A11102 572100

REFERENCE All 1025721 00

01 NBS' P calibration requirements for

EHF SATELLITE COMMUNICATION

SYSTEMS

NBS

PUBLICATIONS

R. C. Baird

W.C. Daywitt

A.C. Newell

S. Perera

A.G. Repjar

D.F. Wait

A.J. Estinf

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

tCyberLink Corporation

Boulder, Colorado 80303

October 1986

-0C

inn

.1156

86-3n58

1986

NBSIR 86-3058

NBS

RESEARCH ' INFORMATION CENTER

CALIBRATION REQUIREMENTS FOR

EHF SATELLITE COMMUNICATION

SYSTEMS

R. C. Baird

W.C. Daywitt

A.C. Newell

S. Perera

A.G. Repjar

D.F. Wait

A.J. Estinf

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

fCyberLink Corporation

Boulder, Colorado 80303

October 1986

Sponsored by

Air Force Satellite Control Facility

Sunnyvale, California 94088

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

EXECUTIVE SUMMARY

This report concludes the present phase of an investigation by the

National Bureau of Standards (NBS) into the calibration and measurement

support requirements of millimeter wave (SHF and EHF) satellite systems such

as MILSTAR, with particular emphasis on the role of Camp Parks Communications

Annex (CPCA) as the prime Air Force Satellite Control Facility (AFSCF) common

user test facility.

Following a general introduction in section 1, sections 2 and 3 review

the needs for and basis of measurements on satellite systems in terms of the

link budgets, the particular role of CPCA, and the new problems that arise

because of operating in the upper SHF and EHF frequency ranges. Section 4

presents an overview of the various means available for calibrating antenna

gain, which is generally the key measurement that needs to be accurately accompl ished.

Section 5 summarizes the calibration and measurement requirements as stated by the MILSTAR program office in the Orbital Requirements Document

(ORD), Annex A (Rev. 3). Section 6 begins to develop an error budget for the

measurement requirements and gives the present status of NBS measurement support capabilities. Finally, section 7 summarizes recommendations to the

AFSCF, and suggests a number of tasks to assist in meeting the calibration

support requirements of the EHF satellite systems.

The report points out three major areas of concern with regard to making

CPCA an SHF/EHF satellite communications metrology terminal. First, without

adequate methods to measure the atmospheric loss, the accuracy of effective

isotropic radiated power (EIRP) measurements in the 20-45 GHz range can be no better than 0.5-3 dB, depending on frequency and antenna elevation angle,

which is inadequate for MILSTAR requi rements . Second, development of required

standards and measurement support services at NBS must be addressed in order

for NBS to have the millimeter wave antenna gain and thermal noise measurement

services needed to support CPCA. Third, if the Sun and/or Moon are used for

measuring antenna gain in the millimeter region, they need to be characteri zed

at those frequencies.

Recognizing that these concerns need to be addressed in order to establish the measurement and calibration capabilities to support AFSCF and CPCA,
NBS has recommended a series of efforts that need to be accomplished. First,

in the general area of atmospheric and noise requirements, laboratory noise

standards and calibration services must be developed to support the SHF/EHF

bands. Second, atmospheric losses must be accurately characteri zed in order

to measure EIRP from a satellite. Third, the Sun and Moon must be accurately

characteri zed at EHF frequencies if they are to be used as known sources for

system calibrations. Fourth, techniques must be developed for evaluation of

connectors and adaptors, since they can introduce significant errors into

millimeter wave measurements.

In the general area of antenna calibration requirements, several efforts

are recommended. First, NBS laboratory equipment for EHF antenna calibrations

must be upgraded and supplemented. Second, modifications in near-field an- tenna calibration techniques that avoid cable flexing problems need to be

developed. Third, swept frequency antenna gain measurement techniques need to

be developed to ensure accurate gain calibrations at all operating frequencies

within a band. Fourth, circularly polarized antenna gain and polarization

standards and associated probes must be developed. Finally, error analyses on gain transfer calibration techniques need to be performed at SHF and EHF.
CONTENTS

Page

1. Introduction 1 2. Link Budgets 3

2.1 Components of a Link Budget 3

2.2 Determination of System Performance 5 2.3 Camp Parks Measurement Role 5 3. Atmospheric Effects 6

3.1 Signal Amplitude Reduction by Absorption and Scattering 6

3.2 Thermal Noise Increase 7 3.3 Interference Increase 7 3.4 Signal Phase Modulation 8

4. Techniques for Determination of Parameters 8

4.1 Natural Extraterrestrial Sources 9

4.1.1 Gain and G/T Measurements Using Solar and Lunar Flux.... 10

4.1.2 Correction Techniques for Atmospheric Effects 11

4.2 Range and Laboratory Methods for Calibrating Antenna Gain 13

4.2.1 Near-field Measurement of Gain 14

4.2.2 Extrapolation Measurement of Gain 15

4.2.3 Compact Range 16

4.2.4 Gain Intercomparison 16

4.2.5 Orbiting Standards Package (OSP) 16

4.3 Summary of Available Gain Calibration Techniques 17

5. Satellite Testing/Calibration Requirements 21

5.1 Satellite Antenna Parameters 21

5.2 Satellite Uplink Performance 22

5.3 Satellite Downlink Performance 22

6. Error Budget Factors 22

6.1 Antenna Gain and Polarization Measurements 23

6.2 Atmospheric Conditions 24

6.3 Receiver Properties 24

6.4 NBS Calibration Support Capabilities 25

7. Conclusions and Recommendations 27

8. References 31

v

Calibration Requirements for

EHF Satellite Communication Systems

R. C. Baird, W. C. Daywitt, A. J. Estin,

A. C. Newell, S. Perera, A. G. Repjar, D. F. Wait

The calibration and measurement support requirements of

millimeter wave satellite systems such as MILSTAR have been investigated. The needs for measurements on satellite systems are reviewed. An overview of the various means available for calibrating antenna gain, one of the key measurements that needs to be

accurately accomplished, is presented. Essentially three new measurement problems arise because of operating in the upper SHF

and EHF frequency ranges. First, without adequate methods to

measure the atmospheric loss, the accuracy of effective isotropic

radiated power (EIRP) measurements in the 20-45 GHz range can be

no better than 0.5-3 dB (depending on frequency and antenna elevation angle), which is inadequate for MILSTAR requi rements

.

Second, standards and measurement support services are not presently available from the National Bureau of Standards and are needed to support millimeter wave antenna gain and thermal noise

measurements. Third, if the Sun and/or Moon are to be used for

measuring Earth terminal G/T, Earth terminal antenna gain, or satellite EIRP in the millimeter region, they need to be appropriately characteri zed at those frequencies.

Key words: antenna gain calibration; atmospheric loss; effective

isotropic radiated power; G/T; MILSTAR; satellite communications;

solar noise flux

1. Introduction

The main purpose of making various types of measurements on a satellite

system following deployment and during its lifetime is to obtain quantitative

engineering information which enables one to anticipate performance levels.

The user can thereby be assured that his requirements, as originally fulfilled

by the systems designer, will continue to be met. In the event of failure or degradation of one or more elements of the system, identification of specific

degraded parameters may generate an opportunity for alternate modes of usage

of the system that will permit at least partial continuance of the mission of

the satellite system. In addition, the agreements between the U.S. government

and vendors of satellite systems, particularly satellite vendors, normally

call for certain bonus payments for achieving stated levels of performance

over specified periods of time. Accurate determination of performance levels

therefore becomes an important factor in the procurement process as well as in

the on-going operations of affected organizations.

1
Broadly speaking, reliable performance measurements are required for

three primary areas of satellite systems operation: a) tracking, telemetry,

and command (TT&C), which may include timing synchronization; b) baseband

signal processing at the Earth sending site, in the satellite, and at the

Earth receiving site, and c) transmission of radiated fields from Earth

station to satellite and vice versa, including the antennas and amplifiers

which generate and detect the fields. This report, as well as the National

Bureau of Standards (NBS) task in support of the Camp Parks Communication

Annex (CPCA), is addressed almost entirely to the last of these three areas.

