OAO Spacecraft

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4 -69 GODDARD .PACE FLIGHT CENTER

Greenbelt, Maryland Phone (301) 982.4955-56.57 After Hours 474-9000

NATIONAL AERONAUTIC. AND .PACE ADMINI.TRATION

OAO SPACECRAFT

Beginning with the observations of the early Babylonian

Egyptian, Chinese, and Greek observers of the heavens, scientific

astronomy has developed in a logical, step-by-step process. The

first real plateau was a comprehensive skymapping of all celestial

objects visible to the naked eye. With the use of radio astronomy

in thiS century, scientific minds have established a second plateau

-a radio-emission map of the heavens. Now another field of

astronomical exploration is possible with the launchings of the OAO's. As in previous ex-

plorations which employed new techniques and equipment, the GAG will first undertake a

skymapping operation.

'l'here exist several narrow windows through which man can view the universe around

him. The earliest, naked-eye observers were limited to a rather restricted view lying

entIrely wItmn tne VlslDle spectrum. 'l'ne Iirst optical telescope served to enhance and

Sharpen this view but did not widen the window. Not satisfied with this limited outlook,

science next developed photographic plates and other photo-detection systems which broadened

me wmaow, gIving tne observer a view 01 tne spectrum whicn extended further into the ultra-

Violet at one ena ana turther into the intrarea at the other. Raaio astronomy opened an

entIrely new ana separate winaow turther along the spectrum,

in our own time we have seen the optical window widened in the ultraviolet direction0 0

from about 3900 Angstroms (A) wavelength down to about 3000 A by photographic and

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photoelectric effects. As an example, the two photographs of Crab Nebula show the

enhancement of our view of the heavens by shifting the spectral window. Widening this

0

window below -3000 A. is expected to further enhance the view. The lower spectral

0

limit at 3000 A has been set, not by our technology, but by a natural limit imposed by

the earth's atmosphere. Energy radiated to us from the heavens at wavelengths below

0

3000 A is masked by the absorption effects of the atmosphere.

From theoretical considerations and rocket-borne observations of a few stars

0

we know that many stars must emit in the 1000 to 3000 A range. This emission is a

natural function of bodies operating at temperatures between 4000 and 8000 Kelvin.

Only by establishing an ultraviolet observatory far outside the earth's atmosphere will

we be able to observe such emissions for extended periods This is the function of the

OAO.

The OAO-A2 contains two experiments (Smithsonian Astrophysical Observatory's

Celescope and the Wisconsin University package), each looking out opposite ends of the

OAO's central tube, and mated at the midsection of the spacecraft. This obs~rvatory

-30°C) with a permissible deviation in temperature of:!: 27°F fors to operate at -22°F

its one-year life. The observatory can look at any point in the sky, except the 450 cone

about the earth-sun line when 300 to the sun. If the telescope is thermally tied to the

spacecraft central tube via radiation interchange and insulated from the space environ-

ment, it will operate within 20 to 5°F of the spacecraft structure temperatures.

Wisconsin Experiment Package

The University of Wisconsin Experiment Package (WEP) will be carried aboard

OAO-A2 along with Smithsonian's Celescope. The primary function is to gather spec-

tral energy distribution information on selected stars and nebulae in the ultra-violet

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0

range (1000 to 4000 A). As a secondary function, WEP will measure time-varying

spectral intensity data on particular stars. This function requires repetitive measure-

menta.

WEP consists of two major packages, the Prime Instrument Package and the

Control Electronics Package. The former is subdivided into seven observing instru-

ments:

Four 8-inch diameter stellar photometers, each of which covers a spectral

0

band of approximately 1000 A with partial overlap; each is equipped with a

0

programmable filter device to further subdivide the coverage into 250 A

bandwidths.

Two scanning spectrometers augment the stellar photometers; one covers

0 0

the range from 1000 to 2000 A, the other from 2000 to 4000 A; the spectrome-

ters may be cycled in 100 steps, thereby yielding a spectral band intensity

0

only 10 or 20 A wide.

