiver. The reference re,•eiverchain is identical, TABLE 1. Transmission characteristics. advised to select one of the three lower sidebands.
Designation
Frequency
Harmonic number
(MHz)
(of 11.4729MHz)
ERP (dBm)
VHF
137.6748
12
+26
UHF
378.6057 390.0786 401.5515 413.0244 424.4973 435.9702 447.4431
33 34 35 36 37 38 39
+ 27 + 26 +30 +27 + 27 + 25 +28
108 252
+ 25 +27
L band
S band
1239.073 2891.171
except for the first mixer, to the various measurement receiver chains, one of which (for one of
the UHF spectrallines) is illustratedin Figure 1. There is one measurement receiver channel for the
VHF signal, one for the L-band, and one for each of the seven UHF signals. In addition, there is an identical measurement channel for each remote
receivingantenna, the number of which is different at the various receiving stations.
The signalin eachreceiverchainfinally is translated to an essentiallyzero-frequencybaseband signal by means of two quadrature detectors. In practice, the output is at the differential Doppler
COMPLEX-SIGNAL TABLE
Station
2.
Ground station locations.
Islands
Stanford, California
9ø24'08"
37024' 10"N
147ø29'14"W 77ø9'00"W 167028' 10"E
122ø10'27"W
169
spectral line (413 MHz). The remote antennasare
Geodeticcoordinates Geomagnetic of main antenna dip Latitude Longitude latitude
Poker Flat, Alaska 65"7'34"N Ancon, Peru 11ø46'34"S Kwajalein, Marshall
SCINTILLATION
65.4øN 0.4øN 4.4øN
42.8øN
optimallydirectedfor a specifiedsatellitepassbut do not track, relative to the main antenna;they are on geomagnetically oriented baselines of 300 m west, 600 m east, and 900 m south. At Ancon, the main (tracking) antenna is a 9.1-m dish and there is a single remote antenna 300 m east of the
dish.Onlya single(9.l-m, multifrequency, tracking) antenna
was used at Stanford.
The receiver outputs are bandlimited in video filters that have selectablebandwidthsof 150, 75,
frequencybetweena specificmeasurement signal or 37.•5 Hz. All operation to datehasbeenat the and the S-band reference. In the case of the remote
maximum bandwidth on all channels. The filtered
antennas,the outputfrequencyis the algebraicsum of the differentialDopplerfrequencyand the relevant interferometerfringe rate. The output signal is modulatedby intensityand phasescintillations. TherearethreeexistingWidebandsatelliteground stations.The first is situatedjust northof Fairbanks, Alaska, for studying propagationeffects of the auroral ionosphere. The station is located at the Universityof Alaska's Poker Flat Rocket Range,
outputsare digitizedon-lineandare recordedmagneticallyundercontrolof a stationminicomputer (HP 2100 with 16k words of memory), which also controlsantennatrackingand receivertuning. To maintainmaximumlinearity, it is the quadrature components of each measurement signal that emergefrom the receiverfor recording;mostdata processing,however, is in terms of intensityand phase.Asidefrom oscilloscope andmeterdisplays, near the town of Chatanika. The location was chosen the only real-timeoutput is a strip chart of VHF to permitcomparativestudieswith soundingrockets intensityand phase, which is calculatedby the andexistingground-based instruments,particularly computerfrom the VHF quadraturecomponents. the incoherent-scatter radar at Chatanika[Leada- An exampleof the real-timerecordsis presented brand et al., 1972]. The other two stations are situatedfor studiesof equatorial scintillation.One
in Figure 2. The record shown in Figure 2 was obtained at
is at the Instituto Geofisicodel Peru's receiving siteat Ancon(nearLima), whichwaschosenlargely becauseof its location beneath the equatorial electrojet.Comparativestudiesare plannedwith the backscatterradarat Jicamarca[Bowles,1963].
PokerFlat, Alaska,on 23 November1976,starting just after local midnight. (Local standardtime at Poker Flat lagsUniversalTime by 10 hours.)The passwasfrom northto south,reachinga maximum elevation of 73 ø to the east of the station. The
Initially, the third Wideband station was located main characteristicof the passis that it provided at Stanford, California, for observation of the a clear-cut example of the subauroralscintillation beacon signalsthrough the relatively undisturbed boundary [Aarons et al., 1969], with the effects midlatitude ionosphere. The Stanford station was of ionospheric irregularities evidentthroughoutthe moved to Kwajalein atoll in the Marshall Islands first half of the passbut absentin the secondhalf.
in September-October1976 to provide a second equatorial station. Comparison of results from Kwajalein and Ancon will permit study of the suspectedmorphologicaldifferencesin scintillation at different longitudesand similar geomagnetic latitudes[Basu et al., 1976]. Rocketsoundings of the equatorial ionosphere also are planned at Kwajalein in conjunctionwith Widebandpasses. Station coordinatesare given in Table 2. At Poker Flat and Kwajalein, there are four receivingantennas'a 3.7-mdishthatprogram-tracks the satelliteand receivesall ten signals,and three remote antennasthat receive only the central UHF
The boundaryis obviousupon inspectionof the intensity record; it is als0 evident in the phase record,whichrevealsa smooth,slab-likeionosphere to the south but many gradientsin total electron content(the line integralof electrondensityalong the radioline of sight),or TEC [Klobuchar,1973], tothe
north.
2.2. Separationo[ scintillations[rom trends. If onethinksof scintillationas arisingfrom relatively small-scale(on the order of a kilometer)irregularities imbeddedin an otherwisesmooth, vertically stratifiedionosphere,then onewouldexpecta phase recordto be easilyseparableintoa smoothlyvarying
170
FREMOUW, LEADABRAND, LIVINGSTON,
ß
:
:
,
:
I
:
:
•
:
,
•
.
,
COUSINS, RINO, FAIR, AND LONG
•
.
,
[ 1013
1014
1015
TIME,
1016
UT
Fig. 2. Exampleof real-timeoutput: strip chart of VHF (138-MHz) phaseand intensityobtainedat Poker Flat, Alaska, on 23 November1976,just after local midnight(AST = UT - 10 hr). Note transition(subauroral scintillation boundary)from irregularto smooth,slab-likeionosphere as satelliteline of sightscanned throughabout64øN magneticinvariantlatitude.
