Skip to main content

Full text of "NASA Technical Reports Server (NTRS) 19750013250: Experiment definition phase shuttle laboratory LDRL 10.6 experiment"

See other formats





ii37o1 



15 OCTOBER 1974 


EXPERIMENT DEFINITION PHASE 
SHUTTLE LABORATORY 

LDRL 10.6 EXPERIMENT 

# 

First Quarterly Report 

( (NASA-eH~143709) EXPERIMENT DEFINITION W75~21322'l 

PHASE SHUTTLE LABORATORY LDRL 10.6 
EXPERIMENT Quarterly Report, 26 Jun. = 

26 Sep. 1974 (Hug hes Aircraft Co.) 45 p HC Unclas 

V$3 . 75 CSCL 14B G3/14 18585 ) 

^ 


NASA Contract NAS 5-20018 



HUGHES i 

I 

U „ ^ — 

HUGHES AIRCRAFT COMPANY 

SPACE AND COMMUNICATIONS GROUP 


Hughes Ref. No, D0824 * SCG 40355R 



I 


15 OCTOBER 1974 


EXPERIMENT DEFINITION PHASE 
SHUTTLE LABORATORY 

LDRL10.6 EXPERIMENT 

# 

First Quarterly Report 



t \ 

j HUGHES ; 

I 1 


HUGHES AIRCRAFT COMPANY 
SPACE AND COMMUNICATIONS GROUP 



Hughes Ref. No. D0824 • SCG 40355R 



CONTENTS 


Page 

1. INTRODUCTION AND SUMMARY 1-1 

2. PROGRAM PLAN 2-1 

3. SYSTEM REQUIREMENT DEFINITION 

3. 1 Mission Requirements 3-1 

3. 1. 1 Transmission Rate 3-1 

3.1.2 Bit Error Rate 3-1 

3. 1. 3 Link Establishment and Maintenance Requirements 3-1 

3.1.4 Lifetime 3-1 

3.1.5 Vibration Specifications 3-2 

3. 2 Parameter Requirements 

3.2. 1 Shuttle to Ground Link Parameter Requirements 3-3 

3.2.2 Shuttle to Molniya Parameter Requirements 3-3 

4. PRELIMINARY SYSTEM CONFIGURATION AND DESIGN 

4. 1 Definition of Packages 4-1 

4.2 Opto-Mechanical Considerations 4-2 

4. 2. 1 Gimbal Selection 4-2 

4. 2.2 Weight Function Relationships 4-3 

4.3 Optical Considerations 4-11 

4. 3. 1 Preliminary Study of System Configurations 4-11 

4. 3. 2 Two-Axis Laser Transmitter and Beacon 

Receiver Using Gregorian Afocal Telescope 4-12 

4.3.3 Optical Design Considerations 4-14 

4.3.4 Data Related to CO£ Laser Transmitter and 

Receiver 4-15 

4.4 Laser Transmitter Package Weight and Electrical 

Efficiency 4-18 

4.4.1 Weight 4-18 

4.4.2 Electrical Efficiency , 4-19 

5. LINK OPTIMIZATION AND ANALYSIS 5-1 

5.1 Objectives 5-1 

5.2 Link Analysis and Optimization Program Description 5-1 

5. 3 System Performance and Weight Handling 5-2 

5.4 Mission Considerations and Assumptions 5-4 

5. 5 Results and Conclusions 5-6 


11 



ERRATA TO 


EXPERIMENT DEFINITION PHASE SHUTTLE LABORATORY 

/ 

1) Page 1-2, Table 1-1: 

Doppler frequency, MHz should read 
-942 instead of 136 

2) Page 3-3, last line: 

Replace _Hl 36 by -942 

3) Page 3-5, Figure 3-2(b); ordinate should read: 

DOPPLER FREQUENCY, MEGAHERTZ xlO 

4) Page 3-6, Replace page: 


New page 3-6 enclosed 



1. INTRODUCTION AND SUMMARY 


This first quarterly report for the Experiment Definition Phase of 
the Shuttle Laboratory LDRL-10.6 Experiment (Contract No. NAS 5-20018) 
covers the activities from 26 June to 26 September 1974. 

The first month of the contract was devoted to the generation of the 
contract program plan. 

The second month of the contract was devoted to establishing system 
requirements and defining preliminary system configurations. Work was 
also started in preliminary system parameter optimization. During the 
second month the PERT diagram for the study was also established. 

The third month's main effort was the preliminary system optimiza- 
tion. During this month, by the initiation of the customer , it became apparent 
that the first experimental deployment of an LDRL-10.6 link will involve the 
shuttle in a low earth orbit to an elliptical orbit (preferably Moliniya orbit) 
satellite and the shuttle to a ground station. The elliptical orbit satellite 
terminal is under current development, and the ground terminals are more 
or less in existence. Under these conditions, priority has been given to the 
experiment definition to be carried by the shuttle. The shuttle terminal 
definition has therefore been the first task to consider. 

From the various packages under study, a two gimbal system, 
termed package B, was selected. This configuration was selected on the 
belief that it will lead more directly to the ultimate deployment of this 
terminal on a dedicated low earth orbit satellite. It was felt that in the long 
run this approach will lead to substantial savings because the shuttle ter- 
minal may be considered to be the engineering model of the actual mission 
terminal. Cost savings will be implemented using the facilities of the shuttle. 
Electronics will be rack-mounted inside the Space Lab Module of the shuttle 
and shorter shuttle lifetime specifications will result in cost savings. 

The key transmit and receive parameters have been subjected to an 
optimization program to pick the best values to provide required performance 
at a minimum weight. The parameters listed in Table 1-1 have been cal- 
culated from this weight optimization study. 