The last instrument is a 16-inch diameter nebular photometer capable of

measuring spectral intensity of star clouds as observed through five pro-

0

grammable filters covering approximately 600 A each; total coverage ranging

0

from 1500 to 3800 A. Each of the seven instruments is equipped with both

analog and digital output circuits. Gain, exposure time and filter position are

controllable from the ground station.

The WEP and the Smithsonian skymapping experiment are each located in one-

half of the center cylindrical portion of the spacecraft,

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Smithsonian Experiment

The SAG, Project Celescope, a term derived from celestial telescope, is designed

to measure the brightness of 50,000 main-sequence stars in the ultraviolet spectrum

0 0

from 1200 A to 2900 A. The Celescope consists of four independent, 12-inch diameter

Schwarzschild telescopes, each employing an imaging Uvicon (UV) system to scan a

0 0 0 0 0 0

particular bandwidth: 1200 A to 1600 A, 1300 A to 1600 A, 1600 A to 2900 A and

0 0

2300 A to 2900 A. The Uvicon is a special TV vidicon designed to operate in the ultra-

violet region. The four UV readings will be used to determine the shape of the spectral-

energy distribution curves for different types of stars. In addition to studying the single

stars, the brighter gaseous and planetary nebulae and interstellar absorption will be

investigated. Planned on a much lower priority is a UV study of both the illuminated

and dark portions of the earth's atmosp~ere.

Typical Experiment Operation

The OAO will point the experimental package toward preselected areas of the sky

as directed by ground station command. Reorientation and fine pointing commands will

be carried out while the OAO is out of contact with, but approaching, the specific ground

station. The Celescope will operate only in real time.

A typical pass commences with execution of a stored series of commands which

slews the OAO into proper orientation and turns on all calibrator lamps five minutes

or more before beginning a ground-station pass. Calibration exposures are made during

the first three minutes of the pass and stored on the Uvicon targets as electrical charge

patterns until the full facilities of the OAO are available to Celescope. First, the

Uvicons are scanned in the digital-direct mode and the first picture, including calibra-

tion information, is transmitted to the ground through the wide-band transmitter; the

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one or more standard C elescope data sequences are commanded, depending on the

length of the pass over the groWld station. Each of these sequences includes a 60-

second exposure and digital-direction scan for each camera. The four exposures are

commenced and terminated nearly simultaneously. The remainder of the pass is used

to send real-time commands to the GAG to control the Celescope sequence of operations

During the remaining minutes of the pass the command memory may be loaded

for operations to be performed during the rest of the orbit including slewing 1.8 degrees

to the next position, and the status may be read out through the wide-band transmitter.

In order to maximize the value of C elescope in the event of premature termina-

tion, the region of the sky containing the constellation Orion, with its large number of

steady-state high stars in the UV, will be surveyed early in the life of the OAO This

area will serve as a prime calibration source for the experiment during later orbits

since it will remain constant while the onboard calibration lamps may deteriorate.

Instrumentation

Optical

The Smithsonian experiment optics consist of four electronically-recording

telescopic cameras serving as broadband photometers. Each uses Schwarzschild

telescope optics with a UV detector television tube at the focal plane. The cameras,

each having a Uvicon tube with high-voltage power supply and video preamplifier, are

mounted in the experiment package which is inserted into the central tube of the

spacecraft.

Structural

The Celescope structural system is fabricated of materials with different coeffi-

cients of expansion, selected to compensate for a wide variation in temperature

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conditions (+ 300 o -50°C) while maintaining correct focus of the optics. Most of the

integrating structure is being provided by Grumman as the experiment container is

essentially part of the GAG. The experiment container is so constructed that the indi-

vidual optical elements and telescope modules can be aligned in the laboratory.

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FUTURE OAO EXPERIMENTS

Goddard Experiment Package (GEP)

Through rocket flights, astronomers have been able to examine the spectra of a0

limited number of stars in the region below 3000 A, and then only with low resolution

The GEP is expected to examine about 14,000 stars a year, at first0

(50 A) instruments.

0

producing resolution of 2 A, which will later be upgraded to resolutions between 0.04

and 0.05 A. GEP will be carried on GAG-B, the third flight vehicle.

