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AFAL-TR-77-158

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§ PROJECT STRESS SATELLITE COMMUNICATION TEST RESULTS

System Development Branch System Avionics Division

July 1977

TECHNICAL REPORT AFAL-TR-77-158

D D C

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Final Report for Period June 1976 - March 1977

Approved for public release; distribution unlimited.

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QNffitAL CONTAINS COl CR PL/.TCS: ALL ODC IÖA0DUCT1QNS WILL BE IN BLACK AND WHITE

■ == AIR FORCE AVIONICS LABORATORY C3 U- AIR F0RCE WRIGHT AERONAUTICAL LABORATORIES

AIR FORCE SYSTEMS COMMAND _^ r« WRIGHT-PATTERSON AIR FORCE BASE, OHIO 45433

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When Government drawings, specifications, or other data are used for any purpose other than in connection with a definitely related Government procurement operation, the United States Government thereby incurs no responsibility nor any obligation whatsoever; and the fact that the government may have formulated, furnished, or in any way supplied the said drawings, specifications, or other data, is not to be regarded by implication or otherwise as in any manner licen- sing the holder or any other person or corporation, or conveying any rights or permission to manufacture, use, or sell any patented invention that may in any way he related thereto.

This report has been reviewed by the Information Office (01) and is releasable to the National Technical Information Service (NTIS). At NTIS, it will be available to the general public, including foreign nations.

This technical report has been reviewed and is approved for publication»

Project En

FOR THE COMMANDER

FRANCIS J, LIBERATOR!, \Jt Cot, USAF Deputy Division Chief System Avionics Division

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UNCLASSIFIED SECURITY CLASSIFICATION OF THIS PAGE (Whin Dot* Entarmd)

REPORT DOCUMENTATION P*'~ ll. aOVT ACCESSION NO.

READ INSTRUCTIONS BEFORE COMPLETING FORM

3. RECIPIENT'S CATALOG NUMBER

(7?

4. TITLE (and Subtitle)

PROJECT SJ SATELLITE ^COMMUNICATION TEST RESULTS t Jl — ■

7. AUTH J c/Frettle^ A. Johnson;» J. Marshall;

v T./Crizinslc R. /swanson

3. 1 TPE UP M&MÜHT A PL PI OP

Final Technical Xejfiortf 6. PERFORMING ORG. REPORT NUMBER June 76 - Mardl 77j ^Fê5iffffTgt^Tn?ÂNi>ûMaERrt)

RDT & E RMSS Code B3220 76462 L 25AAXHX 63511 H2590D (DNA)

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Systems Avionics Division (AA) Air Force Avionics Laboratory Wright-Patterson Air Force Base, Ohio 45433

10. PROGRAM ELEMENT. PROJECT, TASK IBB I k JUJ JWjj JU NIT N-UuflgRJ j S.W.* iKjiRjljUNlT MlUtagRJ

M. CONTROLLING OFFICE NAME ANO AOORESS "T|

'A

Task Work Unit 12272205 ft. RgPORT UAfE""*

Jul£jfcf77 NUMBERS

124 H. MONITORING AGENCY NAME 4 AOORESSf*/ dilfrntttt Irom Controlling Oltico) tS. SECURITY CLASS. To/ thi* report)

UNCLASSIFIED

Mm. OECLASSIFICATION DOWNGRADING SCHEDULE

1«. DISTRIBUTION STATEMENT (of thia Report)

Approved for public release; distribution unlimited

17. DISTRIBUTION STATEMENT (o( tho abatrmct onfrod in Bioek 20, it dlttotont (torn Rapor\

'*. SUPPLEMENTARY NOTES

19. KEY WORDS (Conttnuo on wra* aid* it nmcmaamry mtt idonttly by block numbar)

Barium Cloud Ionospheric Scintillation Satellite Communications UHF Fading

20. ABfTRACT {Continue on ravraa aid* it nacoaamry and idanttty by block numbar)

În order to evaluate the effects of a nuclear disturbance of the ionosphere on ÜHF satellite communication, barium ions were released in the ionosphere. The test, nicknamed STRESS, was sponsored by the Defense Nuclear Agency. The drift of the barium ions across the magnetic field lines cause the ions to consolidate in rods, or sheets, causing diffraction of radio waves passing through the ionosphere. AFAL's Satellite Coomnication System aboard a C-135 type aircraft was flown in the shadow of the cloud and communicated through the ionospheric disturbance to the LES 8/9 UEF satellites. Measurements were made of the fading ^

W DD 1 JAN TS 1473 «DITION OF I NOV «5 IS OBSOLETE

<7I±4>?0 UNCLASSIFIED

SECURITY CLASSIFICATION OF TwiS »AGE »*»n Ddta Snta**d)

f .jÊ^tVi^^^ff^^^^^^^-^M^« vr'~ -T

UNCLASSIFIED SECURITY CLASSIFICATION OF THIS *AG£fW»«n D*tm Enfnd)

%

Key Words Cont'd Block No. 19 Airborne Communication Propagation Anomalies STRESS

Abstract Cont'd Block No. 20

haracteristic3 and bit error rate performance when communicating through the ionospheric disturbance.

«t

UNCLASSIFIED SCCU»«TV CLASS!*iC*T!3»i Of ▼*•* **ö*'*>i«w» Dm*» Emo*»d)

FOREWORD

The effort reported in this Technical Report was accomplished

between 1 June 1976 and 15 March 1977 under Project #12272205,

LES 8/9 Flight Test Effort. The effort was in support of the Defense

Nuclear Agency's Project STRESS.

This Report is a cooperative effort between the Air Force

Avionics Laboratory (AFAL) and ESL, Tncorporated, the latter party

with sponsorship of the Defense Nuclear Agency under RDT & E RMSS

Code B3220 76462 L25AAXHX63511 H2590D.

The tests described herein were supported by the 4950th Test

Wing, MIT Lincoln Laboratory, ESD's Test Management Facility, and an

extensive test team located at Eg!in AFB, Florida responsible for

the rocket launch, aircraft positioning, photo/radar coverage, and

physical analysis of the barium cloud.