trend (caused mainly by the changing pathlength througha slab-like ionosphericlayer) and relatively fast fluctuations, i.e., phase scintillations. In practice, however, it appears that, when structured, the ionosphere contains irregularities on a vast spectrumof scalesextending at least from the order of a hundred meters to at least tens and maybe
hundredsof kilometers [Dyson et al., 1974]. A radio wave passingthroughsucha structuredplasma develops phase perturbations across the same ex-
tendedspatialspectrum[Bramley, 1955]. Intensity scintillation, however, is a pure propa-
gation effect arising from a combination of focus-
ing/defocusingand diffraction pattern development, as will be discussed in section 3.2. As a
consequence,the spectrumof intensityscintillations is limitedat the low-frequencyend by the phenome-
non of Fresnel filtering [Ru[enach, 1971]. The extended nature of the phase spectrum and the Fresnel cutoff of the intensity spectrum require that scintillationbe definedon the basisof intensity. Thus, we have chosen to regard as phase scintillations those variations
that are as fast or faster
than the slowestintensity scintillationsand as phase
COMPLEX-SIGNAL
trendsthosechangesthat are still slower. The spatial spectrumof the ionosphere,of course, is translated into a temporal spectrum of the signal by the scanningmotion of the radio line of sight. The scan velocity in the F layer is about three kilometers per second and that in the E layer is about one kilometer per second. In April 1975, a satellite (P72-2) carrying the first DNA
Wideband
beacon
171
450
e,• 6
was lost due to a launch
300
failure. (The beacon now operating was a backup payload.) In the intervening year before launch of the presentWideband satellite, interim observations were made of the 150-MHz and 400-MHz signals radiated for navigational purposesfrom a number of operational Transit satellites. On the basis of the interim data, a fluctuation period of 10 sec was established
SCINTILLATION
as the demarkation
between
"trends"
and "scintillations." Specifically, it was found that removing intensity trends with Fourier periods longer than 10 sec did not appreciably alter the
150
I
o
lOO8
1010
I 1012
I 1014
TIME,
1016
I 1018
1020
UT
Fig. 3. Intensity and phase trends obtained from record displayedin Figure2 by employingsix-pole,Butterworth,low-pass software filters having 3-dB cutoffs at 0.1 Hz. The manifold 2-tr ambiguity in the starting point of phase is resolved by dispersionanalysisof three UHF spectrallines.
measured valueof the S4 [BriggsandParkin,1963] scintillation index, which is defined as the standard
deviationof the receivedsignalintensity normalized to the mean intensity. The first data processingprocedurein the Wideband satellite experiment consists of separating trendsand scintillationsin the following straightforward manner. First, the phase and intensity of the received signal are each smoothed by means of a low-pass, six-pole Butterworth filter with a (3-dB) cutoff at 0.1 Hz. The smoothed phase is stored as the phase trend and also is subtracted from the point-by-pointphase;the residualis stored as phase scintillation. The smoothedintensity also is stored, and the raw values of intensity are divided by the intensity trend. Thus, the "detrending" process is equivalent to passingthe received signal through a coherent
AGC
circuit.
The intensityand phasetrendsfor the VHF record shown in Figure 2 are displayed in Figure 3. While the (dispersive) phase cannot be measured absolutely in the experiment, it is possible to obtain an estimateof the absolute startingvalue on a given frequency by measuring the second difference of phase among a triplet of UHF spectral lines. The
seconddifferenceof phase,A•q•, is obtainedby forming the phase difference between a "carrier" and an "upper sideband" and again between the corresponding "lower sideband" and the same "carrier" and then taking the difference of the differences.The result is a measureof dispersion
that is related to TEC [Burns and Fremouw, 1970] as follows'
A2•= 2-ire0rnc f3 o Nds
(1)
where
electron charge
permittivityof free space electron
mass
speed of light
"modulation" frequency separatingthe spectral lines of the triad
"carrier" frequency (frequency of the center line of the triad)
and TEC is given by the integral of electron density, N, along the line of sight from the receiver to the satellite.
The individualdispersivephasesalso are proportional to TEC
as follows'
=
'
Nds
• 4*r•ornc[ o
(2)
The frequencies of the UHF spectrum were chosen
to providea sufficientlysmallvalue of [m that (1) can be employedto resolve the manifold ambiguity in (2). The lower part of Figure 3 contains scales for both TEC and an estimate of the absolute value
172
FREMOUW,
LEADABRAND,
LIVINGSTON,
COUSINS, RINO, FAIR, AND LONG
of VHF dispersivephase.The plot indicatesclearly that the radio line of sight encountereda transition between the auroral ionosphere (produced by charged-particle precipitation) and the relatively smooth and depleted lower-latitude ionization "trough" at about 1014UT. That the TEC transition is also the location of the scintillation boundary is evident on careful inspection of Figure 4 (especially the phasechannel), which showsthe detrended phaseand intensityscintillationrecordsobtained from the passbeing discussed. The intensity-trend record in Figure 3 reveals that some intensity-scintillation energy does fall outside the passband of the 0.1-Hz detrend filter. That is, there are random fluctuations of a few
dB in the "trend" during the first third, or so, of the pass. However, comparedwith the scintillations of up to 15 dB (peak to peak) shown in the detrended intensity record in Figure 4, the trend variations would contribute little to the intensity scintillation
index.
Furthermore, careful inspection of the intensity trend in the final third of the passreveals variations of a couple of dB with periods as short as about twenty seconds.The latter variations are thought to result
from
structure
in the transmit-antenna
pattern; use of a longer detrend cutoff would contaminate
the scintillation
records with such ex-
perimental artifacts. Because scintillation is bulkprocessed for statistical analysis, it is beneficial to deal with suchproblemsin the trend component, which is treated more deterministicallyin individual cases.
than a second arise from focusing and defocusing in irregularities behaving as lenses. Such behavior is facilitated by the dominanceof large-scalestructure in the ionosphere's spatial spectrum. Again, the effect is less prominent at higher frequencies because the focal length of such lenses increases with decreasingwavelength, due to the dispersive nature of the ionosphere. 2.3. Parameters routinely measured. The raw-
data tapes consistof time records of quadrature componentsfrom each of the receiver channels(up to 13 channel pairs), each record digitized at the rate of 500 samplesper second. The raw data are retained for such purposesas high-time-resolution spectral analysis of data from events of special interest. Bulk processing, however, is done on reduced-data tapes consisting of the trends and scintillationsseparatedas describedin section2.2. Before detrending, the data are decimated to 100 samplesper second(sps). The detrendeddata are decimated once more to 50 sps for calculation of varioussignalmoments,which are output in tabular and graphical form. Becausethe detrendingprocessis very time-consuming,only nine channelsare detrendedroutinely at each station (six at Stanford), as shown in Table 3. In Table 3, capital letters standfor the frequency band; the subscripts l, u, and c stand for lower sideband,uppersideband,andcenter, respectively; e, w, and s stand for east, west, and south remote antenna, respectively; and numbers stand for sideband orders.
DetrendedVHF strip charts, such as the sample in Figure 4, are generated in the field from each reduced-data tape. The signal moments that are output are identified (in the configurationused for Poker Flat and Kwajalein) in Table 4. In Table
More important, it is often the casethat statistical stationarity does not exist in the records for more than a few tens of seconds,which correspondsto a few times 30 km in the F layer and a few times 10 km in the E layer. This is especially true in
4, ] is a valueof theintensity trend;• is a value
the Alaskan records, where scintillation often is
of the phasetrend; S4 is the intensity-scintillation
"patchy." To use a longer detrend cutoff would complicatestatisticalanalysisconsiderably. The subtletiesassociatedwith intensity scintilla-
index, defined as the normalized standard deviation
tions slower than 0.1 Hz are confined
ofintensity; (•, isa phase-scintallation indexdefined as the standard deviation of phase;p• andp•are
to the VHF
recordsbecausethe Fresnel-filter cutoff frequency increaseswith decreasing radio wavelength. The very existenceof such scintillations, even at VHF, is interesting because the Fresnel-zone radius,
X/-•z, for our VHF signalandsatellitealtitudeis on the order of a kilometer in the F layer and a half kilometer in the E layer. This means that intensity scintillations with periods much longer
TABLE 3.