Princeton University Experiment Package (PEP)

The primary objective of the Princeton Experiment Package is to provide quan-

titative observations of ultraviolet absorption lines. A secondary objective is the study

of the ultraviolet spectra of stars at high dispersion. The telescope has a clear aper-

ture of approximately 32 inches, a speed of f/3 and an effective focal length of 630 inches

The high resolution requirements of both the GEP and PEP experiments necessi-

tate a corresponding high degree of pointing accuracy of the OAO. Since the control

system star trackers of the GAG have a limiting accuracy of one minute of arc, an ex"':

periment fine guidance error signal is provided to the spacecraft. This signal is fed to

the GAG control system which maintains the GAG in a given attitude to an accuracy of

up to 0.1 seconds of arc

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ORBITING ASTRONOMICAL OBSERVATORY (OAO)

FACT SHEET

Spacecraft Manager

National Aeronautics and Space Administration

Goddard Space Flight Center

Greenbelt, Maryland

Prime Contractor

Grumman Aircraft Engineering Corporation

Bethpage, Long Island, New York

GAG Experiments

University of Wisconsin Experiment Package

Consists of four 8-inch stellar photometers, two scanning spectrometers to

augment the stellar photometers and a 16-inch diameter nebular photometer

capable of measuring spectral intensity of star clouds. The package is de-

signed to provide data on energy distribution of selected stars and star

clusters in the invisible light field

Contractor Cook 'l'echnological Center

Morton Grove, Illinois

~mithsonian Astronomical Observatory

Consists of a Celescope, an observation aevice composea of four inaepenaent

~chwarzchi1a telescopes each emplOying a speCial 'l'V tube aesignea to oper-

ate in the ultraviolet region. 'l'hiS paCKage Will measure the brightness ot

50,000 main-sequence stars in the inVisible Ultraviolet portion of the light

fielO an<1 stuoy the Oark portions 01 the t;arth's atmosphere.

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OAO -Q -

OAO Experiments: Smithsonian Astronomical Observatory (Continued)

Contractor -Electro-Mechanical Research. Inc.

Sarasota. Florida

Tracking and Data-Acquisition Stat!ons (Operated under the supervision of OAO Control

Center, Goddard Space Flight Center)

~~cking Stations Satellite Tracking and Data Acquisition Network

Data Acquisition Stations (a) Rosman, N. C.

(b) Quito, Ecuador

(0) Santiago, Chile

(d) Orroral, Australia

(e) Tananarive, Malagasy Republic

Launch V ehicle M~ager

Lewis Research Center

Cleveland. Ohio

Launch Operations at ULO!KSC!ETR

Launch Site Complex 36B, Cape Kennedy Space Center, Cape Kennedy, Florida

Launch Rocket Two stage Atlas/Centaur

Orbit 480 statute mile circular orbit inclined 350 to the Equator

Azimuth 60 degree true

Orbital Period 100 minutes nominal

Window 3:00 to 5:00 a.m. EST -mid November (tentative)

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OAO -10 -

SYSTEMS NOMINAL CHARACTERISTICS

CONFIGURATION AND STRUCTURE:

Weight Total 4400 pounds.

1,000 pounds of the total weight is devoted

to experiments.

Main Body Octagonal cylinder,

7 feet wide across the flats

10 feet in length.

8 solar paddlespendages

.Upper inbd paddle is 5-1/4 ft. long by

4-1/2 ft. wide.

.Lower outbd paddle is 5-1/2 ft. long by

4-1/2 ft. wide.

.Each outboard paddle is 5 ft. long by

2-1/3 ft. wide.

Total solar cell area is approximately

220 square ft.

2 Sunshades on ends of spacecraft,

rated

by a sun sensor to protect

experiment optics from direct rays of

the sun.

Two balance weight booms

-Automatically extended following orbital

injection.

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POWER SUPPLY SYSTEM:

Solar Array Non P-10 .o./cm Solar cells. 107,900 cells

per array. Peak power capability -1800 watts. Charges batteries and supplies

power during light period.

