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DATE: September 1968 

DOCUMENT CONTROL 

BIN: B1F-055-50366-300-1-68 PAGES 66 

COPY NO. 026'. OF 29 COPIES 

ACQUISITION SUBSYSTEM ON-ORBIT 

PERFORMANCE PREDICTION 

DORIAN 

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TABLE OF CONTENTS 

Section Page 

1-3 

2-3 

3-3 

1 INTRODUCTION 

2 SUMMARY 

3 FACTORS INFLUENCING ATS PERFORMANCE . 

3. 1 Background 3-3 

3.2 Factors Presently Being Considered . . . 3-3 

3. 2. 1 Visual Performance of Man . . . 3-4 

3. 2. 2 ATS Hardware Performance . . . . . 3-6 

3.2.3 Scene Characteristics at System Input 

Aperture 3-15 

4 PERFORMANCE ANALYSIS 4-3 

4.1 Original Performance Prediction 4-3 

4.2 Combination of Factors Influencing ATS 

Performance 4-4 

4.3 ATS Performance Prediction 4-13 

4.4 Man's Impact 4-16 

5 CONCLUSIONS AND RECOMMENDATIONS . . 5-3 

6 REFERENCES 6-3 

APPENDIX - ATMOSPHERIC EFFECTS A-3 

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LIST OF ILLUSTRATIONS 

Figure 

Average Day Tri-Bar Resolution in Forward Scan 

Page 

2-1 

Field 2-4 

3-1 Comparison of Visual Thresholds 3-5 

3-2 Static Flight Crew Visual Threshold 3-7 

3-3 Dynamic Flight Crew Visual Threshold 0.25 sec . . . 3-8 

3-4 Dynamic Flight Crew Visual Threshold 0.5 sec (Revised 

Data) 3-9 

3-5 Percent Degradation in Static ATS Performance Due to 

Variations in Scene Brightness from 180 ft/lamberts to 

30 ft/lamberts 3-10 

3-6 ATS's Transfer Function Envelope under Static 

Conditions 3-12 

3-7 Optical Quality Factor-ATS 3-13 

4-1 Original ATS Static and Dynamic Performance 

Prediction 4-5 

4-2 Original ATS On-Orbit Resolution vs LOS Angle . . 4-7 

4-3 Degradation Factor of Resolution with LOS Angle . . 4-8 

4-4 ATS Resolution Dynamic vs Input Contrast Ratio as a 

Function of LOS 4-9 

4-5 Modulation as a Function of Pitch and Roll Angles . . 4-10 

4-6 LOS Angle as a Function of Pitch and Roll Angles . . 4-12 

4-7 On-Orbit Contrast Ratio Based on PAM Atmospheric 

Data 4-14 

4-8 Percent of Useable 'Forward Scan Field as a Function of 

Resolution at 127X 4-15 

4-9 Relation Between Ground Resolution and Scene Contrast . 4-17 

4-10 On-Axis System Performance 2:1 Contrast Ratio . . 4-18 

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Pie 

1-5 

A-5 

A-5 

LIST OF TABLES 

Table 

1-1 Uses of ATS 

A-1 Transmittance 

A-2 Solar Flux 

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ACQUISITION SUBSYSTEM ON-ORBIT 

PERFORMANCE PREDICTION 

SEPTEMBER 1968 

Prepared by: 

W. Guard/ Weissman 

Approved : f4)( 

R. J. Barchet, Mgr. 

MOL Systems Development 

Date:9 4118 

Date: 7 106144. /9a 

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SECTION 1 

INTRODUCTION 

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SECTION 1 

INTRODUCTION 

This report is written to summarize the present status of the Acquisition Subsystem OnOrbit Performance Prediction analysis. It is the second in a planned series of reports 

dealing with this subject. The first report (see reference 1) is recommended reading for 

those who wish to obtain a better understanding of the analytical techniques used for the 

prediction analysis. 

It is important for the reader to recognize that the analytical predictions presented herein 

do not include the effects of all the presently defined factors which may influence alpha 

performance. However, recent customer and in-house interest in alpha performance has 

made necessary the need to document the present status of the effort. This report will 

discuss the major factors that influence ATS performance and will clearly indicate which 

effects are included in the overall performance prediction. It will also outline the work 

yet to be done in those areas where the analysis is not complete. Wherever possible, the 

probable effect of these factors will be discussed in order to give the reader a feeling for 

the sensitivity of overall performance to these factors. 

System performance predictions presented herein are based on the resolving power criteria 

for specifying optical system performance. The standard U. S. Air Force (MIL-STD 150A) 

tri-bar target has been used as a basis for the analysis. This method of specifying resolution is of prime importance in that the significance of the results obtained are thoroughly 

understood throughout the photo-intelligence community. However, to avoid improper 

interpretation, it must be remembered that an ATS resolution number based on a tri-bar 

target is solely a measure of man's ability to resolve that specific geometric configuration. 

