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NASA FACTSVol. II; No . 7

An Educational Services Publication of theNational Aeronautics and Space Administration

NIMBUS

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Nimbus in orbit (artist's conception).

Advanced equipment designed and proposedfo r us e in future weather satellite systems is be-ing tested in NASA's Project Nimbus . The firstNimbus satellite, designated Nimbus I, wa slaunched August 28, 1964.

Nimbus is a second generation meteorological

satellite program. It was preceded by th e successful TIROS project which, by making possibleincreased observations of th e earth ' s atmosphere,contributes to accuracy of weather forecasts an dto the extension of meteorological knowledge.(See NASA Facts- TlROS, No . 8-2-64.)


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'NIMBUS I SENDS FIRST NIGHT-TIMECLOUD PICTURES

Nimbus I provided the first pictures of thearth's surface an d of cloud patterns ever taken

by a weather satellite at night. TIROS satellitesre limited in their observations to the earth ' sunlit side. Th e Nimbus I achievement points

he way to around-the-clock weather coverage by

atellite.

Midnight infrared photograph by Nimbus I shows

heavy clouds over Alps and Central Europe andcattered clouds over Mediterranean Se a and NorthAfrica . (Foot of Italy "boot" at right.)

The excellent quality pictures taken by Nimbust night were achieved by means of it s High

Resolution Infrared Radiometer (HRIR) system.The system detects minute differences in the rela-

vely faint invisible infrared radiation (heat)

mitted by clouds and the earth's surface. It

onverts the data to electrical signals that ar e

tored on magnetic tape for transmission onommand to earth.

When the signals ar e received on earth, they

r e fe d to facsimile recorders that turn them into

strip of pictures. The shades (lightness orarkness) o f images in th e pictures are directlyelated to th e infrared radiation emitted by the

ictured objects . For example, water areassuch as seas, oceans , lakes, an d rivers) appearark because water rapidly gives off heat. Cold

Vol. II, No . 7

Hurricane Ethel in an infrared photograph by Nimbus I.

mountain tops that emit virtually no heat arebright . Land is in various shades of gray,becoming lighter w ith elevation .

Similarly, intensity of infrared radiation emit

te d by clouds decreases as alti tude increases.

In effect, the HRIR system provides three-dimen -sional i n f o r m a t i o n - n o t only length and widthbu t also temperature data from which cloud

heights may be derived.

THREE-CAMERA SYSTEMNimbus is also equipped with an Advanced

Vidicon Camera System (AVCS) consisting of threevidicon cameras (small television cameras) and

related equipment. The three cameras, whichoperate simultaneously, produce an oblongthree-panel photograph covering weather condi

tions over about 750 ,000 square miles of earth.As many as 33 three-panel views can be providedper orbit.

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N I M B U S

AV C S P I C T U R E S

Nimbus I triple-camera coverage from th e NorthAtlantic Ocean to Sahara Desert on August 30 , 1964.

From the design altitude of 575 miles, Nimbuscameras provide a resolution of approximatelyone-half mile . This means that cloud and terrain features as small as a half mile ar e detectable in the photographs. Photographs taken ata lower alt i tude show more detail. The AVeSsystem operates only in dayl ight and turns off

automatically when Nimbus reaches th e earth'snight side . It turns on again when the satellitereaches th e earth's sunlit side. AVeS pictures

ar e stored on magnetic tape for transmission oncommand to ground stations.

Plans call fo the AVeS cameras to be installedin th e TIROS Operational Satellites (TOS). InTOS, however, a single camera is used fo r takingpictures instead of three cameras operatingtogether .

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Nimbus photograph of Nile River (a t left) during flood.Sinai Peninsula is at extreme right.

ADDITIONAL EXPERIENCE PROVIDEDAPT USERS

Nimbus provides valuable additional experience to ground weather services that us e th eAutomatic Picture Transmission (APT) systemAPT wa s first flown on TIROS VIII, launchedDecember 21, 1963.

APT telecasts can be picked up by relativelysimple ground equipment costing about $30,000.As flown on Nimbus, the APT system can providelocal meteorologists with pictures of th e weatherover an area of more than 750,000 square miles,extending more than 825 miles in every directionfrom their stations. It can send each local stationas many as 15 pictures daily.

Like th e AVeS, the APT system operates onlyin daylight. The APT system does not storepictures

fo r latertransmission but starts sending

them within 8 seconds after taking them. Ittakes 200 seconds to transmit a single picture(see below).