The support of an extensive team of engineers/contractors at

Wright-Patterson AFB, Ohio was also Instrumental in the successful

completion of this test.

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TABLE OF CONTENTS

SECTION PAGE

I. Test Objectives 1

II« Test Organization 2

III. Test Concept .... 3

IV. Test Configuration 17

V. Description of STRESS Events 31

VI. Test Results 52

VII. Conclusions 103

VIII. Recommendations For Future Efforts 115

References 119

*******

I. TEST OBJECTIVES

The overall objective of the STRESS (Satellite Transmission Effects

Simulation Study) Experiment was to evaluate the performance of a UHF

satellite communication system in an artificially disturbed ionosphere in

an effort to better predict the expected performance of such communication

systems in a nuclear environment. The specific objectives of the experiment

were:

a. To exercise the techniques used for and to verify assumptions

made in predicting the performance of communications systems

operating through striated plasmas. These techniques involve

gradient drift plasma instability phenomenology for the

determination of the striated environment, multiple thin phase

screen propagation theory, and computer simulations of system

performance that utilize propagation inputs.

b. To obtain data on late-time striation dissipation mechanisms.

To date no theories exist that describe how long striations

from barium or nuclear detonations are expected to persist.

c. To measure the performance of the LES 8/9 UHF command post

force element forward and report-back communication links

operating through a fading environment created by high altitude

barium release and to assess the implications for operations

in a nuclear environment. Since the LES 8/9 systems were chosen

to meet objective (a) and since the system represents a design

phase of future AFSATCCM concepts, an assessment of the

performance of these systems through striated environments is

called for.

II. TEST ORGANIZATION

The STRESS Project is under the direction of the Defense Nuclear Agency

(DNA). The field operations necessary to accomplish the satellite measurements

were planned and carried out by Stanford Research Institute (SRI). The

Electro-magnetic Systems Laboratories (ESL) designed and built special test

hardware for the satellite communication measurements. Tne Air Force Avionics

Laboratory (AFAL) provided and operated the LES 8/9 airborne and grcund

satellite communication equipment. The collection of the satellite communi-

cation data was a joint effort of AFAL and ESL.

A 11st of the field experiment participants and their areas of

responsibility is as follows:

DNA Program Director

SRI Test Planning and Direction, operation of the TV tracking system, FPS-85 radar, and ionosonde.

ESL Construction of special measuring equipment and participation 1n satellite communication system measurements

AFAL Provide and operate airborne and ground satellite communication system

4950th Test aircraft support

TIC Ground photography

SDC Rocket operations

USU Probe payloads

Thlokol Barium payloads

LMSC Optical interferometer

ESO Satellite support

RDA Probe rocket coordination

ADTC Range operation

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III. TEST CONCEPT

The basic concept of the STRESS Experiment involves at least two

communication terminals, a striated plasma in the ionosphere, and a UHF

satellite. In the experiment the two terminals attempt to communicate via the

satellite with UHF signals between one terminal and the satellite traversing

the striated plasma. The properties of the striated plasma perturb the UHF

signals and, thereby, stress the communications link. The two communications

terminals were the AFAL rooftop facility and aircraft C135/662 linked via the

LES 8 (for two releases) or LES 9 (for three releases) satellites.

The first high altitude barium releases provided the plasmas which were

diagnosed using rocket probe, optical, and RF techniques. The five STRESS

releases were preceded by a pre-STRESS release with all bearing girl's names

in alphabetical order as follows: pre-STRESS - ANNE, STRESS - BETTY, CAROLYN,

OIANNE, ESTHER, and FERN. The pre-STRESS release was a field test to determine

the capability of positioning the aircraft 1n the cloud shadow.

The use of chemicals to modify or artificially disturb the ionsophere is

a technique which has received extensive development over the past years. An

artificial barium ion cloud was used to produce propagation path disturbances

during the ARPA SECEDE Program, which involved radar propagation through the

disturbed Ionosphere.

The barium clouds used in the STRESS Test were generated with the launch

of 48 kilograms of barium chips to an altitude of approximately 185 kilometers.

The barium was vaporized by a small thermite explosion. Action of the sun's

ultra violet rays on the barium generated barium 1ons and frt^ electrons. The

barium which did not ionize formed spherical clouds (neutral clouds) which

drifted according to the high altitude winds (30 to 100 meters per second

generally away from the sun). The Ionized barium also formed in spherical

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clouds Initially but soon changed Into elliptical clouds tilted along the

magnetic field lines. The Ionized plasma was confined, and Its diffusion

spread occurred only 1n the direction of the magnetic field lines. Figure 1

from Reference 1 Illustrates the subsequent 1on cloud evolution from two

different views. The bottom row of sketches represents the more typical view

of an 1on cloud 1n the process of strlatlng as 1t would appear from sites with

arbitrary magnetic field line aspect angles. The top row of sketches show

the corresponding appearances of the 1on cloud when viewed up the field lines.

The typical cloud evolution from the elliptical form with the circular cross-

section (labelled "AMBIPOLAR DIFFUSION") to a striated cloud 1s driven by the

neutral wind attempting to drag the denser regions of the barium cloud with 1t

(and thus with the neutral cloud) 1n conflict with the magnetic field confinement

forces (or E x B forces) on the entire 1on cloud. (If the neutral cloud were

shown 1n this figure, 1t would be seen moving to the left.) Initially the

denser portion of the cloud 1s dragged to one side of the cloud forming the

"hard edge," or "BACKSIDE STEEPENING." Further wind drag pulls "fingers," or

"sheets," of dense plasma from the "hard edge" which eventually pinch off to

form Isolated "striatlons." When viewed with a typical magnetic aspect as

1n the bottom row, the appearance of Isolated striatlons embedded 1n a back-

ground plasma cannot be distinguished from the appearance of the overlap of

several sheets of varying thicknesses. Both the sheets and striatlons cause

UHF signal amplitude scintillations while the effect of the unstrlated, or

"smooth," 1on cloud (farthest from the neutral cloud and to the right 1n the

figure) 1s a slight phase .Mft due to the elevated Integrated electron

content through the medium. While the Initial barium release occurs at

approximately 185 kilometers altitude, the free electrons tend to drift up

and down the magnetic field lines between altitudes of approximately 140

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kilometers and 210 kilometers, Figure 2. The development of the striated

Hne might appear as In Figure 3 when viewed across the field lines.