The scope of the report includes the identification of related measurement

requirements, both those characteristic of all satellite systems and those

peculiar to millimeter wave systems. In particular, applications to the 20

and 44 GHz frequency bands are emphasized, where the former is in the SHF band

and the latter in the EHF band.

The primary measure of transmission quality for digital systems is the

bit error rate (or more exactly, bit error probability). For analog systems,

the criterion is more complex, but in an FDM-FM system, voice channel perfor- mance can be deduced from the psophometrical ly weighted noise. Although these

quantities can sometimes be measured directly, in general they are inferred

from parameters of the link power budgets. These parameters include EIRP and

gain to temperature ratio (G/T) with appropriate adjustments for propagation

effects. Interference, both accidental and intentional (jamming), may further

complicate these measurements . Margins for environmental degradation and for

system deterioration may be included as appropriate.

Following this introduction, sections 2 and 3 review the needs for and

basis of measurements on satellite systems in terms of link budget parameters,

the particular role of CPCA, and the new problems that arise because of operating in the upper SHF and EHF frequency ranges. Section 4 presents an over- view of the means available for calibrating antenna gain, which is generally

the key parameter that needs to be determined.

Section 5 summarizes the calibration and measurement requirements as stated by the MILSTAR program office in the Orbital Requirements Document

(ORD), Annex A (Rev. 3). Section 6 begins to develop an error budget for the

measurement requirement and describes the immediate effects. Finally, section

7 summarizes recommendations to the Air Force Satellite Control Facility

2
(AFSCF) and suggests a number of tasks to assist in meeting the calibration

support requirements of the EHF satellite systems.

2. Link Budgets

A satellite communications channel consists of one or more pairs of

links, where each pair is comprised of an uplink and a downlink. The satellite itself connects the uplink and downlink by means of a system which can vary from a simple linear frequency translator to a very complex signal

processor.

The performance of a channel is measured by its carrier-power/noisedensity ratio (C/N0

), and is obtained by combining the separate performances

of the uplink and the downlink. Only in the case of a linear satellite transponder can the overall C/N

Q

of the channel be obtained as a unit. Otherwise,

the C/N

q for each link must be obtained separately, and combined by a reciprocal addition process which is similar to calculating the equivalent of

parallel resistances.

The performance of each link is predicted by its link budget, which can

be derived from the radio range equation, sometimes called the Friis

Transmission Formula. The link budget expression constitutes a convenient

means of calculating the overall figure of merit (C/NQ

) for the link from the

figures of merit of the transmitter and receiver combined with the parameters

of the transmission path. The accuracy to which these elements in the link

equation can be calculated, measured, or otherwise estimated, will directly

affect the accuracy with which performance of the link can be anticipated.

2.1 Components of a Link Budget

The expression for the link budget is given as [1]

C/N

q

= EIRP + G/T - Lpath - Latmos - L rain - (other terms) + 228.6, (1)

in which all quantitites are expressed in variants of decibels.

The left side of the equation is the carrier-power/noi se-densi ty ratio,

expressed in dB-Hz, which is an overall figure of merit for a digital communication link. EIRP is the product of the transmitter power and transmitting

3
antenna gain in the desired direction. It constitutes a figure of merit of

the transmitting station and is measured in dBW. The figure of merit of the

receiving station (G/T) is the ratio of its antenna gain to the combined

temperature contributions of the receiving amplifiers, receiving antenna and

transmission line, and sky (including "standard sky"* and added effects of

rain, clouds, etc.). Its units are dB/K. L path is the free-space loss

associated with the path length and is given by

Lpath = ^0 ^°9l0 ^A), (2)

where x is the free space wavelength and d is the distance between transmitting and receiving antennas, in the same units of length. Latmos and L pa ^ n are the losses attributable to the absorption of the water vapor and oxygen in

a standard atmosphere, and the increased absorption because of the precipitation along the path, respectively. At the discretion of the system

designer, many other quantities can be, and frequently are, included in the

"other terms." These include the effects of pointing errors, impedance mismatches, polarization mismatches, atmospheric and ionospheric depolarization,

feedline losses, edge-of-coverage allowance, end-of-line allowance, stationkeeping tolerance, input and output backoff of a transponder, and other safety

margins as appropriate. The 288.6 term is -10 log k, where k is the Boltzmann

constant.

The resulting C/N0

's for uplink and downlink are then combined by means

of the previously mentioned reciprocal addition process, along with other

similar terms resulting from interference and jamming, intermodul ation , etc.

The result, (C/N

0 )total ’ can then be used for the comP osite figure of merit of

the system. This assumes that the various noise and interference

contributions are statistically independent of each other, which is only

approximately true. A more exact evaluation requires a detailed analysis of

each system.

*A standard sky corresponds to a moderately humid atmosphere (7.5 g/m3 surface

water vapor density) at latitude 45° in July. No precipitation is present.

4
2.2 Determination of System Performance

Service requi rements of a satellite communications system are usually

stated in terms of transmission rate R (bits per second) and an acceptable

probability of error P 0. As a consequence of the techniques and equipment

used in modulation, demodulation, coding, and decoding, the ratio of energy

per bit to noise power density (E

b /N

Q

) can be determined. With appropriate

factors for coding gain, coding redundancy, and margins which may not have

been included in the link budgets, the required value of C/N

Q

is obtained.

This is then compared with the available value, ( c / N

0 )total* The reSL| lting

system performance levels are thereby established. If ( C/

N

Q

) re q is greater

than (C/N0 ) t0ta i, modification of the system is necessary.

2.3 Camp Parks Measurement Role

The function of the Camp Parks facility is to perform measurements to assess the performance of a satellite system or its elements. More

specifically, the performance of the satellite itself is the primary focus of

this activity. Thus, the CPCA does not, and presently cannot, determine the

condition of various Earth terminals used with the satellite and is not an operational communications terminal itself. Of course, Camp Parks is capable

of generating and receiving signals to and from the satellite, but only insofar as needed for measurement purposes. While it is desirable to be able to

determine a number of characteri sti cs of a satellite, such as saturation level

of the output amplifier, gain-transfer characteri Stic of the transponder,

gai n-to-noi se temperature ratio of the satellite receiver, and satellite EIRP

[2], it is only the last characteri Stic that can be measured from the ground

in an advanced nonlinear satellite such as MILSTAR. If a remote measuring

capability were made an intrinsic part of the satellite and its measurement

data telemetered to the Earth, then all of these characteri sties could be

eval uated.

In the absence of such a capability, the only quantity which can be

determined is the satellite EIRP. EIRP is, however, the most crucial of these

characteristics. Associated with EIRP are such quantities as antenna pattern,

sidelobes, beamwidth, boresight properties, and polarization and cross polarization. These are really geographic variants of EIRP and can be measured if

the satellite can be appropriately gimballed or if a mobile measuring terminal

5
can be moved to appropriate spots on Earth while the satellite is held in a fixed orientation. Other boresight properties, such as polarization and cross polarization, could be determined at CPCA with appropriately designed special

antennas.

Calibration of G/T of operational Earth terminals is a separate task, and

will not be considered here. However, calibration of the G/T of CPCA antennas

is an important part of the qualification process of the CPCA mission, and

such a procedure might later be adapted to measurement procedures for other

Earth terminals. The use of Transportable Test Facilities is one way of

accomplishing this.

3. Atmospheric Effects

Atmospheric effects [3] on radio propagation are almost completely

negligible in the frequency range of 1-6 GHz and have relatively little effect

up to 10 GHz. Above that region, however, atmospheric modification of

transmitted signals becomes increasingly important. Near and above 60 GHz,

atmospheric effects on a satellite/Earthlink utterly dominate all other

factors. These effects are attributed to two separate phenomena: molecular

absorption by water vapor and oxygen, and absorption and scattering by discrete particles consisting principally of water droplets. The latter

phenomenon introduces the most difficult problems in operating a satellite

link or in accurately measuring the performance of this link. This is because

the effect of water droplets is not only more profound but also more widely

variable from time to time and from place to place and depends on size, shape,

and distribution parameters of the droplets.

