N8213300mIIIlIIIHIHUlIIilIII

JPL PUBLICATION 81-81

Microwave Attenuation and

Brightness Temperature Duethe Gaseous Atmosphere

A Comparison of JPL and CCIR Values

Ernest K. Smith

Joe W. Waters

to

August 15, 1981

National Aeronautics andSpace Administration

Jet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena, California

https://ntrs.nasa.gov/search.jsp?R=19820005427 2018-05-15T16:41:06+00:00Z

The research described in this publication was carried out by the Jet PropulsionLaboratory, California Institute of Technology, under contract with the NationalAeronautics and Space Administration.

//

TECHNICAL REPORT STANDARD TITLE PAGE

1. Report No. JPL Pub. 81-81 2. Government kce.ion No.ii

4. Title and Subtitle Microwave Attenuation and

Brightness Temperature Due to the Gaseous

Atmosphere: A Comparison of JPL and CCIE Values

7. Authorb)Ernest K. Smith and Joe W. Waters

9. Performing O_anization Name and Address

JET PROPULSION LABORATORY

California Institute of Technolo_

4800 Oak Grove Drive

Pasadena, California 91103

12. Spom_ingAeency Name _ Addreu

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Washington, D.C. 20546

3. Reciplent's Catalog No.

5. Report Date

_,,_,1_t 15. IqRI6. Pel_orm|ng Organization Code

8. Performing Organization Report No

i

10. Work Unit No.

11. Contract or Grant No.

NAS 7-100

13. Type of Report and Period Covered

JPL Publication

14. Sponsoring Agency CodeRD151 J-643-10-03-01-00

15. Supplementary Notes

16. AlxtractAttenuation by the gaseous atmosphere and brightness temperature

(sky noise temperature in a Eiven direction) due to emission from the gaseous

atmosphere are related and should be derived from a common radiative transfer

program in order to assure internal consistency. The Jet Propulsion Laboratory

has developed a sophisticated but flexible radlative-transfer proEram, which

was used to produce the gaseous attenuation and krightness temperature curves

found in two basic reports o : the 1978 ProceedlnE;s of the CCIR [Cam/t6

Consultatlf International des RadlocommunicatlonE (International Radio

Consultative Committee)]. In this report, a com_arlson of each of these

curves is made and a new set is derived from the JPL program, covering a

frequency range of 1 to 340 GHz.

17. Key Worc_ (Selected by Autk.w(s))

Communications

Meteorology and Cllmatology

Navigation, Detection, and

Countermeasures (General)

Eadar Detection

19. Security Cl,,|f. Wf _k ,epo_)

18. Distribution Statement

Unclassified - Unlimited

20. Security Cl_lf. (of this page) 21. No. of Pages

Unclassified Unclassified 56

22. Price

ABSTRACT

Attenuation by the gaseous atmosphere and brightness

temperature (sky noise temperature in a given direction) due to

emission from the gaseous atmosphere are related and should be

derived from a common radiative transfer program in order to assure

internal consistency. The Jet Prooulsion Laboratory has developed

a sophisticated but flexible radiative-transfer program, which was

used to produce the gaseous attenuation and brightness temperature

curves found in two basic reoorts of the 1978 Proceedings of the

CCIR [Comit$ Consultatif International des Radiocommunications

(International Radio Consultative Committee)]. In this report, a

comparison of each of these curves is made and a new set is

derived from the JPL program, covering a frequency range of i to

340 GHz.

iii

PREFACE

In the fall of 1979 the Jet Propulsion Laboratory (JPL), with

support from NASA's Office of Space and Terrestrial Applications,initiated a feasibility study to see whether it was desirable and

possible to develop from a single master computer program the basic

CCIR [Comite Consultatif International des Radiocommunications(International Radio Consultative _ "__ommm_,tee)] information on atten-

uation and sky noise temperature due to the earth's atmosphere.

it was concluded from internal consistency checks that the needexisted. A search for an appropriate computer code uncovered an

excellent one at JPL for use with the gaseous atmosphere (Waters,1976). It was further concluded that inclusion of rain effects was

premature and should be deferred for the time being.

Informal submissions were made by JPL to the Interim Meeting ofStudy Group 5 (June 1980) and most of the inconsistencies in CCIR

Report 719 (CCIR, 1978a) were resolved at the Interim Meeting.

While inconsistencies in the CCIR brightness-temperature curves of

Report 720 (CCIR, 1978b) were pointed out at that time, new curves

were not yet available. These have now been submitted, in the form

of U.S. Study Group 5 documents, through regular CCiR channels tothe final meeting of CCiR Study Group 5 (Geneva August 24-

Sentember ii, 1981), along with the gaseous attenuation modifications.

When endorsed by the forthcoming CCiR XVth Plenary Assembly, in1982, the new texts (CC!R, 1982a, b.) will officially replace the

existing Reports 719 and 720.

Much of the substance of this text was presented in a paper toURSI [Union Radio Scientifique internationale (International Union of

Radio Science)] Commission F at the National Radio Science Meeting

January 14, 1981 in Boulder, Colorado. At that time other organiza-

tions were invited to submit their programs for comparison. Dr. Hans

Liebe of the Institute for Telecommunication Sciences, National

Telecommunications and Information Administration, subsequentlyoffered his comprehensive nrogram on attenuation and dispersion byatmosoheric gases (Liebe, 1981). A comparison of his results for

iv

specific attenuation due to the gaseous atmosphere with those of

the JPL program and the CClR-proposed revised curves indicates

excellent agreement.

V

1

I.i

1.2

1.2.1

1.2.2

1.2.3

1.2.4

2

2.1

2.2

3

3.1

3.2

3.3

4

4. I

4.2

5

CONTENTS

INTRODUCTION

PURPOSE

BACKGROUND -

The Atmosphere

Gaseous Attenuation

CCIR Gaseous Attenuation Curves ......