At the present time a correlation between specific object-of-interest resolution and tri-bar 

resolution has not been established. 

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This report concerns itself only with on-axis performance prediction. Table 1-1 contains a 

brief summary of the major uses of the ATS. Of these, activity detection is the only use 

where an accurate determination of ATS performance is required. This use, however, is 

where man may make his greatest contribution to mission effectiveness. Therefore, a complete understanding of the performance capabilities of man in this capacity is essential. 

Determining ATS performance in terms of tri-bar resolution is a major step towards answering 

the ultimate question, i. e. , how effective is man in utilizing the ATS, thereby enhancing the 

primary photographic mission by detecting the presence of Essential Elements of Information 

(EEI) or of specific activity indicators which may indicate the presence of the EEI 

The Air Force plans to use the results of the work presented herein in an attempt to correlate 

tri-bar resolution to recognition of specific objects of interest. Another possible direct application for the results of this work is in the sequencing of targets for the ATS. It is well to 

note that the analytical techniques developed for calculating ATS performance have considerably 

increased the accuracy of the predictions; however, resolution determination is inherently not 

a highly accurate tool. As an example, inaccuracies are introduced in specifying optical 

system performance with a Modulation Transfer Function (MTF) because phase relationships 

are ignored. Also, the visual threshold of the flight crew used in this study is based on the 

mean performance of the present 14 crewmen, so there exists a statistical variance about 

this mean. It is felt that the absolute accuracy of the performance estimates are within 

five percent, but that the relative accuracy between the various conditions may be considerably greater. For this reason, resolution numbers in terms of ft/cycle are given only to two 

significant figures. 

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Table 1-1. Uses of ATS 

Enhancement of Primary Photographic Mission 

a. Cloud/Haze Detection (16X) 

b. Activity Detection (127X) 

Accurate Determination of Target Location (63X) 

Improvement in Main Optics Pointing Accuracy (63X) 

Backup for Visual Optics 

a. Target Centering (63X) 

b. Hate Nulling (127X) 

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SECTION 2 

SUMMARY 

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SECTION 2 

SUMMARY 

Wi contrast tri-bar target on the ground, and assuming a moderately hazy (average) 

.0"*. atmosphere, a 20 Hz, 0.25 sec (0-Pk) line-of-sight Jitter, and a sun elevation angle of 36 

degrees, the dynamic on-axis ATS nadir resolution is predicted to be 2.6 feet. For a lineof-sight angle of 60 degrees, the dynamic resolution of the ATS is predicted to be 8.0 feet. 

The above information is depicted on Figure 2-1. 

Furthermore, because of the shapes of the ATS and Main Optics performance curves in the 

very low contrast regions, together with the slopes of the visual and film threshold detectability curves, it may be possible that man will be able to detect low contrast targets that the 

Main-Optics will not be able to photograph. 

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SECTION 3 

FACTORS INFLUENCING ATS PERFORMANCE 

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SECTION 3 

FACTORS INFLUENCING ATS PERFORMANCE 

3.1 BACKGROUND 

Less than one year ago, a limited understanding existed concerning the validity of the 

analytical techniques and assumptions used to predict on-orbit ATS performance. Also, 

little consideration was given to the real world conditions under which the ATS will perform. 

For example, Acquisition Subsystem Performance Predictions were based on the intersection 

2, 

of a nominal telescope MTV' curve with an arbitrarily selected visual threshold curve. As 

discussed in Reference 1, the direct intersection of an MTF curve and a visual threshold 

curve is not valid. Also, the visual threshold curve selected to make the original performance 

predictions was based on an extrapolation of experimental results obtained by 0. Schade 

(References 2 and 3). 

The analyses were undertaken for the on-orbit conditions that had been selected by the 

customer to specify Alpha performance requirements: 2 to I. contrast at the input aperture, 

530 ft-L scene brightness, 80 nm orbit, nadir LOS, etc. Jitter effects were incorporated 

using the technique employed to predict the degradation caused by smear in a photo-optical 

system. 

3.2 FACTORS PRESENTLY BEING CONSIDERED 

To overcome the deficiencies in the original performance and analysis, several factors which 

influence ATS on-orbit performance have been identified and categorized according to their 

influence on: 

a. Visual performance of man in the MOL on-orbit environment. 

b. System performance of the ATS hardware. (This includes telescope optical 

performance, control system performance, thermal degradation, structural 

vibration, etc.) 

c. Scene characteristics at the input aperture of the system. 

