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Produced by the NASA Center for Aerospace Information (CASI)
JPL PUBLICATION 85-10

(N>SA -Cii-175647) HICFChAVI ELICKECLIES ECb Ne5-23068

SEICtFOBNE UCEIVIRS (Jet Picipulsion Lab.)

5C p HC AJ3/AF lti 1 CSCL 14B

uucla s

63/35 14808

J

Microwave Blackbodies

For Spaceborne Receivers,

J.M. Stacey

\*6

' r

R , March 1, 1985

4GA

k i

National Aeronautics and

Space Administration

t

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

J '

1

t
JPL PUBLICATION 85-10

Microwave Blackbodies

For Spaceborne Receivers

J. M. Stacey

March 1, 1985

RYSA

National Aeronautics and

Space Administration

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California
The research described in this publication was carried out by the Jet Propulsion

Laboratory, California Institute of Technology, under a contract with the National

Aeronautics and Space Administration.

Reference herein to any specific commercial product, process, c ► service by trade

name, trademark, manufacturer, or otherwise, does not constitute or imply its

endorsement by the United States Government or the Jet Propulsion Laboratory,

California Institute of Technology.
FOREWORD

Microwave blackbody targets are very sophisticated thermal, electrical,

and mechanical structures that simulate in the laboratory and in the

thermal-vacuum chamber the temperatures of objects and gases in space, and in

the atmospheres and on the surfaces of plants. During the calibration and

testing of a spaceborne receiver, the radiating surfaces of blackbody targets

are positioned to be viewed at the input ports of a receiver.

Cryogens are applied to the radiating surfaces of thr targetf to simulate

the colder temperatures expressed by seawater, freshwater, iceforms, and even

the Cosmic Background itself.

When heated, the radiative surfaces of blackbody targets also simulate

the temperatures of deserts, mountains, and heavily foliated regions, like

rain forests.

Considered in this report are the principles of operation of blackbody

targets and the pr.actices involved in their construction and application.

Problems that affect the accuracy of blackbody targets are discussed.

Some new ideas are given for the construction of blackbody targets for use in

both the laboratory and in the thermal vacuum chamber. Suggestions are given

that will improve the accuracy and the practicability of blackbody target

Structures, and also circumvent some of the existing problems.

The structure and operating principles of variable-temperature, blackbody

targets are described, mainly as they are applied to blackbody use in the

thermal-vacuum chamber.

On space missions, or when in Earth orbit, spaceborne receivers carry

blackbody targets for internal temperature standards. Corrugated-horn

antennas are also carried as a receiver component for viewing the coldest

space temperature of all--the Cosmic Background.

ii.i
rr.

K% 

The community of workers who deal with, or who are concerned with, the

application of microwave blackbody targets is small indeed--perhaps only two

or three individuals in a large research organization. Moreover, the targets

are used only briefly during the construction, testing, and calibration phases

of a spaceborne receiver. To this extent, blackbody targets are used,

typically, for only a few weeks in as rrany years--perhaps as infrequently as

every five or ten years.

For this infrequent application, a great deal of manpower and money is

expended to construct, test, and qualify a complex thermal-mechanicalelectrical target structure that is comprised of many different materials and

metals that must interact with a host of cryogens, heaters, and hot gases.

Some targets must also be capable of producing blackbody radiation that is

precisely and variably controlled and has extreme stability at any prescribed

setting.

To produce a blackbody target structure that will operate with high

accuracy in the thermal vacuum chamber, and especially one that must operate

variably and precisely over a large temperature range, requires extraordinary

skills that embrace thermal, mechanical, electrical, and RF engineering--all

operating in consonance as a dedicated team.

The controller that circulates the cryogen through the radiative surfaces

of the lackbody targets is an engineering development in itself and requires

specialized skills and experience in its manufacture. The controller operates

initially to set the radiative surfaces of the blackbody target to a

prescribed temperature and to maintain it at that temperature for as long as

is required. The controller circulates liquid nitrogen and heated nitrogen

gas through the radiative surfaces of the blackbody targets.

The sponsoring organization for a spaceborne receiver development is

importantly affected by the need to maintain, or to have access to, calibration

facilities that involve the use of microwave blackbody targets. To this end,

a large, highly specialized, and very expensive inventory of environmental

test equipment, thermal.-vacuum facilities, and cryogens and their dispensers

must be maintained as well as personnel who are trained in its uGc.

iv
It seems that no one person is ever really comfortable in dealing with

the many and diverse technologies that are embraced by the use of blackbody

targets, especially in an operation where high precision and great accuracy

are required--even the most experienced worker feels, at one time or another,

that his sphere of expertise has been exceeded.

When one considers that the operating requirements for a receiver

calibration sequence involving microwave blackbody targets sometimes specify

that the thermal gradients in a large radiating surface (e.g., 900 cm 2 , or

more) be maintained to less than 0.05 K at any specified temperature setting

from 80 to 320 K, and with the expectation that a controller will circulate a

capricious cryogen in both its liquid and heated gaseous phases in doing so,

it is of little wonder that a truly successful calibration sequence is viewed

by some technicians with considerable awe: Some feel that, at the very least, 

concentrated distillates of legerdemain, blackmagic, and witchcraft--arriving 

from somewhere--must have surely influenced the outcome.

v
vi

--affiLab 

ACKNOWLEDGMENTS

The construction and applical'.on of microwave blackbody targets used to

test and calibrate spaceborne receivers are relatively new and truly esoteric

technologies.

The f a who use them in practice will agree that successful application

is bounded mainly by hands-on experience.

It was long-term, hands-on experience, then, anu' the counsel of Jim

('..J.) Johnston, Pete (F.O.) Olson, Fred S. Soltis, and Richard S. Iwasaki

that made this monograph possible.
'.BSTRACT

The properties of microwave blackbody targets are explained as they apply

to the calibration of spaceborne receivers. Also described are several

practicable, blackbody targets used to test and calibrate receivers in the

laboratory and In the thermal-vacuum chamber.

Problems with the precision and the accuracy of blackbody targets, and

blackbody-target design concepts that overcome suW^ of the accuracy

limitations present in existing target designs, are presented.

The principle of the Brewster-A • igle blackbody target is described where

the blackbody is applied as a fixed-temperature test target in the laboratory

and as a variable-temperature Large, in the thermal-vacuum chamber. The

reflectivity of a Brewster-Angle target is measured in the laboratory. From

this measurement, the emissivity of the target is calculated.