Barium clouds resemble weather clouds 1n that all of the significant

observation light which comes from them Is reflected sunlight; their glow

due to molecular recombination is Insignificant. Barium clouds launched at

sunset are best observed after the time when the sun 1s 6° below the horizon

and before sunset at the 185 kilometer altitude. The spherical neutral cloud

reflects sunlight of a bluish and greenish tint. The Ionized portion of the

cloud reflects sunlight of a pinkish or reddish tint, Figure 4.

For Project STRESS the barium releases occurred at various times relative

to the 6* sun depression angle, Table 1. The barium clouds which were released

early and passed through their early stage of development obscured by the sky

glow became visible well Into their development and remained visible further

Into their development than those launched later. The variety of launch

times allowed optical observation of the late-time cloud development and will

provide data on structure dissipation mechanisms.

The Honest John-Hydac rocket launches carrying the barium payloads took

place from Eglin's launch site, A-15 on Santa Rosa Island. A series of radars

and optical TV trackers were located along the Florida coast at locations

Indicated in Figure 5 to position the aircraft beneath the ion cloud RF

shadow to satellite emissions. A photographic coverage net, one mobile site

omitted, 1s also Indicated in the map.

The concept of using aircraft C135/662 to fly under the barium cloud pro-

jections from LES 8 or LES 9 was Intrinsic to the test concept. Using

satellite ephemerls data and nominal cloud drift assumptions two test windows

for operation of the aircraft with the LES 8/9 satellites were generated

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MAGNETIC ,LINES OF FORCE (FIELD LifJEs)

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ELECTRON MOVEMENT ALONG FIELD LINES

.1 300 KM

■[ 200 KM

100 KM

EQUATOR

NOTE: THIS FIGURE IS NOT TO

SCALE- JUST REPRESENTATIVE.

FIGURE 2 Propagation of Free Electrons Along Field Lines

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TABLE 1: STRESS Events — Summary (Primary Source — T.I.C., Bedford, Mass.)

Event Date

BETTY 26 Feb 77

CAROLYN 2 Mar 77

DIANNE 7 Mar 77

ESTHER 13 Mar 77

FERN 14 Mar 77

Release Time (2) 2352:29 2354:10 0001:10 2301 i8 2246:09

Altitude of Release (Radar)

179 191 186 189 186

Optical Coverage (Z)

0012-0042 0005-0043 0010-0045 0015-0050 0015-0050

Optical Coverage (Release + min)

R+20 - R+50

R+ll - R+49

R+9 - R+44

R+74 - R+109

R+89 - R+124

Radar Track Duration

0047-0258 2358-0202 0004-0149 2304-0237 2246-0109

Duration of Fading

0012-0158 0010-0144 0009-0126 2301-0244 2247-0108

Speed of Drift (m/s) (all clouds moved east to southeast)

"45 "60 46 36 ""20

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before the test and are shown in Figure 6. These windows give a qualitative

feel for the geometry involved. Actual test operations windows differed

somewhat in BETTY and CAROLYN from thos' shown due to cloud drift. The flight

path of the aircraft in the shadow of the cloud was designed primarily to cut

across the striations and to measure the signal fading caused by the dif-

fraction pattern of the striations. Some passes, "parallel runs or end runs,"

were made along the striations to measure their extent and to investigate

propagation phenomena. Figure 7 shows the cross striation aircraft flight

pattern through an idealized cloud shadow. Figure 8 from Reference 2 shows

an aircraft trajectory through an actual projection of the pre-STRESS event

ANNE from LES 9. Figure 9 shows a similar projection, true to shape but not

true to position, of the STRESS event DIAMNE from LES 9 at about release +30

minutes for comparison.

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IV. TEST CONFIGURATION

For the STRESS tests three basic satellite test configurations were

utilized. Test Configuration #1, Figure 10, provided UHF forward downlink

data to the aircraft and a CW UHF uplink probe from the aircraft through the

barium cloud. The uplink probe was sampled at the satellite and sent down on

the K band downlink to the rooftop where it was recorded. The K band forward

uplink was provided either by the rooftop.or by the aircraft.

Test Configuration #2 had been planned to test the report-back link.

However, due to equipment problems this link was not tested during the STRESS

tast.

Test Configuration #3, Figure 11, involved an uplink and downlink UHF probe

between the aircraft and the satellite. The downlink UHF tone was recorded on

the aircraft. The uplink probe was sampled in the satellite and transmitted

downlink via K band to the rooftop. Test Configuration #3 allowed a comparison

of the uplink and downlink UHF fading at frequencies separated by approximately

90 MHz.

One other test configuration was used to evaluate multipath from the aircraft,

Figure 12. The aircraft transmitted a UHF pseudo random (PRN) sequence through

the transponder mode cf the satellite. The UHF PRN sequence was downlinked

from the satellite to the rooftop where a correlation process was used to

indicate the relative strength of the direct and reflected UHF signals. Note

that no barium cloud was needed for this configuration.

The block diagram of the aircraft equipment used in STRESS configuration *1

is shown in Figure 13. The K band received signal was used to measure doppler

from the satellite. A scaled version of that doppler, derived in a divide by

operation in the "frequency unit," was then used to precorrect the L'HF uplink

probe frequency to remove the effect of the doppler. The UHF forward downlink

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was received-and a dehopped version of the received signal used to indicate the

UHF signal fading level. The coarse frequency command signal frcm the dual

modem serial-to-parallel processor to the ARC-171 was tapped, processed, and

recorded for later analyses to be performed in conjunction with recordings of

the dehopped signal. The actual forward link data is received with two modems

and typed out on teletypewriters for later error rate analysis. The signal

strength into one of the modems (the subject modem) is attenuated while main-

taining the same noise floor in order to sweep out performance versus received

signal level. The other modem serves as a control. The block diagram of the

rooftop equipment configuration for STRESS Test Configuration #1 is shown in

Figure 14. The K band signal was received and demodulated. The I and Q

samples were separated, processed, and recorded to obtain phase and amplitude

information.