The APT camera resembles that of th e Av e sexcept that a polystyrene plastic layer is providedto extend the time that its vidicon tube can retainthe image. This image is scanned by an electronbeam in the compartively slow time of about 200seconds as compared to 6 1/2 seconds in the Av e s .


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Hurricane Dora, which devastated Florida an d Georgiain 1964, is pictured by infrared at night (top) and indaytime picture by Automatic Picture Transmission(bottom). Nimbus pictures have made significantcontributions to studying and tracking hurricanes such

as Dora.

The electron beam converts th e image into anelectrical signal that is radioed to earth.

This slow-scan feature permits th e use o f the

relatively simple and low-cost equipment in theAPT ground station. Since data are being trans-

mitted at a slower rate, they ca n be sent on

radio beams of narrower bandwidth. The band

width on which th e APT system transmits is much

narrower even than that o f signals sent to home

television sets .

Vol. II, No.7

Composite of four photographs transmiHed by theNimbus I Automatic Picture Transmission (APT) systemon .August 29, 1964, covers an area from Canada to

South America.

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Strip of pictures made from Nimbus infrared da ta showsclouds (gray) over the South Indian Ocean (black) an dthe icy Antarctic continent (bright mass a t bottom).Locational markings were overprinted a t ground station.

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MAPPI NG THE WEATHER

One o f th e most significant advances in meteorology was th e development o f th e weather

map pioneered by Heinrich Wilhelm Brandes i1 820. Brandes compiled observations made iEurope on March 6, 1783.

This map and further study showed that loca

storms were frequently parts o f great systemscovering large areas. They also indicated tha

storms usually followed th e same paths. Thusknowledge o f th e existence o f a storm, an d thepaths storms generally took, could contribute t

weather forecasting.

In time, meteorologists realized that weatheris global in nature an d world-wide data areneeded to make accurate local forecasts. Toward this end, a vast network of weather observation stations wa s set up around th e globe, an darrangements were made fo r rapid exchange o

information.

Until relatively recent years, weather fore

casters were hampered by serious blind spots itheir observation system. For example, many

severe storms or iginate over areas o f ocean,

desert, polar, and equatorial regions where there

ar e no regular reporting stations.

The launch o f TIROS I on April 1, 1960,marked a major breakthrough in weather sur

veillance. Since that date, th e global observations made by TIROS satellites have been incor

porated in operational weather forecasts.The Nimbus I satellite, a research vehicle,

represents another significant advance in thetechnology o f weather observation by satellite.

Among its other achievements, Nimbus I demonstrated the feasibility of a 24-hour-a-day globalweather watch by satellite.

SPACECRAFT DESCRIPTION

The Nimbus I . satellite, which weighs 83 0pounds, extends 12 feet from its circular base toth e ti p of its command antenna. It has a spano f 111/6 feet across its tw o solar paddles. Themain spacecraft body measures 4 3/4 feet i

diameter a t the base and 10 feet tall.

The command antenna receives orders fromground stations fo r th e satellite to transmit data.

The solar paddles hold 10,500 solar cells which


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are ' p h o t o ~ l e c t r i cdevices that convert sunlight to

electricity. The paddles provide 450 watts ofpower for spacecraft operat ion and rechargeseven storage batteries which furnish eleetricity

when the satellite is not in sunlight. The batteries also power Nimbus from launch until the

solar paddles unfold in orbit an d begin tofunction.

Nimbus is made up of three main sections.One is a sensory ring containing the AVC, HRIR,an d APT Systems described earlier, also thestorage batteries, tracking beacons, an d the te-lemetry equipment which reports on th e space-craft 's condition. A connecting structure, calledan "interconnecting truss," joins th e sensory ringto a hexagonal "att i tude control housing." Thehousing contains the equipment that orients and

stabilizes the satellite so tha t it s cameras and

infrared radiometer point constantly at earth andswivels the solar paddles so that they alwaysface the sun. The system, called one of themost complex ever built, consists of some 5600electronic an d 150 mechanical parts.

Because space has no atmosphere to diffusean d retain heat, spacecraft are subject to sharp

Technicians work on Nimbus.

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temperature changes. Temperatures on a space-craft 's outs id e surface may reach 260 degreesFahrenheit in th e sun an d drop to 240 degreesbelow zero Fahrenheit in the shade . The interiortemperature of th e sensory and attitude controlsections is kept within a range of 10 degreesabove or below 25 degrees Centigrade (7 7

degrees Fahrenheit).