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The factors influencing performance in each of these areas will be discussed in some detail 

below. However, it is well to reiterate that there are several factors for which the required 

analysis and/or experimental measurements have not been completed, and therefore, are 

not included in the overall performance prediction of Section 4. As these data become 

available their effect on Alpha performance will be factored into the analysis. 

3.2.1 VISUAL PERFORMANCE OF MAN 

The performance of man in resolving the tri-bar target, both under static conditions and in 

the presence of sinusoidal jitter, has been investigated in considerable detail. An experiment 

was conducted to determine manes performance in this area. A report is being prepared to 

cover the apparatus and procedures used in the experiment, the data reduction techniques 

employed, and the results obtained. A brief discussion and summary of the results as they 

apply to performance prediction are included here. 

The objectives of the experiment were to: 

a. Determine the static visual threshold for the MOL flight crew using tri-bar targets. 

b. Evaluate the degradation in resolution caused by single frequency sinusoidal jitter. 

c. Determine the influence of variations in scene brightness on resolution. 

d. Investigate the sensitivity of the eye to jitter amplitude as a function of jitter 

frequency. 

Figure 3-1 is a comparison of the visual thresholds obtained from the experiment and 

thresholds derived from published experiment data. This curve shows that: 

a. Utilizing the visual threshold derived from the work of 0. H. Schade produces 

extremely conservative predictions of ATS performance. 

b. The visual threshold based on an experiment conducted by Campbell produces less 

conservative results than the Schade threshold. The experiment by Campbell was 

conducted using a very similar experimental technique to that of Schade. 

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arisonof Visual Thresholds 

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c. The visual threshold obtained from the experiment using ordinary observers 

(employees) with 20/20 vision correlates very clearly with the results obtained 

using the published results of Lowry-DePalma (Reference 4). 

d. The static visual threshold for the flight crew produces better results than do the 

equivalent threshold for the ordinary observer. This appears to be reasonable in 

that the nominal visual acuity for the flight crew is on the order of 20/15 as compared 

to a normal 20/20 for the Itek observers. 

Figures 3-2, 3-3, and 3-4 represent the crewman visual threshold under static conditions 

and in the presence of 20 cycle per second jitter at 1/4 and 1/2 arc seconds (0-P) amplitude, 

respectively. Also shown on these curves are the ± 1 sigma standard deviations about the 

mean. These standard deviation curves are derived from the variations of the individual data 

points from the mean of all the points. Figure 3-5 shows the degradation of resolution due to 

variance in scene brightness from 180 to 30 ft lamberts as a function of contrast. The effect 

of brightness has not yet been factored into the present analysis. The visual threshold curve 

for 180 ft lamberts was used in the analysis. 

It was noted during the experiment that fatigue had a large effect on man's ability to resolve 

a tri-bar target in the presence of jitter. The experiment procedures and apparatus minimized 

the effect of fatigue on performance, therefore, the performance prediction contained herein 

do not consider fatigue effects. 

3.2.2 ATS HARDWARE PERFORMANCE 

There are several factors that should be considered in determining the overall effect of 

hardware on the performance of the system. These major areas are: optical performance, 

control system performance, structural vibration, contamination and thermal distortion. 

These major areas are now discussed. 

3.2.2.1 Optical Performance 

The Acquisition Telescope optical performance is dependent upon the prescription, the 

manufacturing and alignment errors, and the effects of the on-orbit environment. There 

are also some secondary considerations which affect performance such as vignetting at the 

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high forward line-of-sight angles, scattering of light within the tube, etc. Predictions of 

the MTF to date have been based upon ray trace analysis performed by both GE and Itek 

Corporation. 

Figure 3-6 is a chart which shows the results of this analysis. Curves 1 and 2 are the 

prescription physical frequency responses (MTF) calculated by GE and Itek, respectively. 

Curve 3 is the Itek nominal system performance estimate presented in their September 1967 

PDR report (Reference 5) which they use for predicting system performance. Curve 4 is 

a GE worst-case tolerance buildup performance estimate based on previously described 

curves. The probability of designing and building the acquisition telescope to meet the 

performance described by this curve is considered to be relatively high. Therefore this 

curve was used in the present analysis. 

Figure 3-7 represents optical quality factors (OQF) resulting from the ITEK nominal system 

performance estimate and the GE nominal performance estimate. 

It should be noted that relatively large changes in the MTF OQF will not appreciably change 

the performance of the telescope (based on a 2 to 1 contrast input). For example, changing the 

OQF to 60 percent from its present value of 40 percent would improve resolution (for 2 to 1 

contrast target) by only 0.2 foot. Decreasing the OQF to 20 percent from 40 percent would 

degrade system performance by 0.5 foot. The relative sensitivity of system performance 

to changes in the MTF OQF at the 2 to 1 input contrast is due to the slopes of these two 

curves. 