Channels contained on reduced-datatapes.
Poker Flat and
Kwajalein
Ancon
Stanford
COMPLEX-SIGNAL
SCINTILLATION
173
o
1008
1009
-/r,, '. • i 50
1010
Ji •1'i t , i i i ' J'l ,I.... I ,,,'
1011
1012
''-- ,•l,,,, , i
[•'•'l";."'"•"-'":'J'"• .... "'•':'_•';'"5:•'5'"_:1'-':"/'_'•"-";5t'-'-" ';:"t; ','"';'"';':'"•':-;';'"1 .... ;'"; .... ;'"'; .... I'",-•" '" •
•.•-•=•F•
•'a:i•'q"-t-•-•-ffVfq
0 •0•3
t-t4q--•-:l--q
t!
t I I I I tlt
tf
t I / Iit
II
I
I
I
•0•4
•o•s
•0•6
TI•E,
1
UT
Fig. 4. VHF phase and intensity scintillationsisolated from the record displayedin Figure 2 by employingthe trends displayed in Figure 3 as the reference signalto a software coherent AGC processor.
correlation coefficients for phaseand for intensity,
phase scintillation indexes), while the last row
respectively; •a, isthestandard deviation of phase
contains information on second-order statistics,
difference.
namely, measuresof spectral coherence (between the upper and lower third UHF sidebands)and of angle scintillation and spatial coherence (between
Thus, the top three rows reveal trends and firstorder, signal-statisticalmoments (i.e., intensity and
TABLE 4.
Reduceddata tabular outputs(Poker Flat and Kwajalein).
Iu,•
•u,
Iu.,
•u.,
Ius
•Us
P•U3
PlU3
O'•,l, Uew
P• Uew
PlUew
O'•,l, Ucs
174
FREMOUW, LEADABRAND,
LIVINGSTON,
COUSINS, RINO, FAIR, AND LONG
the most widely spacedeast-west and north-south antennas).Each of the entriesin Table 4 represents a columnof data, with one value output for each 20-secsegmentof a pass.Eachdatarow is preceded by marks of time, azimuth, and elevation. Explicit definitions of the scintillation indexes
associatedwith scintillation. Consequently,even a cursory scanningthrough detrended strip charts suchas the one appearingin Figure 4 is instructive in revealingrecognizabletypes of complex-signal scintillation.Examplesof somerecurringtypesare presentedin Figure 5.
and correlationcoefficientsobtainedare given in By far the most common type is that termed (3) through (7), where I and • are understoodto "classical"scintillation, whichconsists of relatively representdetrendedvaluesof intensity(amplitude rapid variations in received signal intensity and squared)andphase,respectively,and() represents slower phase variations, both highly.random in a temporal average over 20 sec. nature. Classicalscintillationusually occursin intervals of a few minutes during a pass, corre$4• - ((i •') _ (i) •')/(I) •' (3) spondingto fields of ionosphericstructurehaving ß
(4)
north-south extents of several hundred km. Similar
behavior occurringover a shorter interval, on the (5) •ab order of a minute or less, correspondingto a patch of less than a couple hundred km in north-south (6) p•Iab = (l. lb)/((l•)(l•)) l/• extent, is referred to as a "classicalpatch." Classi2 (7) cal patchesaboundon passesover Poker Flat and In additionto the above signalmoments,a dB fade are seen less frequently at the lower-latitude staindex is computed for VHF, UHF, and L band. tions. We have found recently that there often is It is definedas the (negative)dB excursion(relative a prominentclassicalpatch of enhancedscintillation to the established trendlevel)of the 98thpercentile locatedat the boundarybetween the auroral ionosphereand the lower-latitudetrough. point during the 20-sec interval, calculated from Anothertype of very randomintensityand phase a sample population of 2000 points (i.e., from a fluctuation that has been observed a number of record having 100 samplesper second). times, mainly at the lower-latitude stations, is It will be noted that the definition of correlation coefficientusedfor intensityis different from that termed "classical slow" scintillation because the usedfor phase.The definitiongivenin (6), in which intensity variationsappear slower to the eye on the mean is not subtracted,is preferable in the stripchartsthando classicalintensityscintillations. inevitablepresenceof systemnoise; in the absence Another characteristic of classical slow scintillation of scintillation, it takeson a valueextremelyclose is that the phasevarianceassociatedwith a given to unity. The coefficient defined in (7), however, level of intensity variation appearsto be smaller than for classical scintillation. In contrast, occareduces to a small value in the absence of scintillasional occurrencesof strong phase variations in tion because system noise is uncorrelated in difthe virtual absenceof amplitude fluctuation have ferentreceiverchannels.The meanvalueof phase is effectivelyremovedin the detrendingprocess, beennoted,promptingthe term "phasesansampli-
however,and there is no steadyvalue of phase analogousto the long-termmean receivedsignal
tude"
scintillation.
In addition to the various types of random scintillation,Figure5 containsexamplesof moreisolattion of correlationcoefficientis used for phase, ed and distinctivesignal fluctuations. First, when out of necessity;it is a meaningfulparameter scanninga record of essentially random scintillawheneversignificantphasescintillationis present. tions, in a few instancesone's eye is caughtby a particularlysuddenphasechangethatpresumably correspondsto a "sharp phase gradient" in the 3. SOME EARLY RESULTS wavefront arriving at the ground. Such a feature 3.1. Typesand severityof complex-signal scin- may be accompaniedby an associatedamplitude tillation. Wideband is the first coherent beacon diffraction pattern. Finally, still more isolated or to providean essentiallyundisturbedphaserefer- "discrete" phasevariationsand corresponding difence and therefore to permit a full and reliable fractionorfocus/defocuspatternsoccur,especially characterizationof the complex-signalstatistics in midlatitude records.
intensity.As a consequence,the zero-meandefini-
COMPLEX-SIGNAL
CLASSICAL
SCINTILLATION
175
Sl-47
LIJ'O
m 50•
C LASSICA L PATCH
P6-8
CLASSICAL
SLOW
A3-39 LLJ -•
m 50 ..... .
PHASE
SANS
AMPLITUDE
SHARP
PHASE
GRADIENT
DISCRETE
I.U 'o
m
50
ß
.........
F1 minq
P5-•
,0
P6-39
Sl-33
Fig. 5. Recurringtypesof (VHF) scintillation. By far the mostcommontype is "classical"scintillation, includingthe subtype called"classicalpatch."The letter preceding the passidentification numberat the upperright of eachrecordstrip denotes the station: S, Stanford, P, Poker Flat; A, Ancon.