3.1 Signal Amplitude Reduction by Absorption and Scattering

The most direct atmospheric effect on the link is a reduction in received

signal level that can be divided into 1) an invariant sky effect due to the

absorption of the "standard" sky (described in section 2.1) which is primarily

a function of frequency, and 2) absorption resulting from rain. Prediction of

the former can be accomplished by standard techniques to within several tenths

of a decibel up to about 20 GHz, and to within a decibel or so between 20 and

45 GHz. In this way, both the performance of the link and relevant parameters

such as EIRP of the satellite can be fairly accurately established. Under

6
conditions of visible heavy clouds and rain, however, establishing these

parameters is much more difficult and less accurate. Calculations from

atmospheric models may not provide the desired accuracies. Thus, either a direct measurement technique may need to be developed and used, or a more accurate way of determining these important atmospheric parameters must be

established. The loss in signal level under severe conditions can be 20 dB or more at 44 GHz, and therefore can nearly obliterate the signal and can com- pletely swamp out its measurement accuracy.

3.2 Thermal Noise Increase

One effect of atmospheric attenuation is to decrease the signal level

received. Further, because the atmospheric temperature is much higher than

the galactic background temperature, atmospheric absorption has the added detrimental effect of increasing the noise temperature at the receiver. In other

words, the C/N

Q ratio at the Earth receiving terminal is degraded both by

decrease in signal level and by an increase in operating noise temperature

.

3.3 Interference Increase

Another adverse effect of the atmosphere on signals above 10 GHz is that

interference from other sources, which are nominally isolated by beam directivity and by orthogonal pol ari zati ons , may in fact be redirected into the

signal channel in question. Water droplets in particular have the relatively

strong effects of defocusing a radio wave and of changing its polarization. A

military system such as MILSTAR is designed to be relatively impervious to

intentional jamming. It is therefore also relatively insensitive to added

interference which may be redirected into its receiving antennas because of

atmospheric scattering. Moreover, MILSTAR does not use dual polarization, and

the small amount of power scattered into or out of the desired polarization is

of minor importance. This phenomenon may therefore be put to beneficial

use. Since the orthogonal polarization mode is not used, it may be possible

to monitor it and, by measuring the amount of signal scattered into that cross polarization, to obtain an estimate of the absorptive attenuation in the

copolarization channel.

7
3.4 Signal Phase Modulation

Phase variations in the transmitted signal can be attributed to two

separate effects occurring in the atmosphere [4,5], Phase delay fluctuations

take place because a wave passes through a medium of variable refracti vity.

These phase fluctuations increase linearly with frequency, but are diminished

as the physical antenna diameter approaches the scale factor of fluctuations,

which is normally about 30 m. Between 20 and 44 GHz, the phase fluctuations

can be of the order of several hundred degrees for an antenna which is one or two meters in diameter.

A second type of phase variation occurs because of wavefront ripple

caused by a "lumpy" medium. These variations in phase are rapid, and are called phase scintillations. They are proportional to the square of frequency

and to antenna size, and decrease with increasing angle of elevation. For

high elevation angles, these phase variations are of the order of ten degrees,

which may have an effect on very high data rate systems. Another effect is an apparent reduction in antenna gain of as much as one decibel.

4. Techniques for Determination of Parameters

In order to have a complete understanding of any radio link, three quantities must be established: 1) the properties of the transmitting station; 2)

the properties of the transmission medium, and 3) the properties of the re- ceiving station. If any two of these are known, one additional measurement

will determine the third. Conversely, if two or more are not known, a single

measurement cannot evaluate the properties in an unambiguous way. The general

procedure will be, in the present requirement, to calibrate the last of these

quantities by independent means, to at least approximate the second, possibly

in real time, and therefore to measure the first at the (Earth) receiver.

This section will address various techniques of calibrating the receiving

station.

The receiving station calibration involves a combination of a voltage or power measurement, usually through a high gain amplifier, and an antenna gain

measurement. Other properties, such as receiver noise level and antenna cross polarization, sidelobes, etc., do not enter directly into the measurement, but

do serve to limit the accuracy that can be achieved.

8
The most important quantity required in the calibration of the receiving

station is the gain of the receiving antenna. Available techniques for making

this calibration include the use of extraterrestri al radiation sources (whose

spectral emissions have been reliably measured) as well as several different

laboratory techniques. The remainder of this section will be devoted to an examination of these techniques, with summaries of advantages, disadvantages,

and quantitative estimates of the accuracies available with each method.

4.1 Natural Extraterrestrial Sources

Several celestial bodies have been used or proposed for use as sources of

known flux levels to calibrate Earth stations [6]. These include such radio

stars as Cassiopeia A, Cygnus A, Taurus A, Orion A, and Virgo A, plus bodies

from within our own solar system such as the Sun, the Earth's moon, and

certain planets.

Below about 10 GHz, the fluxes received on Earth from Cas A and Cyg A are the strongest, and are predictable to an absolute accuracy of between 5 and 10

percent. Above 10-15 GHz, Tau A and Orion A become the dominant stellar

sources, but even they are comparatively weak (below 500 flux units or F.U.

pzr p where one F.U. = 10"^° W/m /Hz) and their intensities have not been well es- tablished. The Sun has long been attractive because it is the strongest

natural source available above 10 GHz, with nearly 6,000,000 F.U. at 15 GHz

and increasing with frequency. There are, however, two restrictions on the

use of the Sun. It is close enough to us that it subtends about 0.5 degrees

of arc and hence its nonuniformity will be resolved by high gain antennas. It

is extremely active and therefore its radio emission varies with time, location on the solar disk, and frequency. Despite these handicaps, this is a potentially useful calibrating source under some ci rcumstances . The Moon is a source of comparable angular extent. It has much lower intensity but greater

uniformity. Planets are far enough away to constitute essentially point sources and may (especially Venus) be bright enough to be useful in the

SHF/EHF frequency range. The disadvantage of planetary sources is that,

depending on the relative orbital location with respect to the Earth, the distance and therefore the received flux varies greatly at different times of the

year.

9
4.1.1 Gain and G/T Measurements Using Solar and Lunar Flux

If the flux from a celestial source such as the Sun or Moon is known,

then it can be used as a reference signal to calibrate satellite EIRP in a comparison measurement. Earth station antenna gain can be determined by measuring the absolute power out of the antenna when the receiving system is

"looking" at the reference source; and the receiving system G/T can be determined [7,8] by measuring the relative power out of the antenna when the system

is pointed at the cold sky background.

Above 5 GHz, the Sun and the Moon are the only celestial sources that are useful if the receiving system has a "small" antenna (less than 55 dB gain).

Below 5 GHz, the flux from the Sun varies greatly over short time spans, so only the moon can be used unless the frequency is low enough (about 3 GHz for

55 dB gain systems) that radio stars such as Cassiopeia A are sufficiently

intense.

For the Moon, a minimum antenna gain of about 40 dB is needed to produce

an antenna temperature (signal -to-noise ratio) great enough for measurement

purposes. The maximum antenna gain is about 55 dB and is determined by the

largest antenna for which the correction for the finite size of the source

(called the "star shape correction factor") can be calculated accurately. So

the Moon is potentially useful as a source for systems with antenna gains

between 40 to 55 dB and for frequencies below about 45 GHz that transmit

through the atmosphere.

The Sun is a more intense source, and antenna gains as low as 13 dB near

1 GHz, or 29 dB near 45 GHz give usable outputs. The maximum antenna gain is

again set by the accuracy of the star shape correction factor and is about

49 dB.

Thus, in principle, one can use the Sun or Moon as a reference source for

measuring Earth terminal G/T and satellite EIRP at frequencies below about 45

GHz. The measurement system requires an antenna with gain between about 29

and 55 dB which can be pointed to the Sun or Moon.