Brightness Temperature

INTERNAL CONSISTENCY

THE SCALE HEIGHT TEST

BRIGHTNESS TEMPERATURE--ABSORPTION COMPARISON

COMPARISON OF CCIR AND JPL VALUES

THE JPL PROGRAM ---

COMPARISON OF GASEOUS ABSORPTION

COMPARISON OF BRIGHTNESS TEMPERATURE

CURVE SETS

JPL CLEAR-AIR ABSORPTION

JPL BRIGHTNESS TEMPERATURE

CONCLUSION

i

I

i

i

4

5

6

9

9

I0

ii

Ii

ii

12

-- 13

-- 13

14

--- 15

REFERENCES 17

Figures

I Atmosoheric reduced-equivalent thickness above

indicated altitude

Variation of optical air mass, with elevation angle,

for a ray traversing the atmosphere

21

22

Precedingpageblankvii

CONTENTS(Continued)

Fi@ures

3

4

6

7

8

9

i0

Ii

12

13

14

Specific attenuation by atmospheric gases

Total one-way ze_ith attenuation t,h_'ough the

atmosphere

Theoretical one-way attenuation from specified

height to the toe of the abmosphere

Sky noise temperature (clear air) for surface

pressure of ] atmosphere and temperaCure of 20°C

and a surface water-vapor concentration of 3 g/m 3

Sky noise temperature (clear air) for surface

pressure of I atmosphere and temperature of 20°C

and a surface water-vapor concentration of I0 g/m 3

Sky noise temperature (clear air) for surface

pressure of i atmosphere and temperature of 20°C

and a surface water-.vapor concentration of 17 g/m 3

Apparent scale height for oxygen absorption in the

atmosphere --

Comparison of apparent and true atmospheric

reduced-equivalent thickness from JPL

radiative-transfer program for quotient

Zenith brightness tem_eraZure for 3, I0, and

17 g/m 3 water vapor

Comparison of specific attenuation by atmospheric

gases

23

24

25

26

27

2_

-- 29

-- 30

-- 31

32

Comparison of zenith attenuation by the atmosphere

(clear air)

Comparison of zenith attenuation for various

specified heights

33

34

viii

CONTENTS(Continued)

F_gures

_5

16

17

i8

19

2O

21

22

23

Compar:tson off CCIR and JPL brightness

temperatures for an arid atmosphere 35

Comparison of CCTR and JPL brightness

temperatures for an average atmosphere 36

Comparison of CC!R and JPL brightness

temperatures for a moist atmosphere 37

Specific attenuation (absorotion) from the JPL

radiative-transfer program for 290 K,

7.5 g/m 3 water vapor and dry (02 ) atmospheres 38

Total one-way zenith attenuation from the JPL

radiative-transffer program for: the 1976 U.S.

standard model atmosphere_ 7.5 g/m 3 water vapor

(2-km scale height) and 02 atmospheres -- 39

Zenith attenuation as a function of station elevation:

_-km height increments from 0 to 16 km 4@

Zenith brightness temperature for 0, 3, !0, 17 g/m 3

of water vapor (2-km scale height) added to the

1976 U.S. standard model atmosphere -- 41

Brightness temperature for 7.5 g/m 3 water vapor

(2-km scale height) added to the 1976 U.S. standard

model atmosphere 42

Brightness temperature for 3 g/m 3 water vapor (scale

height 2 km) added to the 1976 U.S. standard model

atmosphere, I to 340 GHz 43

ix

CONTENTS (Continued)

Fisures

2_

25

Tables

I

2

Brightness temperature for 7.5 g/m _ water vapor

(scale height 2 km) added to the i97 & U.S.

standard model atmosphere, i to 3Li0 _',Hz

O

Brightness temperature for 17 g,"m_ water vapor

(scale height 2 km) added to tropical model

atmosphere (15°N) (Uailey, 1965); ! to 340 :_Hz

Atmospheric Model Used in All Ca!cuT_*ions_ With the

JPL Radiative-Transfer Program Except for Fig. 25

Tropical Atmosphere Model (iS°N Lassitude)

4_

_5

19

2O

X

SE JTION i

INTRODUCTION

i.I PURPOSE

It is frequently difficult to trace the origins of the

Drooagation and noise curves of the CCIR [Comite Consuitatif Inter-

national des Radiocommunications (International Radio Consultative

Committee)]. These curves provide the technical basis of the dis-

cussions that lead to international treaties in radio frequency

allocations. This reoort comoares the present CCiR standard propa-

gation and noise curves pertinent to earth-space telecommunications

with those derived from a sophisticated, uo-to-date radiative trans-

fer orogram (Waters 1976), in an attempt to assess the adequacy

of the oresent set and if necessary offer some possible alternative

curves.

1.2

1.2.1

BA C K@ R0U ND

The Atmosphere

The earth's dry atmosohere consists of oxygen (02) ,

20.946% by volume, nitrogen (N2) , 78.084% by volume, and argon (A),

0.934% by volume. These three total 99.964% and, along with 27 parts

per million of trace gases, are well mixed to a height of nearly

_{] km, where the dissociation of molecular to atomic oxygen begins

to affect these figures. Carbon dioxide (CO 2) accounts for about

333 parts per million and is also well-mixed in the atmosphere, but

it is increasing at about 0.7 parts per million per year (Battan,

1979). This brings the total to 100%.

The principal variable component of the atmosphere is water

vapor (H20). The saturation vapor pressure is a very strong func-

tion of temperature. For example, the saturation partial pressure

of water vapor (over water) is 6.107 mb at 0°C, 12.27 mb at 10°C,

23.37 mb at 20°C, L2.43 mo at 30°C, and 73.78 mb at a0°C.

Relative to the U.S. standard model atmosphere at sea level (15°C,

I

1013.25 rob), we see that water vapor at 100Z relative humidity

constitutes only about _,.7% by volume of _.}_ r_,tmos_here.

The height p,_ofi_e of oressuro an:_ i_n;-_ity of the static dry

atmosphere can be des?ribed ouite adecuat_iij.' !S two fundamental

eouations. The hydrostatic equation descrJ_es, the pressure profi!_,

with height. In differential form it is:

dp : -ogdh (i

where p is pressure, o the atmospheric den_<Jt_, g the acceierat_on

of gravity, and h Zhe altitude.

The relatlonshi<, of oressure and temperature is given by the

perfect-gas law, which may be written in _hc fcrm

o : nkT

_ m- 3z_d_'where n is the nart:Lcle density (2.5a73 _ ..... at i5°C and

1013.25 mb), k is Boltzmann's constant (].380_ ,: 10 -23 J/K), and

T the temperature in kelvtns. Combining '(i' ._nd (2),

dp : _ go- dh - dho n_<_ !i

where the scale height }{ -

mass.

kT

mg

- --, and m : o/n ls the average particle

The signi?_'cance__ of scale height ...._,_'_ _,_ understood by

integrating (3) :

P = PO expmg (4

which, if the variation in g and T is ignored, yields

P n

P no o

- exp (-h/H) (5)

illustrating that the pressure or particle density decreases by a

factor of e in one scale height (about 8 Xm at the earth's surface),

assuming isothermal conditions. Further, by integrating (5):

hn = n exp ( dh : n H (6)

0 0

which can be interpreted to mean that the total number of particles

in the atmosphere may be obtained by multiplying the surface parti-

cle density n o by the scale height H. Hence, the scale height is

frequently equated to the equivalent thickness of the atmosphere.