1-1 



TABLE 1-1. CHARACTERISTICS OF OPTIMAL SPACEBORNE TERMINALS 


Characteristic 

Shuttle Terminal 
Package B 

Molniya Terminal 
Package A 

Aperture diameter, cm 

22.4 

22.6 

Laser output power, W 

0.60 

\ 

NA 

Point ahead angle, maximum pxad 

50 


Doppler frequency, MHz 

- 

136 

Prime power required, W 

165 

152 

Weight, lb 

116 

i 

115 




2. PROGRAM PLAN 


The program plan was included as an appendix to the .first monthly 
report while the associated, finalized PERT was included in the second monthly 
report. These items will not be repeated here. 



3. SYSTEM REQUIREMENT DEFINITION 


The system requirements include mission requirements and parameter 
requirements derived from the particular mission scenario. 

3. 1 MISSION REQUIREMENTS 
3. 1. 1 Transmission Rate 

Transmission rate to be no less than 400 Mbps. 

3.1.2 Bit Error Rate 

The bit error probability is defined by no less than 10 

3.1.3 Link Establishment and Maintenance Requirements 

An acquisition and tracking study is currently being performed. The 
output of this study, in addition to providing an acquisition and tracking 
scheme, will determine bounds and parametric relationships among proba- 
bility of acquisition, probability of false acquisition, mean time to acquisi- 
tion, and probability of loosing track. 

3.1.4 Lifetime 

The lifetime of the shuttle-borne instrumentation is defined with cus- 
tomer consent, to be a maximum of 30 days. In addition, equipment and 
structure attachments must be certified to withstand the crash safety shock 
without breaking loose and creating a hazard to personnel or preventing 
egress from a crashed shuttle vehicle. 

This environmental requirement may be satisfied by the static struc- 
tural stress analysis. Testing should only be performed on those items not 
covered in the stress analysis. The payload design goal for crash safety 
shock has a sawtooth time response with a 40 ±6 g peak value over an 11 ms 
duration. 


3-1 



3.1.5 Vibration Specifications 


Flight 

The Space Shuttle vehicle will be subjected to fluctuating pressure 
loading on its exterior surfaces by engine exhaust, generated acoustic noise, 
and air flow generated aerodynamic noise during powered ascent through the 
atmosphere. These fluctuating pressure loads are the principal sources of 
structural vibration. 

The estimated random vibration for the cabin and midfuselage payload 
interface due to the fluctuating pressure loads is shown in Figure 3-1. These 
vibration levels exist for approximately 29 seconds per mission. The reentry 
vibration environment is negligible. Actual vibration input to payloads will 
depend on the transmission characteristics of the midfuselage, the payload 
support structure, and interactions with each payload’s weight, stiffness, 
and eg. 


Transient Vibration . Events such as gust loading, engine ignition 
and cutoff, and separation and docking will induce low frequency transient 
responses in the Space Shuttle vehicle. The response for each event for 
various locations will be calculated. However, for the interim the overall 
effect of these transient events is accounted for by a swept sinusoidal vibra- 
tion environment imposed in the frequency range from 5 to 35 Hz at an 
acceleration amplitude of ±0. 25 g peak. 

Ground 


The ground vibration spectrum that the payloads are expected to 
experience is a minimum of four sweeps at l/2 octave per minute at the 
following levels (sinusoidal motion): 

2 to 5 Hz at 1. 0 inch double amplitude 
5 to 26 Hz at 1. 3 g peak 

26 to 5 00 Hz at 0. 36 inch double amplitude 
500 to 1000 Hz at 5 g peak 


3. 2 PARAMETER REQUIREMENTS 

The parameters for the two primary links that affect the design of the 
space package include: 

1) Range, Km 

2) Doppler Frequency, MHz 

3) Point Ahead Angle, prad 

4) Azimuth angle in the local tangent plane from true north, degrees 


3-2 



5) Azimuth angle from local vertical, degrees 

6) Elevation angle from local vertical, degrees 

7) Elevation angle rate, deg/min 

8) Total angular rate — vector sum of azimuth and elevation rate, 
deg/ min 

Parameter requirements are based on orbital link deployments. The 
link deployments considered here are the shuttle to ground and the shuttle to 
elliptical orbit of minimal apsidal rotation (Molniya) and with a 12 hour 
period. 


At present the orbits of shuttle and Molniya orbit satellite have not 
been established. The prevailing thought in defining these orbits is to estab- 
lish parameter extremes. Experiment designs will then encompass these 
extremes. 

3, 2. 1 Shuttle to Ground Link Parameter Requirements 

The shuttle is considered to be on circular equatorial orbit at an alti- 
tude of 185 km (100 n. mi. ) which passes directly over the ground station. At 
zero time the satellite is directly over the ground station. 

The time histories of the parameters have been plotted for zenith 
angles up to 60° (30° above the horizon). Azimuth angle and angle rate are 
not plotted as they do not change. Parameter time histories are shown in 
Figure 3-2. It is seen that a maximum of the point ahead angle is about 
50 prad. 

3. 2. 2 Shuttle to Molniya Parameter Requirements 

The Molniya is a minimal apsidal rotation orbit inclined at 63.43° 
with an apogee of 39,438 km and a perigee of 926 km. For maximum doppler 
frequency consideration, the shuttle orbit is taken to be a circular coplanar 
orbit at an altitude of 185 km. As a matter of calculating convenience both 
orbits have been reduced to equatorial (zero inclination). Figure 3-3 gives 
the time histories of the parameters under study. It is seen that doppler fre- 
quency has a maximum of ±136 MHz. 


3-3 



ACCELERATION SPECTRAL DEN 



40356-1 (U) 




DOPPLER 

ZENiTH ANGLE RATE, DEG/M IN FREQUENCY, MEGAHERTZ 





° °,g^ 008 0.1* 

TIME, HOURS x 10' 1 



(d) ZENITH ANGLE RATE 


(el POINT AHEAD ANGLE 


FIGURE 3-2. SHUTTLE TO GROUND LINK 


3-5 


<n)9-sseo* “ (nirsseop 







P01MT AHEftn-MlC 



o_o 



O.tO O.SO 0.60 


I .Off I -10 l.?0 1.30 


d) DOPPLER FREQUENCY 

FIGURE 3-3. SHUTTLE TO MOLNIYA ORBIT 


3-6 


4035S-8{U> ° 40355-1 0<U) 






4. PRELIMINARY SYSTEM CONFIGURATION AND DESIGN 


4. 1 DEFINITION. OF PACKAGES 

The shuttle terminal configurations considered in the experiment 
definition study are defined as packages and are included in Table 4-1. For 
completion, the high altitude transceiver terminal has been included as 
Package A. 

