Nimbus meteorological satellite system for data on worldwide atmospheric processes - real-time weather forecasting and researc

Michaels, J. V.

Press, H.

English

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Although lagging the initial schedule, 
it now sees first spacecraft being assembled 
NimQus spacecraft development 
BY HAR RY PRESS and JACKSON V. MICHAELS, NASA Goddard Space Flight Center 
The Nimbus project is the follow-on 
to Tiros in the NASA meteorological 
, satellite program. As the initial step 
in the development of a global me-
teorological satellite system, the Nim-
bus concept specifically calls for 
worldwiJ:le coverage on a regular basis 
and an increased payload capability 
to provide a platform for an increased 
number of sensors. Preliminary design 
of the Nimbus spacecraft was com-
pleted in 1960.'-' 
More than two years have now 
elapsed, and it will be appropriate to 
describe the approach used in the 
system, to present a brief history of 
the development, and to assess 
achievements to date in the light of 
the original design objectives. 
Nimbus' basic objective has been to 
develop a meteorological satellite sys-
tem that will provide basic data on 
worldwide atmospheric processes for 
l'eal-time use in weather forecasting 
and for research studies. To accom-
plish this objective, the Nimbus sys-
tem was conceived as an earth-stabi-
lized satellite in a near-polar orbit, as 
shown on page 44, capable of full 
global observation on a daily basis, 
with the earth's rotational movement 
providing longitudinal coverage and 
the orbital motion of the spacecraft 
providing latitudinal coverage. The 
initial choice of an 80-deg retrograde 
600-n. mi. orbit, with a local-noon 
equatorial crossing, ideally suits these 
requirements, permitting optimum 
picture-exposure times and keeping 
the orbital plane in the earth- sun line 
HARRY PRESS 
for prolonged periods. As the illustra-
tion shows, this arrangement also 
simplifies the mechanical design of 
the solar paddles, because it requires 
only one axis of rotation for orienta-
tion with the sun. The polar orbit 
allows readout of data from all orbits 
by two ground stations, to be located 
in Fairbanks , Alaska, and Eastern 
Canada. 
An additional objective, fundamental 
in the approach to the Nimbus sys-
tem, was a spacecraft design of in-
herent flexibi lity that would permit 
evolution of the individual subsystems 
and sensors with minimum interface 
and redesign problems. The Nimbus 
spacecraft, shown on page 43, includes 
a completely separable control system 
and a sensory ring incorporating a 
large number of standard-size bays 
to accept individual subsystem 
modules of fixed size and standard in-
terface requirements. Contained with-
in the sensory ring are the powel'-
supply electronics and battery mod-
ules and the clock-command and pulse-
code-modulated (PCM) telemetry sub-
systems. These three subsystems, to-
gether with the controls package and 
the solar paddles, compose the mini-
mum Nimbus spacecraft ensemble. 
Performance of the Nimbus space-
craft is monitored by a 672-channel 
telemetry subsystem having still 
further growth potential. The original 
spacecraft configuration contained 
fully redundant Advanced Vidicon 
Camera Subsystems ( AVeS) , consist-
ing of six cameras, two four-track 
is Nimbus Project Manager at Goddard 
SFC. Previously head of the OARP 
Structures and Operating Problems Oiv. 
at NASA Headquarters, his background 
includes 12 years of research on air· 
craft and missile structural loads at 
Langley Research Center, three yea rs as head 
of its Gust Loads Branch, and, earl ier, 
three years as a USAF weather officer and 
instructor in meteorology at NYU. 
tape recorders, and two S-band trans-
mitters and antennas, plus associated 
electronics and control circuitry. The 
cameras use I-in. vidicons with 800-
line resolution, providing picture res-
olutions of 1.5 mi. at the corners and 
0.5 mi. at the zenith, at an altitude of 
600 n. mi. 
In addition, the configuration con-
tained fully redundant Automatic Pic-
ture Transmission Subsystems 
(APTS), similar in performance to 
the A VCS, but designed for continu-
ous real-time picture transmission, 
rather than for storage and data read-
out to a central data-acquisition sta-
tion. Every 208 sec along the orbit 
path during daylight hours, APTS 
will provide pictures to relatively in-
expensive ground stations equipped 
with facsimile recording devices, thus 
effectively bringing broad real-time 
cloud-cover data to almost every ele-
ment of the wo Idwide meteorological 
community. 