Under low contrast input conditions ( < 2:1) the changes in the system resolution become considerably greater. This will be discussed in greater detail in Section 4. 

3.2.2.2 Control System Performance 

In addition to the introduction by the control system of position and rate errors which would 

result in off-axis viewing (not considered in this analysis) and smear, respectively, the 

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control system will also introduce low amplitude random vibration (jitter). Normally, the 

rate error can be nulled by the crewman; jitter, being random in nature, cannot be negated. 

The noise sources producing control system jitter are: 

Bearing Noise — This is the largest contributor. 

Input Noise Electro-magnetic interference (EMI). 

Electronic Noise - Power amplifier, pre-amplifier. 

Torquer Noise, Gyro Noise, D-A Noise. 

In order to see the effect of jitter on resolution, consider single-frequency, sinusoidal 

motion of the LOS. If this motion is at a small enough amplitude and low enough frequency, 

the eye and brain will be able to follow it; under these conditions there will be no loss in 

resolution. If the frequency is increased above approximately 1.0 Hz, the eye is not able to 

completely track the LOS and the object begins to become smeared. If the frequency of the 

motion were time-varying, the eye and brain would not be able to track as well as with single 

frequency motion; thus jitter, which in practice is random in nature, could be a significant 

source of resolution degradation. Above approximately 20 Hz, smear becomes independent 

of frequency; the peak amplitude determines resolution. 

e--•••• 

This analysis takes into account single frequency jitter at amplitudes (0-P) of 0.25 sec and 

0.50 sec. No data on the effect of random jitter on resolution is available at this time. An 

experiment to determine random jitter effects is being planned for implementation in late 

1968 or early 1969. 

3.2.2.3 Structural Vibration 

Data is presently being obtained to determine the amplitude and frequency of the LOS due to 

structural vibrations caused by such sources as ACTS jet firing, main optics mirror slew, 

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sliding mask slew, camera operation, etc. These data are being obtained using a vibration 

model developed by GE. This performance prediction analysis does not take into account 

structural vibration effects. These effects will be included in a subsequent issue of this 

report. 

3. 2. 2. 4 Contamination 

A possible large contributor to resolution degradation is contamination. The two major 

sources of contamination are exhaust products of the ACTS propellants (various hydrocarbons 

formed from the combustion of N2

0

4 and hydrazine) and the waste products from the lifesupport system. No data is now available concerning the level of contamination on the ATS. 

At this time little is known concerning the effects, if any, of contamination on the ATS. The 

possible effects include change in reflectivity of the mirrors, change in transmittance of the 

window, or perhaps evan a change of the system MTF. 

Contamination effects are not considered in this performance prediction analysis. 

3. 2. 2. 5 Thermal Effects 

Wavefront deformation, resulting in decreased resolution, can occur if the optical elements 

are subjected to a thermal gradient. 

Data is presently being analyzed to determine the extent of this deformation. Preliminary 

indications are that the thermal gradients are sufficiently large enough to cause a degradation of resolution. When more data is available, they will be factored into the performance 

prediction analysis. 

3. 2. 3 SCENE CHARACTERISTICS AT SYSTEM INPUT APERTURE 

If there were no atmosphere between the ground target and the orbiting vehicle, the intrinsic 

contrast of the target would not be attenuated when viewed from orbit. Since there is an 

atmosphere, its effect on apparent contrast and brightness (and therefore on resolution) 

must be determined. 

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On-orbit brightness and contrast is a function of the following: 

a. Relative positions of the Sun, observer, and target. 

b. Altitude. 

c. Target characteristics. 

d. Observing sensor characteristics. 

e. Eye sensitivity. 

f. Solar flux. 

g. Atniosphere scattering characteristics. 

The on-orbit brightness and contrast determined in this performance analysis was obtained 

using the Photometric Atmosphere Model (PAM) (Reference 6) using a tri-bar target of 

reflectances 0.234 and 0.078 giving an intrinsic contrast of 3.0:1. PAM corresponds to an 

atmosphere that is considered "moderately" hazy by EK standards. 

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SECTION 4 

PERFORMANCE ANALYS IS 

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SECTION 4 

PERFORMANCE ANALYSIS 

4.1 ORIGINAL PERFORMANCE PREDICTION 

As has been previously discussed, the original performance predictions were based on the 

intersection of a nominal MTF (Modulation Transfer Function) with a Schade (derived) visual 

threshold curve. Figure 4-1 shows that the intersection of the static and dynamic ATS 

performance curves with the Schade threshold indicates a system resolution of 3.3 and 

3.6 ft, respectively. For a tri-bar target flat on the ground and a constant on-orbit contrast of 2:1 (no atmosphere contrast degradation as a function of LOS angle) the static and 

dynamic resolution as a function of LOS angle is shown in Figure 4-2. 