However, before the Sun and Moon can be used as reference sources above

10 GHz, several problems need to be resolved. Above 10 GHz, the theoretical

models for the flux as a function of frequency and "star shape" correction

factors for the Moon need to be developed. Also, as the frequency increases.

10
the atmospheric effects become more pronounced. Near 20 GHz, water vapor significantly attenuates electromagnetic propagation so that one can expect the

atmospheric absorption to vary significantly if the water vapor profile of the

atmosphere changes during the measurement. Consequently more careful and more

frequent measurements of atmospheric loss are needed than at the lower

f requenci es.

Another problem that needs attention at the higher frequencies is the

effects of rain and clouds on the measurement accuracy. It will be impractical to always make measurements of EIRP on cloudless days.

4.1.2 Correction Techniques for Atmospheric Effects

The atmosphere absorbs and scatters radiation traveling through it, thus

reducing the magnitude of and altering the polarization of a received signal.

In clear weather, oxygen and water vapor are the significant absorbers, with

variations in the absorption resulting primarily from changes in the water

vapor density. The temperature and pressure profiles usually vary little from

hour to hour and day to day within the same air mass but change significantly

across atmospheric weather fronts. These changes occur on a large scale

spanning hundreds of kilometers, mainly affecting the troposphere, with

smaller scale variations of importance near the surface. Diurnal changes in

temperature occur from surface heating and cooling.

The water vapor concentrati on in the lower troposphere is highly

variable, especially within 1 or 2 km of the surface, and occurs both on a large weather mass scale and on the smaller scale associated with individual

clouds. The air within a cloud is generally saturated, while outside lower

humidity values are usually observed.

Depolarization caused by rain attenuates the signal by both scattering

the radiation out of the beam and rotating the linear polarization components.

Thus, in experiments where the orthogonal polarization is also used, crosstalk

occurs. Although the rain attenuation is not directly measurable, it can be

determined by measuring the amount of rain-induced cross polarization of the

received signal, given some knowledge of the drop-size distribution and rainfall rate.

11
The zenith attenuation due to oxygen and water vapor absorption varies

from about 0.05 dB at 10 GHz to over 100 dB at 60 GHz for a water vapor

density of 10 g/m 3 which corresponds to a dew point of + 10°C. The sky emission associated with this oxygen and water vapor absorption varies in brightness temperature from 5 to 300 K over this same frequency range, and competes

with the desired signal reception.

All of these effects can be estimated with more or less accuracy

depending on the state of the atmosphere at the time. Algorithms exist for

determining atmospheric loss and emission even in inclement weather. These

algorithms should be used when measurements must be done in bad weather, but

for really accurate measurements clear, stable weather is necessary. The

algorithms still need to be examined and a rough estimate of their accuracy

determined. The discussion that follows, therefore, assumes good weather.

As far as atmospheric conditions are concerned, a requirement for ac- curate gain and G/T measurement using an extraterrestrial source is the

ability to point away from the source and still encounter the same atmospheric

path loss and radio emission. This roughly defines what is meant in this

report by the phrase "clear, stable weather," even though some variable cloud cover with a small amount of attenuation may be present. Measuring the atmospheric loss at various elevation angles for two different azimuth angles will

produce the same results (within the statistical fluctuations of the

measurement) if the atmosphere is suitable in the above sense. Conditions are sufficiently stable if the loss measured before and after the gain or G/T mea- surements are the same.

Above 10 GHz the atmospheric loss increases, and the magnitude of the

loss must be determined more accurately in order to estimate transmission

effects with the same accuracy as for lower frequencies. Thus, while

calculation of the loss using only surface data suffices below 10 GHz, a more elaborate procedure is required in the higher frequency ranges. Two direct

methods for measuring the loss that do not require a priori knowledge of the

ground station antenna gain are available, the "extinction" and the "tipping

curve" (atmospheric emission) methods [9],

In the extinction method, the relative flux from an extraterrestrial

source (e.g., the Sun) is monitored from zenith to a low elevation angle.

When the logarithm of the source's relative strength, as measured at the

12
ground station, is graphed as a function of the cosecant of the elevation

angle, the slope of the resulting straight-line fit to the points is the

zenith atmospheric loss. This is a slow process, requiring at least five

hours to complete, because the source must travel across a large sky angle.

In addition, the weather may change during this process, thus degrading its

accuracy. The tipping curve method is similar except that the radio emission

of the sky is used instead of an extraterrestrial source. For the present

application, the latter method is preferable since it can be performed at any

time of day or night and at any azimuth angle. When used in conjunction with

the Earth terminal measurement system (ETMS) on an antenna with an 800 K (or

smaller) system noise temperature, the tipping method appears to work up to

frequencies where the zenith loss does not exceed 1 dB (40-50 GHz). Final

evaluation of this method's feasibility and overall accuracy will require ex- perimental tests not yet performed. The details and highlights of this method

are presented in [8], an NBS report on work accomplished as part of this EHF

study.

In order to determine the loss above the 1 dB limit just mentioned (or as

a backup if the tipping curve/ETMS approach proves unworkable), a dual

frequency radiometric technique is available that determines the water vapor

content of the atmosphere by performing measurements at 22.2 GHz and

31.65 GHz. The result is used to calculate the loss at frequencies above the

1 dB limit. Instrumentation for this technique requires a considerable outlay

in time and manpower to construct but may be the best solution in the long

run. Error estimates remain to be determined.

4.2 Range and Laboratory Methods for Calibrating Antenna Gain

The period before and during World War II was marked by enormous steps in

the development of techniques for measuring the characteristics of antennas.

Until the advent of the space age in the early 1960s, however, these advances

were characterized by refinements in accuracy and convenience, rather than by

conceptual changes. Pattern integrators, positioners, and automatically con- trolled signal sources and receivers facilitated data acquisition and presentation, but did not improve the intrinsic quality of the data, nor did they

extend the techniques to electrically (and physically) large aperture

13
antennas then coming into use for space communications. The critical requirements of space communication systems pushed antenna and component designs to

higher performance levels and reduced the margins available for design and

measurement tolerance. As a result, fundamental changes took place in antenna

measurements. Conventional far-field ranges were forced to yield to impossible requi rements. For example, to remain in the far field a 20-meter

antenna operating at C-band would require a separation from its test probe of

at least 10 km. Even a 2-meter antenna at 44 GHz must be over 1 km from its

probe. The combination of more stringent accuracy requi rements , more severe multipath errors, and the high cost of acquiring and maintaining such large

ranges has relegated conventional far-field ranges to a position of reduced

usefulness. Accordingly, new approaches were devised for antenna measurements, including compact ranges, near-field probing techniques, extrapolation

ranges, tapered anechoic chambers, and indirect measurement techniques. Thus

the antenna measurements problem has grown to equal complexity with the

antenna design problem and must be given comparable weight by the antenna

engineer.

In this section, we will address methods of gain determination available

for calibration of Camp Park facilities.

4.2.1 Near-Field Measurement of Gain

Near-field measurements of an antenna [10,11,12] are obtained by careful

sampling of the phase and amplitude of the aperture field in sufficiently fine

detail that any desired radiation property of the antenna can be deduced. By means of an appropriate Fourier transformation, the aggregate near-field data

are used to calculate the far-field amplitude and polarization in almost any

desired direction. If a calibrated probe is used to acquire the data, then

the field amplitudes can be referred to an absolute level. This yields the

gain of the antenna in the boresight direction and also detailed pattern

information. The measurement process is very accurate, but can be complex and

expensive for large antennas; if only boresight gain and polarization are needed, other procedures may be preferred.

A thorough error analysis procedure has been developed for this process.

A gain uncertainty as small as two tenths of a decibel is routinely attainable. A more exact evaluation of uncertainty may be achieved, depending

14
on the specific frequencies and properties of the antenna being measured.

Several problems become more severe for millimeter waves: 1) mechanical misalignments and instabilities are a larger fraction of the wavelength and

therefore phase errors become more important; 2) flexing of cables required

during movement of the probe carriage can introduce disproportionately high

phase errors in the millimeter spectrum, and 3) connectors, both waveguide and

coaxial, must be precisely made. It is currently difficult to obtain con- nectors that provide repeatable connections.