Package D has been rejected from further consideration (see 4.3.1). 
Package C, considered to be either the progenitor of Package B or the modi- 
fied Package A, mounted on a course positioned platform. This last solution 
was examined briefly, but was rejected because it offers no potential for fur- 
ther development into an operational system. Thus the progenitor of Pack- 
age B was selected as the most promising long term solution. In order to 
simplify nomenclature, this package is named Package B. It is essentially 
the brassboard model of the deployed Low Earth Orbit satellite (LEOS) 
terminal. 


TABLE 4-1. PACKAGE DEFINITION 



Packages 



A 

B 





Synchronous 

Deployed 





or Elliptic 

Lower 





Orbit 

Orbit 

c 

D 


Su bsystem 

Satellite 

Satellite 

Shuttle 

Shuttle 

Remarks 

Receiver 

Transmitter 

Optomechanical 

X 

X 

X 

X 


Mod 1 

X 




Pointing coverage 
25° x 25° 

Mod 2 


X 



Pointing coverage 
2 usr 

Mod 2a or la 



X 


Progenitor of B or 
modified A for 
coverage of 2irsr 

Mod 3 
Cooler 




X 

Coude configuration 

Radiation 

X 





Telemetry and 

X 

X 

X 

X 

Common to all 

Command 


1 



packages 


4-1 









4, 2 OPTO- MECHANICAL CONSIDERATIONS 


4. 2, 1 Gimbal Selection 

The opto-mechanical subsystem has the requirement to acquire and main- 
tain the line of sight to the Molniya, or the ground, receiver from alow altitude 
shuttle spacecraft. The mechanical design requirements on the low altitude trans - 
mitter present a difficult task, because hemispherical gimbal coverage is 
required to accommodate the orbital motion of the LEO spacecraft. Hemis- 
pherical coverage requires, in general, three axes or gimbals to prevent 
1f gimbal lock 11 at all view angles. A two-axis system can, however, basically 
meet the pointing requirements in a less expensive way. Thus, for the Pack- 
age B, a two-axis gimbal configuration using an X-Y pattern is being 
considered. 

This choice avoids the complexity of the three -axis system but has the 
disadvantage of blind angular cones at the horizon. Such blind areas can be 
handled in the following three ways: 

1) Pitch the LEO spacecraft twice a day during the period that the 
satellite is behind the earth (i. e. , the spacecraft is the third 
axis) such that the active tracking will not require the blind 
angular zones. 

2) Limit the communications during the obscured period. 

3) Limit the communications during mid-orbit (overhead pointing) by 
using an elevation/azimuth gimbal combination. 

Either of the two-axis combinations (2 and 3 above) has limited viewing on 
only two of the 16 daily orbits. 

The hemispherical coverage in the baseline design includes an inner 
gimbal (tilt axis) that rotates a folding mirror ±75°. This avoids the gimbal 
lock region. The obscured areas, therefore, are ±15°. The outer gimbal 
(roll axis) has limited rotation also, but can slightly exceed ±90° to ensure 
the horizon look angle. With limited rotation in both axes, simple flex elec- 
trical cables can be used instead of slip rings. 

Pointing control for acquisition and tracking is provided by a two stage 
mechanism. Coarse pointing consists of the flat pointing mirror and gimbal 
assembly. The flat mirror is mounted in a yoke on a rotatable pedestal. 
Rotation of the entire pedestal/mirror assembly provides the azimuth or roll 
sweep of the mirror while elevation positioning is obtained by declination or 
tilt of the mirror about its pivot axis in the yoke. Positioning is independent 
in each axis and is accomplished by a stepper motor drive. The positioning 
drive for each axis consists of spur gear reduction from the motor shaft to a 
final output stage which is a single thread worm gear engaging a short, 
matching worm wheel. The use of such a final stage permits utilization of 
a unidirectional spring load at the axis pivot to remove backlash. The over- 
all gear reduction for the tilt axis needs to be twice that of the roll axis, to 
achieve the same effective beam displacement per motor step due to the opti- 
cal doubling effect inherent in the tilt axis. 


4-2 



Fine angular positioning is achieved by means of an image motion 
compensator (IMC) in each axis* These compensators consist of small mir- 
rors within the optical path mounted on the output coil of a piezoelectric 
driver* The image motion compensators provide raster search as well as 
conical scan capability* 

The transmitter opto -mechanical subsystem is composed of the fol- 
lowing elements: 

1) Structure 

2) Gimbal elements including bearings, gear reductions, gimbal 
angle encoders, resolvers, and motors 

3) Optical subsystem 

4) Mounting provisions for optics system and IMCs 

5) Servo electronics and power conditioner 

6) Interconnect cabling 

4 r 2. 2 Weight Function Relationships 

The selection of major parameter values will be made on the basis of 
the lightest weight system. In order to perform an optimization calculation, it 
is necessary to model the components in the package relative to weight. This 
modeling has been done on past contracts for most of the components; however, 
it was updated for this study. The following is that modeling analysis. 

Outline drawings. Figures 4-1 through 4-4, were prepared to identify 
the principal areas of weight growth for large aperture systems of the trans- 
mitter. Tv o possible gimbal bearing configurations for a 10 inch f/2 system 
are shown in Figures 4-2 and 4-3. The outer gimbal bearing arrangement 
shown in Figure 4-2 is the same as the configuration for the 5 inch aperture 
system; however, if after a detailed analysis of bearing loads it is indicated 
that greater capacity is required, one possible configuration is that shown in 
Figure 4-3. 