The "atmospheric reduced-equivalent thickness" (Gast, 1965),

or, simply, the "reduced thickness", is a similar concept to scale

height, interpreted as equivalent thickness as in (6). Atmospheric

reduced-equivalent thickness is used to gauge the air mass, which

must be traversed by an optical ray transiting the atmosohere at its

zenith. It is defined as the equivalent thickness of an isothermal

atmosphere at 0°C, or ib°C and 1013.25 mb surface pressure. For

example, at sea level the reduced thickness at 0°C and i atm is

8.0 km while at 15°C and I atm it is 8.a km. As one goes up in

height the mass above the observer decreases, as shown in Fig. I.

The optical air mass, m, is a companion concep% that relates

an oblique path to the zenith path. Thus

msec 6 for 6 < 75 °m = _-- _= (1% error at 6 = 75 °)

Z

where _ is the reduced-equivalent path at an oblique angle, _ ism z

the reduced-equivalent zenith path, and 6 is the zenith angle. A

plot of m vs 6 (and vs the elevation a:_gle e = 90° - 6) is given in

Fig. 2. The ultimate source of the values is Bemporad

(Smithsonian, ]9_ 7'

!.2.2 Gaseous Attenuation

fin the f_.:-_u_n__._.. .... _, range of ] to _,,,, _<'_._,_,which wi!a be

referred to as microwave frequencies, abse_<,t.'on and emission by

atmospi_eric gases can signif-cantly affect earth-space tele-communications. Absorr:tion by atmospheric F-[a_sezis dominated by

q_9water-vat, or lines at _ :.245 SHz, 183.31 i_'_ii:<,a-_d 325.152 GHz, and

by oxygen lines near (i(),<_Hzand at ]!_.7[J ,i;]{'z.?,;onresonant absorr_-

_r hetion by water vaoor: a_,_loxygen,, als_ has a _.ii;_r._ificant c,'_=__ict ]._] t

window regions between a_d below these !!_us. Ozone has narrow

lines above I00 (}Hz tha¢ to a fLrst apprc.x '_mat:[on can be ignored,

and will be in the mate_;iai to fcl\ow. P'ov _ discussion of these

lines and those o e o.h<:r minor constitue:-:t:._ s_::..:_,for example,

Waters (]976). A thorough study and desc_-'ption of an existing

computer program of water-vapor and oxy_er_ _,tuenuation and disper-

sion pertinent to earth-space communication f(_r frequencies of i to

I000 GHz and altitudes of 0 to i00 km is available in Liebe (!98!).

S<_ectroscopic information on atmospheric c.__stituents including

eoilutants is given in McClatchey, eta]. !{_'.'_,1975), and mope

recently in Poynter and Pickett (.1980).

:n. absorDtlon coefficient for eac _ rno_ocular species is a

comeiex function of temperature, pressure, a_d frequency; see, for

example, Waters '(197u). ilowever, once det<:'vmir:ed, the total absorp-

......_n auite simoly bytion A for a _ath through the atmosphere is. g_._

_o

(T,p,r; dz (7)

where ¥. is the absorption coefficient in ner__ers per meter (or.th

kilometer) or in dB per meter (or kilometer) of the z gaseous

constituent. (Here the neper is used as the unit of optical depth;

consequently, I neper = 4.34 dB rather than the traditional engineer-ing usage of i neper = 8.68 dB.) For the purposes off this paper

only two gaseous absorbers are considered: molecular oxygen and water

vapor. Therefore, (7) becomes

A -- {Yo(T,P,f) + y (T,P,F) dgo (8)

where Yo and ¥ are the absorption coefficients for oxygen and water

vapor respectively, which for the purposes of this report will be

given in dB/km.

1.2.3 CCIR Gaseous Attenuation Curves

Following each Plenary Assembly of the CCIR (which have

recently been taking place every four years), the approved texts

(Recommendations, Reports, Study Programs, Questions, etc.) are

published in three languages: French, English, and Spanish. The

English volumes have green covers and are known as the ,__olR ureen

Books. Because the CCIR is the technical advisory organ of the

[nternationa! Telecommunication Union in matters of radio, these

Green Books provide the technical resource mater:ials for %he Union's

Radio Regulations (which have treaty status). Consequently, the

material in the Green Books tends to be treated as the international

standard for those areas of radfo which are covered by them. The

basic CCIR propagation curves for the gaseous atmosohere are found

in Report 719 (CCIR, 1978a). Three sets of curves were selected

from Report 719 for attention, and are reproduced as Figs. 3, 4

and 5 of this report. In Fig. 3, the attenuation coefficient is

given in one curve for a temperature of 20°C and pressure of

1013.25 mb, for 7.5 g/m 3 of water vapor (43% relative humidity).

The coefficient is given in a second curve for a dry atmosphere

(labeled 02 ) for frequencies of 0 to 350 GHz. Figure 4 gives the

total one-way zenith attenuation through the atmosphere for a

5

frequency range of _).to 300 OHz. The two turves are for the _._m<<_.-

parameters as Fig. 3, namely zero water vat,:or and 7.5 g/m _ water

for a starting e]ev<e_ion from 0 to 16 km invaoor (at the surface_,4-km increments.

1.2.q Brightness Tem_,er_u_e_

When in local Lhermodvn_m_co equil_L_:r_um _LTE'_•,, the emission

from a gas must equal its absorption, and b}; Kirchhoff's law thisapplies for any frequency. The Earth's atmosohere is in LTE for

these purposes and emits radio noise at mit'r,:wave frequencies _n

direct relation to She amount it absorbs. Tke sky-noise tempera-

ture in a given direction is known as the br'ghtness temperature,

TB, and _s given by the oertinent e<_uati_._ _"_'_J,. radiative transfertheory (Waters, !976) :

TB = T(Z i) y(o,w) e-<(_i'°_ dZ + T e (9)

where T(£) is the local ambient temperature, .{(o,_) is the local

absorption coefficient,

£i• (£i,o) = _-(o,w) d£

0

is the optical depth between the emitting e!emen¢ and the receiver,

and T e-'<_ is the temperature from outside the atmosphere, in the

direction in question, reduced by the optical depth through the

m e -Y_atmosphere in that direction. In the discussion to follow,

(normally less than 3 K) will be ignored.