P.adiatively cooled thermal suspensions are discussed as the coolants of

blackbody targets and waveguide terminations that function as calibration

devices in spaceborne receivers. Examples are given for the design of

radiatively cooled thermal suspensions.

Corrugated-horn -antennas used to observe the Cosmic Background and to

provide a cold-calibration source for spaceborne receivers are described.

vii

jo 
CONTENTS

INTRODUCTION

THE CRvOGEN-COOLED DEWAR TARGET . . . . . . . . . . . . . . . . . . . . 4

LIQUID-NITROGEN-COOLED TARGETS WITH FOAM-LOADED RADIATING APERTURES . . 8

AMBIENT-TEMPERATURE BLACKBODY RADIATOR . . . . . . . . . . . . . . . . . 10

VARIABLE-TEMPERATURE BREWSTER-ANGLE BLACKBODY TARGET . . . . . . . . . . 13

RADIATIVELY COOLED THERMAL SUSPENSIONS . . . . . . . . . . . . . . . . . 15

THERMALLY ISOLATED SUSPENSIONS . . . . . . . . . . . . . . . . . . . . . 20

COSMIC BACKjROUND CALIBRATION HORNS . . . . . . . . . . . . . . . . . . 22

REFERENCE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

APPENDIX: THE PRACTICE AND THE HARDWARE . . . . . . . . . . . . . . . . 29

Figures

1 Cryogen-Cooled Target: Absorber Immersed in Cryogen . . . . 5

2 Dewar-Contained Blackbody Target . . . . . . . . . . . . . . 6

3 Cryogen-Cooler; Target: Foamed, Radiative Surface . . . . . . 9

4 Blackbody Target (Zenith Viewing) . . . . . . . . . . . . . . 9

5 Ambient-T_",.^-ra^ure, Brewster-Angle Target . . . . . . . . . 11 

6 Absorber Tiles With Brewster-Angle Prisms . . . . . . . . . . 11 

7 Microwave Blackbody Target: Reentrant Pyramid . . . . . . . 14

8 Variable-Temperature, Brewster-Angle, Blackbody Target

With Cooling Coils Attached . . . . . . . . . . . . . . . . . 14

9 A Two-Channel Receiver With Radiatively Cooled

Waveguide Loads . . . . . . . . . . . . . . . . . . . . . . . 17

10 Radiatively Cooled Target Concept . . . . . . . . . . . 18

11 Thermally Isolated Suspensions . . . . . . . . . . . . 21

12 Corrugated Horn Feed With Extremely Lcw Sidelobes

and Backlobes . . . . . . . . . . . . . . . . . . . . . . . . 24

13 Corrugated, Conical Feed-Horn Pattern . . . . . . . . . . . . 24

14 Receiver Instrument With UVI Domes . . . . . . . . . . . . 26

15 A Cluster of Three Cosmic Background Calibration Horns . . 27

16 A Facing View for the Cosmic Background Calibration Horns . 27

?RM^VIN(; PAGE BLANK NOT FRigF,)

ix

W_
A-1 One Type of Thermal-Vacuum Chamber . . . . . . . . . . . . . 30

A-2 Reentrant Blackbody Target in the Geometrical

Figure of a Wedge . . . . . . . . . . . . . . . . . . . . . . 3

A-3 A Reentrant-Wedge Target . . . . . . . . . . . . . . . . . . 32

A-4 Articulated Antenna Assemblies With Blackbody Targets . . . . 33

A-5 A Multichannel Receiver on a Fixture Prior. to Test in

the Thermal-Vacuum Chamber . . . . . . . . . . . . . . . . . 34

A-6 Blackbody Targets With a Flat Figure and Absorptive Prisms

Nachined on Their Surfaces at the Brewster Angle . . . . . . 35

A-7 Microwave Blackbodies Disposed as Waveguide Terminations 36

A-8 A Three-Channel Receiver With a Radiatively Cooled

Thermal Suspension for Cooling Blackbody Targets . . . . . . 37 

A-9 Deployr.ient of Radiatively Cooled Thermal Suspensions . . . . 38

A-10 Cold-Calibration Horns Assembled on a Multichannel

Receiver With an Articulated AiA enna . . . . . . . _ . . . . 39

A-11 A Cluster of Cold-Calibration Horns Deployed and

Ready for Launch . . . . . . . . . . . . . . . . . . . . . . 40

x
INTRODUCTION

This report is concerned with the design and practical o peration of

microwave blackbody targets as they are applied to quali`.y, test, and

calibrate spaceborne receivers.

Microwave blackbody targets are conceived as thermal-mechanical-electrical

structures that confcrm to the requirements for blackbody radiation. In

practice, blackbody targets are positioned at the input ports of receivers to

simulate signals and to test input-output relationships.

The average power (flux) radiated by a microwave blackbody target is

expressed by 

PTar (v ,T) = by W/Hz (1)

KT

e - 1

where

h = 6.62520E-34 joules, Planck's constant.

k = 1.38046E-23 joules, Boltzmann's constant.

T = Temperature of -he blackbody in kelvins.

v = Frequency in hertz.

T'he flux radiated by a blackbody is a nonlinear function of its

temperature. The deviation from linearity is expressed by the exp (hv/kT)

term in (1).

A blackbody target that operates at 300 K (ambient temperature) radiates

a flux level of 412.91 fW into a predetection bandwidth of 100 Mhz, which is

centered at 37 GHz. At a target temperature of 77.36 K (ligl'id nitrogen), the

flux level reduces to 105.57 fW. The nonlinearity is slightly less than 1%

over this temperature range.

1
A trivial mathematical manipulation simplifies and reduces (1) to

P. ? kT W/Hz (2)

_ar

for the case where b y /kT is small, as it is in the microwave region.

Expression (2) is a very good approximation of (1). The importance of

(2) is that it gives a simple, conceptual, operating relationship between

radiated flux and blackbody temperature.

By the princi-)le of reciprocity, blackbodies absorb and radiate microwave

power. The emissivity is equal to 1. The radiated flux is randomly polarized.

The only natural blackbody is the Cosmic Background, whose measured

temperature_ is 2.7 K with i 1-sigma uncertainty of 0.15 K (circa 1980).

One important application of the blackbody target, as it is used to

qualify spaceborne receivers, is to produce a variable signal that moves over

a prescribed interval of the dynamic temperature range of a receiver.

Blackbody targets are sometimes used as signal sources to produce variable

flux levels that are applied to the input ports of receivers. From the

input-output responses of a receiver, the mathematical form of the transfer

function may be derived.