Remaining subsystems of the origi-
nal configuration were a five-channel 
Medium-Resolution Infrared Radiom-
eter (MRIR), similar to that flown 
in the Tiros spacecraft, and a High-
Resolution Infrared Radiometer 
(HRIR), designed to be the night-
time cloud-cover complement of A VCS 
and responsive to the 3.4- to 4.2-micron 
region of the infrared spectrum. In-
cluded, too, was a Low-Resolution In-
frared Radiometer (LRIR), designed 
to examine the earth's heat budget. 
Inability to achieve required perform-
ance goals caused the elimination of 
JACKSON V. MICHAELS 
Numbus Spacecraft Systems Manager at 
Goddard, previously headed the Tech-
nical Staff of the Army Signal R&D Labs' 
Astro Electronics Div., engaged in 
technical planning for Courier, Score, 
Tiros, and Advent. He has taught 
courses in space technology, and is 
co·author of the book " Introduction 
to Space Communication Systems," to 
be published by McGraw·HiII . 
ReprintedJrolll ASTRONAUTlCS AND AEROSPACE ENGIl\TEERING, April 1963 
Copyright, 196:3, by the :\ merican Institute of Aeronautic and .\ stronautics and reprinted by permission of the c0pyright owner. 
the LRIR subsys'cem earh in the pro-
gram. 
The equally basic design objective 
of long life (six months in orbit) dic-
tated the use of conservative design 
practices, extensive subsystem re-
dundancy, and intensive subsystem 
and spacecraft qualification. A high 
margin of safety is generally secured 
during prototype development by 
qualification under conditions more 
severe than the actual launch and 
space environments. Flight hardware 
is qualified under the nearest possible 
simulation of the anticipated flight 
levels. Before sUbsystem integration, 
all thermal and mechanical system 
conclusions are validated by checking 
and testing full-scale thermal and 
mechanical models to prototype quali-
fication levels. 
NIMBUS SPACECRAFT 
SOLAR PADDLE--~ 
SOLAR PADDLE 
SUN SENSOR 
PNEUMATIC TANK 
THERM.AL CONTROL SHUTTERS 
With the completion of the initial 
design in 1960, there was defined a 
basic management approach to the 
system development, consisting of the 
government (specifically, NASA's 
Goddard Space Flight Center) assum-
ing the role of prime contractor and 
system manager, with the responsi-
bility for selecting industrial contrac-
tors to develop and fabricate the sub-
systems, and for providing technical 
and managerial direction of the sub-
system development efforts. A project 
manager and four system managers 
(for the spacecraft, launch vehicle, 
data-acquisition, and data-utilization 
systems) were appointed to head the 
over-all project effort. 
The modular aspect of the Nimbus 
concept allowed development of the 
subsystems to proceed concurrently 
STABILIZATION AND 
CONTROL HOUSING 
with preparatory spacecraft develop-
ment. Substantial advanced mechani-
cal and thermal analyses, and corrob-
oration of these analyses by full-
scale model testing, were conducted 
with good results. However, the price 
paid for this flexibility has been the 
need for close supervision of subsys-
tem development and tight control of 
interfaces. 
In late 1960 and early 1961, indi-
vidual contracts were let for parallel 
development of each of the subsys-
tems, calling for detailed design, 
breadboard development, and proto-
type and flight-hardware construction. 
The contracts also covered extensive 
subsystem environmental qualification 
testing in regard to humidity, acceler-
ation, vibration, and vacuum-thermal 
cond itions. 
I:--- - COMMAND ANTENNA 
>I'i"~:!tt'~¥,l,,w,i\-- HORIZO N SCAN N ER 
BEACON AND 
TELEMETRY ANTEN NA -----t/;ftiH++-tttittttffiffi1tt~Uill 
____ SENSORy RING 
S-BAND ANTENNA 
HIGH RESOLUTION 
INFRARED RECORDER 
TV RECORDER 
MEDIUM RESOLUTION 
~INFRARED RADIOMETER 
DIRECTION IN ORBIT 
MEDIUM RESOLUTION 
INFRARED ANTENNA 
AUTOMATIC PICTURE TRANSMISSION CAMERA 
~~ __ J 
I 
L 
In addition to development of the 
mdividual subsystems, a contract was 
let for integration and testing of the 
complete spacecraft. This contract 
covered construction of the basic 
spacecraft housing structure, integra-
tion of the various subsystems into a 
full prototype and into flight space-
craft , over-all spacecraft system per-
formance testing, and, finally, a vigor-
ous spacecraft environmental-qualifi-
cation testing program. Noteworthy 
is the development of a system which 
simulates, within the confines of a 
vacuum chamber, the operation of the 
spacecraft in orbit (see page 46) . Dul'-
ing vacuum-thermal qualification, the 
full Nimbus spacecraft floats on a 
film of air, tracking an artificial hori-
zon with communication only by radio 
to the checkout equipment outside the 
chamber. 