Estimated weights of major subsystem categories for the receiver 
and transmitter were curve fit, and the resultant computer generated curves 
are presented in Figure 4-5. The following conclusions may be drawn from 
these data. 

Structure Versus Primary Aperture 

In both cases, structure weight increases are the direct result of 
primary mirror diameter changes. Package B, however, is less sensitive 
in the structure category with the larger impact in the gimbal, motors, and 
steering mirror category. 

Optical Telescope Versus Primary Aperture 

The relationship of weight increase to aperture diameter is basically 
a cubic for both Package A and B. The higher weight of Package B is due to 
the additional folding mirror required for the transmitter. 


4-3 



1.8 



FIGURE 4-1. PACKAGE B - 5 INCH OPTICS F/2 SYSTEM 


40355-1 1<U) 



40355-1 2 ( U) 











OPTICAL TELESCOPE WEIGHT, P0UNC 

J..00 8-00 16-00 24.00 32.00 


to 

n 


« 

ai 

01 



.00 


0-04 (MM o'. 12 0^16 Q 20 0*-24 

PRIMARY flPERTURE DIAMETER •METERS 
b) TELESCOPE 


O’. 26 0.32 


FIGURE 4-5. ESTIMATED SUBSYSTEM WEIGHT 


4-8 


(nm 



GIMBALS. MOTORS. STEERING MIRR0R WEIGHT .P0UNOS SERV0 ELECTRONICS WEIGHT .P0UNOS 

J.OO 9.00 10.00 12.00 14.00 16.00 J..60 2.40 3.20 4..00 4.80 6.60 



c) SERVO ELECTRONICS 



d) GIMBALS AND DRIVES 

FIGURE 4-5 (CONTINUED). ESTIMATED SUBSYSTEM WEIGHT 


4-9 


40355-1 7{U) <r> 40355-1 8<U) 




Q— 



•} SERVO POWER CONDITIONING 



f) SERVO ELECTRONICS POWER 


FIGURE 4-5 (CONTINUED). ESTIMATED SUBSYSTEM WEIGHT 


4-10 


40355-1 9 (U) 40365 20(U) 





Servo Electronics Weight and Power 


The greater weight and power required for Package B over that of 
Package A is specifically a result of the added requirement to drive and 
process resolvers and encoders for both axes. The wide gimbal angle 
requirement of Package B along with the condition of a fixed error sensor 
relative to the gimbaled mirrors means the servo system must compute 
coordinate transformations. 

Gimbals, Motors, and Steering Mirror 

Package A weight is primarily affected by mirror and yoke type, 
whereas Package B sizing for bearings, ring gear, and steering mirror is 
directly related to the optical clear aperture, which in this case goes through 
the inner gimbal bearing bore. For Package B, two bearing configurations 
are possible. The optimized bearing size, however, will depend upon a 
detailed analysis of bearing loads. 

Servo Power Conditioner Weight 

This reflects the added requirements of the servo electronics. The 
higher weight of Package B servo power conditioner is a result of converter 
and voltage regulation for the resolvers and encoders not used in Package A. 

Image Motion Compensation Subsystem 

Estimates for IMC systems were included in the weight data; however, 
a single size device now exists and, therefore, it may be necessary to limit 
the acquisition field or design a new IMC device. 


4.3 OPTICAL CONSIDERATIONS 

4 . 3 . 1 Preliminary Study of System Configurations 

A number of optical design solutions have been examined with regard 
to their applicability to the transmitter subsystem. The conclusions of this 
study are as follows. 

Cassegrain Telescope With Coude Focal Positions 

This type of telescope design generally has an alti-azimuth mount, 
and the viewing axis is stablized along the polar axis. For space applica- 
tions, this design approach does not offer advantages over a Gregorian sys- 
tem (see next subsection), although the basic telescope design is compact. 
The compactness of the Cassegrain design comes about because it does not 
forma real primary image, but this can be a disadvantage when the system 
is required to be well baffled. 


4-11 



In order to maximize the output power of the laser transmitter, it is 
important to minimize the central obscuration of the telescope. In the case 
of the Cassegrain system, this implies that the focal length of the secondary 
mirror must be minimized with respect to that of the primary, and that a 
fast relay system will be needed to relocate the exit pupil to a convenient 
position so as to accommodate the image motion compensators. The addi- 
tion of this relay system renders the Cassegrain telescope approach less 
attractive as compared with a simpler Gregorian design. 

Gregorian Telescope With Cofocal Paraboloids 

The Gregorian configuration has been studied in detail, and this type 
of telescope appears to offer the best design solution for laser transmitter. 
The output from the telescope is reflected out of the Gregorian system using 
a large folding mirror located with its center close to the focus of the pri- 
mary mirror. This folding mirror has a small cutout in the center so as to 
permit light from the secondary mirror to reach the primary. This design 
approach minimizes the central obscuration of the output beam. The 
Gregorian configuration also permits the exit pupil to be positioned exter- 
nally without the need for a separate relay system. Subsection 4. 3. 2 pre- 
sents an example of a laser transmitter and beacon receiver system using 
the Gregorian telescope. 

In designing an efficient Gregorian laser optical system, it is impor- 
tant to take into account the following: 

1) The dependence of central obscurations on the f number of the 
primary mirror 

2) The telescope FOV during acquisition 

3) Methods for maximizing the output power in the presence of a 
given central obscuration 

The above topics will be discussed in 4. 3. 3. 

4. 3. 2 Two-Axis Laser Transmitter and Beacon Receiver Using 
Gregorian Afocal Telescope 

The optical system shown in Figure 4-6 consists of a pointing mirror 
in front of a Gregorian afocal telescope subassembly. The afocal telescope 
consists of two cofocal paraboloids. A large folding mirror reflects the 
laser beam out of the system and directs the incoming beacon beam toward 
the primary mirror. A pair of image motion compensators are placed very 
close to the exit pupil of the telescope. This exit pupil is also the aperture 
stop of the system. It is relayed out of the telescope by a small folding mir- 
ror located in front of the secondary. Two folding mirrors direct the trans- 
mitted and received energy through the center of the outer gimbal. Behind 
the outer gimbal is a beam splitter. This beam splitter reflects the 0. 9 pm 
beacon and transmits the 10. 6 pm laser beams. 