Brightness temperature in the CCIR is given in Report 720(CCIR, 1978b). Figures i, 2, and 3 of that report appear asFigs. 6, 7, and 8 of this report. They apply to surface water-

vapor densities of 3, I0, and 17 g/m 3 respectively _or a surfacetemperature of 20°C.

SECTION 2

INTERNAL CONSISTENCY

The curves of Figs. 3 through 8 form an interrelated set in

that all are functions of atmospheric absor_<tion. Two tests for

internal consistency can be designed for such a set. ]first, the

zenith attenuation for the dry (!32 ) atmosphere of Fig. 4, when

divided by the specific attenuation of 02 at the surface (Fig. 3),

should yield the oxygen-absorption scale height (about 5.4 km) away

from absorption lines. Second, for optical depths, T, much less

than one (say less than i dB), the brightness temoerature may be

approximated by the eauation (derived from the first term of the

series expansion of exp[-_]):

_ 60 [A (dB)] T << I (10

and for larger values of _ by

T _ T (I - e -r_.: j (iio m

where T is a mean temperature for the atmosphere, say, 280 K, andm

T = A(dB)/4.34.

2.1 THE SCALE HEIGHT TE_T

When the zenith attenuation of curve B (dry atmosphere) in

Fig. 4 is divided for each frequency by the specific attenuation for

02 from Fig. 3, the resultant equivalent thickness of the a_mosphere

(or scale height) for 02 absorption is given in ];_ig. 9, and can be

seen to fluctuate between 2.3 and 9 xm (away from the 60- and

llS-OHz absorption lines]. When the same thing is done to the

equivalent data derived from the JPL radiative-transfer program

(Waters, 1976), the curve of Fig. !0 results, where away from the

lines the scale height varies between 5 and 6 km. Comparison of

Figs. 9 and i0 leaves little doubt but that the lattem is the more

Precedingpageblank

reasonable.

2.2 BRIGHTNESS TEMPERATURE--ABSORPTION f;OMPARISON

When the zenith curves are selecCed f':_omeach of Figs. 6, 7,

and 8 and clotted on a single chart, as _n ?'__. Ii, a brightness

temperature may be computed using (ii) a_d values read from

curve A (7.5 g/m 3 _,,_atervaoor, at the surJa___i: o_ Fig. 4. This

informal, ion is plotte0_ on the same chart a_ ":!uezenith brightness-

temperature curves in i}_ig. !!. 1!: car_ b_ __e:-_ that agreement is not

bad below 50 ,_Hz, but that above about 85 _H:: the agreement deteri-

orates rapidly.

I0

SECTION J

COMPARISON OF CCIR AND JPL VALUES

3.1 THE JPL PROGRAM

The JPL radiative-transfer program (Waters, 1976; Poynter and

Pickett, 1980) incorporates the Gaut-Reifenstein (1971) empirical

adjustment for water vapor in the wings of the absorption lines

(an additive correction term proportional to the square of the fre-

quency). It also makes use of the Rosenkranz {1975) expression for

the oxygen absorption coefficient that includes first-order coherence

effects of overlapping lines (Waters, 1976).

3.2 COMPARISON OF GASEOUS ABSORPTION

While it is clear from Figs. 9 and i0 that there must be sig-

nificant differences between the CCIR and JPL values for either

specific attenuation or zenith attenuation, it is not obvious for

which one. The natural assumption is that these differences would

be in the zenith attenuation, as specific attenuation can be deter-

mined from ground-basbd measurements. This did not turn out to be

the case, as is apparent in Figs. 12 and 13. As can be seen in

Fig. 12 the JPL values are 70% higher in the wings of the H20

(7.5 g/m 3) curve of specific attenuation, and the 02 curve above

120 GHz decreases much more sharply as a function of increasing

frequency for the JPL case than it does for the CC!R one. Further,

it is clear in Fig. 13 that zenith attenuation is in pretty good

agreement for curve A (7.5 g/m of water vaoor), but that She

"oxygen only" values (curve B) are significantly different above

120 GHz (as in the case of the values for specific attenuation].

The comparison of zenith attenuation for steps of 4 km in

elevation above the surface (Fig. 5) is given in Fig. 14. An

interesting feature of this comparison is that the percentage sepa-

ration between the CCIR and JPL models increases with the elevation

of the earth station.

ii

3.3

For the brightne_.s-tem erat._,e c is ., p ,, om_:,ar on the JPL

radiative-transfer code was programmed fo= _ the 1962 U.S. standard

model atmosehere (COESA, 1962) and for a .wster-vacor scale height of

2 km up to fh = tro:__:cpa_se The _esuit. s are sh.own in _--

and 17 for surface _ens_o_es of wat_-__ ".:_c_ _ ::.!'_, i0, and 17 g/re'

respectively. The arid climate case (_ g m', portrayed in Fig. 15

is seen to be in pretty good agr_emen_ u_ to <[:_ ,JHz, and to d__ _ig::

for low elevation an_tle_ (e < 20°'J for n_r__r frequencies. For _he

'..... 16 <intermediate and moist atmosphere pases oz :_gs. and 17 0 g,/m_'

and 17 g/m 3) divergence becomes significant above 20 i]Hz.

12

SE _mTn_T 4

CURVE e_,-e

4.1 JPL CLEAR-AIR ABSORPTIOII

The JPL radiative-transfer program can accommodate some

eight model atmospheres. Two model atmos_.heres have been used in

connection with this report. They are:

(I U. q 1962 _tanaa_d model atmospher_ (_E_A, ]9 )

(2 Tropical (15°N) model atmos_>here (Valley, 1965:

Table 2.2)

The characteristics of these two model atmospheres are

given in Tables ! and 2 res_JectJvely. The U.S. standard model

atmospheres for !962 and 1976 are identical up to 50 km. According

to Fig. 2, less than 10 -3 o _ the mass of the atmosphere is above

50 km. Therefore, the difference between the two model atmospheres

would not show up until the fourth decimal place, and would not be

discernible in the nlots given in this renort. Hence the data

derived from these two model atmospheres are used interchangeably.

The tropical model atmosphere (15°N) is brought _n to allow for a

17 g/m 3 density of water vapor. The two U.S. standard model

atmospheres have surface temperatures of 288.1 K (15°C), which has

a saturation water vapor density of 12.85 g/m 3. The tropical

model atmosphere has a surface temperature of 302.59 K (29"44°C)

with a saturation water-vapor density of nearly 30 g/m 3.