4.2.2 Extrapolation Measurement of Gain

The extrapolation measurement [13,14,15] is specifically designed for

determining the on-axis (boresight) gain and polarization parameters. NBS has

developed a rigorous means for determining the far-field gain products of two

antennas which are located in each other's near fields. If three antennas are permuted in pairs through such measurements, the gain of each antenna can be

separated out. On an extrapolation range, this process is conducted in the

near-field zone of the antennas, and then the effects of proximity and multiple reflections are rigorously removed. The axial ratio, tilt angle, and sense of polarization for each antenna can also be determined by measuring the

amplitude and phase changes which occur when one antenna (of each pair) is

rotated about its boresight axis.

This measurement technique is not as sensitive to mechanical misalignments and instabilities as the near-field scanning method described in section

4.2.1. Further, movement of the antennas with respect to each other occurs

only in one dimension, rather than two. Therefore, rails and trolleys can be

made more rigid, thus keeping positional uncertainties to smaller values. As

a result, the extrapolation method does not have a readily discernible upper

frequency limit and should be quite useful for evaluating millimeter wave standard antennas. The most severe constraint is that the available extrapolation ranges are somewhat limited in the weight and physical size of antennas

that can be handled. Also, the length of an extrapolation range must be sufficient to permit the separation distance between the antennas to vary from

about one-fourth to one-half D^/X at the minimum to about 2 ^,e

maximum.

15
4.2.3 Compact Range

The compact range [16,17] simulates a far-field range by illuminating the

antenna under test with a collimated beam, while surrounding the entire

assembly with absorbent material. The collimated beam is formed by illuminating a precision paraboloidal surface with a point source antenna located at

the focus of the paraboloid. This beam can be assumed to have a plane wave configuration in the central 6-10 percent of its cross section. Calculations

indicate that a surface roughness tolerance of 0.007 x in the reflector can result in 0.5 dB amplitude error. Other effects such as scattering from the

reflector edges and stray radiation from the point source further degrade the

results obtained by this technique. Absolute gain measurements with a compact

range are not feasible. Comparative gain measurements are possible but their

accuracy limitations have not been analyzed.

4.2.4 Gain Intercomparison

If an antenna which has been previously calibrated is available, a gain

comparison technique can be used to determine the gain of an unknown antenna

by simultaneously comparing the signals received from a satellite (or

celestial body) by both antennas [18]. If the antennas are co-located, this

has the further advantage of eliminating atmospheric effects since both

received signals will have traversed paths having essentially identical

characteristics.

The gain intercomparison technique requires a relatively small amount of

special instrumentation; however, one must compensate for various possible

sources of error. If the gains of the reference antenna and the antenna under

test differ by more than about 30 dB, a significant source of error can be

introduced into this technique. It is not expected that this problem will be

encountered with the antennas under consideration for MILSTAR Earth terminals.

This technique will become more important at higher frequencies and may be the

only reliable and practical method of calibrating the larger Earth stations.

4.2.5 Orbiting Standards Package (0SP)

Near-field scanning techniques require elaborate instrumentation and personnel with expertise in near-field theory and measurements. Conceivably, it

is possible to develop transportable near-field facilities to perform

16
measurements at a field site, but the process would be expensive and difficult. Consequently, a new concept, the Orbiting Standards Package (OSP) was investigated at NBS toward the late 1970s for use in conjunction with field

measurements as well as routine antenna testing at manufacturer' s facilities.

As was stated in section 4.2, far-field measurements of an electrically large

antenna on a terrestrial range require extremely large separations which

introduce major problems of undesirable ground reflections. However, if the

remote terminal of such a range could be located on a satellite, the requirements of separation and multipath elimination could both be met. Developments

in metrology over the previous decade now make practicable the tasks of remote

measurement, control, and recalibration. The advantages of the OSP are: 1)

both uplink and downlink measurements can be made without loss of accuracy

caused by transference; 2) atmospheric effects can be directly compensated;

and 3) testing of the system can be designed so as to exercise it under many

configurations. It does mean, however, that the OSP must be designed into a satel 1 i te.

4.3 Summary of Available Gain Calibration Techniques

The various regions of applicability of available techniques of calibrating antenna gain are shown in figures 1 and 2. The three most important

parameters of an aperture antenna, any two of which are independent, are its

frequency, physical diameter, and ratio of diameter to wavelength. These

parameters also serve to set limits on reasonable configurations of such

antennas. On the frequency scale, the bounds of this region are approximately

200 MHz to 75 GHz. Physical size limits the top of the region to well under

100-m diameter. Electrical diameter (diameter expressed in wavelengths or D/x) cannot be less than about one-half and rarely is higher than about 500

because of the limitations on surface precision of the reflector and the difficulty in pointing a beam which is sharper than about 0.1 degree. These are not theoretical limitations, but merely represent common practical bounds.

These constraints define the region in which aperture-type antennas are found

and the heavy line delineates this region.

17
Antenna

Diameter,

Meters

IOC*

+D/K

o.oi

Frequency,GHz

Figure 1. Regions of applicability of range-type antenna gain measurements

18
The usefulness of these charts is that, for a given antenna size and frequency range, one can identify feasible methods of gain calibration. The

techniques which are included in this chart are those methods of primary

calibration that have been discussed in section 4 of this report. They do not

include: gain comparison, because that is a secondary calibration; OSP,

because the concept has not been implemented and tested; or conventional farfield ranges, because their usefulness depends on the terrain and height of

towers available.

Near-field forms of measurement are constrained to basically triangular

regions on this chart. The right side of the triangle results from a limit in

phase uncertainty defined by mechanical precision of the trolleys and tracks

used to move the antenna under test or the test probe. The upper side of the

triangle is determined by the largest antenna the range is capable of

handling. The diagonal side of the triangle is limited by the sidelobe radiation from the antenna. At small values of D/x, the sidelobe radiation

increases enough that errors caused by multipath effects begin to dominate and

degrade accuracy. Notice that with these near-field measurement techniques, a large value of D/x is not in itself a limiting factor.

Calibration by natural extraterrestrial sources is shown in figure 2.

Stellar flux measurements are limited in the range of frequencies between 1 and 15 GHz. At the lower frequency end, nonthermal galactic radiation begins

to dominate the sky and interferes with specific stellar sources. At the

upper end, stellar flux levels become too weak for reasonable accuracy. The

electrical size of the antenna is significant because too large an antenna has

a beamwidth smaller than any available star and, therefore, resolves the

structure of the star. Too small an antenna does not have a sufficiently high

G/T ratio to detect the star. A variability of D/x from about 30 to 500 can

be successfully handled with Cas A.

The solar disc is a very bright source which approaches blackbody radiation characteristics in the millimeter range. Its frequency limitations are from 5 GHz, below which highly variable coronal radiation dominates, to about

45 GHz, where atmospheric uncertainties become unacceptably high. Acceptable

values of D/x vary between 20, below which sidelobe levels are excessive, and

150, above which the main beam resolves details of the solar disc.

19
4

3

2

098

7

6

5

4

3

2

1

9e

7

6

6

4

3

2

Figure 2. Regions of applicability of

celestial -type antenna-gain measurements.

20
The Earth's moon, although having weaker radiation than the Sun, is useful over a wider frequency range, from 1 to about 45 GHz. The minimum antenna

D/x is about the same as for the Sun because of the sidelobe problem, but the

maximum D/x increases to about 200 because some resolution of the more uniform

lunar disc is acceptable. However, because of the weaker radiation from the

Moon, systems with high noise temperatures may find the Moon less useful than

the Sun. Both Cas A and the Moon encounter a Y-factor limit which is a function of the achievable receiver temperature at the operating frequency,

the gain of the antenna, and the brightness of the source. This limit

accounts for the curved boundary at the lower edge of these regions.