Batteries 3-20 AH batteries to supply all power during

dark and be recharged by solar arrays during

light period.

Power Control Unit Performs logic and control functions of the

battery charge system.

Power Regulating Unit Regulates power from Solar array to battery

and vehicle.

Regulator/Converterand Inverter

Converts and regulates power from battery

to :I:: 8v, +18v and :l::10v DC and to 1, 2, and

3 phase AC power.

Spacecraft Power Utilization 422 watts nominal.

Nominal System Voltage 28v DC

COMMUNICATIONS AND DATA

TRANSMISSION SYSTEMS:

Wideband Telemetry Type -PCM/NRZ/FM. (1 Kilobit, 50 Kilo-

bits) PCM/Split Phase/FM (Tape

Recorder input) 66 Kilobit

Two 7 watt 400.550 MHz RF transmitters

(redundant) .

Two data handling units. Spacecraft and Ex-

periment Tape Recorder (Spacecraft Data)

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NO MINAL CHARAC TERISTIC SYSTE MS

(Continued)OMMUNICATIONS AND DATA TRANSMISSION SYSTEMS

PCM/PSKypearrowband Telemetry

Two 1.6 watt 136.260 MHz RF transmitters

(redundant)

Radio Command Four command receivers (dual redundant

pairs with combiner in each redundant pair)

Two 160 mw 136.400 MHz RF transmitters

(redundant) .

Tracking (CW

Antennas:

VHF 2 Slot antennas in upper Solar Paddles.

Radiation pattern is omnidirectional.

Polarization is linear on direction per-

pendicular to the direction of slot.

2 Pitchfork Antennas

omnidirectional.

Radiation pattern isHF

DATA PROCESSING SYSTEM

Primary Processor andData Storage (PPDS)

Quadruple component redundancy at the logic

circuit level in parallel arrangement, or

triple modular redundancy at the functional

level.

Total weight is 244 pounds and 4.62 cubic ft

volume.

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SYSTEMS NOMINAL CHARAC;'fRRTR'fT~R

DATA PROCESSING SYSTEM (Continued)

Primary Processor andData Storage (PPDS) (Continued)

Functions to verify received signals, provide

experiment data storage, accurately positionthe gimballed star trackers, control space-

craft attitude changes (slews), issue com-

mands in Real Time and Delayed Modes to

the experiment and to other spacecraft equip-

ments, and store commands for Delayed

Mode.

System Clock Provides timing signals for all spacecraftequipment and elapsed time reference for

stored command execution.

Receive and Verify Unit Provides on-board verification and error

detections of commands received from the

ground station.

Command Decoder and Distributor Accepts, decodes, and distributes verified

commands as determined by the operation

and selection codes contained in the first

command word.

Command Storage Unit Employs four random access memory

arrays, each array having 30 planes of a

16 x 16 core arrangement to obtain dual

redundant storage of 256 commands.

Data Storage Unit Employs two 25 plane arrays, each plane

having a 64 x 64 core arrangement. Pro-

vides sequential address storage for 4,096

25-bit words in a redundant mode or 8,192

25-bit words in a nonredundant mode.

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SYSTEMS NOMINAL CH AR A~'1'RRT~'1'T~~


Programmer and Star TrackerSignal Controller (PSSC)

Control System Programmer Sequencer provides signals to the stabiliza-

tion and control equipment.

High-Level Jet Controller provides control

signals to the pitch, yaw, and roll high-level

jets.

Star Tracker Signal Controller Mode Selector generates control signal for

each Star Tracker selecting either the

Command or Track Mode.

Error Inhibit logic generates a signal that

either inhibits or enables the use of Star

Tracker error qy the Stabilization and

Control System.

Star Presence -Counter provides counts of

two, three, and four in the Roll Search Mode

for determining star pattern and star acqui-

sition.

Orbit Counter provides counts of four, three

or two as an indication for determining the

number of stars required for star patterns

and star acquisition.

Restabilization Reset Generator provides a

reset signal to the Orbit Counter due to loss

of all star presence signals after star pat-

tern acquisition has occurred.