Receivers produce internal noise waveforms whose statistical properties

are characterized as stationary. That is, the mean and variance are

significantly invariant with time. The signals that receivers must deal with,

howe^cr, are characterized by nonstationary statistics; that is h e mean,

F,andard deviation, and variance change with time depending upon the character

and the pattern of the media being observed.

Noise waveforms with nonstationary statistics can be simulated in the

laboratory by a blackbody target that radiates variable flux levels. For

example, as the temperature of a blackbody target is lowered from ?00 to 80 K,

the mean and the variance of the radiated noise waveform decrease even as the

process progresses. To this extent--albeit imperfect--a nonstationary

noiselike signal is simulaLed.

2
Microwave blackbody targets importantly serve as secondary temperature:

standards for use in the laboratory; mainly, they are cooled by liqui-1

nitrogen, solid carbon dioxide, and ice water. Where elevated emission

temperatures are desired to produce higher flux levels, the radiating surfaces

of targets are affixed with electric heaters. Alternatively, some are

designed to circulate hot inert gases.

Typically, the emissive surfaces of targets are imbedded with very

precise thermometers, such as platinum-resistance el(. 2nts or thermisters.

Blackbody targets are frequently contained within the mechanical

structure of a receiver to functior, s a stable reference temperature or an

internal calibrator.

Variable-temperature blackbody targets are operated in a vacuum chamber

to prevent icing and frosting of their emissive surfaces. In practice,

variable-temperature targets are required where it is necessary to chance the

temrerature of the emissive surfaces (e.g., from 80 to 320 K) and to

accurately and stably set the operating temperature :a• ywhere within a r;;n^-,e.

From this capability, the inpat-output transfer function of the receiver, or

its linearity, may be determined.

Sometimes in the testin 7 and verification of spaceborne receivers,

blackbodies may, of necessity, have to interact with one another. This is

especially true for blar':budfes contained within a microwave structure, such

as an RF modulator switch, or in any low-loss microwave cavity where

blackbodies are inextricably coupled together by their electromagnetic

fields. In particular applications, blackbodies may be in direct view of one

another and in addition be coupled by their elecLromagnetic fields.

For these interactive situations, it is informative and useful to

consider g hat blackbodies are controlled by the Laws of Thermodynamics in the

manner in which they radiate and absorb blackbody radiation.

3
When two blackbodv radiators are in thermal equilibrium with a third,

they must also be in thermal equilibrium with each other. This is a

consequence of the Zeroth Law of Thermodynamics. A practical manifestation of

tt,.s law is suggested by the example of an RF modulator switch that contains

more than one blackbody termination, with the expectation that all of the

blackbodies are mutually coupled by at least their electromagnetic fields and

some ma y also have a direct view of one another. Ii for any reason one or

more of the blackbodies changes its temperature in any way, the temperatures

of all other blackbodies will be ineluctably affected in a matter of degree.

An actual example fur this case is where solar irradiance propagated through

an open-hole aperture of a waveguide section and caused local heating of one

of the blackbody terminations in an RF switch. Because of this, the output

response of the radiometer was affected in a spurious way by the corresponding

change in temperature of a different blackbody termination within the body of

the switch structure.

Microwave flux is transferred from a blackbody at a higher temperature to

one at a lower tempera t ure, and never the other way around. Flux that is

intercepted by the lower-temperature blackbody can never return to the highertemperature blackbody. This is a restatement of the Second Law of

Thermodynamics as it applies to blackbodies. A practical example of this law

is suggested when two reentrant-pyramid targets are oriented to face each

other. If one target is elevated in temperature with respect to the other,

the target at the higher temperature radiates flux--the lower temperature

target absorbs it.

THE CRYOGEN-COOLED DEWAR TARGET

Tile most common laboratory target structure is the cryogen-cooled Dewar

containing a microwave absorber as the blackbody radiator. See Figures 1 and

2. In this concept, the antenna, or, more typically, the feed aperture of the

receiver, views the absorber through the liquid-nitrogen cryogen. The 

emission emperature of the blackbody radiator immersed in the liquid nitrogen

is given by the boiling temperature of the liquid nitrogen, which is also

affected slightly 1 , atmospheric pressure:

4
REFERENCE-TEMPERATURE I CIRCULATOR

LOAD 1L SWIICH

U

ANTENNA

METAL FOIL

CRYOSTAT

WA!_L

F

CRYOGEN, LN2"

'BLACKBODY

'RADIATOR:•

Figure 1. Cryogen-Cooled Target: Absorber Immersed in Cryogen

5

w
r

0P1GOVAL FMA- 'S

OF POOR QUALITY

Figure 2. Dewar-Contained Blackbody Target

The practical embodiment of a cryogen-cooled (LN 9 ), microwave, blackbody

target, as it is contained in a Dewar for general-purpose laboratory use.

A flat, blackbody target is positioned on the bottom of the Dewar where

it is viewed by a feed horn (antenna). A ray passing between the blackbody

target and the feed horn executes a 90-degree turn at the flat mirror and

thereafter passes through the microwave-transparent, white-foam window.

The cylindrical diameter of the Dewar and the window diameter for the

feed (in the frustrum of the cylindrical cone) are 50 and 18 cm, respectively.

On the lower deck of the table, and in the geometrical figure of a

rectangular prism covered with white foam, is a chamber where the liquid

nitrogen is heated to form a dry gas that continuously purges the volume

within the Dewar above the level of the liquid nitrogen.

The electric, heat-controller unit for the purging nitrogen gas is shown

on the top deck--it is set to maintain the temperature of the gas at

approximately 268 K.

6 
TLN2 = 77.36 + 0.011 (P a - 760), K

where Pa is the ambient pressure of the atmosphere in mm of mercury.

The accuracy of the cryogen-cooled, Dewar-type target is affected by

several practical problems that diminish the target's general acceptability.

The space above the liquid nitrogen must be free of all moisture to prevent

the formation of frost and ice crystals on the surface of the liquid nitrogen

and on the sides of the Dewar. In practice, it is constantly purged with dry,

warm, nitrogen gas.

Another problem is that the emission from the reference-temperature load

or other dissipative components in the front-end of the receiver is reflected

from the dielectric boundary layer of the liquid nitrogen and reenters the

receiver as an error contribution to the blackbody radiation from the

absorber. This component is shown by bold arrows in Figure 1.

The dielectric constant. of liquid nitrogen a*. microwave frequencies is

1.454.