4-12 



FIGURE 4-6. LASER TRANSMITTER AND BEACON RECEIVER OPTICAL SCHEMATIC 


40355-21 <U> 


The transmitted laser beam will be suitably expanded (see 4. 3. 3) to 
match the input of the afocal telescope. Some details related to the expansion 
ratio of the laser beam expander, the central obscuration and the f number of 
the telescope will be considered in the following subsection. 


4, 3, 3 Optical Design Considerations 

The Gaussian beam profile of a CO 2 laser has its highest energy con- 
centration in the center of the beam. This central portion of the beam would 
be lost in the presence of central obscuration. Figure 4-7 shows the percent 
energy obscured for uniform and Gaussian beams using a 12 cm aperture, 
the Gaussian profile being truncated at the l/e^ point. 


The central obscuration of the Gregorian telescope is essentially 
determined by the inside apertures in the folding mirrors. The sizes of 
these apertures are in turn dependent on the telescope acquisition FOV, 0. 
This is because these inside apertures must be sufficiently large so as not 
to obscure any part of the image field of the beacon beam during acquisition. 
The size of the image field, r\, is given by 


q = 0 • f 


where f is the focal length of the primary mirror. For the proposed laser 
transmitter system, 0 = 17.45 mr (i. e., 1°). Thus, for a given 0 and pri- 
mary mirror diameter, D, the central obscuration can be minimized only if 
f is minimized. This implies that the f-number ( = f/D) of the primary must 
be small if the power loss resulting from central obscuration is to be 
minimized. 

Based on practical experience, the f-number of the primary should 
not fall much below f/1.5, lest the misalignment tolerances become extremely 
small. Here the advantage of having a very fast primary could easily be 
cancelled by the performance degradation resulting from residual misalign- 
ments. It is therefore recommended here that mirrors faster than about 
f/l.5 should not be considered as candidates for the laser transmitter. The 
effects of the resulting larger central obscuration upon the output power will 
have to be reduced by techniques other than the one using very fast mirrors. 

There are at least two methods for reconstituting the Gaussian beam 
profile so as to minimize the output losses resulting from the central obscur- 
ation. These methods are outlined below: 

Method One — This method requires the Gaussian beam from the CO^ 
laser to be suitably expanded so as to overfill the aperture stop of the 
telescope (see Figure 4-6). By broadening the beam profile, the high 
energy central region will now spread more, and consequently a given 
central obscuration will cut off a smaller portion of the laser energy. 
The increase^n the output power is achieved by truncating the low 
power l/e regions of the beam profile and passing more of the high 
power regions. 


4-14 



Method Two — This method is based on the axicon, an energy redis- 
tribution device shown in Figure 4-8. Such a device can increase the 
far field peak irradiance by a factor of 2 when the diameter obscuration 
ratio is 0. 25. Figure 4-9 compares antenna gains as a function of 
obscuration ratio for an optical telescope illuminated with a uniform 
irradiance beam, a Gaussian beam, and a Gaussian distribution modi- 
fied by an axicon such as shown in Figure 4-8. The alignment toler- 
ances for this device tend to be quite critical. The increased gain 
anticipated from its use must be weighed against possible degradation 
produced by its misalignment in the system and/or any wavefront 
deformation due to residual manufacturing errors. 

Both methods will be examined in detail, and one of these solutions 
is expected to be incorporated into the final design. 

4.3.4 Data Related to CO 2 Laser Transmitter and Receiver 

Typical optical losses for both the CO 2 laser receiver and the trans- 
mitter are presented in Tables 4-2 and 4-3, respectively. The telescope 
design configuration for each system is the Gregorian. 

Table 4-4 gives weight and loss estimates for three transceivers. 
Graphs of weight versus primary mirror diameter are shown in Figure 4-5b. 


TABLE 4-2. TYPICAL OPTICAL LOSSES FOR C0 2 LASER RECEIVER 


Net transmission associated with eight surfaces 
in the optical train (assumed transmission/ 
reflection loss at each surface to be 1%| 

92.3% 

Diplexer transmission of input beam 


97% 

Diameter obscuration of 20% results in net 
transmission of detected signal 


92.5% 

Mixing efficiency at detector 


72% 

Overall receiver transmission 

(0.923){0.97)(0. 925) (0.72) 
0.596 

Overall receiver loss 

2.25 dB 



I 


4-15 





OBSCURATION DIAMETER, CENTIMETERS 


FIGURE 4-7. PERCENT ENERGY OBSCURED FOR UNIFORM AND 
GAUSSIAN BEAMS USING A 12 CM APERTURE 



DOUG HNUT SHAPED 
OUTPUT BEAM 


FIGURE 4-8. ENERGY REDISTRIBUTION TECHNIQUE 


4-16 


<n)cz-9seo* 




FIGURE 4-9. NORMALIZED ANTENNA GAIN FOR 
OPTICAL TELESCOPE ILLUMINATED WITH UNI- 
FORM INTENSITY WAVEFRONT (G U ).TEM 00 LASER 
MODE OUTPUT (G g ), AND GAUSSIAN DISTRIBUTION 
REDISTRIBUTED WITH AXICON (G p ) 


4-17 


40355-244 U) 



TABLE 4-3. TYPICAL OPTICAL LOSSES FOR C0 2 LASER TRANSMITTER 


Net transmission associated with four reflecting 
surfaces in optical train (assumed reflection loss 
per surfacfe to be 1%) 

96.0% 

Dipiexer transmission of outgoing beam 

91.0% 

Diameter obscuration of 20% results in net 
transmission of Gaussian amplitude beam 

71% 

Transmission of a four lens zoom system for 
beacon 

$1.4% 

Overall transmitter efficiency (0.96) (0.91 ){0.71)(0.914) 

0.567 

Overall transmitter loss 2.46 dB 



TABLE 4-4. C0 2 LASER TRANSCEIVERS WITH WEIGHT AND LOSS ESTIMATES 


System 

Primary 

Mirror 

Diameter, 

cm 

Optics Weight 
With Mountings, 

lb 

Transmitter 

Losses, 

dB 

Receiver 

Losses, 

dB 

Three- Axis 

14 

4.7 

2.46 

2.25 

Two-Axis 

18 

6.7 

2.46 

2.25 

Two-Axis 

27 

16.0 

1 

2.46 

2.25 


4.4 LASER TRANSMITTER PACKAGE WEIGHT AND 
ELECTRICAL EFFICIENCY 

4.4. 1 Weight 

The package weight for the transmitter laser, modulator, and modu- 
lator driver is a function of the transmitter output power. For the particular 
case of a 300 Mbps system, an expression for the approximate package weight 
can be stated. 