The block diagram of the aircraft equipment used in STRESS Test Configuration

#3 is shown in Figure 15. The K band receiver determined the downlink djppler

which was scaled to correct the UHF downlink *nd precorrect the UHF uplink

signal to remove the effects of the doppler. The block diagram for the rooftop

equipment used in STRESS is very similar to that used in Test Configuration »1,

as shown in Figure 16. Again, the K band received signal was separated into I

and Q channel samples, processed, and recorded for further analysis of the phase

and amplitude variations.

The block diagram of the aircraft PRN sequence equipment used in the multi-

path test is shown in Figure 17. The 125 KHz (8 microseconds per symbol)

pseudo random sequence (length 127) was transmitted from the aircraft ARC-146

UHF transmitter at a 1 kilowatt level. Various transmit antennas were used

during the test to determine the isolation each provides between the direct

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and each reflected signal. The block diagram of the rooftop PRN equipment is

shown in Figure 18. By eliminating occasional bits in the repetitive sequence

the rooftop correlated the locally generated PRN sequence with first the

direct path PRN sequence and at a later time with the reflected PRN sequence

coming from the aircraft through the satellite.

The frequency plan used during STRESS is shown in Table 2. Shown is the

nominal frequency of the uplink tone used 1n each of the five releases and

the nominal frequency of the downlink tone used simultaneously with the uplink

tone during ESTHER. These tones were doppler corrected using AN/ASC-22 20 MHz

plus doppler estimates derived from received K band signals from the LES 9

dish at 36.84 GHz or from the LES 8 dish at 38.04 GHz. The doppler correction

ideally would divide the K band doppler by the ratio of the K band frequency

to the UHF frequency to produce an estimate of UHF doppler. In the actual

hardware realization the divide-by ratio was limited to integral values. The

ratio chosen for each release is shown with the parenthetical entries indicating

offsets from ideal. The suboptimal choices for the BETTY and CAROLYN uplink

frequency allows a small component of the aircraft-to-satellite doppler to

enter into the phase data. While changes in aircraft heading are obvious in

the data with a 400 Hz change in uncorrected doppler producing a 0.7 Hz change

in the doppler corrected signal, the phase data corruption produced by

bumpiness of flight on straight and level data runs is insignificant.

Shown in Table 2 are the synthesizer settings used to adjust the uplink

and downlink frequencies. The 1 Hz settability of the HP S660 frequency

synthesizers used is reflected in the table with parenthetical entries

indicating the fractional offset required for a zero frequency demodulator

offset. Ideally the resulting measurements should reflect these offsets,

but the long term drift of the aircraft rubidium standards and of the satellite

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clock apparently caused the observed demodulator offsets to deviate from

Ideal.

Also shown in the table are the satellite telemetry display values in

octal for the downlink and uplink synthesizers. It should be noted that the

choice of the UHF frequency synthesizer settings in the satellite for the

uplink and for the downlink are not independent, and that the ESTHER downlink

frequency shown corresponds to the uplink frequency used simultaneously in

ESTHER.

31

J

V. DESCRIPTION OF STRESS EVENTS

The dates and launch times of the five STRESS events are listed in Table

1.

The first barium release on 26 February 1977, BETTY, occurred at 2352:29Z

at an altitude of 179 kilometers. Radar returns from the ion cloud were

received as late as 0258Z. However, fading was observed only as late as 0153Z,

indicating either a problem with the radar positioning of the aircraft or a

dissipation of the barium cloud. Radar track of the ion cloud did not commence

until 0047Z, although fading was observed much earlier as the aircraft

maneuvered in the vicinity of the expected projection location. BETTY moved

in a general eastward or southeastward direction, as did all the STRESS ion

clouds, at a moderate velocity (40 meters/second). The BETTY ion cloud was

unusual in that it was exceedingly narrow as viewed up the field lines during

times when it was optically visible. Whether this narrowness was due to

improper venting of the barium vapor at release is not known. In most other

aspects BETTY was a normal cloud. Strong fading was observed on at least 5

of the 29 total passes.

The second barium release, CAROLYN, occurred on 2 March 1977 with a

release time of 2354:10Z at an altitude of 191 kilometers. Radar returns were

received from the cloud until 0202Z. However, radar positioning as indicated

by fading at the aircraft was valid only until 0144Z. CAROLYN moved at a

relatively high velocity (60 m/s). In most other respects it was a nominal

cloud. Good up-the-f1eld line photographs were obtained from CAROLYN at

times later than those taken up to that date in previous barium release

programs. A total of 21 data passes were made by the aircraft with the strong

Rayleigh-like fading observed en 6 of them. Some fading was obvious on a

total of 16 passes.

32

The third barium release of STRESS, DIANNE, occurred on 8 March 1977 (7

March local time) at 0001:10Z at an altitude of 186 kilometers. Radar track

was maintained until 0149Z with fading observed until 0126Z. Of the total

18 data passes made by the aircraft some fading was observed on 15 with strong

fading, either early-time-like or Rayleigh-like, being observed on 11 passes.

DIANNE was unusual in that the ion cloud developed a right angle bend as viewed

up the field lines. The cause of this bend is currently believed by plasma

phenomenologists to be high altitude wind shear because of a deformation of the

neutral barium cloud that was also observed. Some of the strongest fading to

be observed during the series was seen in DIANNE, which may be atrributed to

its unusual geometry.

The fourth barium release, ESTHER, occurred on 13 March 1977 with a

release time of 2301:08Z and an altitude of 189 kilometers. This release occurred

earlier than the preceding three by more than 50 minutes. The cloud drifted

at a slower rate than the previous releases, 36 m/s. Optical coverage extended

from 74 to 109 minutes after release, late into the cloud development, and

may reveal information about late-time strlation dissipation mechanisms. Radar

returns for cloud tracking were received as late as 0237Z, three hours and

thirty minutes after release. The aircraft by maneuvering in the proper

vicinity observed fading until 0244Z. Of the total 45 data passes made by

the aircraft fading was observed on 42 with early-tim^ or Rayleigh-lik^ fading

on 29 passes. An unexpected patch of fading was fortuitously observed at

release because of the aircraft's proximity to the Initial release point

projection. While most of the 1onizat1on 1n the Ion cloud 1s produced by

solar ultraviolet nominally 30 seconds after release, seme of the barium is

ionized thermally by the heat of the thermite explosion that initiates

33

release. Structure in this thermally produced ionlzatlon was observed to

cause fading and phase effects as early as 10 seconds after release, Figures

19 and 20.