The power reflected from a dielectric in air at normal incidence is given

by

1 - rc 2

P reft 1 + e (3)

where

c = the dielectric constant

The power reflection -oefficient for liquid nitrogen at normal incidence

is 0.0087, which proGuces an additive 2.5-K component to the nitrogen-immersed

blackbody radiator, assuming that the temperature of the passive radiation

element in the receiver is in thermal equilibrium at 290 K.

7

a J
Another annoyance provoked by the reflection propensities of both the

cryogen liquid and the imperfect absorber is the incident local oscillator

power emanating from the feed. Where local oscillator emissions and their

harmonics are not effectively suppressed, or are internal to the receiver, or

averted in some manner, the accuracy of the target becomes suspect.

Because of the problems associated with icing in the upper chamber area,

dielectric reflections at the air interface, and the imperfect properties of

flat absorbers, some investigators have abandoned the LN 2 cryogen-cooled

target. Nevertheless, for purposes of setting gain-scale factors in receivers

and for estimating temperature resolution, it is still retained as a

practicable and low-cost embodiment of a cooled target.

LIQUID-NITROGEN-COOLED TARGETS WITH FOAM-LOADED RADIATING APERTURES

Two variations of the laboratory-type, cryogen-cooled, blackbody radiator

(Figure 1) are shown in Figures 3 and 4. These target configurations have

advantages, mainly because the air-dielectric interface is eliminated in the

path between the blackbody radiator and the feed. The radiation from the

reference-temperature load and the local oscillator in the receiver is also

absorbed without reflection.

The reentrant-pyramid figure for the blackbody radiator is superior to

flat radiators because surface reflections are importantly absorbed by

multiple reflections. 

The foam on the surfaces of the absorbers suppresses the formation of ice

and frost.

1 An innovative approach for the reduction of reflections from the dielectric,

and for improving the transfer of power in the air-liquid nitrogen to the

blackbody interface, is conceived by E. J. Johnston (JPL). In this concept,

the surface of the dielectric is agitated with a periodic or aperiodic waveform to improve the impedance match at the air-liquid interface.

8
0

o ^o o ° 0 0

0 0 ° 0 0 0

I

-, I

WR AivAL. PAi:e, w

OF POOR QUALIT`c'

^- WAVEGUIDE

ANTENNA

METAL FOIL

VENT 

DRY-NITROGEN

PURGE GAS

CRYOSTAT

LOW-LOSS FOAM

FERRITE-PRISM

ABSORBERS

PLATINUM (REENTRANT-CONE

RESISTANCE OR PYRAMID)

THERMOMETERS LN2 CRYOGEN

TARGET SUPPORT

MATERIAL

(FOAM)

Figure 3. Cryogen-Cooled Target: Foamed, Radiative Surface

FOAM DEWAR WITH METAL LINERS

CONICAL TARGET

METAL BASE PLATE

FOR ABSORBER

FERRITE-PRISM

ABSORBERS

MUTIFREQUENCY

FEED HORN

IOMETER

kbody Target (Zenith Viewing)

9

LN2 CRYOGEN

^- METAL I I NER S

— PLATINUM RESISTOR

TEMPERATURE SENSORS

FOAM-LOADED INTERIOR

WARM AIR

PLENUM

\METAL FOIL SHROUD
1

a

Both linear polarizations in a dual-1 'arized feed are absorbed by the

reentrant pyramid design, albeit imperfectly and not identically. The

reentrant pyramid is importantly adaptable to the principle of absorption at

the Brewster Angle.2

' 1t

The Brewster Angle is expressed by 0 B = tan Notably, the

Brewster-Angle absorption conce p t is implemented both in the prisms that are

machined on the absorber surface and in the taper of the walls to the apex of

the pyramid.

AMBIENT-TEMPERATURE BLACKBODY RADIATOR 

A general-purpose blackbody target that is mainly intended to be viewed

I by a feed aperture for laboratory tests at ambient temperatures is shown in

Figure 5.

The geometrical configuration of the target is a reentrant pyramid with

ferrite-epoxy absorber elements serving as the emissive surfaces. Each

absorber element that comprises the target area is individually shaped to

absorb energy at the Brewster Angle. The dielectric constant of the material

in this particular design is = 20.

The walls of the pyramid are also fixed at the Brewster Angle with

respect to the incident radiation to further enhance the absorptivity of the

target on multiple reflections. The figure of a reentrant pyramid was chosen

over a conical figure mainly for ease of construction and lower cost.

i

The Brewster-Angle absorber eL=ments are moulded as ferrite prisms on

aluminum plates to form a tile as shown in Figure 6. The the dimensions are

10 cm on a side, as illustrated. The tiles are cut and fitted to form the

I

i

2The concept and development of the Brewster-Angle Reentrant-Pyramid Target,

as shown and discussed here, is credited to R. S. Iwasaki (JPL).

I

10
"Page missing from available version"
1 

sides of the pyramid. Heavy capper plates form the sides of tte pyramid and

the absorber tiles are secured to them by machine screws. To improve the

thermal contact between the aluminum baseplates of the absorber tiles and the

copper walls, thermal grease is applied.

I

The tile structure, as shown in Figure 6, has been elaborately tested to

survive repeated thermal cycling from 320 to 80 K. Very cursory tests also

show that the tile survives repeated shock immersions into liquid helium

without apparent damage to its physical structure. From these preliminary

ltests, the tile shows promise as a candidate absorber for a variable—

temperature blackbody to operate at helium temperatures, i.e., 4.2 K. 

Platinum resistance thermometers are imbedded into the apexes of the

ferrite prisms to improve the measurement accuracy of the thermometric

temperature of the ferrite material near the prisms - absorbing surfaces. The

target may be adapted for operation at elevated temperatures by affixing

heater elements to the copper _`aceplates.

The measured emissivity of this target is >0.9995 as based on VSWR

measurements at both horizontal and vertical polarizations. 

The power reflection coefficient is expressed by

2

P =

VSWR —1 (4)

refR VSWR +1

where

1 VSWR is expressed by a number greater than 1.

i

Emissivity is defined by

E = 1 — P refR (S)

12
D 1 

.1

'I

1

I

The emissivity is computed from a V IJWR measurement of 1.05 at 5.6 GHz

where the VSWR measurement was determined to be the residual VSWR measurement

i

capability of the reflectometer.