Power Conditioning 

The power conditioning weight is approximately 10 pounds for every 
watt of laser output power. Weight = 10 P where P is the modulated laser 
output power. 

Laser 

The minimum weight of the laser is about 5 pounds including the 
modulator and modulator driver. This weight increases with the laser output 
power because the laser is larger and the modulator driver is larger. It is 
approximated by weight ~ 5 = (2P)^. 


4-18 





Opto- Mechanical Factors 

Structure weight also scales with the laser output power in that greater 
strength must be provided in the structure to support the heavier laser modu- 
lator and modulator driver. This additional weight is approximated by 
weight = 5 + P. The net weight of the transmitter as a function of laser 
transmitter output power is shown in Figure 4-10 and is represented by 

2 

Total weight = 10+ 11P+4P pounds 


4. 4. 2 Electrical Efficiency 

Laser transmitter efficiency is determined by a power tradeoff analy- 
sis of the laser and modulator driver. Optimum pressures and gains have 
been determined for any given laser circulating power, and the tradeoff 
between modulator driver lever and laser circulating power is used to deter- 
mine the overall optimum power division between the laser and driver. Data 
for these estimates are preliminary and must be updated as the program 
progresses. The resulting efficiencies as a function of laser transmitter 
output power is given in Figure 4-11. 


4-19 



w« 

o 



FIGURE 4-10. LASER TRANSMITTER WEIGHT VERSUS OUTPUT POWER 



4-20 


40355-27 (U) S 40355-26<U) 




5. LINK OPTIMIZATION AND ANALYSIS 


5. 1 OBJECTIVES 

In principle, the specified Laser Data Relay Link (LDRL) performance 
requirements can be met by a continuum of similar systems each with an 
appropriate combination of transmitter and receiver aptertures (Dq and Dr) 
and transmitted power (Pq). Even within the realm of sensibly realizable 
combinations, wide variations in these fundamental system parameters are 
possible. From these possibilities then, one such combination must be 
chosen which (by some meaningful criterion) results in a "best" system. For 
spaceborne systems, one meaningful measure of "best" is lightest, all else 
being equal. The determination of the combination of aperture sizes (hence, 
transmitted power) which results in the lightest weight LDRL system of 
specified performance was the objective of the optimization effort described 
here. The minimum weight LDRL is determined by a computer program which 
uses a direct search technique to minimize system weight expressed as a 
function of the parameters to be optimized. The same program then generates 
a detailed system weight tabulation and a link gain-loss summary table for the 
optimized system. 


5.2 LINK ANALYSIS AND OPTIMIZATION PROGRAM DESCRIPTION 

The optimization computer program is written in FORTRAN V for the 
UNIVAC 1108 and uses a variation of Powell's conjugate direction algorithm 
to minimize a function which determines weight for a LDRL of specified 
performance. The program link model performance is specified in terms 
of information bandwidth and received signal to noise ratio. For any mission 
environment these system performance parameters are uniquely related to 
the optimization variables of interest (transmitter and receiver aperture sizes 
and transmitted power). In the program, the weight of each system constituent 
is functionally related to transmitter or receiver aperture diameter, trans- 
mitter power, information bandwidth, or combinations thereof. The required 
information (output) bandwidth and signal to noise ratio are specified by the 
user, and the program judiciously chooses the aperture diameters so that the 
combined total spaceborne system weight is minimized. 

Explicit program inputs which specify link parameters and component 
characteristics are presented in the following section. Implicit program 


5-1 



inputs are the subsystem weight functional relationships illustrated and dis- 
cussed in Section 4. 

Program outputs are a tabulation of optimum aperture diameters, 
transmitted power, and weights of all major system constituents for each 
optimization, as well as a link gain-loss summary table. The optimized 
outputs may be plotted as a function of intermediate frequency (IF) bandwidth. 


5.3 SYSTEM PERFORMANCE AND WEIGHT HANDLING 

The LDRL signal to noise ratio (S/N) (hence, bit error probability) 
and output bandwidth (therefore, data rate) are related to the optimization 
variables (Drp, Dj^, Prp ) through the range equation, 



where 



= S/N in B q 

- detector gain 

= detector quantum efficiency 

= electronic charge 

= Planck's constant 

= optical carrier frequency 

= detector load resistance 

= output or baseband bandwidth 

= received signal power 

= received background power 

= detector dark current 

= Boltzmann’s constant 

= postamplifier noise temperature 
= local oscillator power 


5-2 



The received signal power is given by 


P 


S 


- p T G T G R 'n A n t^r^p 



( 2 ) 


where 

Pp = transmitted power 

G t = G t (Dp), transmitter aperture gain 

G r = G r (D r ), receiver aperture gain 

= atmospheric loss 

r|p = transmitter losses 

r| = receiver losses 

R 

Tp = pointing loss 

R = range 

\ = wavelength 

while the background power is 

P B = We R B l A R T1 R 

where 

W = background spectral radiance 
0~ = receiver field of view (solid angle) 

x\ 

Bj = optical bandwidth 

A d = receiving aperture area 
K 


The component performance parameters and losses which characterize 
the LDRL optimization program performance model are summarized in 
Table 5-1. The LDRL weight modeling assumptions for each subsystem are 
discussed in detail in Section 4 and the resultant functional relationships are 
illustrated. The weight modeling procedure in most instances consisted of 
fitting a power law relationship ( r n ^3* to actual subsystem 