The fifth and last release, FERN, occurred on 14 March 1977 with a

release time of 2246:09Z at an altitude of 186 kilometers. Radar returns

from the cloud were received as late as 0109Z. Fading was observed at the

aircraft until about the same time. Of a total of 33 passes made by the air-

craft fading was observed on 29 with Rayleigh-like, or early-time, fading

observed on 22 passes. The optical appearance of FERN (release plus 89 to

release plus 124 minutes) is enigmatic. The ion cloud resembles none of

the barium ion clouds observed in the past. The drift of FFRN was the slowest

of the releases (approximately 20 meters per second). Several of the late-

time passes produced fading usually typical of early-time fading. The

interpretation of FERN phenomenologlcal data may be complicated by sporadic

E at the end of the test.

The aircraft trajectories with fading occurrences indicated and the

radar cloud track projection positions are shown in Figures 21 to 36 for

each of the five barium releases.

34

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S

VI. TEST RESULTS

A summary of the results for each pass of each test is contained in

Tables 3 through 7. The fading is characterized according to early-time

like, Rayleigh-like, or Rician-like, and parallel pass fading. Some

characteristic examples are taken from aircraft strip chart data on downlink

fading from FERN and from semi-processed rooftop uplink phase and amplitude

fading from FERN and ESTHER. System effects are also discussed.

A. Early-Time Fading: The downlink received signal strength for Pass

1 FERN, Figure 37, shows a classical diffraction pattern for a single,

spherical or cylindrical cloud. The UHF signal level had a sharp rise of

approximately 10 db as the aircraft reached the edge of the cloud shadow and

then fell rapidly with a very broad fade lasting 80 seconds. As the aircraft

flew out from under the shadow, the characteristic signal enhancement occurred

and the signal returned to the unfaded signal level. A similar result was

observed during the next pass, Pass 2 FERN, with a ringing, or multipath,

occurring at the end of the pass, Figure 38. Pass 4 also shows the characteristic

decrease of signal with a broad multipath-like ringing occurring at the end of

the pass, Figure 39. The ringing is caused by interference between the rays

passing outside of or near the edge of the ion cloud with rays diffracted outward

from the portions of the cloud with steepest gradients of ray path integrated

electron contents. The ion cloud evolution of its "hard edge" is reflected in

the results of FERN, Passes 2 and 4, by the fact that this multioath-like

effect occurs predominantly on one side of the cloud, presumably, the edge

where the gradient steepening is occurring. Similar results are seen in

Figures 40 through 43, showing uplink amplitude and phase for Pass 1 cf FERN

and Pass 2 of ESTHER. These plots represent unfiltered data and the hashy

appearance is due to a combination of thermal and digitization noise. The

53

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classical diffraction pattern is unmistakable, however, and easily correlated

with the barium induced phase. In the early time runs these phases strongly

resembled Gaussian curves (barium phase effects increasing with decreasing

values on the plot producing inverted Gaussians) with the phase shifts due to

integrated content effects as high as 96 cycles.

Pass 31 of FERN also behaved like an early-time pass which is unusual

at this late period in the cloud evolution, Figure 44. Comparison of Pass 31,

Figure 44, to the theoretical drawing in Figure 45 shows the theoretical curve

to be reproduced very faithfully by the barium cloud structure.

B. Rayleigh-Like Fading: As the cloud developed into a series of indi-

vidual irregularities, rapid and deep fading was produced often with a ringing

type multipath caused by edge diffraction effects at the beginning or end of

the pass as seen in FERN, Pass 8, Figure 46. The downlink received signal level

during Pass 9 of FERN showed a broad decrease at the initial part of the pass

with rapid Rayleigh-like fading toward the end, Figure 47. Figure 48 shows

similar results during FERN, Pass 10. Figure 49 shows two additional examples

of Rayleigh-like downlink fading lasting a little over 60 seconds each.

Excellent examples of Rayleigh-like fading are seen in the uplink ESTHER, Pass

8, data, Figure 50. The diffraction edge in this pass is obvious on the left

at the start of the pass. The phase shown in Figure 51 indicates vestiges of

the smooth Gaussian early-time behavior. It is, however, corrugated both by

changes in path integrated electron content and by diffraction effects which

can produced phase jumps associated with deep fades.

C. Parallel Pass Fading: Typically the aircraft flew cross-striation

passes, but occasionally a maneuver was flown parallel to the projection of

the striations. The fading on these passes differed from that observed

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during the cross-striation patterns in that it was much slower. Figures 52

and 53 illustrate the uplink and downlink fading observed respectively

during Pass 20 of ESTHER, which was <x parallel pass. Figure 54 illustrates

the downlink fading observed during a similar pass, Pass 20 of BETTY.

D. Rician-Like Fading: A weaker form of amplitude fading categorized

as Rician-like was observed often. Figure 55 shows an example of this type

of fading occurring on the uplink during Pass 14 of DIAMNE. The fading is

noticeably less intense than the Rayleigh fading seen in Figures 46 to 50.

Weaker fading such as this is observed more often later in the cloud develop-

ment and may be attributable to weaker striations and/or poorer cloud tracking.

The phase observed on this DIANNE pass, Figure 56, indicates a less intense

cloud than seen earlier in the release.

E. Frequency Oecorrelation — Test 3 & Downlink Hop: Comparison of the

uplink and downlink tone indicates considerable decorrelation of the fading

due to the frequency difference of 90 MHz. Examples of the test configuration

#3 uplink and downlink fading for ESTHER, Pass 18, are shown in Figures 57

and 58. Many of the gross features are duplicated in the plots. However, the

actual fading generally appears decorrelated. A cross correlation of the

received powers en the uplink and downlink produced the plot in Figure 59. A

peak of .16 rises significantly out of the noise (with typical peaks of .08)

with a relative delay of .9 seconds,versus a completely correlated value of

about 1. Degradable by ncise on either link, the .16 value reflects considerable

but not complete decorrelation.