. . . . 1OI.0ItIl

Principal equipment of Nimbus .

ORBITAL INFORMATION

Nimbus satellites ar e designed to be launched

into nearly circular polar (traveling north an d

south) orbits about 575 miles above earth . A

polar-orbiting satellite provides north to south,or lati tudinal, coverage by it s fl ight path. Itprovides east-west, or longitudinal , coverage

because of the earth's rotation.Nimbus I, launched from the Western Test

Range, Point Arguello, Calif., attained an initial

orbit ranging from 25 2 to 578 miles above earth.Its period was 98.3 minutes; i. e . , it completed anorbit every 98.3 minutes. Its elliptical, insteadof a planned circular orbit, was caused when thesecond stage of it s Thor-Agena launch vehicle

ceased firing prematurely. Despite th e un-planned orbit, the satellite ' s camera and radiometer systems sent thousands of h igh quali typictures of the earth an d cloud patterns.

Nimbus I stopped operating on September 23 ,1964 , when it s solar paddles locked into one

position. Unable to swivel an d follow the sun,the solar paddles could not provide the electricalpower needed to keep the spacecraft functioning .


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Thor-Agena launch vehicle with Nimbus payloadawaits launch.

GROUND FACILITIES

Several kinds of ground stations constitute thesurface facilities taking part in the Nimbus ex

periment . The Nimbus Technical Control Center,located at NASA's Goddard Space Flight Centerin Greenbelt, Md., evaluates the spacecraft ' s performance an d determines ' commands to be sent it.

The commands are radioed to Nimbus by eitherof two Command and Data Acquisition Stationslocated at Rosman, N . C. , and Gilmore Creek(near Fairbanks), Alaska. The 8S-foot diameterantennas of these stations also pick up dataradioed from the camera an d radiometer systems

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of Nimbus . The information is fe d to an d processed by Nimbus Data Handling Facilities atGilmore Creek an d Goddard .

The Data Handling Facilities record the cameraand infrared data on magnetic tape similar to

that used to record live television shows fo r presentation at later dates . The ca mera data ar ealso fed to kinescopes (small television

sets)which turn them into pictures that are photographed on 70 mm. film. The infrared data arechanneled to facsimile recorders that convert

them into strips of pictures.N A S A ' s global Satellite Tracking and Data

Acquisition Network tracks N imbus an d providesorbital predictions. This information is funneledto the Nimbus Technical Control Center and processed by the Nimbus Data Handling Facilities atGoddard.

About 60 Automatic Picture Transmission(APT) stations se t up by government and commercial organizations in this country and abroadparticipated in th e Nimbus I exper iment. Theprincipal equipment of an APT station is a manually-operated radio receiving antenna , a com-

8S-foot diameter antenna a t Gilmore Creek, Alaska.


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APT antenna.

mercially available radio receiver, an d a facsimile

recorder comparab le to those tha t make th e

wire photos appearing in daily newspapers.

Goddard supplies each APT station with in -

format ion abou t Nimbus ' orbi t and when an d

where to ai m its antenna to receive the APT

picture. As an additional aid, Nimbus transmitsa radio signal to alert th e stations to its

approach.

NASA FACTS fo rmat Is des igned fo r bu l l e t in -boa rd displayuncut , or fo r 8 x 10Yl l oose lea f no tebook Inser t ion whencu t a long dot ted l ines an d fo lded a long sol id l ines . Fo rno tebook ring inser t ion, punch a t solid dots In th e margins.

The National Weather Satellite Center, U.S.

Weather Bureau, receives Nimbus data from the

Nimbus Technical Control Center. The Weather

Satellite Center analyzes th e data and dissemi-

nates information derived from it to weather

services in the United States an d abroad. The

Weather Satellite Center also archives the data

used in meteorological operations.

NASA FACTS is a n educa t iona l publ icat ion of NASA ' s Educat ional Programs and Services Office. It will be mailed toaddressees who reques t it f rom: NASA, Educat ional Publicot ions Distribution Cen te r, AFEE-l, Wash ing ton , D.C., 20546 .

U . S . GOV ERNMEN T PRINTI,,\ G O FF IC E : 1964 0 " - 75 6 -3 6 5

For sale by the Superintendent of D o c u m e ~ t. ... U.S. Government Printing OfficeWashington , D .C., 2 0 4 0 2 - Price 15 cents pe r c o p y

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