Based on this original prediction of ATS performance as a function of LOS angle, one can 

easily see that considering the contrast degradation properties of the real world atmosphere 

would make the apparent on-orbit performance considerably degraded, probably in the order 

of 8-9 feet at 40 degrees and in excess of 20 feet at 60 degrees. 

Figure 3-3 shows the intersection of the system performance curve for 2:1 and 1.3:1 

contrast input. Drawing a family of these system performance curves over the region 

between 5:1 and 1.1:1 allows the generation of a curve which describes the performance of 

the system based on optical system and observer performance. This curve can then be 

extended to represent system resolution as a function of LOS angle. This scale factor is 

directly proportional to the percent change in LOS distance from the vehicle to the ground 

as the LOS angle varies. This assumes that there is no degradation in resolution caused 

by change of aspect angle WRT the plane of the ground. This is considered separately. 

It is felt that dividing by the cosine of the angle that the LOS vector makes with the normal 

to the Earth's surface at the point of intersection is overly harsh due to the three dimensional 

characteristic of the real world. 

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Considering the real world and the orientation of the Standard USAF tri-bar, the worst 

case degradation was found to occur when the projection of the LOS formed a 45 degree 

angle with both sets of bars in the group. Figure 4-3a represents the degradation factor 

(that applies to nadir resolution) as a function of LOS for a target oriented perpendicular 

to the LOS. Figure 4-3b is an additional factor to account for the target laying flat on 

the ground oriented so that the bars form a 45 degree angle with the horizontal projection 

of the LOS. Both factors were used in this analysis. 

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4.2 COMBINATION OF FACTORS INFLUENCING ATS PERFORMANCE 

In Section 3, the many factors influencing system resolution were enumerated. In this 

section the method of combining these factors to arrive at the ATS performance is described. 

The performance curves of the ATS as a function of input modulation were obtained by convolving the MTF of the ATS with the input scene. The procedure is explained in Reference 1. 

The intersection of the performance curve for a particular input aperture modulation with 

the dynamic flight crew visual threshold curve gives the ATS dynamic resolution for that 

modulation. 

Figure 3-3 shows intersections of these curves for contrasts of 2. 0:1 and 1.3:1 (modulation 

= O. 333 and 0.130, respectively). The jitter amplitude of Figure 3-3 is 0.25 sec (0-P); 

the jitter frequency is 20 cps; the LOS angle is 0 degrees. 

Figure 3-4 differs from Figure 3-3 in that the jitter amplitude is 0.50 e. 

Thus, it is possible to generate a curve of resolution vs contrast ratio for the 0 degrees 

LOS case. This is shown as the top curve of Figure 4-4. 

In order to generate resolution vs contrast ratio curves for LOS angles other than 0 degrees, 

the data of Figure 4-3 was applied. The result of applying the factor ( both curves of. Figure 

4-3) of resolution degradation as a function of LOS angle is shown as the family of curves 

of Figure 4-4. Figure 4-4 describes the relation between ATS dynamic resolution. LOS, 

4-4 SECRET/DORIAN 

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and contrast ratio. Thus, Figure 4-4 in conjunction with Figures 4-5 and 4-6 allow 

the generation of Figure 2-1; ATS dynamic resolution as a function of roll and pitch 

angle. 

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0 /0 20 30 40 50 60 

LINE OP SIONT ANGLE (DEC) 

/6 

1/41 

:, 

aC 

4 a 

4 

6 

2 

0 

Figure 4-2. Original ATS On-Orbit Resolution vs LOS Angle 

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I I 

AIA8N/PIC.47/0N- /27X 

CONTRAST- 24 (81/-ORSIT) 

Al7/1-00E -80 MN 

ffilTER- 0.2S Sqa (0-P) 

DYNAMIC 
CURVE A 

/0 ■ 

9 

-41 

2 Cul? VE B 

I 

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0 • 

0 no 20 50 40 50 60 70 

L. O. S — DECREES 

Figure 4-3. Degradation Factor of Resolution with LOS Angle 

4-8 

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2 

3 

4 

5 

7 

TIC/- BAR Resoc

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(fTICYCI-E4NO) 

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LOS ANILE 

0' 

IO. 

20. 

30. 