5-3 



TABLE 5-1. SYSTEM CONSTANTS USED IN 
LDRL OPTIMIZATION 


DETECTOR QUANTUM EFFICIENCY# PERCENT 
DETECTOR GAIN- 
DETECTOR tOAD Rt SI STANCE] 




LOCAL 

local 


5 lifier 

OSClLLA I OR POWER, 
rjSCILLAtQR 
R ‘ 


it DEGREES K 


r OHMS 

dirleJerVoss, percent 

MIXING EFFICIENCY ““ nl ~ 

.RANSMlTTtR OPTICS EFFIC] 

TRANSMITTER DtPLEXEH LOSE,. 

BEACON ZOOM OPTICS EFFICIENCY, PERCENT 
RECEIVER OPTICS EFFICIENCY# PERCENT 
RECEIVER DIPLEXER LOSS f PERCi-NT „ 

RECEIVER DIFFRACTION LQSS.PERCENT 

BACKGROUND RAD I ANCE # WATTS/SO M*MICRON*STERADI AN 

DETECTOR FIELD UF VIEW, MICRORADIANS 


^ fic Y# pIrJeNT 

^PERCENT 


50 , 

‘“SIS? 

.005 

% 

91j 


.001 

84 . 


preliminary design weights for appropriate independent variable {D'p , Dr, or 
Pj, etc) values in the range of interest. Other relationships were used where 
indicated by physical considerations (as in the case of laser weight versus out- 
put power). The optimization procedure requires only that the system weight 
function to be minimized be continuous and well behaved. 

The weight associated with the prime power requirements (Figure 
5-1) corresponds to the approximate specific weight (lb/watt) of solar power 
systems in near-earth space with appropriate energy storage, power con- 
ditioning, and control to provide a regulated bus. The additional laser power 
conditioning weight is accounted for in the laser weight model. Other dedica- 
ted power conditioning weight is specified explicitly. 


5.4 MISSION CONSIDERATIONS AND ASSUMPTIONS 

The LDRL optimization has been performed only for the link communi- 
cation function; acquisition performance is not explicitly considered. A 
beacon acquisition system is included in the detailed weight breakdown, but 
its weight is constant and so does not affect the optimization. Only the space - 
to-space link optimization is presented here. It has been assumed that point- 
ahead angle control is implemented by beam deflection so that the system 
is always operating on-axis. For point -ahead angles encountered by the 
LDRL (*50 microradians) the additional weight required to do so is much 
less than that associated with the alternative off-axis operation. The commu- 
nication range of 46,720 km assumed is approximately the maximum between 
a 185 km orbit shuttle and a Moliniya terminal. A received S/N of 20 dB was 
dictated by the specified probability of bit error and margin. Other mission 
considerations such as line of sight angular rates and doppler frequency shifts 
are considered implicitly in the design and the corresponding weight depen- 
dencies of sensitive system elements. 


5-4 




0 


40.00 


80.00 120.00 100.00 200.00 240.00 280.00 


SYSTEM PRIME INPUT P0MER #HflTTS 


320.00 


FIGURE 5-1. LDRL PRIME INPUT POWER VERSUS POWER SYSTEM WEIGHT 




5.5 RESULTS AND CONCLUSIONS 


The LDRL optimization results are indicated in Figures 5-2 through 
5-4 and Tables 5-2 and 5-3. 

Table 5-2 consists of a detailed weight breakdown for Package A and 
Package B as optimized for an IF bandwidth of 600 MHz (corresponding to 
approximately 300 Mbps). The indicated optimized values for Package A and 
B aperture diameters minimize the total (Package A + Package B) "Associated 
Weight Burden," which includes the prime power system weight. Certain 
reliability critical components in the weight tabulations are seen to be followed 
by a parenthetic redundancy factor permitting the number of such components 
included for improved reliability to be specified. 

t 

Table 5-3 is a tabulation of link gains and losses that expresses the 
range equation (Equation 1) in logarithmic (decibel) form. The pointing loss 
entry includes degradation from ideal gain due to nonuniform aperture illumi-. 
nation and secondary obscuration and so is nonzero even though the actual 
pointing error is assumed to be negligible. 

Finally, Figures 5-2 through 5-4 depict optimized parameters of 
interest for Package A and B as a function of IF bandwidth, for the same lin^ 
(communication range = 46,720 km, S/N = 20 dB). The interaction of the 
various weight dependencies of Section 4 may be perceived in the comparison 
of optimized aperture diameters versus IF bandwidth (Figure 5-2), At 
smaller aperture diameters, the optimized Package B aperture is smaller 
than that of Package A because of B's stronger optical weight dependence 
on aperture. At larger apertures, this disparity is transcended by the 
stronger Package A structural weight dependence on aperture, and the 
Package A optimum aperture becomes the smaller. Similar, though less 
obvious, interactions between all LDRL weight constituents combine to 
determine the optimum combination of Dx and Dp (hence, Pj) which results 
in the minimum weight system. 