Star Pattern and Star Acquisition Detector

samples the count of both the Orbit Counter

and the Star Presence-Counter.

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Spacecraft Data Handling

Equipment (SDHE)

Organizes, encodes, and transmits space-

craft data received from all OAO systems

to ground stations via the narrowband trans-

mitter or to data storage in the PPDS, or

the tape recorder.

SDHE receives digital data, hi-level data,

analog data, and NRZ signals.

Analog signals are encoded and transmitted

as an eight-bit binary coded output.

Analog Multiplexer 12 groups of 22 channels, each group having

an output isolation AND-gate, and 12 groups

of 2 channels, each group having an output

isolation AND-gate.

Analog-to-Digital Converts an input of zero to + 5 volts, or 0

to + 200 mv analog to an eight-bit digital

signal.

Digital Multiplexer 16 AND-gates feed one OR-gate for each of

the 24-bits in the system.

Outputs from the OR-gate are fed to the

Word Generator where the information is

stored until readout time occurs.

Gimbal Storage Register Converts gimbal serial input information

(commands) to broadside readout to the Word

Generator via the Digital Multiplexer.

Holds gimbal error and Star Tracker status

information.

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NOMINAL CHARACTERISTICSYSTEMS


Spacecraft Data Handling

Equipment (SDHE) (Continued)

Word Generator Contains a 25-bit shirt register, 25 output

buffer amplifiers, parity counter, code flip

flop and drivers.

Control unit for the entire SDHE system.Programmer

Contains a storage register and counters

synchronized by the PPDS 50 kc clock.

Power Pack Comprised of a DC-to-DC converter and

various filter networks.

Converter isolates and converts 28 volts VL:

primary power to the various DC voltages

required for operation of the SDHE system.

Experimenters Data Handling

Equipment (EDHE)

Assembles and formats analog and digital

data from the experiments for transmission

to the ground in the Real Time Mode or to

data storage in the STORE Mode.

Operation is determined by the lJata Hanoling

Command Word and discrete commands

from the experiments.

Consists of analog gates, digital gates,

analog-to-digital encoders, programmers

shift registers, and a clock.

Analog Multiplexer Three groups 01 ten ChannelS, eaCh navrng

an output isolation AND-gate.

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GAG -17 -


(Continued)ATA PROCESSING SYSTEM

Experimenters Data Handling

Equipment (EDHE) (Continued)

Analog-to-Digital Converter Converts an input of zero to + 5 volts analog

to an eight-bit digital signal.

Digital Multiplexer 12 AND-gates feed one OR-gate for each of

the 25-bits in the system.

Word Generator Contains a 25-bit shift register, 25 output

buffer amplifiers, parity counter, code flip-flop, and two serial-code output AND-gates

and drivers.

Pro grammer Contains storage elements to store all

Primary Processor commands, except the

eight STORE-DIGITAL-WORDS, until the

proper time to execute a given command.

Proper command execution time is deter-mined by the 50 kc oscillator, binary

counters, and timing matrices.

Composed of a DC -to-DC converter and

various filter networks.

Power Pack

Converter isolates and converts 28 volts DC

primary power to the various DC voltages

required for operation of the EDHE system.

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Signal Conditioning Unit (SCU) Most circuits are primarily resistive

voltage dividers.

Power is supplied to the SCU at a 10 volt

level and the maximum total consumption

is 6.0 watts.

Physical size is 3.75 cubic inches and

12-1/2 pounds.

SCU prepares transducer signals indicating

the status of the OAO and delivers these to

the SDHE for transmission to the ground or

to storage.

Control Command Junction Box

(CCJB)

Controls ON and OFF switching to space-

craft equipment by relays which are ener-

gized by either a control command pulse

that has been amplified by a pulse stretcher

or by a signal from specific units within the

OAO.

Tape Recorder (l'R) (GFE) To record spacecraft (SDHE) data on a con-

tinuous basis and transmit to ground via

Wide Band Transmitter.

Provides surge current limiting during SAO

turn-on.