40° 

50° 

60° 

J 

141142NIF/C/ITION0.4, 

1271( 

ITTER-0.25 SEC 

-AXIS 

/.0:/ 20:/ 5.04 404 

/1/PUT CONTRAST RATIO 

Figure 4-4. ATS Resolution Dynamic vs Input Contrast Ratio as a Function of LOS 

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A4: 0.28 

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so 

W 

40 

4,1 

30 

/0 

41/44X-BASIII 

AMAX#441//I 

ROLL ANGLE (DEC) 

Figure 4-5. Modulation as a Function of Pitch and Roll Angles (Based on PAM Atmospheric 

Model and 3:1 Contrast Ratio Flat on Ground) 

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It is now possible to determine resolution as a function of contrast ratio for any LOS angle. 

What remains to be done is to determine what is the functional relationship between LOS 

angle and contrast ratio with respect to real world parameters. 

The real world parameters are introduced by means of the Photometric Atmospheric Model 

(PAM) (Reference 6). PAM calculates contrast ratio ( — modulation) as a function of the 

following: 

a. Relative position of the Sun, observer, and target. 

b. Altitude 

c. Target reflectances 

d. Characteristics of observing sensor. 

e. Eye sensitivity. 

f. Solar flux. 

g. Scattering characterisitcs of the atmosphere. 

For this performance prediction, a Sun angle (elevation angle) of 36 degrees was assumed 

together with ground tri-bar target reflectances of 0.234 and 0.078. This choice of 

reflectances result in a ground (intrinsic) contrast ratio of 3:1 (modulation = 0.50) and an 

average reflectance of 0.156. The target was assumed to be flat on the ground for 

all cases. A detailed explanation of the PAM is given in the Appendix. 

Figure 4-5 shows a map of constant modulation contours as a function of roll and pitch 

angle. Figure 4-5 was obtained from PAM output data. 

Figure 4-6 gives the LOS angle as a function of pitch and roll angle. 

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110., ••■• =mono ami■• ••■••• emm= NI■ne •■■ ammo limonono ••■ awasmo 7– —I— ••■ - 

LOR/ION 

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40' 

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AMU- 

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20' ‘111%91pV 

4■ 

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.4/ 

70 

20 

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0 

60 

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20 30 40 So 

Parli Am9LE (DECREES) 

Figure 4-6. LOS Angle as a Function of Pitch and Roll Angles 

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Figure 4-5 together with Figure 4-6 allows the generation of Figure 4-7, on-orbit contrast 

ratio vs LOS angle. The parameters involved in this curve are given in paragraph 4.3. 

Figure 4-7 together with Figure 4-4 produce the desired result; average day tri-bar resolution 

in the forward scan field. This is Figure 2-1. 

Figure 4-8, the percent of useable forward scan field as a function of resolution for target 

recognition describes the cumulative percent of the forward field that yields resolution 

better than the value of the abscissa. 

4.3 ATS PERFORMANCE PREDICTION 

Using the methods described in paragraph 4.2 it was possible to obtain Figure 2-1, the 

Average Day Tri-Bar Resolution in the Forward Scan Field. Nominal values were chosen 

for the pertinent parameters involved: the tri-bar reflectances were 0.234 and 0. 078 

(3. 0:1 at the ground; the target was flat on the ground and always oriented 45 degrees 

to the horizontal projection of LOS; the sun elevation angle was 36 degrees, a yearly 

average over the latitudes of interest; the nadir altitude was 80 nm; the other pertinent 

data used in the PAM program is described in the Appendix; the crew visual threshold 

for 0.25 "Ca (O-PK) and 20 Hz jitter was used with the ATS performance curves obtained 

from curve 5 of Figure 3-6. 

The dynamic nadir on-axis resolution (roll angle = 0, pitch angle = 0) was found to be 2.6 

ft. This represents a significant change from the original prediction of 3.6 ft. At 60 

degree pitch angle, 0 degree roll angle, the resolution has degraded to 8. 0 ft; based on 

the original performance prediction one would have anticipated a value in excess of 20 ft 

at a 60 degree angle. 

Figure 4-8 indicates than on-axis resolution for 50 percent of the forward scan field is 

better than 4.4 It, resolution for 70 percent of the field is better than 6.1 ft. 

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I. 