The downlink UHF hopping signal received on the aircraft was recorded on

magnetic tape for further analysis of the spectral decorrelation across the

hopping band. By processing the received amplitude wich the grcss frequency

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command the decorrelation across the hopping band can be assessed. Visual

inspection of this data indicates a potential decorrelation across the band

during the early-time passes in ESTHER for fading corresponding to the

multipath-like ringing.

F. Systems Effects: The teletype copy received on the UHF down forv/ard

link was protected against moderate fading by powerful error-correction

coding techniques employing a half-rate code with feedback decoding and full

message interleaving. The teletype copy remains basically error free until

a channel binary symbol error rate of between 5 and 10 per cent was reached.

At that time the teletype copy either exhibited a few errors or did not print

due to a print/no-print threshold which excludes all messages exhibiting more

than a limited number of errors. The observed range in UHF signal level

between the perfect copy and no message copy is rather narrow» on the order

of 3 or 4 db for performance in white noise only. Figure 60 is an example of

error free copy, while Figure 61 shows a limited number of character errors.

Figure 62 shows a larger number of character errors, and Figure 63 is an

example of a STRESS pass where most messages were not copied due to the high

error rate.

Figure 64 shows a plot of the percentage of messages received correctly

versus received signal strength (C)-to-thermal noise power density (NQ)

£1n inverse Hz] for all passes with significant fading in ESTHER. Also plotted

1s the system performance without fading that depicts the narrow range between

print and no-pr1nt. In spite of the full message interleaving (which ideally

would average out the received bit energies through fades and focuses) a loss

of at least 5 db can be ascribed to the barium induced fading. This loss

appears to be dependent upon the nature of the fading, the slower fading

characteristic of a late-time barium cloud causing a greater loss.

86

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6. System Effects - Frequency Decorrelation: A comparison was made

on the UHF down forward link between a help mode which is a fixed downlink

frequency and a hopping mode which hops over a relatively narrow frequency

bandwidth. On Pass 8 of the fifth test (FERN) the forward downlink was in

the fixed frequency mode (help). Figure 65 shows the received signal level

for Pass 8. Also shown on Figure 65 are the UHF link quality for each message

and the message character error rate. Approximately 5 minutes after the Pass

8 data the satellite was put in the hopping mode, and data from Passes 9 and

10 can be compared with Pass 8. From Figures 66 and 67 it is obvious that

the fading structure remained very similar to that encountered during the

fixed frequency Pass 8. The UHF link quality showed no improvement which

could be attributed to the frequency diversity of the frequency hopping mode.

Likewise, the character error rate per message was no better than that for

Pass 8. Table 8 shows a comparison of the average link quality over the 60

seconds of fading for each pass. These results show no improvement for the

frequency diversity effect of the frequency hopping on Passes 9 and 10. If

the propagation medium exhibited frequency decorrelation, some performance

improvement would be expected in the hop mode. Lack of an obvious improvement

indicates that the FERN propagation environment traversed was not strong

enough to produce significant frequency decorrelation across the downlink

hop band.

H. Multlpath Test Results: Good quality multlpath data was taken

during the ESTHER and FERN test flights using LES 9 while the barium fading

measurements were being simultaneously performed using LES 8. Ouring ESTHER

the aircraft transmitted a PRN sequence from either one of two antennas,

the crossed slot and the bottom blade, while during the FERN flight test

92

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Fern 14 March 1977 Pass 3

FIGURE 65

Comparison of Signal Level, Link Qualify and Character Errors

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TABLE 8: Comparison of UHF Link Quality For Olfferent Modes

96 1

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only the bottom blade was used. The PRN correlation receiver at the rooftop

facility (RTF) received over 600 returns during the seven-hour ESTHER flight

and over 220 returns during the five-hour FERN flight. The nature of the

ESTHER returns is summarized in Table 9, which gives the antenna type used

and the terrain flown over. Four types of returns were received: crossed

slot land returns, crossed slot sea returns, bottom blade land returns,

and bottom blade sea returns.

The land returns received during ESTHER from the crossed slot antenna,

Figure 68, consisted of one or two peaks rising out of an approximate -146 dbm

noise level. The first peak received was the main path signal, typically

arriving at the receiver pre-amp with a -115 dbm level. Link calculations of

power level received by the RTF helix from the LES 9 low power transmitter

give similar numbers indicating that the main path signal-to-noise ratios at

the input to the satellite transponder is greater than unity as expected. The

main path correlation peak typically had no noise fluctuations greater than

the resolution of the strip chart pen. The second peak, when observed, was

the land bounce multipath or reflected signal. This signal typically had a

-141 dbm received signal level with ±2 db instantaneous fluctuations. The

reflected path peak shape was typical of the upper portion of the main path

shape indicating an effectively (to the limitation of the noise) specular

reflection. Typical delays between the main path and reflected signals were

of the order of 12 seconds representing propagation time delays of 40 to 50

microseconds, consistent with aircraft altitude and satellite elevation.

The sea returns received from the crossed slot, Figure 69, were similar

to the land returns 1n nearly all aspects. The prime difference was that

the sea multipath returns were received at a level of -133 cbm with ±3 db

Instantaneous fluctuations in contrast to the -141 dbm land multipath value.

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The land returns received from the bottom blade, Figure 70, were similar

to the crossed slot returns 1n that two similar peaks could be identified.

The main path peak represented signal leakage around the aircraft fuselage

(with subsequent propagation to the satellite) and Its level fluctuated slowly

from -118 dbm to -135 dbm on a return-by-return basis. A signal level of -123

dbm represents a nominal return. Typically the main path signal did not

fluctuate Instantaneously. The multlpath peak was typically received with a

-128 dbm level with ±3 db Instantaneous fluctuations. The onset of this peak

corresponded to a 50 microsecond delay from the onset of the main path peak.

Nearly 8 microseconds after the onset of the multlpath peak a constant signal

level 3 db out of the noise was received for more than ICO microseconds.