Approximately seven different Earth station antennas are planned for use with MILSTAR. These range in size from 0.15 to 2.30 m. These antennas have

been indicated on figures 1 and 2 for both uplink and downlink use.

5. Satellite Testing/Calibration Requirements

This section summarizes the relevant on-orbit testing and calibration

requirements as delineated in the MILSTAR ORD, Annex A, Rev. 3. These re- quirements are to be imposed on the AFSCF CPCA and other related units. The

primary function of the requirements is to provide initial on-orbit calibration of communication payloads. The second function is to support anomaly

testing as directed by the Satellite Test Center (STC). The third function is

to support any additional satellite test efforts undertaken by the STC. The

requirements fall into three categories: satellite antenna parameters, satellite uplink performance, and satellite downlink performance.

5.1 Satellite Antenna Parameters

The satellite transmit and receive antennas operate in circular polarization, and have a number of steerable and fixed beams available. The

requirement for polarization measurement is stated in terms of axial ratio,

with tilt angle and polarization sense being unnecessary . (It is quite

logical to ignore these last two polarization parameters, for sense is either

"right" or "wrong," and tilt angle becomes indeterminate in the limit of perfect circular polarization.) Other antenna characteristics such as beamwidth,

beam pointing, and antenna coverage, as was mentioned earlier in this report,

21
can be determined by using test equipment based on an equivalent of CPCA capability in a mobile land or airborne configuration.

5.2 Satellite Uplink Performance

The basic requirement of uplink performance is to obtain BER information

at the receiving Earth terminal (CPCA) while the satellite is illuminated with

different Received Isotropic Power (RIP)* levels. This requirement is specified in terms of uncertainties of the RIP which depend on the particular

hopping configuration of the uplink. In particular, the allowable uncertainty

for normal hopping is ± 0.5 dB ; for partial band measurement it is ± 0.3 dB;

and for TRANSEC zero** mode, ± 0.2 dB. At present, the only means of

complying with this requirement is with a single channel power meter of

unknown characteristics which is supplied by the satellite vendor.

A satellite G/T parameter is also desirable, but specific requirements

are not stated in the ORD.

5.3 Satellite Downlink Performance

Satellite downlink performance is specified in terms of its EIRP for

various beams. This requirement is also stated for the same three modes of

hopping operation mentioned in the previous paragraph: ± 0.5 dB, ± 0.3 dB,

and ± 0.2 dB, respectively.

6. Error Budget Factors

In this section we shall convert the requirements of section 5 into mea- surement system specifications. As a first approximation, we assume that the

variances of the different elements in the measurement are statistically

independent and therefore are additive. This is equivalent to stating that

the uncertainties combine by the root-sum-quare process. The 0.2 dB overall

uncertainty requirement identified in section 5 suggests a goal of 0.1 dB for

the measurement uncertainty of each element. The antenna and propagation re- quirements are essentially independent of the mode of hopping. For this

*Flux density at the satellite times X

2 /4tt, where \ is the wavelength; or equivalently, the power received by an isotropic antenna.

**TRANSEC zero is a stop-hop mode, used for default.

22
reason, the tolerances on those elements must be tied to the tightest of the

requirements, ± 0.2 dB. The associated terminal equipment, receivers, and

power meters will have their own intrinsic performance limitations. This is

particularly true of the satellite receiving terminal, where the characteristics of the equipment are not under the control of AFSCF.

Basic measurement support capabilities available from NBS will be addressed as appropriate.

6.1 Antenna Gain and Polarization Measurements

A gain uncertainty requirement of ± 0.2 dB on the associated test an- tennas places their calibration at the limit of the present state of the art,

which still may not be adequate to satisfy other error criteria. For example,

in the on-board power detection system, the error in the uplink received isotropic power measurement is bounded by ± 0.2 dB. This will require antenna

gain accuracies well within ± 0.15 dB since error uncertainties in

attenuators, reflection coefficients, connectors, etc. are also contributing

factors to the total error. In light of this, error budgets need to be

constructed that reflect a more realistic understanding of the specific

measurement involved.

A second requirement is to measure the transmit and receive axial ratios

of the satellite antenna to an uncertainty of 0.2 dB. Assuming that the axial

ratio of the antenna under test is about 0.5 dB (or equivalently its cross component is 30 dB below its main component), one would require the measuring

antenna to have an axial ratio in the order of 0.2 dB if circularly polarized

probes were utilized. A linear probe could be rotated for the axial ratio

measurement, but in either case depolarization due to atmospheric effects

needs to be understood for an accurate error budget determination.

These requirements impose additional conditions on the quality of the

measuring antenna, as well as on the accuracy of its calibration. Under these

circumstances, it may prove necessary to design and build a special measurement antenna, rather than use one of the standard Earth station units being

procured for operational use. Once such an antenna is obtained, however, the

calibration of its axial ratio can be accomplished in conjunction with the

gain calibration by the extrapolation technique described in section 4.2.2.

23
6.2 Atmospheric Conditions

With a goal of limiting the atmospheric contribution to the error budget

to ± 0.1 dB, very stringent controls must be exerted on the meteorological

conditions under which measurements are made. If clear air conditions are present, zenith attenuation from sea level is about 0.3 dB at 20 GHz and about

0.5 dB at 44 GHz. The standard deviation of these quantities over all seasons

and at many locations is several tenths of a decibel, but can be considerably

reduced by using additive corrections for local water vapor density and temperature at the surface. In addition, a geometrical correction factor must be

used for angle of elevation.

The atmospheric corrections for cloudy or rainy conditions are much

larger, and are far from accurate enough to satisfy the goal of an atmospheric

correction to ± 0.1 dB. Therefore, measurements must be strictly limited to

clear sky conditions, or a new and involved means of correcting for clouds and

rain must be developed and used.

6.3 Receiver Properties

In evaluating received power levels, either at the Earth terminal or at

the satellite, measurements are made of either the IF or video detected power

which has been processed by the receiver, converters, and amplifiers. The

Earth Terminal Measurement System (ETMS) designed by NBS specifically compensates for receiver imperfections and thereby makes a power measurement which

is accurately related to the power received at the antenna. If other types of

power measurements are used, especially at the satellite, receiver properties

such as linearity and dynamic range, short- and long-term gain stability,

variation of the detection law, and possibly VSWR must be known for a reliable error evaluation. For the hopping and partial band measurement modes, the

receiver response time is also needed. The test-signal/noise-temperature

ratio and receiver bandpass characteri sties affect the accuracy of

measurement. These quantities may also require corrections that depend on the

particular hopping pattern.

Particularly at the satellite, there may be significant drifting in the

receiving and power measuring systems because of aging, environmental conditions, or the stresses of launching. For this reason, the NBS 0SP would

24
include remote self calibration procedures so as to retain confidence in the

accuracy throughout the life of the satellite.

6.4 NBS Calibration Support Capabilities

NBS measurement services now provide only spotty coverage of parameters

of interest to the AFSCF in the EHF region. Table 1 is a summary of the

available services for power, attenuation, reflection coefficient, antenna

character!' sties (gain, pattern and polarization), and thermal noise sources.

It is apparent from the table that there are several significant gaps in

services in the MILSTAR bands. The table also includes the priority recom- mendations of the Promote National Microwave Standards (PNMS) Committee. This

is an IEEE sponsored group, independent of NBS, composed of representatives

from major U.S. industries who have identified industry needs and lack of NBS

capabilities. One of the PNMS committee goals is to make people aware of the

importance of and need for NBS standards and metrology support throughout the

active microwave and millimeter wave frequency bands. The committee's highest

priority is on the development of measurment support services in the 18 to

50 GHz bands.

The tri-services Calibrations Coordination Group (CCG) of DoD is another

organization which is concerned with obtaining adequate metrology support from

NBS. This group is responsible for identifying calibration requirements

throughout DoD and then making sure that the needed measurement capabilities

exist in DoD calibration labs. Since DoD calibrations must be traceable to

NBS, CCG works closely with NBS and encourages the timely development of

measurement services required to support new and existing military systems. A

major concern of CCG's is the lack of NBS standards and metrology capability

in the 18 to 60 GHz bands needed to support the MILSTAR program.