While it is intended to study scintillation morphology systematicallyin terms of the' signal moments•especially the phase scintillation index• defined in section 2.3, a visual inspection scale of (intensity and phase) scintillation level has been establishedas a means for quickly obtaining an overview of scintillation activity at the various stations.The overview is provided by scanningthe detrendedVHF strip chartsto determinethe severity and extent of intensity and phase scintillation, by comparisonwith records selectedas archetypes of seven activity levels ranging from "extremely quiet" to "extremely active." Figure 6 illustrates the archetypes that define the extremes and middle of the seven-level activity scale. To give some initial indication of the level of complex-signalscintillationobservedover a period of about five months (June through October 1976) in the presentepochof minimalsolaractivity, Figure
7 shows the record count by activity level and ground station for passesinspected thus far. Note that the Stanford data base is from the end of May to early September only and that the Kwajalein data base is primarily from the month of October. It is interesting to note that only the equatorial stationshave yet reported either extreme of activity. It is well-known that extreme scintillation activity is encounterednearthe geomagneticequator [ Skinneret al., 1971]. It is rather remarkableto encounter records from the same region that imply an ionospheric smoothness on the order of one electrical degree at VHF (i.e., on the order of a centimeter or less for horizontal distances up to 10 km or more) at night as well as in the daytime. 3.2. Signal statistics of classical scintillationw First-order statistics. Since the majority of scintillation is of the type termed classical (and its subtype, classicalpatch), it is this type that is most
176
FREMOUW,
LEADABRAND,
EXTREMELY
LIVINGSTON,
COUSINS, RINO, FAIR, AND LONG
ACTIVE
A4-29
m .50 0"
MODESTLY
ACTIVE
EXTREMELY
QUIET
S1-27
50
A3-13
•...*,,* ,%;-•%,, ;;-.½• • ,•••'- ..-? •.., '•:• •.•.• ....................
•...•:•... ;:-•;•.•:• :,:•:;; •4•:•-;;;• -•,:-.:'::'•
,11 mi. I Fig. 6. DetrendedVHF recordschosenas typicalto definethe extremesand middleof a seven-levelscalefor quickdetermination of the combinedseverityand extent of complex-signalscintillation.
important to characterize signal-statisticallyfrom the point of view of modeling the transionospheric communication channel. Perhaps the most useful engineering information to be gleaned centers around the frequency dependenceof scintillation. As an example of frequency behavior, Figure 8 displays detrended strip charts of intensity and phase obtained from a pass over Poker Flat that was categorizedas very active on the basisof visual inspection of the VHF record. The Same features are notable at VHF, UHF, and L band, ranging from fades as deep as 40 dB and phase excursions of several xr at VHF to barely discernibleintensity scintillations(a couple of dB) and phase scintilla-
track each other remarkably consistently. The intensity scintillation indexes exhibit somewhat more variability from frequency to frequency. The same characteristics carry over to the fre-
tions of less than ___xrat L band.
a remarkably ordered frequency dependence.The only departuresfrom a strictf-• dependenceshown by the raw data points occur at L band, and these departuresare entirely instrumentaleffects stemming from phase scintillation of the S-band reference signal.
The intensityand phasescintillationindexesmeasuredduring this pass are shown in Figures 9 and 10, respectively. Again, the same features are identifiable on the VHF, UHF, and L band frequencies.Indeed, the standarddeviationsof phase
quencydependences of S4 and (r,, which are illustrated (respectively) in Figures 11 and 12 for
two selected20-secintervals.The S4valuesdisplay a moderateamountof scatteraboutan f-1.5 dependence. They also exhibit a "saturation" as the curves approach the limiting value of unity that one would expect if "fully developed" intensity scintillation obeys a Rayleigh distribution of amplitude.
In comparison with S4, thevaluesof (r, exhibit
COMPLEX-SIGNAL
90
SCINTILLATION
177
channels, the measured standard deviation will be
Numbe'r
related to the true standard deviation
as
80
•,. = •,(1 - n-:) 7O
aST^.O.D
I
Inverting (9) and applying it to the raw data points
in Figure 12 resultsin a negligiblechangeat all
KWAJALEIN 29I
frequenciesexcept L band, for which the resulting correction is shown by the squaredata point. Thus, Figure 12 implies: (a) that diffraction did not change the frequency dependence of phase scintillation from that expected in the near zone of a phase-perturbing screen, and (b) that there were phase scintillations on the S-band reference
Total i
z
(9)
30
channel that were correlated
20
with those observed
at L band. Both of these conclusions 10
are consistent
with productionof phasescintillationby ionospheric structure having a spatial spectrum dominated by large-scaleirregularities, to which we shall return in section 3.3. 0
•
0
•u
0o
0
o
Durability of phase statistics in the presence of diffraction doesnot necessarilymeanthat the spatial pattern of phase itself is not changed. Furthermore, diffraction sufficiently strong to perturb the f-•
•
Fig. 7. Passactivity level for recordsinspectedasof 9 December 1976. Recordedbetween the end of May and the end of October (Stanford to mid-Septemberonly; Kwajalein mainly during the month of October).
For refractive-index irregularitiesof sufficiently large scale that propagationeffects within the ionospheric scatteringlayer itself may be ignored, the phase at each point on the output plane is given
dependence of •r, hasbeenencountered. An example is presented in Figure 13, which shows the frequency dependenceof phase scintillation during two 20-sec periods of a pass over Ancon shortly before local midnight. (Local standard time at Ancon lags UT by 5 hours.) In the less disturbed of the two periods, the L-band and UHF data points show the same ordered dependencenoted in Figure
12, but the VHF point falls beneaththe [-1 curve.
In the more disturbed period, the VHF point is by (2). Taking the standarddeviation of • across still more depressed and the UHF points display that plane would result in an [-l dependencefor some scatter. (In all cases, the phase-screentheo•r•. As an intensitydiffractionpatterndevelops retical dependence has been passed through the in post-scattering propagation (preferentially at central UHF point.) lower frequencies), one might expect disruptionof If phase fluctuations on a measurement channel the[-• frequencydependence of phasescintillation and those on the reference channel are uncorrelated, (with•r• beingdecreased at thelowerfrequencies). then their variances add. The result is that, for Figure 12 showsno such disruption. The phase,•, measuredat the output of the an inherent [-q frequency dependenceof scintillation, the measured standarddeviation in the prescoherent receiver is derived from the measurement
frequency'strue phase,•, and that of the reference frequency,•r, as •,. = • - •/n
(8)
ence of an uncorrelated reference true standard deviation as
•,= = •,[1 + n-2('+q)] '/2
is related to the
(10)
where n is the ratio of the reference frequency to the measurementfrequency.Given an f-] dependenceof phasescintillationand correlation between
In spite of the point scatter in the more disturbed data set in Figure 13, there is little doubt that the underlying frequency dependencein the UHF to
the fluctuations
L-band regimeis closeto [-1. Thus, the correction
on the measurement
and reference
178
FREMOUW,
LEADABRAND,
LIVINGSTON,
COUSINS,
RINO,
FAIR, AND LONG
I
I
I
I
i
i
i
i
i
i
i
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
TIME,
I
UT
Fig. 8. DetrendedVHF, UHF, andL-bandstripchartsfrom a passrecordedat PokerFlat on 29 May 1976.