The JPL absorption curves for clear air consist of the follow-

ing set:

Figure 18. Specific absorption (in this case synonymous

with specific attenuation)

Fi$ure 19. Total one-way zenith attenuation

Figure 20. Zenith attenuation with 4-km height increments

13

4.2

curves :

_T qJPL BRI,._HTI,_ES<,TEHPERATURE

m fo_,he brightness t _ _ s ur families:,mpera _ure et ....

Figure 21. Zenith brightness temnerature for 0, 3, !0, and

17 g/'m3 water vaeor

- • _ - _'_ 7.5 g/m 3 water vapor_igure 22 Br_ightness tempera_r_ _:_,_, ,

for frequencies of 1-50 <3Hz

Figure 23. Brightness temperature for 3 g/m 3 water razor for

frequencies of 1-340 GHz

Figure 2_. Brightness temperature for 7.5 g/m _ water van,or

for frequencies of 1-340 OHz

Figure 25. Brightness temperature fop 17 g/m 3 water vapor

for frequencies, of 1-340 OHz

The tropical model atmosphere is used for Fig. 25; the

others use the U.S. 1962 standard model atmosphere (taken here to

be equivalent to the 1976 one).

14

SECTION 5

CONCLUSION

The atmospheric attenuation and brightness temnerature data

of CCIR Reports 719 and 720 (CCIR, _978a,b) have been shown to have

internal consistency problems. A comparable set prepared from the

JPL radiative-transfer program does not suffer these problems,includes more modern information, and is documented. The JPL set isrecommended over the CCIR (1978) set.

The C_J_IR, of course, does not stand still. The problems with

the 1978 versions of Reports 719 and 720 will be corrected in 1982

versions. Also, more sophisticated approaches have been incorporatedin the proposed 1982 texts, so some of the CCIR curve sets which are

used for comparison here are slated to disappear in the future(CCIR, 1982a,b).

15

REFERENCES

Battan, L. J. [197_ Fundamentals of Meteorology, Prentice-Hall,

New York.

CCIR _978J "Attenuation by Atmospheric Gases," Report 7]9,

Proceedings of the CCIR XIVth Plenary Assembly , Kyoto_

1978--Volume 5: Propagation in Non-lonized Media, Inter-

national Telecommunication Union, _.$eneva, 1979, pp. 97-102.

(The CCIR "Green Books" are the English-language version.)

CCIR [1978b] "Radio Emission Associated with Absorption by

Atmospheric Gases and Precipitation," Report 720,

Proceedings of the CCIR XIVth Plenary Assembly; Kyoto_

1978--Volume 5: Propagation in Non-lonized Media, Inter-

national Telecommunication Union, Geneva, 1979, Pp. 103-107.

(The CCIR "Green Books" are the English-language version.)

CCIR _98_ Report 719 (Revised), Proceedings of the CCIR XVth

Plenary Assembly_ Geneva_ 1982 (to be published).

CCIR _982b] Report 720 (Revised), Proceedings of the CCIR XVth

Plenary Assembly_ Geneva_ 1982 (to be published).

COESA [1962] U.S. Standard Atmosphere, 1962, U.S. Committee on

Extension to the Standard Atmosphere (COESA), U.S. Govern-

ment Printing Office, Washington, D.C., 278 pp.

Gast, P. R. _965] "Irradiance Within the Earth's Atmosphere,"

Section 16.1.2 of Handbook of Geophysics and Space Environ-

ments, S. L. Valley, ed, McGraw Hill, New York.

Gaut, N. E. and Reifenstein, E. C. [197_ Environmental Research

and Technology Report No. 13, Lexington, Massachusetts.

Liebe, H. _98_ "Attenuation and dispersion by moist air between

I and i000 GHz for heights 0 to I00 km," to be published in

Radio Science.

Precedingpageblank17

McClatchey, R. A., et al. [1973] "AFCRL Atmos<_herlc Absorption Line

Parameters Calculation," Environmenta_ Research Paper

No. _34 (available from APGI_, Ha__scom AFB, MA 0!731).

Updated in Applied Optics 15, <. fdl_i <i976), and 17,

p. 509 and p. 3517 (1978).

NOAA [1976] U.S. Standard Atmosehere, 197r_, I.]%AA, NASA, and USA?,

_" ¢- c, - •

SuDer=noendent of Documents, ....;_. <}oy_-:_nment Printing Off_.ce,

Washington, D.C.

Poynter, R. L. and Pickett, H. H. [1980] 3ut:m__llimeter_ Mi]!imezer_

and Microwave Spectral Line Catalogue, JPL Publication 80-23,

Jet Propulsion Laboratory, Pasadena, <"aiifornia, June i,

1980.

Rosenkranz, P. W. [1975] "Shape of the 5-ram oxygen band in She

atmosphere," IEEE Trans. Antennas and Pro_., AP-23,

pp. 498-506.

Smithsonian [1951] Smi_hsonian Meteoro!o_zisal Tables, 6th Revised

Edition, Smithsonian Institution, Washington, D.C.,

Table 137.

Valley S. L. (ed.) [1965] Handbook of Geophysics and Space Hnviron-

ments, McGraw Hill, New York, by agreement with the United

States Air Force.

Waters, J. W. [1976] "Absorption and Emiss-on by Atmospheric

Gases," Chapter 2.3 of Methods of Experimental Physics,

Vol. 12B (M. L. Heeks, ed. ), Academic Press, New York.

18

Table i.

75,0072.5070,0067,5065,0062,5060,0057,5055.0052,50SO,O04?,5045,0042,50aO,O037,503S,O032.5030,0027,5025,0022,5020,0017.5015,0012.5010,00

7.50S.O02.50

.00"2.50

Atmospheric Model Used in All Calculations With the JPL

Radiative-Transfer Program Except for Figure 25. Mode_

Represents: - U.S. Standard Atmosphere, 1976 (0 to 50 km)

- U.S. Standard Atmosphere, 1962 (0 to 75 km)

Water vapor added to the model is 7.5 g/m J at the surface

with 2-km scale height to 12 km, standard water vapor

abcve.