VARIABLE-TEMPERATURE BREWSTER-ANGLE BLACKBODY TARGET

A variable-temperature target configuration is adapted from the

room-temperature target shown in Figure 5. The details of the structure, when

affixed with cooling coils and when instrumented by platinum-resistance

thermometers, are shown in Figure 7. Typically, about 16 platinum

thermometers are imbedded near the apexes of the ferrite prisms, as shown

within the pyramid structure. In practice, an equilibrium temperature is

declared when the therr,:jmeters Stabilize to less than 0.1 K of each other.

The outside copper walls are affixed with coils for circulating nitrogen both

in the liquid and heated-gas states. A key element in the variable target

design is the servocontroller that regulates the temperature and the flow rate

of the nitrogen through the coils. The controller permits a specified

temperature to be set, and the platinum resistance thermometers that are

imbedded in the prisms serve as the feedback element to stabilize the target

at the prescribed temperature setting. Higner target temperatures are

achieved by circulating heated nitrogen gas through the coils. Because of the

thermal inertia of the target, an hour or more is required for the target to

stabilize to a target-temperature change of 25-K in the thermal-vacuum

chamber. The target, as shown, operates at any specified temperature from 80

to 325 K. The accuracy of the setting is determined by the platinum resistance

thermometers operating as feedback elements.

The target operates in an air-evacuated (thermal-vacuum) environment to

prevent frosting cif the emissive surfaces.

The use of the variable temperature target is sine qua non for the

measurement of subtle nonlinearities present in the signal transfer responses

of microwave receivers.

13

1 fir% •_ _
e = 2011^ ^Zj^

SIDE INSIDE VIEW

VIEW

OF

PRISM

GEOMETRY

OF POUR 4UAL-if, ti

COOLING COILS (LN 2 CRYOGEN)

(COVERED BY STYROFOAM)

PIGTAILS THERMOMET`_D ACCURACY,

O.US K (lo ), SECU IED I N PLACE

BY THERMAL GREASE.

Figure 7. Microwave Blackbody Target: Reentrant Pyramid

Figure 8. Variable-Temperature, Brewster- An oiu , Blackbody `target With

Cooling Coils Attached

14

.fir — s • • ,_ -_ --__ - - -- ^ 
Figure 8 depicts a multifrequency feed assembly that is positioned near

the opening of a reentrant-pyramid blackbody target. The feed is wideband and

operates over 2.5 octaves in frequency and at five discrete frequencies with

both horizontal and vertical polarization at each frequency--basically, a

ten-channel feed assembly. The figure illustrates the methodology for

calibrating all ten channels of a spaceborne receiver, simultaneously, by

using the variable-temperature blackbody radiator.

The temperature of the target is varied from 350 to 80 K by circulating

liquid nitrogen gas through the coils that are affixed to the outside walls of

the reentrant-pyramid target. The target is operated in the vacuum chamber

where the temperature increments are changed (typically) in 25-K steps over

the total temperature range of the target. For any particular temperature

setting of the target temperature controller, the emissive surfaces of the

target are declared to be in equilibrium when the platinum thermometers are 

within 0.1 K. The accur-icy of the platinum thermometers is traceable to a

primary temperature standard.

The emissivity of the absorptive surfaces of the target is greater than

0.9995 based on a 6.6 GHz measurement.

RADIATIVELY COOLED THERMAL SUSPENSIONS

The signal or antenna temperatures received by Earth =orbiting receivers

commonly range from about 100 K when viewing smooth water surfaces to about

310 K when viewing hot land masses. To calibrate this range of signal

temperatures, it is desirable to provide calibration temperatures that are

within the same temperature range as the signal temperatures. In this way,

extrapolation distances are minimized and the curvature in receiver

input-output responses is more closely accounted for. The need then arises

for a cold-calibration temperature that operates suitably within this rang

The radiatively cooled blackbody target emerges as a candidate calibration

source for the circumstances where calibration temperatures are satisfied

within the range from 100 to 200 K. The radiative:y cooled blackbody target

15
concept, as it is adaptable to serve the needs of a cold-calibration source,

is illustrated in Figure 9. This is, in fact, an operational, radiatively

cooled blackbody that is designed to cool several waveguide terminations. The

waveguide termination sections are clamped to the aluminum plate (18 x 18 cm)

that is in turn cooled by a radiative cooler (infrared radiation) in the form

of a matrix of two-surface mirrors. The mirrors operate as a cooler to

transfer heat in the infrared to the Cosmic Background by the principle of the

Second Law of Thermodynamics.

The term "radiatively cooled" deserves more explanation. In the context

that the term is used here, the heat in the metal plate (a thermal suspension)

is transferred to the Cosmic Background by the principle of the Second Law of

Thermodynamics. That is, the heat is always transferred from a warm to a cold

body and never the other way around.

Stainless-steel waveguides are affixed to the waveguide terminations on

the plate to minimize thermal conduction from the plate to the receiver

chassis. The plate itself is supported by long thermal insulators in the form

of fiberglass rods.

A more efficient design from the standpoint of providing lower operating

temperatures is shown in Figure 10. Tn this concept, the radiatively cooled

plate supports microwave targets that thermally equilibrate to the plate

temperature. The important consideration illustrated here is that the

waveguide transmission lines are not in physical contact with the plate or the

cooled targets and the conductive thermal path is therefore eliminated. The

plate can, therefore, be reduced to lower calibration temperatures. A variety

of targets and target configurations can be affixed to the plate to

accommodate several types of target-viewing antennas.

One interesting target configuration is illustrated in Figure 10; it is

in the form of a reentrant cone with tapered prism corrugations that run

longitudinally along the inside cone surface. The absorptive material in the

target is machined to absorb the incident radiation at the Brewster Angle both

16
ON POOR

mo

STAINLESS-STEEL

(GOLD - PLATED)

WAVEGU IDES --

a

COQ L* ED WAVEGU I DE

TERM INAI IONS ------

to

.mb

FI BER GLASS j

SUPPORT RODS--

. ^.