SAC Buffer Box

Buffers all timing signals which the BAO

experiment shares with other spacecraft

components. (32 bit gates, 4 clocks, 1 com-

mandable)

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Command Control Switching (CCSU)

(GFE)

Expands the spacecraft control command

capability for controlling ON and OFF

switching of spacecraft equipment and

heaters. (An extension of the CCJB)

Government Furnished EquipmentInterface (GFEI) (GFE)

Provides signal conditioning and control

circuits for the tape recorder.

A free running clock is provided to give a

non-resetable spacecraft time.

A synchronous hi-level subcommutator is

provided such that 120 hi-level channels are

arranged into five output groups.

A command verification counter is provided

to count the number of spacecraft ground

commands received and verified.

STABILIZATION AND ATTITUDECONTROL SYSTEMS

Sensors

SunSensors Eight coarse sensors (four on pitch axis

and four on yaw axis).

Disable eye

Rate Gyros Three gyros, one for each axis, the voltage

output of each being proportional to the

angular rate about its sensing axis.

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OAO -20-

NO MINAL C HARAC TERI~'r lC ~YSTEM~

(Continued)TABILIZATION AND ATTrrUDE CONTROL SYSTEM

(Continued)ensors

Star Trackers Six Gimballed Star Trackers, each mounted

on a two-degree-of-freedom gimbal system.

One Boresigilted Star Tracker aligned with

the experiment optical axis.

Three (one per axis) which sense earth's

magnetic field.

Mag1letometers

Actuators

Primary Gas Jets Six high-pressure jets used during initial

stabilization and restabilizations.

Six low-pressure jets which reduce inertia

wheel speed.

~econ<1ary ias Jets Six high-pressure jets for backup during

initial stabilization and restabilizations and

for use with RAPS.

lnertia WneelS:

Three wheelS (one lor eacn aXIS) usea to

slew the spacecraft upon command.

Coarse w tleelS

Three wheels (one for each aXis) used to

control spacecraft attitude.Fine WheelS

By magnetically torquing the spacecran, it

removes long term disturbances.

Magnetic 'l'orquer tlars

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DAD -21 -

STABILIZATION AND ATTITUDE CONTROL SYSTEM (Continued)

Digital Logic Unit (DLU) Process signals from PPDS for distribution

to six Gimballed Star Trackers.

Controls Star Trackers gimbal motion

Decodes error information from Star

Trackers (D/ A conversion) for distribution

to STSP.

Star Tracker Signal Processor

(STSP)

Averages, weighs, and amplifies Star

Tracker error signals for distribution to

fine wheels.

Sensor Signal Processor (SSP) Sums, averages, weighs, and amplifies error

signals from all Sun Sensors and Rate Gyros

for distribution to system actuators (Fine

Inertia Wheels, Low Thrust Jets, and High

Thrust Jets).

Fine Wheel and Jet Controller(FW&JC)

Controls all Fine Inertia Wheels, Low

Thrust Jets, and High Thrust Jets using

error signals supplied by the STSP and SSP.

Coarse Wheel Controller (CWC) Controls Coarse Inertia Wheels used for

spacecraft reorientation.

High Torque Controller (HTC) Controls torque mode (high and low) of Fine

Inertia Wheels.

Magnetic Unloading System(MUS)

Processes magnetic and wheel speed data

to correct for disturbance torques.

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OAO -22-


STABILIZATION AND ATTITUDE CONTROL SYSTEM (C ontinued)

Rate and Position Sensor (RAPS)

Sun Sensors Eight sensors (four on yaw axis, two on

pitch axis, one null and one anti-eye).

Gyro Unit Three gyros, one for each axis, the voltage

output of each being proportional to the

angular rate/position about its sensing

axis.

Gyro Electronics Process signals from gyro unit for distribu-

tion to the RAPS Controller. RAPS Signal

Processor (roll only) and FW&JC .

RAPS Controller Controls six secondary gas jets for sun

bathing (power stand-by attitude) or attitude

hold.

RAPS Signal Processor Process signals to roll FW&JC.

Pneumatics N 2 at 3250 psi stored in 8 tanks; about

64 lb N2 total, primary and secondarysystem.

GPO 874-936

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OAO Spacecraft