Initial plans called for the sequen-
tial development and construction of 
the initial flight spacecraft at six-
month intervals, in what was envi-
sioned as an orderly research and de-
velopment activity. It was thought 
that the first spacecraft development 
cycle would require about 1 1/2 years, 
one year being allotted to subsystem 
development and 6- 9 months to space-
craft integration and qualification. 
This time schedule turned out to be 
overly optimistic. 
The progress of Nimbus was accel-
erated rapidly when, in May 1961, the 
President presented a special message 
to Congress requesting an accelerated 
Nimbus program "to give us at t he 
earliest possible time a satellite sys-
tem for worldwide weather observa-
tion." Later that year, t he Congress 
appropriated additional funds to the 
Weather Bureau to implement this 
objective. This new requirement es-
sentially resulted in revisions and ac-
celerations of the program aimed at 
an earlier worldwide operational capa-
bility. Additional spacecraft funds 
were included, as well as funds for 
additional ground stations, in order 
to assure a worldwide weather-obser-
vation capability. NASA, working in 
conjunction with the Weather Bureau, 
moved rapidly to supplement the 
existing R&D program to meet this 
objective. 
Nimbus spacecraft development has 
encountered significant delays due to 
technical difficulties in the develop-
ment of a number of major subsys-
tems. Although these delays have been 
time-consuming and expensive, the 
problems were not insurmountable. 
The table on page 45 compares major 
system performance achievements 
with the initial design objectives. In 
most cases design objectives have been 
attained. The achievement of an 
AVeS with the desired resolution and 
high linearity (1/2 % ), using a wide-
band frequency-multiplexed system, is 
a significant accomplishment in stor-
age systems. The development of 
APTS is also considered a major ad-
vance in real-time meteorological sys-
tems. In two cases-stabilization ac-
curacy and the HRIR resolution-
some compromise of design val ues was 
found necessary, but this should not 
seriousl/' -aifect early flight missions, 
and the design values will be subject 
to later improvement. 
All major subsystems have been 
built and fully qualified in prototype 
hardware, with two exceptions. Flight 
hardware also nears completion. The 
two exceptions are the stabilization 
and control subsystem and the solar 
paddles. Each of these has posed seri-
ous development problems. 
In the control subsystem, a major 
redesign of the infrared horizon-scan-
ning system used for attitude sensing 
was required to achieve reliable atti-
tude data. The redesigned system has 
been built and has performed satis-
factorily in tests, although sensor ac-
NIMBUS TELEVISION COVERAGE 
NIMBUS 
SPACECRAFT 
FIRST ORBIT 
-----
-----
- -
..-
~ -
SUN RAYS 
FIRST ORB IT 
- -------
--- - --- -
----- ---
4--- - ---__ 
- ---- - - -
SUN RAYS 
SECOND ORBIT 
r ------ --
PRESENT ~IMBUS PERFORMANCE 
I 
DESIGN OBJ ECTIVE 
WEIGHT 650 LB 
STAB . ACCURACY ! 1% 
POWER ( MAX ) <450 WATTS 
AVCS ( RES) Y, MILE 
PCM TELEMETRY 672 CHANNELS 
HRIR ( RES ) 2 MILES 
MRIR (RES ) 30 MILES 
COMMANDS 128 
APT S ( RES) IY, MILE 
curacy does not yet provide the full 
pointing accuracy desired. 
Development and construction of the 
solar paddles was also seriously de-
layed by a decision in late 1962 to 
change from the initial p-on-n solar 
cells to more-radiation-resistant n-
on-p cells. This decision arose from 
the unexpected creation of an artifi-
cial radiation belt at Nimbus flight 
altitude by a high-altitude nuclear-
explosion test in mid-1962. New and 
satisfactory cells are now being pro-
cured and integrated into Nimbus. 