PRE$, TEMP, DENS, VOLUME MIXING RATIO8(MB,) (K) (CH*_-3) H20 02

,2417-01 208,_0 ,8_00÷t5 .2837-05 ,2100+On,3S78-0| 213,33 ,1215÷16 ,4126-05 ,2100÷00,5220-01 219,60 ,1722+16 .6000-05 ,2100+00

.7654-01 226.45 .2_48÷16 .6000-0S .2100_00

.1103,00 _33.25 .3425,16 .6000-05 .2100÷00

.1566÷00 2U0,12 ,u725÷16 .6000-05 ,2100÷00,2195÷00 247,00 ,643B÷16 ,6000-05 ,2100÷00.308B÷00 253.87 .8810+16 .6000-0S .2100÷00.4265÷00 260.75 .II90+17 .6000-05 .2100÷00.$673÷00 267.62 .15q0÷17 .6000-05 .2100÷00,7974÷00 270,60 ,2135÷17 .6000-05 ,2100÷00.109S÷01 269.67 .2942÷17 .6000-05 .2100÷00,1504÷01 264.15 ,4124+17 .6000-0S ,2100÷00

.2073,01 257.27 .5838,17 ,6000-05 .2100÷00

.287090! 250.30 ,B307÷17 .6000-05 .2100÷00,4074÷01 243,42 ,1212+18 ,544q-OS .2100÷00,5808+01 236.50 .'1779÷18 ,4949e0S ,2100+00,8324÷01 229,80 ,2624÷18 .449Sm0S ,2100÷00,1197÷02 226,50 ,3828÷18 ,4082-0S ,2100÷00,17§9÷02 224,00 .5687,18 .3708e05 ,2100÷00.2580÷02 221,$5 .8434÷18 .3367-05 .2100÷00.3778÷02 219.10 .1249,19 .3058-05 .2100÷00.$528÷02 216.60 .184q÷19 .3000-OS .2100"00.82S8÷02 216.60 .2762÷19 .3000*0S .2100÷00.1226÷03 216.60 .4099÷19 .3000-0S .2100÷00,1808÷03 216,60 ,60_8÷19 ,7956-0S ,2100÷00.2650÷03 223,30 .6S97÷19 .1967-03 .2100÷00.3853_03 23q.4S .||66_20 .5063-03 .2100_00.5445÷03 255.70 .1543÷20 .1335-02 .2100÷00.7505÷03 27|,9S ,1999÷20 .3sq7-02 .2100÷00,1013÷04 288.10 .2S47÷20 .q854-02 ,2100÷00

19

Table . Tropical Atmosphere Model (15°N. Latitude).

Water vapor added to the model is 17 g/m 3

at the surface with 2-km scale height to

12 km, standard water vat,or above that.

MT

(KM)

72.5070.00bY.SObS.0nb2.5nbO.O057.5055.0052.5O

50,00_7.50OS.On_2.50

aO.O037._n35.0032.5030.0027.5n

?5.00_2.5n20.001705015.00I_.SOlO.On

7.505.on2.5n

.0oo2.5o

PRES.(M_,)

TEMP, DENS, VOLUME 'IIXING RATIOS

(K) (CM**-3) MY() 02

.3338.01 20e.flq .tlbg÷tb ._12b-o5 .2100+00

.5n02-01 _15.b_ .lbSO+tb .60nO.O_ .2100÷00

.7575.0t 224.3q .2381÷18 .bOnn.o5 .2100+0_

.1071÷00 _3].1_ .33_+1b ._OnO-05 ,2100+00,1_35÷o0 _Ul.90 ._5q7÷1_ ._00-05 .2100+00,2171÷0') 250.b5 ,6275÷Ih ._O00-n5 .2100÷00

,3038÷n_ 2_7.15 ._559+16 .600n-n5 .2tO0+O0.a222+00 262,15 ,II_7÷17 .6N00.05 ,2100÷00.Sfl$O+no 267,t5 .15_1÷17 .6on0-o5 ._10o+00.800b+O0 P70,15 ._1_7÷17 .6000.o5 .2100÷00.10qg+01 270.15 ,2qU5+17 .bnO0-O5 ,21_0+0,,

,151n÷ot _65.75 ._115+17 ._nnn.O_ ._lOO+Oh.2080÷n1 2b0.25 .581u+17 .8onh-05 .210n+O0

.2q11+Ol P_u.75 ._27_17 .60o0-05 .PIO0÷O0,U085+01 _Uq.25 .lIA7+tfi ,5_q-05 .2100÷0n.577_+01 2a3.75 .t717+tR ,ao_q.N5 .FIOO÷NO.8_33+01 23fl.25 .2503,1fl ._uOb.n5 ._lO_÷O*.118_÷02 232.75 .3b_3+18 .a082-0_ .21nn+O0.171q_02 _27.25 .q_8+lfl .3708-0_ .?tOO+On,2510÷02 _21.75 .81_8+18 .3367-05 .2100÷(JO.3707+02 _1h.25 .12_.1_ .$05_-05 .2100+00._5_5+o2 207.15 .l_3q÷l_ .3000-05 .?100÷00.8_61+n2 197.15 .3100_1_ .3_00-05 ._100+00,l$Ob÷O_ P03.20 ._bS5÷19 .30o0-05 ,2100+On.1q55+05 21_._5 .6_1+Io .70Sb-05 .2100+00.2_35+03 _3b.bO ._O_l÷lq ._15-03 .2t00÷00.a020+03 253.10 .11_0÷20 .t1_3.0_ ,2100÷o0.557_+o3 Pbq.bO .1_8÷20 .3118-02 ,_lO0+On.75_0+0_ 28b. Oq .lqlq+20 ._ql-02 .21OhiO0.1o13÷0_ $02,5q .2_2A÷2n .23_5-Ol .2100÷00

2O

1 T I I fll I I I t I I IIIJ 1 I I I II[11

6O

2O

10

0.01 0,.01 0.,1 I 10

ATMOSPHERIC REDUCEDEQUIVALENT THICKNESS, km

Figure i. Atmospheric reduced-equivalent thickness above indicated

altitude, surface at NTP (0°C, 1013 mb). Data from

Valley (1965), Table 16-5.

21

ZENITH ANGLE Z (DEGREES)

80 88 87 86 85 83 80 75 70 60 5040 20 0IOOL , , ,, ,,i,, , , ,, , ,,_

80f !60

40_

20 -

N2_ 10 -

1 2 4 6 810 20 40 60 80

El FVATION ANGLE(DEGREES)

Figure 2. Variation of optical air mass (m), with elevation

angle, for a ray traversing the atmosphere, com-

pared to sec @ and the Chapman Function for

Q = I000. Data from Gast (1957) and Allen

(1976). (lllustrative only of refraction, due to

the fact that water vapor is different at radio

and optical frequencies.)

22

4O

2O

I0

Scadl8 A:

S_Je B:

J°o

m

0,5

i 0,2

0,1

I IIllo,

I Illf l

III II

lit[Ii

/,II'I

I //I _II flI II I

l_r i l

Jrl I I

_H20

/\!

