AD-A098 498 AIR FORCE WRIGHT AERONAUTICAL LABS WRIGHT-PATTERSON AFR OH FIG 17/2.1SCINTILLATION FADING SIMULATION(UMAR 81 W T HUNT

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O SCINTILLATION FADING SIMULATION

Wade T. Hunt

Information Transmission BranchSystems Avionics Division

March 1981TICI ECTE

TECHNICAL REPORT AFWAL-TR-80-1 187 4i MAY -7 1981~

Final Report for Period November 1977 to December 1979 A

Approved for public release; distribution unlimited.

L

AVIONICS LABORATORYAIR FORCE WRIGHT AERONAUTICAL LABORATORIESAIR FORCE SYSTEMS COMMANDWRIGHT-PATTERSON AIR FORCE BASE, OHIO 45433

81 5 06 129- -- i-,---

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6

NOTICE

When Government drawings, specifications, or other data are used for any purposeother than in connection with a definitely related Government procurement operation,the United States Government thereby incurs no responsibility nor any obligationwhatsoever; and the fact that the government may have formulated, furnished, or inany way supplied the said drawings, specifications, or other data, is not to be re-garded by implication or otherwise as in any manner licensing the holder or anyother person or corporation, or conveying any rights or permission to manufactureuse, or sell any patented inventio' that may in any way be related thereto.

This report has been reviewed by the Office of"Public Affairs (ASD/PA) and isreleasable to the National Technical Information Service (NTIS). At NTIS, it willbe available to the general public, including foreign nations.

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

WADE T. HUNT CHARLES C. GADDERProject Engineer Chief, Information Transmission Sr.Information Transmission Br.

FOR THE CONANDER

RAYNOND E. SIFERD, Colonel, USAFChief, System Avionics Division

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Copies of this report should not be returned unless return Is requized bg SrItyconsiderations, contractual obligations, or notice on a sEpeIfic domont.

AIR FORCC/170/15 April 1981 - 100

I.

SECURITY CLASSIFICATION 0f THIS PAGE (Ihaft9, tet r40,

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S. PRFO RENG ORIAT IO NAEANDA.OE AGE PRORA L PRET. TASG0JfT ~ ~ RE AC98NJO3 ORE UNIATALOG UMER

AAnc Laortry(AWL/AD)prjctN

AINForceAWIght FAerNauia LabToraore (ASC Tiak NAo 1tWrgtPtesoSi oc asOi 53 Work Unit N74o. eek 722703

9I. CERORMLING OFFAICEI NAME A94 DDRESS10PRGAE- ISEC TK

Avionics Laboratory (AFWAL/AAA) Projec N8Air Force Wright Aeronautical Laboratories (AFSC) Tas AGESZR

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Approved for public release; distribution unlimited.

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IS. SUPPLEMENTARY NOTES

I9. KEY WORDS (Coalthw an revere, Side it necessary ad Identify' by Mlock nuotber)

Scintillation FadingSimulationIonospheric ScintillationSatellite Communication

30. ABSTRACT (Continue an reerrs Sid* It necessay ad identify by block imoet)

--'tis difficult to determine the long term effects of ionospheric scin-tillation on a particular modulation/coding scheme since the scintillationcharacteristics change rapidly. In order to measure the effect of thescintillation fading on the acquisition time, it would be necessary torepeat the acquisition test several hundred times. Since the acquisitionof a modem such as the Dual UHF modm my take 1 to 5 minutes, it wouldbe necessary to have scintillation fading for a period of 4 to 8 hours.

DO I " ANTS 1473 EDITION OF I Nov o5 IS 00SOLCTE

SECURITY CLASSIPICATION4 OF TNIS PAIOWN Data. bet.

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SECURTY CLASIFICATION OF THIS PAGZIMbII Die 4xftDw*

The alternative Is to record the UHF signal level of a simulator and repro-duce a short segment of fading over and over again. This was done using theAvionics Laboratory Comunication Systems Evaluation Laboratory (CSEL).

SIlCURITY CLASSIVICATION OF ?T** AO5[ fI WO a En.

;,- - -.

AFWAL-TR-80-1187

FOREWORD

This report was prepared by Mr. Wade T. Hunt (AFWAL/AAAD) of the

System Avionics Division of the Avionics Laboratory, under Project 1227,

"Communication Systems Concepts and Technology." The work was done to

demonstrate the value of being able to simulate satellite communication

links in a controlled laboratory environment such as CSEL. The simula-

tion can be repeated many times which cannot be done in flight testing

in a cost effective manner.