The sea returns received from the bottom blade, Figure 71, differed from

the land returns 1n a few ways. The main path signal was received with a

level that was lower than the land received signal (probably a geometry effect),

-132 dbm. It differed from the land main path signals in that ±1 or ±2 dbm

Instantaneous fluctuations were evident in addition to the expected returns by

return fluctuation. The most obvious difference was that the reflected

received signal level was a ^try strong -117 dbm typically with ±2 db fluctu-

ations. Also, the multlpath delay profile neither appeared as consistently

as In the land return , nor persisted to such long time delays.

101

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VII. 'CONCLUSIONS

During the five STRESS tests a wide variety of ionospheric scintillation

fading was encountered. Fades during the early development of the cloud

usually consisted of a single, long defocus, roughly 10 db deep with a single

enhancement (focus) prior to and following the defocus. The mid-time fading

was Rayleigh-like, usually with rapid fades from 15 to 30 db deep with enhance-

ments of 5 to 10 db. During the late-time the fading was often patchy and

Rician-like with slower fading.

The effect of the fading on the teletype character error rate has been

estimated for various fading models. Figure 72 shows a series of curves

depicting the expected symbol error rate into the decoder for the cases of no

fading, flat Rayleigh fading, and selective Rayleigh fading. The difference

in symbol energy required to achieve a 5 X 10"2 symbol error rate both in no

fading and in flat fading is a representative figure of the expected system

performance loss in the barium environment. At the 5 X 10"2 symbol error rate

into the decoder the message error rate is expected to be approximately one-

half. This loss can be seen to be about 8 db in Figure 72 which is close to

the performance loss of 6 db actually observed in the data reduced to date

(ESTHER). Sources of error in this comparison are: 1) the assumption of

Rayleigh fading, the barium fading used to determine the 6 db loss figure may

have been less intense than Rayleigh; 2) the assumption of 5 X 10~2 symbol

error representing a message error rate of one-half; and 3) the assumption

that the fading was flat, some frequency selectivity could be mitigating the

fading. Further investigation should be fruitful in this area.

Regarding Assumption 3), the assumption of flat fading, there is some

quick-look evidence that the barium induced fading during FERN is indeed non-

selective. The effect of frequency decorrelation on the forward UHF downlink

104

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SIMULATION POINTS

0 BARIUM

-

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A NUCLEAR

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1 I 1 1 1 1 1 1 l\ ! ! ! 1

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FIGURE 72 Average Character Error Rate Probability, Slow Fading 105

-..;,-.r,--, ''"•- ■'-■---'''.■. ■■v>^--:^':'.'*"V'J-rf"/- ■■:-:?">:7"^ . ^- | ;'.^^|WI^JW^ll»J¥«*^^

appeared to be insufflcent to change the fading performance during FERN. The

error rate 1n the hopped, or frequency diversity mode, appeared to be almost

identical in the fixed frequency "HELP" mode. Mote, however, that decorrelation

of the uplink and downlink tone, which are separated by 90 MHz, was evident.

Frequency diversity of the order observed over the 90 MHz could be used to

improve system performance if used in conjunction with an error correction

coding system similar to that employed in the LES 8/9 system.

The ionospheric irregularities produced during the STRESS test by the

barium cloud produced fading quite similar to that encountered in the equatorial

ionosphere. Figures 73 through 77 show examples of fading encountered during

an AFAL ionospheric scintillation test in the area of Lima, Peru during March

1977. The rate of fading and depth of fading appeared quite comparable. The

physical extent of the fading was naturally much smaller for the STRESS test.

In the equatorial region scintillation is often encountered for periods of

hours. The barium cloud scintillation also appeared to have a more regular

repetitive structure than that encountered in the natural equatorial scintillation

situation.

One of the main objectives of the STRESS test was to obtain fading

information or to characterize the fading situation of a disturbed ionosphere

for extrapolation to a nuclear environment. For this reason it is appropriate

to make a comparison between the barium and the expected nuclear environments

with considerations to the validity of a systems test, as is summarized in

Table 10. The barium induced propagation disturbance was observed to cause UHF

Rayleigh fading. This aspect of barium induced fading is a key point of

commonality since the signal fading expected to result from nuclear detonation

induced ionospheric irregularities also has a Rayleigh amplitude distribution.

Apart from this important aspect, the fading from barium will differ from those

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predicted for a nuclear environment in one major and two minor areas. The

major difference is that the nuclear environment is expected to cause long-

lived signal absorption. Barium clouds produce no absorption of significance,

and thus, they cannot be straightforwardly used to simulate nuclear induced

system effects. However, it is believed that a meaningful test can be con-

ducted if the predicted values of nuclear induced signal absorption are

artificially Injected into the system channel. Signal attenuation of the

UHF system signals received by the subject modem was used in the STRESS experi-

ment for this purpose. The two minor differences between the barium fading and

nuclear environments are that generally the barium fading is expected to be

slower than the nuclear fading and that the barium fading is expected to be less

frequency selective. Although these differences will exist for most nuclear

geometries, it should be noted that geometries through the nuclear environment

with weaker or less extensive irregularities that have fading rates and

frequency selectivities similar to barium are not ruled out. It should also

be noted that for the range of fading rates and fading correlation band-

widths Involved in irost nuclear modelling, those presented by the barium

environment are worst case conditions for error correction coded frequency

hopping systems (LES 8/9 forward and report-back 1n particular). Oecoders

and hard decision demodulators in such systems generally perform better for a

given average channel symbol error rate as the channel fading they must

correct for causes a more random channel symbol error pattern. A slow fade

rate causes an undesirable organization of channel symbol error patterns in

time as does a large fading correlation bandwidth, which to a wideband hopped

signal presents the same fading from hop to hop. If the system's effects *ere

less severe when produced by slower fading or large fading correlation band-

widths, then some doubts might prevail with regard to the ability to use

113

lyiJ^inii^ - ^

barium in a systems test. However, because of the worst case conditions of

these differences, it I* felt that systems tests with barium are meaningful

1n the assessment of nuclear detonation induced systems effects, given that

the attenuation has been set according to the expected absorption.