As mentioned previously, a major 
weight problem was encountered in 
spacecraft development. The design 
objective was a weight of 650 Ib for 
the spacecraft, to permit compatibility 
with the medium-sized Thor-Agena 
booster. Although every effort was 
made to meet this objective, it was 
readily apparent by mid-1961 that the 
full Nimbus spacecraft with the 
planned redundancy would exceed de-
sign weight by a substantial amount, 
and would exceed the launch vehicle 
capability. It was accordingly con-
cluded that the early development 
flights would not include the desired 
l·edundancies. 
A mod ified Nimbus configuration 
was defined in late 1961 and pro-
g rammed for the early flights. Subse-
quent subsystem development gave 
rise to additional weight increases, so 
that the basic spacecraft now weighs 
April 1963 
NIMBUS 'A" WEIGHT HISTORY 
AC HIEVEM ENT 
750 LB 
! 2 % 
400 WATTS 
Y, MILE 
NIMBU S AND T1ROS C O?l1 P-'R~D 
672 CHANNELS 
SMILES 
TIROS I NIMBUS 
GEOMETRY PILLBOX DUMBBELL 
30 MILES 
WEIGHT (I b ) 300 650 
128 ORBI TAL ALTITUDE (noutlcal miles) 3 80 600 
I Y, MILE ORBITAL INCLINATION 48- EQUATORIAL 60"' POLAR, RETROGRADE 
STABILIZATION SPIN-STABILI ZED 3·AXES EARTH ORIENTED 
EARTH COVERAGE 10 TO 25% 100% 
CA MERA RESOLUTION 500 LINES/FRAME 800 LINES/ FRAME 
TV RESOLUTION (miles) 1/2 
MAXIMUM POWER AVAILABLE 'wolfS) 20 400 
IR SENSORS (rI!Sotu110n. mile,) MRIR (30) MRIR (30) 
about 750 lb, but can still be launched 
by the Thor-Agena to an altitude of 
500 n. mi. 
In the present configuration, the 
spacecraft contains one three-camera 
A VCS, one single-camera APTS, and 
the HRIR, in addition to the minimum 
spacecraft clements described previ-
ously. Although the original payload 
goals appear optimistic in light of 
subsequent developments, the weight 
history at the top indicates that the 
sensory subsystems account for about 
20 '70 of the total weight, constituting 
a reasonable payload efficiency. Struc-
ture, which includes wiring harnesses, 
thermal controls, and antennas, is less 
than 25 % of the payload. Later flights, 
which plan to incorporate full redun-
dancy, will require a launch vehicle of 
increased capability. 
In parallel with the spacecraft de-
velopment, the Nimbus project is also 
developing a Command and Data Ac-
quisition (CDA) system to provide 
near real-time readout of data from 
all orbits. As indicated earlier, full 
orbital coverage can be achieved with 
two high latitude stations, one at 
Fairbanks and the other in Eastern 
Canada. The construction of the Fair-
banks station is well underway and 
the station should be operational by 
early fall in ample time for the initial 
flight. The station will include an 85-ft 
tracking antenna with auto-track ca-
pabi lity as well as an extensive data 
HRIR 15) 
processing system for data evaluation 
and data transmission over wide band 
microwave links to the GSFC and to 
the Weather Bureau. The sensory data 
will thus be available for direct ap-
plication to weather forecasts. The 
construction of the second CDA station 
in Eastern Canada has been delayed. 
Meanwhile, NA A plans to use the 
antenna system at Rosman, N.C., as a 
second and backup station for the 
initial flight. 
In summary, the development of the 
Nimbus spacecraft has proceeded at 
a slower pace than initially planned, 
but the technical problems are now 
almost completely solved and the first 
flight spacecraft assembly is under-
way. A great deal has been learned 
about the technical problems implicit 
in a meteorological satellite with the 
capability of Nimbus, and the prob-
lems inherent in the management ap-
proach taken. Initial fl jghts should 
prove the validity of Nimbus design 
concepts and our ability to realize 
them in flight systems. This flight ex-
perience, moreover, should provide the 
decisive tests on which to base the 
growth of a national meteorological 
satell ite system. 
References 
1. Press, H . and tampH, R., "Nimbus Space-
craft System," Aerospace Engineering, July 1962. 
2. Stampfl, R. A. , "The Nimbus Spacecraft 
and lts Communication System," NASA Tecb-
nical Note D-1422. Jan . 1963. • • 


NIMBUS SPACECRAFT DEVELOPMENT

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