The help and suggestions by Mr. Allen L. Johnson in writing and

organizing the text is greatly appreciated. Also, the help of Mr. Arnold

Feineman, Mr. William Jardine, Mr. Steve Thompson and Ms. Susan Pfaad,

all formerly with Systems Research Laboratory (SRL), and Mr. James T. Zurn

(TRW) in making the measurements is greatly appreciated.

SRL has a contract for service and maintenance of the CSEL facility

and TRW has a service contract for service and maintenance of the modem.

i.

AFWAL-TR-80-1187

TABLE OF CONTENTS

SECTION PAGE

I INTRODUCTION 1

II TEST PROCEDURES 2

III TEST RESULTS 6

IV SUWIARY AND CONCLUSIONS 7

REFERENCES 8

i

PRcDi. A

i1-~ 7TT~7i ~im~

AFVAL-TR-80-1187

LIST OF ILLUSTRATIONS

FIGURE PAGE

1 Picture of CSEL 9

2 Forward Link Scintillation Fading Simulation 10

3 Spectrum and Interference Generator 11

4 Samples of Rapid, Medium, and Very Slow ScintillationFading 12

5 Rooftop Facility 13

6 UHF Modem 14

7 Programmable Signal Processor 15

8 Error Free TTY Message Copy 16

9 Message Containing Errors 17

10 Simulation Without Fade 18

11 Ionospheric Scintillation Fading from LES 9 19t 12 10 Minute Sample of Medium Fade Data 20

13 Simulation with Fading 21

14 Forward Link Percent Message Copy 22

15 Baseline Acquisition Test Results 23

16 UHF Modem Acquisition Time in Seconds Vs. No.of Occurrences 24

17 UHF Modem Acquisition Time Frequency Distributions 25

18 Acquisition Time Statistics Without Fade 26

19 Acquisition Time Statistics with Fade 27

vi

- \ . \"" ~ ~q. r ,,,

AFWAL-TR-80-1187

SECTION I

INTRODUCTION

Amplitude fluctuations believed to be caused by variations In iono-

spheric structure are termed scintillation fading. It is dependent on

radio frequency, geographic position, and geomagnetic conditions.

Since scintillation fading characteristics change rapidly, it is very

difficult to determine the long term effects of ionospheric scintillation

fading on a particular modulation/coding scheme. In order to measure the

effects of scintillation fading on bit-error-rate or acquisition time it

Is necessary to have a consistent type of scintillation fading pattern for

a long enough period of time to get an adequate data sample.

Because the acquisition time of modems may take from one to five

minutes, it would be necessary to have scintillation fading characteristics

to remain constant for a period of four to eight hours in order that a good

sample might be obtained.

The alternative is to record the UHF signal level of the scintillation

fading and use the recording to vary the signal level of a signal simulator.

This would allow reproduction of short, consistent segments of fading over

and over again in order to get sufficient sample size.

1)

.I

AFWAL-TR-80-1187

SECTION 11

TEST PROCEDURE

The Avionics Laboratory Communication Systems Evaluation Laboratory

(CSEL) is equipped with a Lincoln Experimental Satellite number 8 & 9 (LES

8/9) satellite simulatorb Figure 1, which can generate a UHF downlink signal

with the LES 8/9 modulation. The downlink signal is a 660 bit encoded

message at 150 bps data rate. The MD 1004 UHF modem decodes the message

and outputs a 40 character, 75 bps teletype message (Reference 1). A

drawing of the test setup for the scintillation fading simulator is shown

in Figure 2.

For the forward link simulation the concept is to generate a forward

link message in the LES 8/9 simulator and then attenuate the output using

a digitally controlled pin diode attenuator driven by a reproduction of

the UHF scintillation fade signal level. The 660 bit encoded/interleaved

forward link message is repetitively sent from the LES 8/9 simulator to

a Spectrum and Interference Generator (SIG) (Figure 3). In the SIG, the

signal is down-converted to 560 MHZ and attenuated with a pin diode

attenuator. The scintillation information is originally recorded on a

14 channel analogue magnetic tape recorded during a flight test typically

to the polar or equatorial region. Ten minute samples of the data are

selected in order to give a representation of rapid, medium, or very slowscintillation fading as shown in Figure 4 (Reference 2). The data is

transcribed from the tape recorder and stored on a disk pack and fed into

a PDP-11 computer. The PDP-11 computer derives the necessary output

voltage to operate the pin diode attenuator and reproduce the scintilla-

tion fading. After the pin diode attenuator, the signal is mixed down to

the UHF frequency and output. The output signal is fed into the Rooftop

Facility (Figure 5), and then through a manual attenuator to adjust the

absolute signal level. Then the signal is fed to a UHF transceiver in

the Rooftop Facility. The UHF transceiver dehops the frequency hopped

signal and down converts it to a 70 MHZ IF where it is fed to the UHF

modem (Figure 6). The modem completes the dehop procedure, deinterleaves,

2i.