O 

O 

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SOLAR ALTITUDE 

TARGET REFLECTANCE 

CONTRAST - 

ORBIT ALTITUDE 

- 36° 

B MAX 

- .234.078 

: I 

B MIN 

80 N. MI. 

0 /0 20 30 40 SO 60 

LINE OF SIG/4T ANGLE (DEGREES) 

Figure 4-7. On-Orbit Contrast Ratio Based on PAM Atmospheric Data 

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00 

70 

SCAN F/E

LO (%) 

$0 

60 

A MOUNT 

OF USE

A BL E 

40 

30 

20 

/0 

0 

2.0 3.0 4.0 .570 6.0 70 8.0 

ATS A'ESOLUT/ON (Pt) 

Figure 4-8. Percent of Useable Forward Scan Field as a Function of Resolution at 127X 

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4.4 MAN'S IMPACT 

Figure 4-9 is a plot of normalized resolution for both the ATS and the Main Optics as a 

function of input contrast ratio. Both performance curves have been normalized to intersect 

at the 2:1 input contrast. The flatness of the curves show that the man-ATS system performance is much less sensitive to input contrast than is the Main Optics. This may, in 

part, be due to the fact that the Main Optics may have been optimized for a 2:1 contrast 

at the expense of performance at lower contrasts. However, a primary factor is the shape 

of man's visual threshold curve. The shape of the threshold curve is steep in the region 

where the 2:1 performance curve for the ATS and Main Optics intersect their respective 

thresholds for the eye and for film. Figure 4-10 illustrates this fact. It is a plot of 

modulation as a function of normalized spatial frequency for both the Main Optics and the 

ATS. They have been normalized so that the intersections of the system performance 

curves and the threshold curves both occur at the same normalized spatial frequency. 

Figures 4-9 and 4-10 indicate that man's ability to resolve low-modulation level targets 

with the ATS is possibly greater than that of the Main Optics/Camera. 

For relatively large objects which exhibit low on-orbit contrast ( r:11.1:1) man may, under 

certain conditions, be able to detect their presence, whereas a Main Optics photograph of 

the same object might not reveal their presence. 

It must be understood that this section (Section 4) is very preliminary and much further 

investigation should be undertaken to prove its validity. 

4-16 Tf DORIAN 

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SECTION 5 

CONCLUSIONS AND RECOMMENDATIONS 

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SECTION 5 

CONCLUSIONS AND RECOMMENDATIONS 

The major conclusion to be reached from this study is that the on-axis dynamic tri-bar 

resolution is far greater than originally anticipated. This is due primarily to the use of 

new experimental visual threshold data that differed greatly from that originally available. 

Though the resolution of 2. 6 ft at nadir looks very satisfying, it must be remembered that 

many factors were not taken into account. These are: random jitter, structural vibration, 

contamination, thermal gradients and non-average haze. 

A secondary conclusion is that for very low contrast (1.1:1) the man-ATS may detect 

targets that the Main Optics/Camera cannot resolve. 

It is recommended that the following areas be investigated: non-nominal haze conditions; 

random jitter effects on visual thresholds; man's ability to enhance mission through 

observation of low contrast conditions; performance prediction based on real world 

statistics of target location, sun elevation and target contrasts; off-axis performance; 

contamination effects; thermal gradient effects; structural vibration effects. 

It is further recommended that a stronger interface be established between performance 

prediction and simulation oriented contractor and customer personnel. Also, it is highly 

desirable that a regular working group be established including General Electric, Eastman 

Kodak, Itek, Air Force, Aerospace and the Intelligence Community in order to exchange 

information and solve problems. 

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SECTION 6 

REFERENCES 

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SECTION 6 

REFERENCES 

1. Acquisition Subsystem EAR, General Electric, dated 12 February 1968, Section 2. 

2. Journal of Optical Society of America Vol. 46, September 1956, Page 721, 

"Optical and Photoelectric Analog of the Eye, " by 0. H. Schade. 

3. Journal of Society of Motion. Picture and Television Engineers, Volume 73, 

February 1964, Page 81 by 0. H. Schade. 

4. Journal of the Optical Society of America, Volume 52, March 1962, Page 325 

by DePalma, J. J. and Lowry, E. M. 

5. Acquisition Optics Subsystem PDR, ITEK Corporation dated 25 September 1967. 

6. Photometric Atmospheric Model (PAM) by James Pierson. 

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APPENDIX 

ATMOSPHERIC EFFECTS 

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APPENDIX 

ATMOSPHERIC EFFECTS 

The degradation in resolution of a tri-bar chart caused by atmospheric effects was determined 

through use of the Photometric Atmospheric Model (PAM) computer program. PAM calculates 

on-orbit brightness and modulation as a function of the following: 

a. Relative positions of sun, observer and target. 

b. Altitude. 

c. Target characteristics (reflectances of light and dark bars of trl-bar). 

d. Characteristics of observing sensor, such as wave length dependence of optics. 

e. Wave length response of the eye. 

f. Solar flux. 

g. Scattering characteristics of atmosphere. 

PAM corresponds to an atmosphere that is considered moderately hazy by EK Standards. 

In PAM, the target is assumed to be a lambert diffuser specified by a pair of reflectances 

(PAM'S calculations do not take into account the background in which this pair is submerged). 