Therefore, given specific (and compatible) guidelines by both the private

and military sectors, NBS is directing its efforts into developing the most

needed services, as resources allow. AFSCF needs are (fortunately) on that

path of development. However, the time frame within which the needed EHF

standards and measurement support services will be established depends upon

the total level of support NBS receives for the development of microwave

metrology.

25
TABLE

1:

NBS

CALIBRATION

CAPABILITIES

IN

THE

EHF

REGION

THERMAL NOISE

SOURCE

CQ 2 CQ CQ CQ 2 QOC o O o o o o (®

SVNN31NV

gain

pattern &

polar!

z.

of

arbit.

antenna E

r—1 £H £H s ur-H **-) C_) 2 E 03i—i •*-> • • • • o o • N . E

t, n)

m

co c t: >, o O CQ. or £«

60

j=60

f-i

sz60

—-t xT 03

60 »-> CD 2

JC 03 60

Q.

V• o

0*5 73

4-> • C 01 c

•H C. (5 Ih O M O £ • • • • o o •

CiT tu

CxJ OO

<C 2 O) d> <c 2

CD

<*-.

O

J

&uU

cc

OO • • o o (® 2O1-0 E—

«S

ZD

cd 2 oo oo

CQ 2 o 2WH< o o • • o o 0

CC

Ci3 3

CQ 2

XD

co

XD

CO c 2 o

XD

O

CL. o O • • o o

FREQ.

[GHz] 3.5

mm

(18-26) E O E ^T1 in vo . C\J co w / . \o

CM CM =T I CO

CC r- 3 w

CD oO .rr CM I M3

CC CM 3 ^ WR

22 (33-50) 0

O' M31 O CC jq- 3 WR

15

(50-75)

OO <cX 3<>U1GQHQU

X3

0) CQ C. 73 **H \ 3 CQ

CQ cr •o

•o <0

X c CQ

N

in •o

X o 0) •o t— O • t- Cl) CQ • CQ ? o XD 73 ?

73

U c0 in

O M3 o c. T— in | • d) 60 • • t M3 CQ

c. c CQ. ?c. in 73 3 c 3 3 o O o O *D in o o 0) • o d)

CO X) C. CO

m 60 • c 3 d) c • o o O • XD c0 ? N +J o N o C. n CO •H r-H • t. **H 0) c. • c.

cO o 73 c c0 d) cr 3 H <u CD •H rH 73 d) o o S- d) C0 o f-t c. o

o. Ou

C 60 Q. 0) <u CO

XD •H i-> 3 H E c o

73

d)C

(a) —i

CQ

CO 73

<o <0

CD CO

O •H ,

N

4-> >»

X

•^t 4J 03 o

t. •—1 73 CO o t, o /—N o in • in

X

c. t-H 3 o o M3 73 a. c. S * i r r—

Ou o in in • CQ CO o CQ 1 o in o 1 f ? 

73 CO o N—. 2 c (r* .—• o • a. « '—

O o • o d) • 2 » mo^ 60 t- R i ^ +i C 3

«. >. CQ.

O 1 I cO

O 2 aD o 3 i. O « CO CO • • C0 w d) £- C. d) C C • • §> 3 0) 60 3 3 cr C

CD

O 3 COO d) tH

m •H

O

O CO

O

O c CO o C C0 a C C0 CO V) 60

«k

H CQ < n

aaa

n

a

n *

<C co JQ

O 73 0) 60

26
7. Conclusions and Recommendations

For SHF/EHF satellite systems (MILSTAR), it will be the primary function

of the AFSCF test facilities to provide the on-orbit calibration of the com- munications payloads. To meet these requirements, test terminals must be

sized to provide calibration capabilities for the EHF uplink and SHF downlink

parameters.

A number of testing and calibration requirements for such systems have

been examined in this study. For example, in the on-board power detection

system, the error in the uplink received isotropic power measurement is

bounded by 0.2 dB. This will require antenna gain accuracies and power meter

accuracies to be well within ± 0.15 dB since error uncertainties in attenuator, reflection coefficients, connectors, etc. are also contributing factors

to the total error. Compliance with this requirement will be difficult, if

not unattainable. In light of this example, the test facility must also have

the capability to measure not only the EHF uplink EIRP parameters and SHF

downlink G/T parameters within stringent error bounds, but also various

parameters for the satellite transmit and receive parameters including

polarization, axial ratio, beamwidth, beampointing, antenna coverage, bit error rates, etc. within strict error bounds.

These requirements appear to be based largely on earlier satellite

systems (such as DSCS) which are less complex and also operate at lower frequencies. These two factors have profound effects on measurements and calibrations at EHF/SHF frequencies in that the achievable accuracies are yet to

be determined for individual parameter measurements. Some of these measurements are greatly affected by the atmosphere whose state has not yet been es- tablished accurately.

Two principal reasons for embarking on a program supporting calibrations

and measurements are: first, to ensure the continued operational health of the

system during its life and to be able to identify modifications and repairs

which permit operation in the event of degraded performance or failure; and

second, to establish the level of satellite performance that permits com- pliance with incentive payments to the satellite vendor. Because of the con- siderable difficulty and expense associated with measurements at and beyond

the state-of-the-art, a careful trade-off analysis should be conducted between

stated requirements and the commitments needed to achieve that level of

27
capability. For example, if measurement of the uplink behavior of the system

must be known very accurately to adequately monitor system performance, then

weight, power, and cost must be assigned out of the spacecraft budgets during

the satellite design phase in order to meet this goal. If the uncertainty of

measurement must indeed be no larger than 0.2 dB, then three steps should be

undertaken. First, system testing must be given a priority on allocation of

system time, so that relatively rare opportunities of ideal meteorological

conditions can be utilized for test purposes by preemption. Second, a program

of atmospheric research should be initiated so as to be able to precisely

identify atmospheric states and to make loss corrections of sufficient accuracy to meet the requirement. Third, calibration techniques and standards at

SHF/EHF frequencies for the specific parameters need to be evaluated and/or

developed to assure the highest accuracy, or, equivalently, the lowest error

bounds.

Recognizing that these recommendations are in themselves difficult to

implement at the present stage of systems development, we are including a suggested series of efforts that need to be accomplished in order to establish

the measurement and calibration capabilities required to support AFSCF and

Camp Parks in their mission.

Atmospheric and Noise Requirements

1. Development of laboratory noise calibration services in the waveguide sizes

required to support the 20 and 44 GHz frequency bands

Table 1 of this report shows that there has been to date sufficiently

little demand for these services that NBS resources have been directed

elsewhere. The MILSTAR application identifies a single requirement for

this service, and therefore does not justify using general purpose funding

to the detriment of broader national programs now underway.

2. Accurate characterization of the atmosphere

To measure EIRP from a satellite at EHF frequencies, the atmospheric

loss must be known. The atmosphere will cause measurement errors of about

0.5 dB for ideal conditions and errors up to the order of 3 dB for cloudy

or rainy conditions. More precise atmospheric models and adequate

measurement methods need to be developed.

28
3. Accurate characteri zati on of Sun and Moon at EHF

If the noise flux from extraterrestrial sources is well known, these

sources can be used as far-field test standards for calibrating Earth

terminals. They are particularly attractive for use in remote locations

where it may not be feasible to transport calibrated antennas and other

standards. Moreover, calibrated extraterrestrial sources can be used

effectively to evaluate and develop improved atmospheric loss models.

Since the Sun and Moon are potentially the most useful sources at EHF

frequencies, they must be accurately measured and characterized.