neededto accountfor the effect of reference-phase bar through the L-band point represents the range fluctuations on the measured L-band phase is of uncertainty caused by reference-phasefluctuabracketed by (9) and (10) with q = 1; the vertical tions. There is only a 3% difference between the raw data point and the bottom of the uncertainty range (uncorrelated condition). Drawing an f-• line ß 138MHz POKER FLAT through the raw L-band data point would, in fact, result in a rather satisfactory fit to the cluster of UHF points. Thus, it seemslikely that there were S4 S-band fluctuations(of a little over 20ø rms) during this observation that were largely uncorrelated with the L-band phase scintillations. The remaining unß""--x._._./ ........ 'v'-x........... certainty about the correlation and the L-band 0.011 uncertainty bar in Figure 13 represent fundamental 1212 1214 1216 1218 1220 1222 1224 1226 TIME, UT limits of the experiment, stemming from the differential nature of the phase measurement. The intensity measurement, of course, is not Fig. 9. Intensity scintillation indexes measuredat VHF, UHF, and L-band during the pass displayed in Figure 8. differential in nature. The frequency dependence
10 1
,
1.0!ß413 MHz29 MAY 1976
o.,!
COMPLEX-SIGNAL
10 __
100
ß138 MHz POKER FLAT l
ß 413 MHz 10
I
I
I
SCINTILLATION
I I I Ill
I
•
29 MAY 1976 1000
ø1239 MHz
_
i
I
179
I I Ill
POKER FLAT
•
29 MAY 1976 --300 200
T
100
10
0.1•
6O
1.0
1
0.01
,
1212 1214 1216 1218 1220 1222 1224 1226 TIME,
UT
-
Fig. 10. Phase scintillationindexes measuredat VHF, UHF, and L-band duringthe passdisplayedin Figure 8.
-
of intensity scintillationobservedduring the same two periods discussedin conjunctionwith Figure 13 is shown in Figure 14. The "fully developed" nature of the intensity scintillation at VHF and the five UHF frequenciesanalyzed,duringthe more
e•l ßCORRECTED FOR PHASE SCINTILLATIONS ON S-BAND REFERENCE CHANNEL (see text)
i
O.Ol
•
i lilll
o.1
I
I
i
i
I
FREQUENCY
-
3
--
2
--
1
iii
1.0
disturbedperiod, is apparentin the departureof those six data points from the [-•'• dependence that holds between $band and L band. An interest-
10
6
io
--
GHz
Fir. 12. Frequency dependence of phase-scintillation index,
ing point in the less disturbedperiod is the fact during two 20-sec periods of the pass displayed in Figure 8, comparedwith an J'-' dependence. than an S4 value of 1.20 was measuredat VHF. This is the largestvaluenotedin recordsinspected to date, and it is 20% largerthan the limitingvalue predictedfor a Rayleighdistributionof amplitude. Careful inspection of the VHF time record for this 1.0
•' •' ' ' '''•1 , , , , , ,,,• hancements 20-See period reveals three prominent signal el'land one very deep fade which suggests -\ N -
POKER FLAT -[ focus effects caused byparticular ionospheric ir-
'33UT 29MAY1976
lO•,,,• [ [ [[][[i [ [ ][[[[[I '[ [[ I-- •. ANCON •. S4 0.1 =_ l•
THRESHOLD I \
0.01
\
NOISE DUETO
I ':' •. I •
_1
•
lO .--
'•. •
•,
--200
UNCERTAINTYDUE TO -PHASESCINTILLATIONS
•
100
o. s-m^N•.EFE.E.CE- e0 I
••--•.CIRRF•.TI::r•-• 04õ2:/:20'• N•C•HANNEL ,see text)•• 20 30 \
l- ........... •,eN•._ f-1
o,I,,,",?;',7, ,"',,,,,,,, ' o.1
1.o FREQUENCY
I-•
16DECEMBER 1976-- 300
L% • ee• 0448:40 UT
I •.o I•
-APPROXIMATE, , , MEASUREMENT •-- -- --•'---- --•- -- --
N•
1.o
FREQUENCY--
10
lO
GHz
GHz
Fig. 13. Frequencydependenceof phase-scintillation index durFig. 11. Frequencydependenceof intensity-scintillationindex ingtwo 20-secperiodsof passrecordedat Anconon 16December during two 20-sec periods (starting at the times noted) of the 1976, compared with an [-' dependencearbitrarily passed passdisplayedin Figure8, comparedwith an [-,.s dependence. throughthe 413-MHz data point.
180
FREMOUW,
I
LEADABRAND,
I I I I IIII I
1.0
,
I I I I IIII I
X ' X X
LIVINGSTON,
ANCON
COUSINS, RINO, FAIR, AND LONG
I I I II
16 DECEMBER 1976
'• 0448:30 UT
S4 0.1
•PPROX'MATE \
\
MEASUREMENT I •.
X
THRESHOLD _ f----'•-- ..... --•0.01
DUE,T9 ,N,O!S,E,, I , \
0.1
, l ill I I\
1.0 FREQUENCY
That even the slow complex-signalscintillationsare not pure phase variations, however, is illustrated in Figure 16, wherephaseand intensityfluctuations _- having Fourier periodsbetween 2.5 and 10 sec have been isolatedby meansof the first step in a second _ "detrending" procedure,in which a low-passfilter havinga very sharpcutoff hasbeenemployed.The
,
•
signalshownin Figure16 is the referencesignal
_ -
appliedin the coherentAGe operationthat constitutes the final stepof the seconddetrendingproce-
-
dure.
-
The scatter diagram at the top of Figure 16 shows clear evidence of signal enhancementsand fades
I , l, •
10 --
GHz
ß ..-:;: • •,•.-'• '"•: •.' ß'"'"4':'-• -•'-"
ß '-' - .-' ",•:;-'"
Fig. 14. Frequency dependenceof intensity-scintillationindex during two 20-sec periods of the pass partially displayed in Figure 23, comparedwith an [-l.5 dependence.
' . e
.:-.:'.- '1' ; '. ¾.'
o•
~ ':'
X
-:?"' .:'?.'.":.
-;::• e"
'•' 0 "IRe
-•'
::½-.-•C'.'
ß -"!..•...'•. -• I^x,s .:•_•",'
ßß .-.:.•: .ß • . ','•.'/• •'-'• oo: .:.'.:.•_] :'
.
of the former
:.' :.-•_•:..-.
:.
::...-•..'..•.":.
regularities that may have dominated the signal behavior during this short time interval. More detailed analysisof first-order signal statistics has suggestedto us the presence of a focus component as an integral part of ionospheric scintillation. Following loss of the first Wideband beacon, some complex-planescatter plots were made of the VIlE signalreceived from operationalTransit satellites,usingthe coherentUHF signalas a phase reference. The scatter diagram obtained after removal of phase and intensity trends with Fourier periods greater than 10 sec was basically arc-like becauseof dominationby phasefluctuations. Without knowing whether the (UHF) reference phase fluctuations were correlated with the (VHF) measurement-channelfluctuations, it was not possible to know in detail the effect
I
2(IEI2) ',$
..