In this study, the reflectance values were al.= 0.234 and a2 = 0.078. These values are 

assumed to be independent of wave length. Thus, the ground contrast was al/a2= 3:1. 

The average reflectance was al + a2 = 0.156 

2 

Table A-1 gives the values of transmittances as a function of wave length (X) used in the 

program for the human eye and for the ATS optics. The transmittance values for the ATS 

optics are based on extrapolated and interpolated data from values furnished by ITEK. 

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Table A-2 gives the solar flux values relevant for light incident on the top of the atmosphere. 

PAM is basically an empirical scattering model and allows for aerosol scattering and molecular scattering. The zenith angle of the Sun, A1, and the nadir angle of observation, A2 

(the angle between the nadir and the target as viewed from the orbiting vehicle), determines 

relevant optical depths for light attenuation in the upward and downward directions. The 

apparent brightness seen by the observer may be considered to be composed of two parts: 

1. Indirect or scattered light (haze). 

2. Directly transmitted light which has been attenuated by its double traverse of 

the atmosphere. The direct light is proportional to the reflectance, a of the 

target. Thus, B (apparent) = B (indirect) + aB (direct) where B (apparent) is 

the on-orbit brightness. B (direct) is calculated by: 

B (direct) =

X

IT

3(X) T2(R) 

F

R) 

T

1(X) 

COS (A

1

) + F

(X) 

S

(X) 

V

4 

where 

• wave length values from Tables A-1 or A-2. 

T

3(X) 

- 

optical transmittance of sensor (Table A-1). 

T2(A) 

• 

attenuation of light on its outward traverse of the atmosphere. 

F

(X) 

solar flux (Table A-2). 

T

1 (X) 

• 

attenuation of light during its inward traverse of the 

atmosphere. 

S 

• 

a function depending on and A (zenith angle) governing 

R) the amount of flux incident on de ground due to light which 

has been scattered. 

V(X) 

• 

visual sensitivity. 

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Table A-1. Transmittance 

X (Microns) 

0.40 

0.45 

0. 50 

0. 55 

0.60 

0.65 

0. 70 

0. 75 

0. 80 

Visual Sensitivity 

V

2 

0. 0012 

0. 0422 

0. 3766 

0. 9516 

0.6384 

0.1280 

0.0065 

0. 0002 

0. 0 

Sensor Optical 

Transmittance 

T

3X 

0. 0564 

0.1926 

0.2947 

0. 3374 

0. 3286 

0.2898 

0.2510 

0.2124 

0.1834 

Table A-2. Solar Flux 

A Microns Solar Flux (watts/m2 

- Ao) 

F

X 

0.40 0.1310 

0.45 0.2090 

0. 50 0.2000 

0. 55 0.1930 

0.60 0.1810 

0.65 0.1620 

0. 70 0.1440 

0.75 0.1270 

0. 80 0.1127 

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B (indirect) is a complicated function depending primarily on the scattering angle A3 

(cos A

3 = cos Al 

cos A3 + sin A2 cos ti.) where ik = azimuth angle), and the wave length 

dependence of aerosol and molecular scattering. 

Since we are dealing with a tri-bar chart which has two reflectances aland a2, we define 

the apparent contrast above the atmosphere as B1/B2 

where 

B1 = B (indirect) + alB (direct) 

B

2 = B (indirect) + a2 (B direct) 

a

l 

> a

2 

The apparent target modulation is defined as M = (B1- B2) (Bi+ B2); the average brightness, 

B

AV is defined as (B1 + B

2

)/2. An example is now given. 

Assume a Sun elevation angle of 36 degrees (A1 = 54 degrees), with the target at nadir 

(roll angle = pitch angle = 0 degrees). For these parameters B (indirect) is computed 

to be 119.29 ft lamberts and B (direct) = 998.74 ft lamberts. In this study al= 0.234 and 

a

2 = 0. 078. Therefore, a /a = 0.234 1 2 - - 3. 0:1 = intrinsic contrast. O. 078 

Then, B1 = 119.29 + 0.234 (998. 74), BI= 352.995 ft lamberts. 

B

2 = 119.29 + (0.078) (998.74) = 197.192 ft lamberts. 

The apparent contrast = 352. 995/197.192 = 1.79:1. 

The apparent scene modulation is 352.995 - 197.192/352. 995 + 197.192 = 0.283 = M. 

The average Sun elevation angle over the latitudes of interest (based on a yearly average) 

is 36 degrees. Figure 4-5 shows modulation and contrast ratio as a function of roll and 

pitch angle for the values of the parameters described in the above example. Figure 4-6 

gives the line-of-sight (LOS) as a function of roll and pitch angle. 

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