Figure 9. A Two-Channel Receiver With Radiatively Cooled Waveguide Lo--.d.,,

17
PLATINUM

THERMOMETER

MOUNTING

SCREWS

FERRITE-EPDXY

CONE

E=20

RADIATIVELY-COOLED

PLATE

— REENTRANT-CONE,

BLACKBODY TARGET

TWO-SURFACE

•^

MIRRORS

WAVEGUIDE 1

LOW-EMISSIVITY

THERMAL BLANKET

WAVEGUIDE 2

E

_ HEAT RF-SWITCH

RADIATION MIXER IF ASSEMBLY 0n I

TYPICAL

INSTALLATION

,

,

•

(^

U^=-w - WAVEGUIDE N

LOW-EMITTAN _E,

SUN SHIELD ^

THERMAL INSULATOR

SUPPORT

"___ CHASSIS

REENTRANT-CONE

TAPERED-PRISM

CORRUGATIONS

BREwl . LR-ANGLE

CONE TAPER

COLD-LOAD TARGET

BREVISTER-ANGLE

TAPERED FERRITE

PRISMS

2" DIAM

24.6° -;--J

24.6°

0 F v 0

TARGET 'DETAIL

Figure 10. Radiatively Cooled Target Concept

18
in the cone angle and in the apex angles of the prisms. Alternatively, where

a less-efficient target is tolerable, flat prism sections as shown in Figure 6

are simple and sometimes suitably efficient. By expectation, the conical

target configuration can provide emissivities greater than 0.999 in the

microwave region. The flat target is considerably less absorptive and is more

polarization sensitive.

A low-emittance sunshield is required to direct the infrared radiation

from the mirrors in a preferred direction to avoid intercepting the infrared

irradiances from structures on the spacecraft and to minimize the occurrence

of solar irradiance on the cold plate itself.

The radiatively cooled blackbody target concept, as shown in Figure 10,

may provide cold-calibration temperatures in the range from 150 to 200 K.

Actually, the radiatively cooled target is somewhat resistant to solar

irradiance because the two-surfaced mirrors offer high reflectivity to

incident sunlight, when and if this may briefly occur in a given orbit. Even

so, if the solar irradiance is sustained uniformly on the mirrors for a length

of time, the temperature of the targets will be affected.

The radiatively cooled blackbody target is invulnerable to manmade radiofrequency interference arising from its companion spacecraft sensors or from 

ground-based transmitters. This is a considerable advantage and may

importantly influence the adoption of this class of calibration device in

certain applications.

A critical design cai,sideration in the construction of any cooled

suspension (plate) is to ensure that the thermally conducting paths from the

plate to the instrument ^hassis are minimized. In fact, it is this very

feature that controls the minimum temperature that can be achieved by the

plate.

19
The radiatively cooled plate' shown in Figure 9 is severely penalized

for achieving a temperature lower than about 200 K because the large stainlesssteel waveguides that interconnect the plate with the chassis conduct an

undesirable amount of heat into it. Also, the long fiberglass rods that

provide mainly the thermal insulation are cumbersome and undesirable because

they adversely affect the vibration properties of the cooled structure.

THERMALLY ISOLATED SUSPENSIONS

A superior design for a radiatively cooled plate is shown in Figure 11.

The cooled plates (thermal suspensions) are shown suspended by monofilament

fiberglass bands, which provide the thermal isolation from the base mounting.

When the bands are firmly tensioned, the plates become a very stiff mechanical

structure. It is stable in six dimensions and will qualify for the most

stringent shake and vibration tests. Actually, this particular suspension has

been environmentally qualified and flown in space with thermoelectric coolers.

In the practical implementation of the thermally isolated suspension, one

side of the plate is cooled by either a thermoelectric device or by radiation

to the Cosmic Background by the principle of the Second Law of Thermodynamics.

Second-law coolers operate in the infrared. When second-law coolers are

attached, they must also be supplied with a sunshield and properly oriented so

that the radiation from the cooler is not obstructed by spacecraft structures.

3 The radiatively cooled thermal suspension (plate) shown in Figure 9 is

credited to James W. Stultz (JPL). This device cools waveguide terminations

on a multichannel spaceborne receiver that has been in orbit for over a

decade; it has consistently maintained a waveguide termination temperature

of 200 K +1 K (3 sigma).

4 The radiatively cooled thermal suspension as it is illustrated in Figure 11

is crediter' to Robert M. Bamford (JPL).

1 20
URIGWAL PA(A IS

OF POOR QUALITY

Pure 11. Thermally Isolated Suspensions

21 
Thermally isolated suspensions and their associated cooler mechanisms are

capable of producing stable temperatures in the range from 100 to 200 K

depending upon a host of factors such as the size of the targets to be cooled,

the type of cooler me(-hanism, and even the way the suspension itself is fitted

to the instrument that carries it.

COSMIC BACKGROUND CALIBRATION HORNS

The Cosmic Background calibration horn is manifestly the most widely used

of all cold-calibration devices as evidenced by its prevalent usage on

spaceborne receivers. The mechanical structure of the horn is simple and it

possesses the additional attributes of low weight, low volume, and in a

relative sense as compared to other cold-calibration devices, low cost.

Because the temperature of the Cosmic Background is so low (2.7 K), the

dissipative losses within the calibration horn and the transmission components

that follow it importantly determine the magnitude of the calibration

5' temperature as it is introduced into the input terminals of the spaceborne

receiver.

As an example, consider a Cosmic Background calibration horn that is

connected to the input terminals of a spaceborne receiver through a short

length of waveguide. If the dissipative losses of the horn and the waveguide

are both only 0.2 dB, and if they are maintained at a temperature of 300 K,

the 2.7-K temperature of the Cosmic Background appears as a cold-calibration

temperaCure of 29 K at the input terminals cf the spaceborne receiver.

One disadvantage of using the Cosmic Background as a calibration source

is that the 2.7-K temperature is cumbersome and expensive to simulate in the

laboratory, or in the thermal vacuum chamber--and especially if a variabletemperature target is used. Liquid helium is required to produce a 2.7-K

temperature in the test chamber.

22

It
MY

In practice, because of the technical difficulties and costs of simulating

the Cosmic Background temperature in the laboratory, it is typically not done.

Liquid-nitrogen temperature targets (80 K) are substituted as the lowest

calibrated temperature with the expectation that the spaceborne calibration

accuracy of the receiver is compromised to some indeterminate degree.

In the practical installation of the cold-calibration horn on the spacecraft, considerable attention is given to maintain its field of view in 2

preferred attitude that sustains an unobstructed view of the Cosmic Background.

This requirement is more easily achiev-d if the calibration horn projects a

very narrow beamwidth. Further, the sidelobes and backlobes of the horn must

be extremely low to prevent spurious radiation from the spacecraft and the

_j Earth from entering the calibration horn.

The corrugated horn feed meets all of these requirements well. An

i

.i improvement on the conventional corrugated horn is an extension to the

corrugations around the feed aperture; this extension sustains the length of

the corrugations and operates as an RF choke section to further reduce surface

currents. See Figure 12.