,,.
..-
•; -._•j
-
on the
recovered scatter diagram. However, there was clear evidence of correlation between phase and intensity. Successful launch of Wideband, with its much higherreferencefrequency,now permitsa definitive look at the complex-signalstatisticsassociatedwith scintillation. Figure 15 illustratesthe nature of the VHF complex-signal statistics obtained during a
0
30
60 TIME
--
90
s
shown at the top of Figure 5. For even a modest intensity scintillation index of 0.28, some of the associatedphasevariations(having Fourier periods
Fig. 15. Three representationsof VHF complex-signalfluctuationsduring90 secof the "classical" scintillationrecord appearing at the top of Figure 5. Top to bottom: scatter plot on the complex plane; real and imaginary parts of the complex signal; amplitudeand phaseof the complex signal.This data segment was selectedby eye as displayingreasonablestatistical stationarity; the average intensity and phase scintillation indexes for
of 10 sec and less for a l(X)0-km orbit) exceed _+aT.
thesegment are,respectively, Sn = 0.28and•r, = 1.58rad.
90-sec interval of the "classical scintillation"
record
COMPLEX-SIGNAL
on time scales comparable to the slowest phase variationsretained in the first detrendingprocedure (i.e., those with periods between 2.5 and 10 sec). Furthermore, the intensity scintillationsare correlated with the phasescintillations.That is, a phase advance is accompaniedpreferentially by a weak fade, and a phaseretardation is accompaniedpreferentially by a weak signal enhancement; this is precisely the behavior that would be expected becauseof focusingand defocusingin the near zone of a phase-perturbingirregular ionosphere. The signal residual of the second detrending procedure (i.e., the output of the second coherent AGC operation) is illustrated in Figure 17. The elliptical cluster of points is consistent with the generalized gaussian signal statistics [Rino and
SCINTILLATION
181
'/•--
2 ¾•
_'/• -
uJ
uJ
0
I
I
30
60 TIME
90
s
Fig. 17. The scattercomponent(containingintensityand phase fluctuationswith Fourier periodsshorter than 2.5 sec) of the scintillatingsignal shown at the top of Figure 5, isolated by employingthe signalshownin Figure 16 as the coherent-AGC reference in a seconddetrendingof the data shown in Figure 15.
0
30
60 TIME
--
90
s
Fig. 16. The focus component(containingintensityand phase fluctuations with Fourier periods between 2.5 and 10 sec) of the scintillating signal shown at the top of Figure 5, isolated
by applyinga 10-pole,Butterworth,low-passfilter to the intensity and phaseof the signalshownin Figure 15.
Frernouw, 1973] that might be expected in the intermediate zone of a phase-perturbingscreen, in which the diffraction fields from several irregularities comparable in size to and smaller than the Fresnel zone are contributing to the complex-field fluctuations. The observed tilt of the ellipse arises from correlation between the quadrature components of the received signal. Experimental isolation of the signal components illustrated in Figures 16 and 17 prompts us to postulate that the first-order signal statistics of scintillation can be characterized by the following
two-componentmodel:
E = E•Ef = (x•+ iy•)exp(Xf+ i•t)
(11)
182
FREMOUW, LEADABRAND,
LIVINGSTON,
COUSINS, RINO, FAIR, AND LONG
are understandablein terms of propagationtheory. First, the level of perturbation in both components shows the expected strong dependenceon frequency. Second, the scatter componentshows a trend from near-zone (highly non-Ricean) to farzone (Ricean) behavior as the frequency is de-
VHF,S4 = 1.04, o• = 5.69rad'
creased (i.e., as the Fresnel-zone size is increased).
Finally, the focuscomponentdisplaysan orderly trendfrom essentiallycompletedominanceby phase fluctuations at L band, through developmentof weak focuses and.defocuses
UHF,S4 = 0.36,o• = 1.98rad
correlated
with
the
phase perturbationsat UHF, to a condition that may representa kind of statistical"focal plane" behaviorat VHF. The compositeVHF scintillating signal probably approximates that propagated through a Rayleigh fading channel, although systematic goodness-of-fit tests have not yet been made.
L-BAND,S4 = 0.06,o• = 0.55rad Fig. 18. Scatter diagramsof the focus (left) and scatter (right) componentsderived from the VHF (top), UHF (center), and L-band (bottom) records obtained during 20 sec (starting at 1220:33UT) of the passdisplayed in Figure 8.
where xs and Ys are jointly gaussianvariates,as
arextand•t, andwhereE sandEtare statistically
STANFORD, % = 1.58,S4 = 0.28
independent(by virtue of the sharp second-detrend
filter). We supposeE s to arise from diffractive scatter by irregularitiescomparableto and smaller
thanthe first Fresnelzoneand Et to arisefrom refractive focusing and defocusing by larger-scale irregularitiesacting as lenses.If statisticalhypothesis testing proves out the above model, then it will be possible to fully characterize the first-order, complex-signalstatisticsof classicalscintillationin terms of the "six sigmas,"
POKERFLAT,% = 0.80,S4 = 0.28
O'xs • O'ys•O'xsYs• O'x?O',bi•O'xf,b!
which are the (co)variances of the subscripted variables.
Preliminary indications of the durability of the two-component scintillationmodel are containedin Figures 18and 19. Figure 18 showsscatterdiagrams of the focus and scatter components recorded at VHF, UHF, and L band during 20 sec of a pass over Poker Flat under conditions of strong VHF scintillation.
There
are trends in both the focus
and scatter componentsthat, at least qualitatively,
ANCON,% = 0.35,S4 = 0.30 Fig. 19. Scatterdiagramsfor the focus(left) and scatter(right) components isolatedfrom segments of VHF recordsobtained at a midlatitude(top), auroral(center),and equatorial(bottom) station.
COMPLEX-SIGNAL
Figure 19 contains one VHF data set from each of the three latitudinal regions being probed by means of the Wideband experiment, selected for their similar intensity scintillationindexes. A striking feature of the three data sets is that while they all exhibit an S4 index of approximately0.3, the standard deviation of phase ranges from 0.35 to 1.58 rad. Correspondingly, the focus and scatter components, respectively, display internally consistent behavior
trends
from
near-
to intermediate-
and
intermediate-zone
and far-zone
behavior.
SCINTILLATION
183
Second-order statistics. The time structure of radio-wave scintillation data contains information
that can potentially reveal both the location and size distributionof the irregularitiesin the transmission medium. Intensity records alone, however, provide only limited information. As noted in section 2.2, the Fresnel filtering effect suppressesthe contribution of large-scale structures. Moreover, the intensity scintillationlevel varies with changing irregularity strength as well as with distance from the receiver.