4. Techniques for evaluation of connectors and adaptors

Measurement and calibration techniques developed in a primary standards laboratory use the highest quality connectors available in order to

achieve the lowest levels of uncertainties in the results. Such con- nectors are not usually used in field and space applications because of

fragility, size, cost, or other considerations. Therefore, a calibration

performed in the standards laboratory must be physically transferred to

the field environment through appropriate adaptors and connectors. In the EHF region, this can be accompanied by severe degradation in accuracy

unless techniques are available for identifying and correcting for the

imperfections in these devices. This is an area in which comparatively

little research has been done [19], but under the combined pressure of

greater degradation at these frequencies and more stringent requirements

for advanced systems, such work is needed.

Antenna Calibration Requirements

1. Upgrade and supplement laboratory equipment for 44 GHz antenna calibrati ons

As with noise calibration, there has been no specific demand for

antenna measurement services at this frequency. Thus, NBS is not

properly equipped to do these calibrations without specific support.

Among other prerequisites, this means that the presently available range

of attenuator calibrations must be extended to cover a dynamic range of

at least 40 dB.

29
2. Develop modifications in extrapolation and near-field antenna calibration

techniques that avoid cable flexing problems

Highly accurate antenna calibrations at EHF are thrusting against

the limits of basic technology. The near-field and extrapolation

techniques developed at NBS for accurate antenna gain measurements

require considerable physical movement of the antenna under test while it

is connected into the system. Therefore, cables whose attenuation and

phase transfer characteristics are unstable become limiting factors in

the accuracy of the resulting calibration. Present capability is

adequate up to about 30 GHz, but the cable problem will have to be

resolved in order to meet the requirements at 44 GHz.

3. Develop swept frequency antenna gain measurement techniques

This is needed because calibrations are not normally performed at

exact operating frequencies but at fixed points with a swept frequency

calibration to aid interpolation. In order to retain high accuracy, new equipment and revised techniques are needed.

4. Develop CP antenna gain and polarization standards and associated probes

Most present antenna calibrations are done with linearly polarized

fields. Although circularly polarized antennas can be handled to a limited degree with these techniques, maximum attainable accuracy and

efficiency of calibration requires a modification of technique to operate

directly in circular polarization. This in turn requires antennas

designed and constructed for this purpose, rather than simply using

available antennas which have been developed for other applications.

5. Perform error analysis on gain transfer calibration techniques, including

effect of imperfections in standard antennas

Gain transfer measurement techniques for antennas are likely to play

an important part in support of the MILSTAR system. Because of the difficulty of calibrating the gain of all MILSTAR Earth station antennas, it

is likely that gain standards will be calibrated at NBS and these used to

transfer gain measurements to CPCA and to other sites. Moreover, having

a gain standard on site can provide an additional means of evaluating

atmospheric loss in real time.

30
8 . References

[1] Bhargava, V. K. ; Haccoun, D. ; Matyas, R.; Nuspl , P. Digital

communications by satellite. New York: John Wiley & Sons; 1981.

[2] Kreutal, R. W. , Jr.; DiFonso, D. F.; Mahle, C. E. Satellite system

measurements. Proc. IEEE 66(4): 472; 1978 April.

[3] Ippolito, L. J.; Kaul, R. D. ; Wallace, R. G. Propagation effects

handbook for satellite systems design. NASA Ref. Pub. 1082(3); 1983

June.

[4] Yokoi , H.; Yamada, M. ; Satak, T. Atmospheric attenuation and

scintilation of microwaves from outer space. Publ. Astr. Soc. Japan

22(4): 511-524; 1970.

[5] Yamada, M. ; Yokoi, H. Measurements of Earth-space propagation

characteristics at 15.5 Ghz and 31.6 GHz using celestial radio sources.

J. of the Inst, of Electronics and Communications Engineers of Japan 57-

B(2); 1974 February.

[6] Wait, D. F.; Daywitt, W. C.; Kanda, M. ; Miller, C. K. S. A study of the

measurement of G/T using Cassiopeia A. Nat. Bur. Stand. (U.S.) NBSIR 74-

382; 1974 June.

[7] Wait, D. F. Error predictions for 1-60 GHz G/T measurements using

celestial radio sources (to be published).

[8] Daywitt, W. C. 10-60 GHz G/T measurements using the Sun as a source.

Nat. Bur. Stand. (U.S.) NBSIR 86-3046; April 1986.

[9] Falcone, V. J., Jr.; Wulfberg, K. N.; Gitelson, S. Atmospheric emission

and absorption at millimeter wavelengths. Radio Science 6(3): 347; 1971

March.

[10] Newell, A. C.; Crawford, M. L. Planar near-field measurements on high

performance array antennas. Nat. Bur. Stand. (U.S.) NBSIR 74-380; 1974

July.

[11] Yaghjian, A. D. Near-field measurements on a cylindrical surface: A source scattering-matrix formulation. Nat. Bur. Stand. (U.S.) Tech. Note

696; 1977 September.

[12] Wacker, P. F. Nonplanar near-field measurements: Spherical scanning.

Nat. Bur. Stand. (U.S.) NBSIR 75-809; 1975 June.

[13] Newell, A. C.; Baird, R. C.; Wacker, P. F. Accurate measurement of

antenna gain and polarization at reduced distances by an extrapol ation

technique. IEEE Trans. Antennas Propag. AP-21: 418-431; 1973.

[14] Newell, A. C.; Kerns, D. M. Determination of both polarization and power

gain of antennas by a generalized 3-antenna measurement method.

Electron. Lett. 7: 68-70; 1971 February 11.

31
[15] Newell, A. C. Improved polarization measurements using a modified three

antenna technique. Session 15, 1975 June 2-4; Urbana/Champai gn , II.

Proc. IEEE Int. Antennas and Propagation Symp.; 1975. 337-340.

[16] Johnson, R. C.; Ecker, H. A.; Moore, R. A. Compact range techniques and

measurements. IEEE Trans. Antennas Propag. AP-17: 568-576; 1969

September.

[17] Repjar, A. G. ; Kremer, D. P. Accurate evaluation of a millimeter wave compact range using planar near-field scanning. IEEE Trans. Antennas

Propag. AP-30(3); 1982 May.

[18] Kanda, M. Accuracy considerations in the measurement of the power gain

of a large microwave antenna. IEEE Trans. Antennas Propag. AP-23 ( 3

)

:

407-411; May 1975.

[19] Estin, A. J. Scattering parameters of SMA connector pairs. IEEE Trans.

Instrum. Meas. IM- 25 ( 4

) : 329; 1976 December.

32
U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 86-3058 October 1986

4 . TITLE AND SUBTITLE

Calibration Requirements for EHF Satellite Communication Systems

5. AUTHOR(S)

R. C. Baird, W. C. Daywitt, A, C. Newell, S. Perera, A. G. Repjar, D. F. Wait, A. J. Estfin

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

Air Force Satellite Control Facility

Sunnyvale, California 94088

10. SUPPLEMENTARY NOTES

] Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The calibration and measurement support requirements of millimeter wave satellite

systems such as MILSTAR have been investigated. The needs for measurements on satellite systems are reviewed. An overview of the various means available for

calibrating antenna gain, one of the key measurements that needs to be accurately

accomplished, is presented. Essentially three new measurement problems arise because

of operating in the upper SHF and EHF frequency ranges. First, without adequate

methods to measure the atmospheric loss, the accuracy of effective isotropic radiated

power (EIRP) measurements in the 20-45 GHz range can be no better than 0.5-3 dB

(depending on frequency and antenna elevation angle), which is inadequate for MILSTAR

requirements. Second, standards and measurement support services are not presently

available from the National Bureau of Standards and are needed to support millimeter

wave antenna gain and thermal noise measurements. Third, if the Sun and/or Moon are

to be used for measuring Earth terminal G/T, Earth terminal antenna gain, or satellite EIRP in the millimeter region, they need to be appropriately characterized

at those frequencies.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon

antenna gain calibration; atmospheric loss; effective isotropic radiated power;

G/T; MILSTAR; satellite communications; solar noise flux

13. AVAILABILITY

[j£| Unlimited For Official Distribution. Do Not Release to NTIS

I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[Xj Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

40

15. Price

W U.S. GOVERNMENT PRINTING OFFICE: 1986-773-373/65009 USCOmm*
.

.