%I _-,-- _7 I

e,os i / I I

/ I I/ / I1

°'" / / I Io,o,/I , !

O,N$

o,0o4_" I I I10 20 SO

1 2 S

//

!% !

mmmi

lOI10

IO2

/1

J/

I\\

.,o IIIiinI11II11

IIIJIIII

II 1',Ii

W1

I11!11

[III:!

!II1I

I1

J20O 3.50

Frequency (GHz)

Pzemure: I mi_ (1013.6 mb)

Tmlxn_: 20"C

Wmt_ _ density: "/.5 IJm _

Figure 3. Specific attenuation by atmospheric gases--Fig, i of

CCIR Report 719 (CCIR, 1978a). Use scale B for

oxygen absorption below I0 GHz.

23

woo

zoo

lOO

0

to

0

s0

,_ o,,5

0

0,2

o,1 / /F

i / 8

O,OlO

IVII

[Ill i

i I IN |I III 1I !11 1

IIII ifill

Jill]

IItllIllll11 ] I l

LI I NI[I I ILII |IIli I!1

m u ltlB] I-, , t\'.J

II] 1 ] I

!! I I I %fl !! I 1 II_ ql I I \k.//I I I '

I

i 1

1 1 III I |

Ill[ 1 I

tllN

J/

J

|

I!

\\

\

/!

I

/

,i

/

J

_ _ A

B

1_ 200

Fn_quency (GiIz)

A: 7.5 g/m 3 at ground

B : dry atmosphere (0 g/m 3)

R: range of values due to free structure

rio

J

300

Figure _. Total one-way zenith attenuation through the

atmosphere--Fig. 2 of CC!R Report 719.

24

'o

I0 j

I

J,mo

$

I0-

I

;--o kin-'-!

4 km

I

10- _ _ll km_

12 km ,-.

!_ -!6 k.--

t 2

! I ! 11Io,! !!p --i • •

I I I I I

ill II,--Okm. iio,

II• j

4 km

i8kin "_-

111'kin _

t

r _16 km

• Is I • ! ] •l._ml • I • I

./'-i _'iii i l iI Jtdi I l I I

Nil !! !Illll ! ! I

Jlll_|l !1 III111 I I !:= :-: • . ; ;i,w._ I i..... 1;

" Ilrllll I I ••" II lib 11 •

iailR IIIIIIil IllHIiI_ II/! vJl 11'LIJti

... l__ . :., _--H_O ;;; ......ill I ! I • l

i ID I Ill I ! i/'_ IIII I !1% I I III

I _ A IH 111 kllllll

/ lU I i1_!'14/i _-!/11 Jill IIIll

._ /11 I _ _11 IllHI .... ! ! I !1 _ i II

I I i ! _ ! ! ! ] ! Ill

• • i 11 I ! I • il'kN_ _" IN l • kllOn

J

//

J II i • XI IHn

/ iJ I I_ I_VI II

/ rA !!_ I!1|\A II I ! _1 II t

/./! 11 .__ I

J ! I i ! I1 1 1 II

I I 1 !1

I11 11I II I!I ! I II

I0 20 50 I00 200

v

J

ILL,,,

h

,1 rt_'-

/

.,,_r

/ 't1"

Frequency (GHz)

400

A: Suurtinlheilh_ O.m)

B: Mkabnum values for piths 8tarlb_ tt indicated heights Ocm)

C: Ranle of vaJuu for the ptth from the 8urfaoe to 80 km altitude doe to f'me mu©ture

Figure 5. Theoretical one-way attenuation from specified

height to the top of the atmosphere--Fig. 3 of

CCIR Report 719.

25

3O0

20O

100

5 10 20 50

f (GHz)

I00

Figure 6. Sky noise temperature (clear air) for surface pressure

of i atmosphere and temperature of 20°C and a surface

water-vapor concentration of 3 g/m3--Fig, i of CCIR

Report 720.

26

IO0

@! 5 10 20 50 tO0

Figure 7. Sky noise temperature (clear air) for surface pressure

of ! atmosphere and temperature of 20°C and a surface

water-vapor concentration of i0 g/m3--Fig. 2 of

CCIR Report 720.

27

A

3O0

100

0

! 2 5 10 20 SO

[(GHz)

'I00

Figure 8. Sky noise temperature (clear air) Scr surface pressure

o_ i atmosphere and temperature of 20°C and a surface

water-va_or concentration of 17 g/m_--2_g. 3 of CCIR

Report _

28

z_v o

u___Z" 100_'0

>Z5"'

au

aua

a. bl

_I_ II

I I I0 100 200 300 400

FREQUENCY, GHz

Figure 9. Apparent scale height for oxygen absorption in the

atmosphere, derived by dividing the magnitude of

curve B (dry atmosphere, i.e., 0 g/m 3 water vapor)

of Fig. 4 by the attenuation coefficient (dB/km)

at i atmosphere and 20°C for 02 from Fig. 3.

29

E

m

-T-

_w,,,r_

w_

-r

O

F--

F--Z

g_

0.(3-

100

10

I I I I I

c_ _ THICKNESS, Ir

O...<)-<)w_ "-_'K:)-C_.O_C>K>W:_ --

ATMOSPHERIC REDUCED EQUIVALENT

.... O- .... -'C

0 I I I I I

0 '50 100 150 200 250 300

FREQUENCY IN GHz

Figure i0. Comparison of apparent and true atmospheric reduced-

equivalent thickness from JPL radiative-transfer

program for quotient: zenith dry-atmospheric

absorption by the specific absorption of 0 2 from

the same program.

3O

7°I6O

IO

I I

SURFACE WATER

VAPOR

CONCENTRATION

17g/,n 3

10g/m 3

/m 3

/m 3

I

3

O

3g/m

1 I I I I l20 30 40 50 60 70 80 90

FREQUENCY (GHz)

I00

Figure ii. Zenith brightness temperature for 3, i0, and 17 g/m 3

water vapor--taken from Figs. i, 2, and 3 of Report

720 (CCIR, 1978b)--compared to brightness tempera-

ture derived from curve A of Fig. 2 of Report 719

(CCIR, 1978a).

31

40

20

i0

6

4

E 2

r-,

"= i

z 0.6W

U 0.4M-I.I..

0.2

o.1I'--,-, 0.06e_o 0.04e_

":: 0.02

0.01

0.006

0.004

0.002

0.001SCALE A: 10

SCALE B : ]

Figure 12.