For instance, the equatorial (Ancon) scattercomponent displays a very Ricean characteristic, while
By measuringthe full complex-signal•structure, the irregularity size distribution can be determined
that obtained at midlatitude (Stanford) is highly
essentially undistorted by diffractioneffects.In-
non-Ricean.
deed, our phase-scintillationdata show no measurable Fresnel-radiusdependenceexcept for the most severelydisturbedpasses.This is indicated, in terms of first-order statistics, by the fact that the signal-
Figure 19 is suggestiveof a latitudinal trend in the signal-statisticalnature of scintillation, which may'be significantboth geophysicallyand for communicationchannel modeling. It is consistent, for
phasevariancevariesquadraticallywith wavelength
instance,•th the possibilitythat equatorialscin-
through substantialchangesin Fresnel radius and
tillation is producedby irregularitieshavinga greater mean, or centroid, height than those at higher
scintillation level, as described in section 3.2.
It is also suggested, in second-order statistical
latitudes.It is prematureto drawsucha conclusion, terms, by the fact that our measuredphase spectra however,until manymoredata setsare analyzed generally show a power-law form over the medium anduntilgeometrical factorsate takenintoaccount. frequency range, which is unaffected by our phase 3.3.
Signal statisticso[ classicalscintillationm
BEST
FIT
ß , (v) = T v-r,
-10
• (•')= 0.008 •,-2.2
N
detrendingoperationand the noisefloor. To characterize the phase spectrum, we use a log-linear, least-squaresfitting procedure to determine the strength and slope parameters, T and p respectively, defined such that
(12)
The fitting is done over the frequency range 0.5 Hz 3). •e stren•h parameter, T, is numefi•ly •u• to the •wer spectraldensity of phase at a fiuct•tion rate of 1 •. U•ike the phase v••, T is un•fected by detrending. For the intensity s•ctra, we have derived a measureof the intensity scintillation rate by esti•ting the diffraction-inducedturnover at the lowfrequency end of the s•ctmm. •e esti•te is implementedby 1ogarith•c, least-squ•es fiHing -10
-20
-30
o
-40
•50
TIME,
•52
1
UT
Fig. 22. Spectraldecorrelation behaviorof UHF intensityduring portionof passrecordedat Ancon on 16 December 1976. (Data points placedat centersof 20-seecalculationintervals.)
of a cubic function to the imensity spectrum between 0.2 and 5 Hz. We then differentiate
function
to determine
the cubic the local maximum which
we take as the scintillation-rateparameter,re. An example is shown in Figure 21.
The scintillation-rateparameter,vc, is affected bothby the Fresnelradiusand by the relative motion of the irregularitiesand the radio line of sight. The satellite motion, which is known, usually is the majorcontributorto the relative motion.In addition, the rate parametertends to increaseunder strongscatter conditions, becauseof a broadeningof the spectrum. In our routine data processing,we measure the spectral summary parameters, T, p, and general morphology. Second-ordersignal statistics in the spatial and spectral domains are being characterized in terms of correlations between signals received, respectively, on spacedantennasand at various frequencies in the UHF spectrum. As an example, we present results on loss of signal coherence across the band of transmitted UHF spectrallines during a passover Ancon in Decemberof 1976.In principle, one would like to characterize spectral coherence in terms of the complex mutual coherencefunction (E(jt)E*(J + A•)). Becauseof the dispersivenature of the ionosphere to radio waves, however, the
._>
-50
I •,
•
vc, to determinetheir geometrical dependence and 0.35 Hz
,,-
0.5
-60
mutual coherence
-70
0.1
1.0 v--
10
IO0
Hz
can take on a reduced
value causedby a deterministicphase trend across a band of frequencies, having little to do with correlation
Fig. 21. Power spectrumof VHF intensityfor a 10 sec portion of Poker Flat pass7-35, 27 August 1976, 1,244UT.
function
of the statistical fluctuations
of interest
in characterizing scintillation. Consequently, we have chosento work directly with the real correla-
COMPLEX-SIGNAL
0446:30
0447:00
SCINTILLATION
0447:30
TIME,
185
436
MHz
390
MHz
379
MHz
0448:00
UT
Fig. 23. Portionsof four (detrended)UHF intensityrecordsused in correlationanalysispresentedin Figure 22. Recordedat Ancon on 16 December
tion functions defined in (6) and (7). Figure 22 shows the imensity correlation coeffi-
cients,definedin (6), for two pairsof UHF spectral lines, as functionsof time duringthe first part of the pass in question. As might be expected, the more widely separated pair (379 and 447 MHz) consistentlydisplaysgreaterdecorrelationthan does the less widely separatedpair (390 and 436 MHz).
1976.
data decimated to 50 samplesper seconddoes not result in a continuous track of all 2•r crossings.
The data presented in Figures 13 and 24 were processedat the full 500 sample-per-secondrate, but it is still possiblethat some 2•r crossingswere missed. This may account for some of the less ordered behavior of phase moments as compared
with their intensitycounterparts.(The basic depar-
Alsoshownonthefigureisa graphof (1 + S•) -•/•, which is the value that the correlation
coefficient
defined in (6) would take under conditions of
uncorrelatedintensityscimillations.Thus, throughout the event there is partial correlation between the intensity fluctuations recorded on the UHF channelsanalyzed. A portion of the four intensity recordsusedin the correlationanalysisis presented in Figure 23 for visual inspection. Compared with the ordered behavior of the intensity correlation coefficients, the behavior of the phasecorrelationcoefficientsfor the samefrequencies, defined in (7), was more erratic, as illustrated in Figure 24. There is some question about the accuracyof statisticalmomentsof phasecalculated under
the
most
severe
scintillation
conditions.
Under such conditions,the routine processingof
0.8
390MHz 436
MHz
ø'11X 0.2
0LI
I 0448
I
1
I
0450 TIME
t
I
0452 UT
Fig. 24. Spectraldecorrelation behaviorof UHF phaseduring portionof passrecordedat Anconon 16December1976.(Data pointsplacedat centersof 20-seecalculationintervals)
186
FREMOUW, LEADABRAND, LIVINGSTON, COUSINS, RINO, FAIR, AND LONG
tureoftr, froman1-1behavior illustrated inFigure 13is thoughtto be a truediffractiveeffect, however, sincethere was virtually no difference in the VHF values calculated at the two data rates.)
Spatialdecorrelationanalogousto the spectral decorrelationillustrated in Figures 22, 23, and 24 also has been observed. Measurable spatial decorrelationhasbeenobservedmore frequently, in fact, thanspectraldecorrelation.Under suchconditions, the spaced-receiverrecords can be employed to
gleaninformationaboutthe heightand motionof thescintillation-producing irregularities.The results of suchanalysis,includingfull cross-spectral processing,will be reportedseparately.
Wideband satellite. Detailed analysis of the data is presently under way. Several conclusionscan be drawn from the preliminary observations,however.