AFWAL-TR-80-1187

decodes, and prints out the teletype message. The 40 character teletype

message used in the simulation is "SCINTILLATION FADING SIMULATION AFAL

AAD". The decoder generates a link quality indication which identifiesthe number of channel errors corrected by the decoder. The link quality

is a measure of the channel error rate prior to decoding. The link

quality is available visibly on a LED display and can be recorded on

magnetic tape for later analysis.

The Programmable Signal Processor (PSP), Figure 7, which is used to

simulate the LES 8/9 satellite configuration, was set to generate a 700

MHZ IF signal that is converted to 560 MHZ with the frequency converter

In the Spectrum and Interference Generator (SIG). The 560 MHZ signal

is converted to L Band in the SIG and then converted to the UHF frequency

of 242 MHZ in the signal combiner. The output (242 MHZ) from the signal

combiner is sent to the Rooftop by coaxial cable where the signal is

attenuated to a level of -120 dBm for reference. The UHF signal is

converted to 70 MHZ by the radio and fed into the single modem. At a

level of -120 dBm, signals from the single modem are received error free.

Figure 8 is a sample of an error free copy of the received message. The

signal was then reduced in 3dB steps and the link quality was recorded

at each 3dB step until the modem lost lock. Figure 9 is a sample copy

of a message containing errors. A curve for simulation under nonfading

conditions is shown in Figure 10.

Fading was then added to the signal using a 10 minute sample ofscintillation fading signals (taken during the 17 March 1977 flight test),

Figure 11, which drives a digitally variable attenuator to cause the

output UHF signal to vary in accordance with scintillation fading rate

and depth. When set up for the fading condition, the signal level into

the single modem was set at a -107 dBm level.

The fading data used is a 10 minute sample of medium rate fading

(periods of 2 to 5 seconds). The data (amplitude variations) was digi-

tized by the 4950th Test Wing data reduction facility on a 7-track

3

AFWAL-TR-80-1187

digital tape at a sample rate of 20 samples per second. The tape is then

converted to a 9-track digital tape which is compatible with the CSEL tape

playback equipment. The 9-track tape is used to load the disk memory in

CSEL. The fade duration used in the simulation lasts for approxiw.!tely

10 minutes. The disk information Is used by the PDP 11/50 computer to

generate the analog voltages to drive the pin diode variable attenuator

in CSEL in one dB steps. This 10 minute sample of data is shown in Figure

12.

The link quality is measured for a -107 dBm signal level after which

the signal level to the modem is reduced in 3dB steps and measurements are

made until the power is reduced to a level where the single modem cannot

maintain lock. Figure 13 is a curve of simulation with fading added to

the signal (Reference 3). The link quality numbers on the graphs of Fig-

ures 10 and 13 are generated by counting the information bit and paritybit errors. This count is used as a "quality" indicator for the communi-

cation link. A count of 00 indicates no errors and a large number indi-cates many errors. The maximum number that can be displayed is 77 octal

which equals 63 decimal.

Due to the steepness of the Bit Error Rate (BER) curve as the signallevel is decreased (attenuation increased), the link quality curve changes

from a convex shape to a concave shape as the BER threshold is reached

(around 37 dB attenuation on Figure 10 and around 30 dB attenuation on

Figure 13).

Figure 4 is a plot of the Forward Link percent Message Copy for both

fading and non-fading conditions (Reference 3). Curves of actual datataken during a flight test during March 1979 are shown for comparison

(Reference 5).

The signal acquisition test consisted of setting up an unfading sig-

nal level and measuring acquisition time. The initial reference level

was set at -107 dBm. Next, the signal level was reduced in 3dB steps

4

iI

AFWAL-TR-80-1187

until the acquisition time became extremely long and finally the UHF

modem was unable to maintain a lock to the signal. The signal was then

set to its nominal level of -107 dBm and fading was added to the signal.