The choke sections, in combination with a compatible design for the feed

and its internal corrugations, operate to drastically reduce the sidelobe

levels, especially at the far-angle sidelobes. This is of considerable

importance in the cold, calibration-horn application to minimize spurious

emissions and radiations from the spacecraft, the earth, and from the sun.

The calculated and measured antenna patterns (Ref. 1) for the corrugated horn

(Figure 12) show sidelobe levels (Figure 13) that reduce to -80 and -90 dB at

off-axis angles exceeding 90 degrees.

When the calibration horn suffers an exposure to solar irradiance at some

time in the orbit, it requires elaborate protective measures. Ultraviolet- 

visible-infrared (UVI) irradiances produce extraordinary heating and thermal

gradients in the machined s • irfaces of the feed. Ultimately, by thermal 

conductivity, heat is transferred into the waveguide structures and RF switches 

23

a 
Figure 12. Corrugated Horn Feed With Extremely Low Sidelobes and Backlobes

0

10 -7--- 1 1 -------7V --- T------ nnrnciiorn

%OKA'L. PF

F POOR QllP

30

m

a

s 40 W

a 50

W

'- 60

70

80

9U

100

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

ANGLE, deg

Figure 13. Corrugated, Conical Feed-Ilorn Pattern (Low-Sidetohe Sky Horn)

24

1V

20
that control calibration and signal inr.ensities. UVI irradiances that enter

the collecting cone of a machined aluminum horn may increase the surface

temperature by several tens of degrees in a few minutes of time. In addition,

UVI irradiance may propagate down the infrared reflective inside walls of the

waveguides and affect the temperatures of waveguide terminations and RF switch

elements. All of these effects militate to degrade the calibration accuracy

and to vitiate the measurement of signal temperatures.

The vulnerability of the cold-calibration horn to UVI irradiance is

eliminated by the installation of a UVI-dome as shown in Figure 14.

Dome structures are fabricated from materials that are transparent to

microwaves and resist the penetration of UVI. Notably, and importantly, the

UVI-dome covers all metallic surfaces of the sky horn and the signal feed.

Multichannel receiving systems that require individual cold-calibration

horns can add considerable complication to the RF section of the instrument. 

Figures 15 and 16 illustrate the microwave component interactions of the cold

calibration horns, their attaching waveguides and coaxial cables, and the

multifrequency signal horn. In this multifrequency receiver application, five

frequencies are serviced by three cold-calibration horns. Several of the

horns combine two fr,2que«cies.

25
I

OF POOR QUALITY

y

. T

' t ^

r

t

Figure 14. Receiver InsLrument, With UVI Domes

I

26
UkIG1fdHL {..^ ,;. ,

OF POOR QUALil

Figure 15. A Cluster of Three Cosmic Background Calibration Horns are

Shown on the Left. The two large horns each operate at two

frequencies. The multifrequency feed that illuminates an

offset parabolic antenna is shown on the right.

Figure 16. A Facing View for the Cos-nic Background Calibration Horns

27

a
REFERENCE

1. M. A. Janssen, S. M. Bednarczyk, S. Gulkis, H. W. Marlin, and G. F. Smoot,

"Pattern Measurements of a Low-Sidelobe Horn Antenna," IEEE Transactions

on Antennas and Propagation, Vol. AP-27, No. 4, July 1979.

1

i

28

r
API NDIX A

THE PRACTICE AND THE HARDWARE

Blackbody targets, radiatively cooled thermal suspensions, and antennas

that view the Cosmic Background (sky horns) serve to calibrate spaceborne

receivers.

In practice, blackbody targets are generally cooled; occasionally they

may serve as reference-temperature standards that operate at ambient or at

elevated temperatures.

Depending upon the need, blackbody targets may be located either within

the spaceborne receiver, as a component, or externally where they are viewed,

on command, by an articulating antenna.

This appendix shows actual applications of blackbodies where they serve

as calibration devices and operate in the environment of space..

•

29
ORIGIV.HL PAC --

OF POOR QUALITYL

riguru A- I. Ont Type of Thermal- `JCUUm Ghamher.

In l.rinciple, the thermal-vacuum chamber simulates the vacuum and thermal

environment of space as it will be experienced by an instrument on a

spacecraft.

Ithe thermal-vacuum chamber shown here is a cylinder with discoid-type end

caps. The caps are attached to the body of the cylinder by steel rods. The

rods slide into cylindrical bores within the body of the cylinder and permit

the end caps to be retracted for access to the chamber or closed to seal it

off.

This chamber is 3 m in diameter and 3 m long. It is regarded as

intermediate in size. The test instrument mounted on the plat f orm will be

positioned within the chamber when the end cap is clo-jed. Coils that

circulate a liquid cryogen are secured to the inside body of the cylinder.
GRIGHNAL PA(*

OF POOR QUALITX

Figure A-2. Reentrant Blackbody Target in the Geometrical Figure of a

Wedge.

The target is an adaptation of the reentrant pyt:•.nid. The advantage of

the wedge is that is extends the blackbody surfaces especially in one

dimension so that manv feeds or antenna assemblies that occur in a line array

or in clusters may view the same target simultaneously.

The absorptive surfaces of the reentrant wedge target are constructed

from ferrite-loaded-epoxy prisms oriented to intercept the incident microwave

radiation at the Brewster Angle. The entire aperture of the target is filled

with a white foam that is transparent to microwaves. The foam prevents

frosting of the absorptive surfaces and also keeps then clean.

The apex angle of the sides of the wedge is also formed at the Br-caster

Angle. The reentrant wedge--similar to the reentrant pyramid--has the

advantage of absorptive properties that are improved by multiple reflections

within the wedge.

Coils are attached to the outside copper surfaces of the target fo: the

circulation of nitrogen in both its liquid and gaseous states. The absorptive

surfaces of the target may be varied in temperature from 80 to 320 K.

31
OF POOR QUALITY

Figure A-3. A Reentrant-Wedge Target (Laboratory Model).

The smail target is shown on the top dick of the table where if is part

of a laboratory test; it is viewed by a receiver with two synchronized and

offset antenna assemblies. The antennas are affixed to each side of the

receiver. The hyperbolic offset reflectors direct the blackbody radiation

from the reentrant-wedge target into the corrugated horns when the antennas

are oriented to view the wedge. When the antennas are rotated to view the

flat blackbody targets that are mounted directly below the reflectors, the

ambient temperature radiation from the flat targets is directed into the

corrugated ho r ns. The fla —disk targets are integral components of the

instrument design- they are affixed with platinum-resistance thermometers and

serve as t e ambient-+_zmperature calibration function for the instrument.