First, a large majority of ionosphericallyimposed scintillation can be adequately modeled in terms of an equivalentphasescreen[Bookeret al., 1950]. On the other hand, propagation theories based on assumptionof weak, single scatter (e.g., the first Born approximation) seldom are appropriate. That is, even moderate intensity scintillation is usually accompaniedby phase scintillation (in the same fluctuation-spectralregime)comparableto or greater than a radian. Furthermore,
under conditions
of even rather well developedintensity scintillation, there usuallyis little or no decreasein phasevariance 4. CONTINUING OBSERVATIONS with increasing Fresnel distance, such as would A largevolumeof dataon transionospheric signal be predicted, for instance, if one were to apply statisticsis being collectedat the three Wideband the Rytov formalism[ Tatarskil, 1971] to calculation experimentreceivingstations.Presently,plansare of phase statistics. firm only for one year of observations,ending in All of the foregoingis consistentwith production May 1977. Active considerationis being given, of scintillation in plasma-density irregularities however, to continuingobservationsat Kwajalein whosespatialspectrumis dominatedby large scale throughSeptemberand to openingthe Artconand sizes. Indeed, the limited spectral analysiscarried Poker Flat stations in the September-October- out thus far on the Wideband signalsis consistent November equinoctialperiod. The routine observ- with a power-law model for the spatial spectrum. ing scheduleat each of the stationscomprises The power-law exponent, however, is not a univeroperationsduring local nighttimehours on Tues- s& constant for all ionospheric structure. In pardays,Wednesdays,and Thursdaysand duringlocal ticular, there appears to be a consistenttransition daytimehourson Wednesdays.Thisresultsin about in spectral index associatedwith enhancementof six recordingsper week at eachof the two low-lati- irregular structure at the subauroral scintillation tude stations and about twelve per week at the boundary. This boundary enhancement is a data feature we intend to emphasizein future analysis. high-latitudestation. The beacontransmitscontinuouslyand is expectWhile dominanceof the ionosphericspatial speced to continue doing so for several years. It is trum by large-scalestructurerendersphase-screen very likely, in fact, that the transmissions will modelinguseful most of the time, vivid departures continue for the better part of the current 11-year from the signalbehavior expected in the near zone solarcycle,althougha reductionin duty cyclemay of a phasescreendo occur on occasion.Especially be necessaryafter a few years. The orbit is so under conditionsthat produceintensity scintillation nearly circularand its sun-synchronism so stable at gigahertzfrequencies,the signalstatisticsat lower that several observersdo, and many more could, frequencies are decidedly altered by diffraction receive the Wideband satellite signals(with rela- effects. The alterations include disruption of the
tively broadbeamantennas)without a need for accurateorbital updates.Information to facilitate
simplef-• frequencydependence of phasevariance, and decorrelationof UHF signalsas closelyspaced
such observations is available from the authors.
as 11.5 MHz.
5.
CONCLUSIONS
We have presentedhere a few examplesof the results being obtained from observationsof the
The preliminary Wideband results, therefore, suggestcaution in applying differential-phasemeasurementsperformed at lower frequencies to prediction of propagation conditions at gigahertz frequencies, for two reasons. First, under conditions
COMPLEX-SIGNAL
of interest, one would not know if phasescintillations on the reference channel in a VHF/UHF coherent receiver were correlatedwith phasescintillations in the measurement channel or not. Even
acceptingthis uncertaintyin the measurement,one could not safely apply a universalscalinglaw to apply VHF phasemeasurements to predictionsof
L-bandphasescintillation. An [-2 scalingof phase variance, for instance, would underestimatethe L-band varianceby an unknownamount.This may be of some relevance, for instance,in planningfor L-band imaging systemssuch as SEASAR (the syntheticapertureradar on SEASAT).
A topic to be emphasized in our future analysis will be the variability of scintillation frequency
dependence, includingcomparison with theoretical calculations.We intend to explore also•probably in many of the same records•conditionsunder which frequencydiversitymightconceivablybe a viable means of combating scintillation, because of spectraldecorrelation.The strong diffraction effects contained in such records occur both at
auroraland equatoriallatitudes. For the most part, however, the greatestextremes in scintillati•n• both the most disturbed and the most benign• conditions occur near the geomagneticequator.
SCINTILLATION
187
REFERENCES
Aarons, J., J.P. Mullen, and H. E. Whitney (1969), The scintillationboundary,J. Geophys.Res., 74(3), 884-889. Basu, S., S. Basu, and B. K. Khan (1976), Model of equatorial scintillations from in-situ measurements, Radio Sci., l/(10), 821-832.
Booker, H. G., J. A. Ratcliffe, and D. H. Shinn (1950), Diffraction from an irregular screen with applications to ionosphericproblems,Phil. Trans. Roy. Soc., A242, 579-607. Bowles, K. L. (1963), Measuringplasmadensityin the magnetosphere, Science, 139, 389-391.
Bramley, E. N. (1955), Some aspectsof the rapid directional fluctuations of short radio waves reflected at the ionosphere, Proc. Inst. Elec. Eng., 102, 533-540.
Briggs,B. H., and I. A. Parkin (1963), On the variation of radio star and satellite scintillations with zenith angle, J. Atmos. Terr. Phys. 25(6), 339-366. Burns, A. A., and E. J. Fremouw (1970), A real-time correction
techniquefor transionosphericrangingerror, IEEE Trans. AntennasPropagat., AP-18(6), 785-790. Dyson, P. L., J.P. McClure, and W. B. Hanson (1974), In situ measurements of the spectralcharacteristicsof F region ionosphericirregularities,J. Geophys.Res., 79(10), 1497-1502. Klobuchar, J. A., (Ed.) (1973), Total electron content studies of the ionosphere,AFCRL-TR-73-0098,Air Force Survey in Geophysics,No. 257, Air Force CambridgeResearchLaboratories, L. G. Hanscom Field, Bedford, Mass. Leadabrand, R., M. J. Baron, J. Petriceks, and H. F. Bates (1972), Chatanika, Alaska, auroral-zone incoherent-scatter facility, Radio Sci., 7(7), 747-756.
Phelps,A.D. R., and R. C. Sagalyn(1976), Plasmadensity irregularities in the high-latitude topside ionosphere, J.
Geophys.Res., 8/(4), 2059-2064. Rino, C. L., and E. J. Fremouw (1973), Statisticsfor ionosphericallydiffractedVHF/UHF signals,Radio Sci., 8(3), 223-233. Acknowledgments.The Wideband satellite experiment is beingperformedfor the DefenseNuclearAgency(DNA) under Rufenach,C. L. (1971),A radioscintillationmethodof estimating the small-scalestructure in the ionosphere,J. Atmos. Terr. contract DNA001-75-C-0111. The authors acknowledge with Phys., 33, 1941-1951. appreciationthe many contributionsmade by D. E. Evelyn, of DNA, to the developmentand fielding of the experiment. Skinner, N.J., R. F. Kelleher, J. B. Hacking, and C. W. Benson (1971), Scintillationfadingof signalsin the SFH band, Nature The receivingstationat Ancon,Peru, is operatedby personnel Phys. Sci., 232, 19-21. of the Instituto Geofisico del Peru, under the direction of J. Tatarskil, V. I. (1971), The Effectsof the TurbulentAtmosphere Pomalaza. The location of the receiving station at the Poker on WavePropagation,472pp., NationalTechnicalInformation Flat Rocket Rangenear Fairbankswas made possibleby the Service, Springfield, Va. GeophysicalInstituteof the University of Alaska.