The process was repeated by reducing the signal level in 3dB steps until

no acquisition occurred. Figure 15 is a plot of the results. Also,

tests were conducted with a fixed level of -124 dBm and samples of data

under non-fading and fading conditions were taken. A range of acquisi-

tion times was determined for both the fading and non-fading situations.

I

5!

' ! l klimm i-i i)i:~ I- '

..... I :

ArWAL-TR-80-1187

SECTION III

TEST RESULTS

Using the UHF scintillation fading simulator a variety of output

data is obtained. For the forward link measurements the curves of the

channel error rate, as indicated by the link quality indicator, are shown

in Figures 10 and 13. Figure 10 is a baseline curve without fading and

Figure 13 is a curve showing the effects of the simulated scintillation

fading. A curve of the percent error-free messages received versus

average signal level is shown in Figure 14. This curve indicated the

difference between no fading and scintillation fading. A plot of actual

flight data is also included on this curve.

Figure 15 is a curve of the acquisition time for a particular UHFmodem at a nominal signal level with and without fading.

Acquisition times varied from 17 seconds to 58 seconds with no fad-

ing on the signal. The mean for the sample was 30 seconds. With fading

on the signal, acquisition time varied from 18 seconds to 358 seconds.

The mean was 57 seconds. A frequency distribution for both the fading

and non-fading conditions are shown in Figures 16 and 17. Figures 18

and 19 are also plots of acquisition time statistics indicated (Reference

4).

6

V

AFWAL-TR-80-1187

SECTION IV

CONCLUSIONS

In order to validate the simulation, a comparison was made between

actual bit-error-rate and channel quality (link quality) data taken during

the equatorial scintillation testing and during the simulation period.

In Figure 14 a dashed curve, derived from actual data taken in equatorial

tests is superimposed on the simulation data. The accuracy with which

the simulated data reproduces the actual data is very close. A baseline

was established and measurements were made using a sample with medium

scintillation. The acquisition time data seems reasonable for medium

rate scintillation. Future testing will involve slow and fast scintilla-

tion ds well as more testing on medium scintillation. The results are

for the simulation of amplitude effects of the scintillation only. A

phase measurement capability has been installed in CSEL where multipath

effects as well as amplitude effects may be evaluated.

7

AFWAL-TR-80-1 187

REFERENCES

1. Allen L. Johnson, UHF Scintillation Fading Simulation System,AFAL-Th-78-1-AAD, Air Force Avionics Laboratory, WPAFB, Ohio,3 January 1978.

2. Allen L. Johnson, The Effects of Ionospheric Scintillation Fadingon Aircraft-to-Satellite Communications,' AFAL-TR-78-171, AirForce Avionics Laboratory, WPAFB, Ohio, February 1979.

3. Wade T. Hunt, Scintillation Fading Simulation JRepgrt fl), AFAL-Th-78 12-MAD, Air Force Avionics Laboratory, UPAFB, Ohio, 3 March 1978.

4. Wade T. Hunt, Scintillation Fading Simulation (Re~rt #2 ), AFAL-Th-78-24 AAD, Air Force Avionics Laboratory, WPAFB, Ohio 231'May 1978.

5. Johnson, Allen, et al., South American Test Report, AFAL-Th-79-14-MAD, Air Force Avionics La-boratory, WPAFB, Ohio, 9 July 1979.

AFWAL-TR-80-1 187

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9

AFWAL-TR-80-1187

SIGNAL INTERFERENCE GENERATOR

LES /9

SATELLITE SIMULATOR TOO mHz

GENERATES 660 BIT FREQUENCY HOPPEDENCODED/INTERLEAVEDFORWARD LINK MESSAGE

PROGRAMABLESIGNAL PROCESSOR 560 mtz

PIN DIODE ATTENUATOR

PDP/1l UDC/1COMPUETER D/A CONVERTER

O-10V OUTPUT

24-2.6 mHz

DISK FILEUHF FREQUE! CY HOPPEDOUTPUT WITI FADING

LINKQUALITYINDICATOR

MANUAL AN'TENUATOPTO ADJUST LEVEL j

OUTPUT CAN BE HARDWIREDUHF TO ROOFTOP LAB OF

TTY UHF MODEM TRANSCIEVER RADIATED VIA UHFANTENNA TO AIRCRAFT

Figure 2. Forward Link Scintillation Fading Simulation

10

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AFWAL-TR-80-1 187

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AFWAL-TR-80-1187

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Figure 4. Samples of Rapid, Medrium an1 ey-lwSiniltondn

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AFWAL-TR-80-1 187

Figure 7. Programable Signal Processor

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