The PRN multipath testing brought forth several conclusions. Multi-

path returns obtained while using the crossed-slot antenna indicated that the

isolation of the direct from the reflected propagation path was of the order

of 17 db ±3 db over sea and 30 db ±5 db over land for 30* to 40* satellite

elevation angles. The sea isolation value 1s in good agreement with data

taken from the crossed-slot antenna on the 16 January 1977 test flight. The

values of multipath Isolation set confidence limits on the determination of

the depth of barium induced fading from crossed-slot antenna data. Although

fades deeper than the isolation may be observed, it 1s not possible to

attribute them solely to barium cloud effects. (Thermal noise sets similar

confidence limits on fading data.) The sea multipath isolation of any

antenna 1s dependent upon three parameters: the direct path antenna gain,

the sea state reflection coefficient, and the antenna gain (due to leakage

around the aircraft fuselage) in the direction of the received multipath. The

first two parameters are relatively uniform over the set of upward looking

topside UHF antennas on the aircraft C135/662. The last parameter may change

moderately and, as a result, the multipath isolation of the topside upward

looking antennas may vary by 5 or 10 db from antenna to antenna. The best

sea multipath Isolation available 1s 25 to 30 db from Dome-Margolin hybrid

antenna based on the results of pre-STRESS, STRESS, and the 16 January 1977

test flight. This antenna was used in STRESS for transmission of the uplink

probe.

114

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Returns obtained while using the bottom blade Indicated that sea

specular reflections cause a loss of only a few (more than 1 less than 10)

db of signal strength. In contrast land reflections cause a greater (more

than 10 db but less than 20 db) loss. The strong sea multlpath returns

point to the possibility of the use of the sea bounce path as a spatial

diversity path through Ionospheric fading. Conventional spatial diversity

techniques are not Implemented 1n airborne receiver platforms to mitigate

Ionospheric fading because of the large antenna separations typically re-

quired. Use of the multlpath bounce channel would overcome the separation

problem with the sacrifice of some signal level.

In the bottom blade results a clearer picture of the multlpath delay

profile 1s available than previously available 1n the topside antenna

results of pre-STRESS. This picture Indicates a multlpath profile that 1s

predominantly specular to the 8 microsecond resolution of the PRN measure-

ment. Some diffuse energy arrived after this specular return 1n both sea

and land results. However, 1t was observed more consistently 1n the land

results. In the land results diffuse energy arrived at the satellite as

late as 100 microseconds after the start of the main path reflection 1n

many returns. This delay corresponds to energy arriving with a 30 kilometer

longer path length over and above the extra 15 kilometer specular multi-

path length.

115

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VIII- RECOMMENDATIONS FOR FUTURE EFFORTS

Additional reduction and analysis of the STRESS data is planned. As

discussed previously, the downlink hop data can be reduced along with

coarse frequency command recordings to give information on the frequency

decorrelatlon of the downlink hop propagation medium. It 1s also possible

to back-propagate the recorded propagation probe data to determine the true

integrated electron content of the barium cloud for phenomenological pur-

poses. Such a technique has been used with downlink data from Pass 16,

ESTHER, the amplitude fading of which 1s shown 1n Figure 58. Figure 78

shows the scintillation Index (S4) of the data versus back propagation

distance. The minimum of about 165 kilometers indicates that most of the

fading effects seen at the ground were due to barium structure at this

altitude. The dotted curve 1n Figure 7S 1s the phase of the field at this

altitude (contrast the phase at the ground in the solid curve) which is a

very good indicator of the actual Integrated electron content. Back

propagation analyses such as these are planned to assist phenomenological

Interpretation of ion cloud behavior and to check propagation prediction

techniques. A thorough data reduction effort is planned to extend through

June 1978.

Further Ionospheric testing is planned in the future, STRESS multi-

path measurements indicate that signals reflected off the ocean provide a

signal path through the ionosphere both spatially distinct from main path

signals and strong enough to establish a spatial diversity gain through

Ionospheric fading. The 6C0 meter spatial correlation lengths of ionospheric

fading prevent the use of conventional spatial diversity techniques on

airborne force elements. Plans exist fc future testing of the sea rrultipath

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spatial diversity path against equatorial fading using LES 8 or LES 9

to evaluate the usefulness of the concept. Such a test could occur in the

Fall of 1977.

Additional testing/simulation is planned to further evaluate the

relationship between fading level, acquisition time, and bit-error rate.

Magnetic tape recordings have been made of the UHF signal levels received

during the STRESS test and during natural, equatorial ionospheric scintil-

lation fading tests. These tape recordings will be used to generate a

fading signal for the additional evaluation. AFAL's Communication System

Evaluation Laboratory is equipped with a LES 8/9 satellite simulator.

This satellite simulator can produced a UHF forward downlink signal which will

be modified by the signal level traces recorded during the STRESS and

equatorial scintillation tests. The UHF satellite receiving system utilizing

the single UHF modem cr the dual UHF modem can be subjected to repeated

tests using a selected fading pattern and varying the median signal level.

This technique will allow meaningful evaluations of acquisition times

during various fading structures with various signal level margins. These

tests are planned for Fall 1977.

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REFERENCES

1. D. R. McDaniel, editor. Project SECEDE Final Program Review, Vol. II, SRI 2-5462, October 1972, p. 19.

2. L. M. Linson. Private Communication, February 1977.

3. C. Prettie, et. al. Final Report of the Feasibility Study for the STRESS Experiment, DNA Report by ESL; Sunnyvale, California, 20 August 1976.

4. J. Marshall. Preliminary Quick-Look Data Summary Communication/Propagation Test Data For Project STRESS; ESL, Sunnyvale, California, 15 March 1977.

5. D. R. McDaniel. STRESS Field Operations Plan; SRI Report for DNA; SRI, Menlo Park, California, February 1977^

6. Joint Report On Peru Scintillation Tests; AFAL/AFGL Report, AFAL: flrTgfit Patterson~EFB, Ohio; 28 February 1977.

7. Dr. C. Prettie. Project STRESS Barium Release Experiment; ESL SR-226; ESL; Sunnyvale, California, 29 April 1976.

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