5-6 




FIGURE 5-2. LDRL PACKAGE A AND B WEIGHT AND OPTIMIZED 
APERTURE DIAMETER VERSUS IF BANDWIDTH 


5-7 






40355~30(U) 








FIGURE 5-4. LDRL PACKAGE A AND B INPUT POWER AND PRIME POWER 
SUPPLY VERSUS IF BANDWIDTH 


5-9 


40355-31 (U) 



(6MM 


#V*,Od h-US AS 9 3‘37 *37d 

d3*Gd 13N7d 'lUdii‘-'U3 
ad^od adiiiwSwvax *0373* 
a1*fld wSiSASbOS OMxMud 
a 3 o d $3l*Dai y jaij adAiaj.-id 
aawod iftd.Ni doivpuso "i v 3 c n 

sxiv* NOliviObvi di«Od v aovovd 


0 U ■ u l 
ou‘ l 
OS *o 
UU ***l 
6^*** 


»3i$AS H 30?>O7ri 

adrtUd 13*7d IQaiwDO 
ddilUd aiAliOda Nu5f3b 
bih v Gd ^jdlfcAfcflfiS (mllMOd 
cihgo SJl^uaiOjjli d3xli*S*7fci 
ll'idhl cJS7l d3iXlw6*7yi 

ill ?.* Mill* 1i!ti?i *3*Ud d 3f)v>jffd 


Naaand ih 913* oaxrxDussv v 3t>7>09d 
IWOl'd* h3XSAS 3wlad Uii713t)SS7 

mala* nvioi 7 397*370 

I 

sfiu admass i* 

S*0i33NMD3 U* 7 tmfl73 
*3XSA£>dnS nOIx7S*3hwQ3 nuIxuw 3f)vwl 
*$ 1 * 7103 * snooou n30N7dxa 

aiiooo dui^Mjao a** duxodxjij 
(i) yas7'i fccuvimso ivoui 
(I) d^svl NUJVd^ 

ONI OIUCINU3 O^Ud a3*io 
ONlNOl 1 10^03 di^ud OAddS 
(I) S3INQ&13313 9NlS5330dd 17N9IS UN7 ti3Ai3J3d 

SDlNGdiJaia 0 A * 3 $ 
aDttdlH SNIHJ 3 XS CIN 7 *SaGiOK*$l 7 ii*I^ 
$ 9 NliNPGH H 1 I rw "ddUJS 3 l 3 l IV'JildO 
1 M 9 1 3 * 3diU3naiS 

fiONnDd«Nail7 Irtdfl i H tl X U ^ ? 397>37d 


i Id 
on * 00 i 
irvw 

s<S" u * 
$*• i 
Xf 4 i 

ui> # 
*u’i 
os, ei 

£ U d 

iu # *i 

IL'ti 

»o* n 

Ui*s? 


-MiCjdMt AHt/ia-. UJ19I3USS7 u 397x37d 
IH9X3* *lXSAb dd>.Ud d*icid U31V130SSV 

InlfidT T7iCi B 397X370 

snoa^iiaasi* 

S6ul3dN*03 ON V SilWVO 

"dlSAbti.lS i\U X X V SOomO NGlXU* d9V*I 
bdIi r jfci33‘H u *i» V iJiidO d'3AI33ad *0373$ 

(i> A laaHS ddMOd ur<7 £*133x3^ *U373j 
SJi uioiJi u *UIlvZl1Ifl7iS d3SV* 
OlidJ SPOdWTlOSIw On 7 dd<3ldIG 
9*-; 1 "JUX II U03 tj 3 ^ 0 d b3Hiu 
yNi,*ullluh-JJ $3*dd UAfl3S 
(U ddAida oJiviiijur <ji<v*aui\nnuo**tt3S7i 

$01-10*113313 uAd3fc 
a u ft ti I * 9 , 1 ft 3 3 a s JN7*$duiUK , S'l7bwI9 
Stn-Hiw.u.'. ^ X A v i 3duJb313l I73lld3 
iH‘JI3w 3oni3,Odifi 

S i^r!L ,a*ujii7‘int7x ihijia* 397*073 


5crAw X;-i33s 3d* a JN31 31 ddi a3xilwbN7hi 

tcts* Sxi7 v *dJfMjd indi^u d3xxl^bN7dX 

>> * odi i\, V i <J (adAAdJdd) 7 397x3Vc 

^«is'di3wtlU fiMNilht IojX X I hSN 7fa X J 0 3*37X3 Vd 

S 3 I* 17 A 032I«Ud0 

■ 0 u Sr *HilHUN 7 b il 

Ml'JX^ONVd dl iA S ft 3 l 3 »•• V C V C? * i . I ’+ 


(3P 02 = N/S ‘^>1 0Zt'9P = 3DNVH) 

Nonvinavi uaiaiNvavd qnv ihoihm iaan aaziirtiiido - 2-s anavi 


S2 

M 

A 
( 





TABLE 5-3. OPTIMIZED LDRL SUMMARY (IF BANDWIDTH = 600 MHz, 
RANGE = 46,720 km, S/N = 20 dB) 


OPTICAL S T GN AL/NO ! $F PA9A'«FTF*S.nP 

T P A M s ■ ' T T T F R P (*! !*i p D , >) 

T PANS!" M TFP OPTICS PFFTrTr'!rY 
TftANS>'TTTFR 40FPTUPF CitM 
TH4NSMT TTKR DTPLFVFP l.OSS 

POlNtl^r. LOSSES 

4t(-0SPHFP!C LOSSES 

PROP A f; ATT 0 N L.PSS 

R F A C 0 M Z n O* OPTICS LOSS 

ycpptVFP APpRTilPF K A T ! - : 

p f C F f v F P P I F F P 4 C T I n »; (CSS 

SFffciVFR OPTICS EFFirTFvrY 

RECEIVER LOCAL 05CTU- ATOP OTF’lFXFE 

o £ T F f T o P <’ T X I ‘ 0 LOSS 

PFCFT VpF rlPlFXF» ' “‘S' 5 

otter top degradation 
PLANCk ' S CONSTANT 
CAPRTFP F'PFD'tF'T, Y , HZ 
n E t F r T d F 1 quantum f f f t r t f n c y 
a| 0 f SF RAAiD^IDTHf P7 „ 

SIOK-SI TO NOTSF « A 1 T 0 


LOSS 


-2.22 
-.18 
96.95 
- . 9 1 
-) .99 
.00 
> 274. *7 

-.39 

96 . 5 ? 
-.39 
- . 59 
- . 09 
- 1.93 
-.15 
-.57 
331 .78 
■ 139.52 
- 3.01 
- 89.77 

*"?o!cc 


ORIGINAL PAGE B 
OF POOR ^UAXiM. 

V'Tf 


5 -