I I

P = 1 atmT = 20°C

Pw 7.5 g/m 3

H20 /

02

02

I

20 50 100

2 5 i0

FREQUENCY, GHz

WATERS (]PL)

CClR RPT 719

I I

0

O2

/., /

(

\\

\

/ \/ H,,O \x

\

Z/I /

200 350

Comparison of specif[c attenuation by atmospheric

gases as _, _ _,r, _- ,-,o_s,_n in <_IR Repo .... i' iq (!_'ig. 3 of this

re_ort)--solid line--and as ob,-ained __o_ th_ J_=L

radiative tpansfer program--dashed and dot-Jashed

lines.

32

1000600400

2OO

100

6040

20

2u")"' I< 0.6

0.4

0.2

0.1

0.06O.04

O.02

O.010

I I I I I

A: 7.5g/m3 AT GROUNDB: DRY ATMOSPHERE (0g/m3)

R: RANGE OF VALUES

A

"\._. B

_'_._BI I ! I .-...__i

100 1.50 200 250 300

FREQUENCY (GHz )

--.-- WATERS(JPL)

CCIR RPT 71g

Figure 13. Comparison of zenith attenuation by the atmosphere

(clear air) as given in CCIH Report 719 (_:ig. 2 of

this report)--solid line--and that obtained from

the JPL radiative-transfer program--dashed line.

33

/

Ii 2 5 10 20 50 200 400

-310

100

FREQUENCY (GHz)

Figure 14.

WATERS (JPL) ----- CCIR RPT 719

Comparison of zenith attenuation for various specified

heights--Report 719, Figure 3 (CC!R, 1978a)--for CCIR

vs the JPL radiative-transfer program. Water vapor is

7.5 g/m 3 at the surface with 2-km scale height. Sur-

face temperature is 20°C. The JPL program uses the

U.S. 1962 standard model atmosphere. (COESA, 1962).

3q

A

v

300

20O

I00

' I '

P = I atmT : 20°C

Pw : 3 glm 3

O : 5 C

02 5

f (GHz)

•-o-. O = 90° (WATERS, JPL)•-c>--O = .5° (WATERS, JPL)

----- CCIR

2O

-.a,--0 : 0°

-'<>'-0 : 10°

RPT 720

: 90°

50 I00

Figure 15. Comparison of CCIR--Report 720, Figure I 19?_b)--and

JPL brightness temperatures for an arid atmosphere

(3 g/m 3 H20 at surface with scale height of 2 km).

35

30O

2OO

100

0

P = 1 atmT = 20°c

3p = 10 g/m

W

O = ELEVATION ANGLE

0--"I0°---', I

0

1 2 5 10 20 50 100

f, GHz

--D-O ; 90° (WATERS,•--'_--0: I0°

1980) --o--O = 5° (WATERS, 1980)"'-O-"O = 0°

CClR RPT 720

Figure 16. Comparison of CCIR--Report 7J_J,8,,Figure 2 (1978b)--and

JPL brightness temperatures for an average atmosphere

(i0 g/m ° H2© at surface with scale height of _' kin).

36

300

2OO

P -- 1 atmT - 20°C

= 17 g/m 3Pw

e : ELEVATION ANGLE

A

vv

I--

100

O = e = 10°

]

) ; 90°

1 2 1005 10 20 50f (GHz)

-<>- e - 5° (WATERS, 1980) --<)--e = 0° (WATERS, 1980)

-o-e = 90° (WATERS, 1980) --_-- e = 10° (WATERS, 1980)

CClR RPT 720

Figure 17. Comparison of CCIR--Report 720, Figure 3 (1978b)--and

JPL brightness temperatures for a moist atmosphere

(17 g/m 3H20 at surface with scale height of 2 km).

37

40

2O

10

0,01

0,005

0,002

0,001SCALE A: 10SCALE B: 1

0 2

O 2

H20 /

O 2

0 2 (f <10GHz)

(f < 10 GHZ)

20 50 100 200 3502 5 10

FREQUENCY (GHz)

Figure 18. Specific attenuation (absor_.tion) from the JPL

radiative-transfer program for 290 K, 7.5 g/m 3

water vapor and dry (02 ) atmospheres, 1020.5

mb for the moist, and 1013 mb for the dry

atmosphere (02) ; I to 340 GHz.

38

Figure 19.

I l I I

3

)-

50 100 150 200 250 300

FREQUENCY (GHz)

Total one-way zenith attenuation from the JPL

radiative-transfer program for: the 1976 U.S.

standard model atmosphere, 7.5 g/m 3 water

vapor (2-km scale height) and 02 atmospheres.

Surface temperature 288 K, pressure 1020.5 for

water vapor, 1013 for 02 atmosphere, i to 340 GHz.

39

100.0

O. 0010 16 km

Figure 20.

O.00010 I0 20 30 40 50 60 70 80 O0 100 II0120

FREQUENCY, GHz

Zenith attenuation as a function of station elevation:

4-km height increments from 0 to 16 km. 7.5 g/m 3

water vapor at surface with 2-km scale height added

to the 1976 U.S. standard model atmosphere

(2_8 K at surface).

t0

300

280

269

240

%f 220

I

140

(,/I 12oi..i.iZI-- too"I-

c_cc_

IIi

o II5ot

A!

0 20 4O

#,l_ v ti J

iLl" /_,, k3 1

i'-//1[I / ---t_j,, Ill ,/k //lILY

80 80 100 120 140 160 180

/i/

/

t\

i/

200 220 240 260 280 300 320

FREQUENCY -GHz

Figure 21. Zenith brightness temperature' for 0, 3, i0, 17 g/m 3 of

water vapor (2-km scale height) added to the 1976

U.S. standard model atmosphere (surface temperature

288 K). I to 340 GHz. Note curve A is dashed

because it is not physically realizable.

41

0 5 10 _ 20 _ 30 35 40 8 J0FE_CY GHz

Y.__m3

10

Figure 22. Brightness temperature for 7.5 g/m 3 water vapor

(2-km scale height) added to the 1976 U.S.

standard mode_ atmosphere (surface temperature

288 K), i to 60 GHz.

42

Figure 23. Brightness temperature for 3 g/m 3 water vapor (scale

height 2 km) added to the 1976 U.S. standard model

atmosphere, I to 3_0 GHz.

_3

Figure 24. Brightness temperature for 7.5 g/m 3 water vapor

(scale height 2 km) added to _he 1976 U.S.

standard model atmosphere, i to 340 GHz.

44

Figure 25. Brightness temperature for 17 g/m 3 water vapor

(scale height 2 km) added to tropical model

atmosphere (15°N) (Valley, 1965); i to

340 GHz.

._-ja-c_.. L,..c.,,, 45