Electric heaters are mounted on the copper sideolates of the wedge to

elevate the temperature oC its radiative surfaces.

The ferrite-epoxy prisms that form the absorptive surfaces of the target

and the copper side plates are both set at the Brewster Angle.

The instrument calibration is performed at two different target

temperatures: ambient temperature (290 K) when the flat targets are %viewed by

the antennas, and 325 K when the wedge is viewed.

32

web, ,
Ur^JvHVAL Pilo

OF POOR QJALI

Figure A-4. Articulated Antenna Assemblies With Blackbody Targets.

The three assemblies are offset from their resrecLive corrugated-horn

fee,is, and their hyperbolic reflectors rotate ar^und the periphery of the feed

aperture. The flat, blackbody targets operate ar_ Ehe ambient temperature of

the instrument.

When operating in space, the antennas are positioned by rotation to view

the Cosmic Background; this technique provides a cold-calibration temperature

for the receivers.

33
Figure A-5. A :Multichannel Receiver on a Fixture Yriot to Test in the

Thermal-Vacuum Chamber.

The receiver is shown with three separate reflectors that functionally

rotate around their respective corrugated feeds. The white-foam material

protects the absorptive surfaces of the targets from solar irradiance.

The blackbody targets are affixed with a multiplicity of platinumresistan-e thernometers to accurately monitor the temperature the

absorptive surfaces and the thermal gradients as they may occur.

When spaceborne, the reflectors rotate to allow viewing of the Cosmic

Background; from this view, the receivers are provided two calibrated

temperatures: 290 and 2.7 K.

OF POOR QUALITY

34
O04GIIXAL RVA*r, iS'

OF PouR QUALITY

M2'.

Figure A -o. Blackbody Target:, With a Flat Figure acid Absorptive Prisms

Machined on Their Surfaces at the Brewster Angle.

Two of the blackbodies are covered with a white, low-loss, protective

cover (expanded polystyrene) to protect the prisms from solar irradiance and

to ke p i) them clean. One target is shown without the Brewster-Angle prisms.

35

I
OF POOR 4

Figure A-7. Microwave Black';odies Disposed as Waveguide Terminations.

The geometry of a blackbody structure is shown as it functions as a

waveguide termination in an RF switch. The blackbody is the disk-cylinder in

the end of a waveguide section that has been unbolted from the body of the

modulator switch for the purpose of viewing it. This particular blackbody

termination, as shown, functions as the reference-temperature load (RTL) in

the modulator switch as it is applied to the needs of a modulated radiometer.

An identical blackbody structure--again in the form of a waveguide

termination--provides an internal calibration Lemperature.

Small variations in the temperatures of these blackbodies critically

affect the performance of a radiometer. For this reason, platinum-resistance

thermometers are imbedded within the absorptive surfaces of the blackbody

through a hole drilled In t!!e metal wall of the waveguide. Sometimes the

thermometers are cemented directly to the outside wall of the waveguide

opposite the blackbody.

The open waveguide section on the left side of the switch shows the

signal-input port.
OF POOR QUA :Tf

Figure A-8. A Three-Channel Receiver With a Radiatively Cooled Thermal

Suspension for Cooling Blackbody Targets.

The aluminum plate (18 X 18 cm) mounted on the top of the receiver

chassis is radiatively cooled by the two-surface mirrors affixed to one of its

surfaces. The mirrors radiate infrared heat from the plate to the Cosmic

Background by the Second Law of Thermodynamics. To facilitate this function,

the plate is a thermally isol.atc!d suspensir^n: it is supported by fiberglass

rods to minimize heat conduction.

Waveguide sections with blackbody terminations are clamped to the reverse

side of the plate--they serve as a cold-calibration temperature for each of

the three receivers. Stainless-steel waveguide sections are used to minimize

heat conduction from the plate.

This particular rhermal suspension maintains the tem perature of the

blackbody terminations in the waveguides at 200 K +1 K while operating in

space.

TLe antenna apertures use fixed hemispherical lenses to shorten the iucal

Length of the antennas, thereby reducing the volume of the receiver. The

boresikht for the antennas is fined at the nadiral path of the spacecraft.

An internal waveguide termination in an RF switch with'i the receiver

provides an ambient-temperature calibration.

37
k^

r j

Ok1GKtL. P,h -

CF Poor? Q'J),Lj

Figure A-9. Deployment of Radiatively Cooled Thermal Suspensions.

A Nimbus spacecraft is shown suspencicJ from an overhead beam in a

spacecraft test-and-assembly area. On the underside of the spacecraft is the

instrument complement in the launch configuration.

Near the outer periphery of the central band of cooiing-louver panel

sections, slightly to the left of center and on the underside, are two

spaceborne receiver modules. One module has two hemispherical lens antennas

and the other has three.

The radiatively cooled thermal suspensions for the two receiver modules

are shown as black rectangles. They are positioned immediately outboard, and

adjacent, to the receiver modules. From this coign of vantage, the Cosmi,

Background is viewed without physical obstructions.

38

OV 
OMtt AL PACk :4,

OF POUR QUAL11Y

Figure A-10. Ccld-Calibration Horns Assembled on a Multichannel Receiver

With an Articulated Antenna.

The figure displays a portion of a :Nimbus spacecraft. Slightly to the

l-ft of center is a multichannel microwave receiver module with an

articulated, parabolic reflector attached to its scanning mechanism by pods.

A cluster of corrugated-horn feeds are seen on the extreme left edge of

the receiver. The collecting apertures of the feeds are covered with white,

disk-shaped, plastic covers that are removed prior to launch.

The cold-calibration horns are positioned for an unobstructed view of the

Cosmic Background when in Earth orbit.

39
1 ! ! 1 if

E VOWij

,

1 ^ ^

I

Figure A-I1. A Cluster of Cold-Calibration Horns Deployed and Ready for

Launch.

A multichannel microwave receiver with an articulated antenna system is

shown in its launch configuration. This is the same receiver shown in the

previous figure, but mounted on a different spacecraft. The receiver section

is located in the lower center of the figure where it is oriented within the

shroud of the spacecraft.

The multifrequency feed assembly projects through a thermal blanket

(painted vilite) where it illuminates the articulated collector. Affixed to

the front of the receiver module are cooling louvers for the entire receiver

module. To the left of center is a cluster of cold-calibration horns oriented

to view the Cosmic Background from orbit.

uF..

40 NASA.-jn-a,m. LA. c.r