Radiometric sky temperaturemeasurements at 35 and 89 GHz

A.D. Sayers, B.A.. A.M.I.E.E.

Indexing terms: Radiowave propagation (millimetre wave), Remote sensing

Abstract: A set of radiometric sky temperature measurements at 35 and 89 GHz is presented in the paper. Thesystem used to make the measurements is described as well as the method used for calibration. The tem-peratures were measured as a function of elevation twice a day over a period of about six months. Examples ofthe results obtained are presented, together with a cumulative frequency graph of zenith temperature for theentire period. The results are converted to one-way attenuation through the atmosphere, and correlation withmeteorological conditions is discussed.

1 Introduction

Much interest has recently been shown in the use ofmillimetre-wave receivers in the atmospheric absorptionwindows at 35 and 89 GHz. In some applications, the use-fulness of such systems depends on sky temperature. Thispaper describes a set of radiometric sky temperature mea-surements carried out by Philips Research Laboratoriesover a six-month period from 26 July 1984 to 25 January1985. Radiometric sky temperature was measured twice aday in both W-band and Ka-band against elevation. Anoutline of the theory of radiometer receivers relevant tomillimetre wavelengths is given in References 1 and 2.Information on the physical origins of sky noise may befound in References 3 and 4. The measurements werecarried out over the above-mentioned period using a dualfrequency radiometer and automatic measurement systemsituated at Philips Research Laboratories (PRL). Mostwere carried out in vertical polarisation, as sky tem-perature was found to be independent of polarisation.Weather information, including cloud cover, cloud base,temperature and humidity was obtained from GATWICKVOLMET, a weather station 3.1 km from PRL.

2 Description of measurement system

A brief description of the radiometer measurement systemused to obtain the sky temperature results in this paperwill now be given. It is not intended to give a full descrip-tion, which may be obtained from Reference 5.

Fig. 1 is a photograph of the system, which consists oftwo receivers operating at 35 and 89 GHz, placed on amovable platform, and of a rack with equipment forcontrol and data acquisition. The central unit is an HP85desk computer, which is connected to an HP6940 multi-programmer. The HP6940 provides interfaces to datalines, control lines and stepping motors. During operationthe platform is capable of scanning a predefined raster,while data from both radiometers is read into the com-puter. However, the particular measurements performedhere only require a single scan in elevation, and thus theraster capability was not used. The two radiometeroutputs were then examined on a graphical display beforerecording on computer tape, thus acting as a system con-fidence check. Specifications of the radiometer heads them-selves are given in Table 1.

Fig. 1 Radiometer measuring system

Table 1: Radiometer parameters

35 GHz 89 GHz

Noise figureIF bandwidthLF integration times (T)PolarisationMRTD (T=1 sec)

8.0 dB 8.4 dB1 GHz 2 GHz32/64/147 ms 57/102/474 ms(horizontal or vertical)0.087 K 0.090 K

MRTD = minimum resolvable temperature difference

In the W-band radiometer the incoming signal is down-converted using a mixer and a Gunn oscillator. A descrip-tion of the W-band radiometer is given in Reference 5.

During this particular experiment, the integration timeof the radiometers were set to 64 ms for the 35 GHz radi-ometer and 102 ms for the 89 GHz radiometer, givingRMS noise levels of 0.34 K and 0.28 K, respectively.

2.1 AntennasThe antennas in the radiometer measurement system wereCassegrain, the parameters of which are given in Table 2.

Table 2: Antenna parameters

35 GHz 89 GHz

Antenna diameter 460 mm 300 mmAntenna beamwidth 1.3degs. 0.8 degs.Peak antenna sidelobes -18.2 dB -18.9 dB

Paper 4545H (Ell), received 14th November 1985 and in revised form 20th January1986

The author is with Philips Research Laboratories, Cross Oak Lane, Redhill, SurreyRH1 5MA, United Kingdom

3 Calibration of the radiometers

Calibration of the radiometers, which both had linearresponses, was achieved by measuring the response of two

IEE PROCEEDINGS, Vol. 133, Pt. H, No. 3, JUNE 1986 233

reference temperatures. Plessey AFP absorbent foam wasused at ambient temperature and the temperature ofboiling liquid nitrogen (77 K). The calibration was per-formed by putting a block of the foam at ambient tem-perature in front of the antenna feed, noting the output ofthe radiometer. A separate block of the foam, which hadbeen soaking in liquid nitrogen, was then placed in front ofthe feed. The nitrogen would immediately boil, giving asecond radiometer output that was stable for a fewseconds, while there was still liquid nitrogen on the foam.In practice, this second calibration point was found to beremarkably stable and consistent.

The calibration described above was performed beforemost measurements taken with the radiometer. On othermeasurements, average values of the calibration param-eters were used. This was necessary because it was not pos-sible to perform the calibration in bad weather conditions.

Factors affecting the accuracy of the calibration were:(a) The temperature of the absorbent foam. At the high

temperature calibration point the foam may have been at atemperature different from that of ambient temperature,because it had previously been in the warm environment ofthe laboratory. For simplicity, ambient temperature wastaken as 290 ± 5 K. The accuracy of this calibration pointwill affect the results when the radiometric temperature ishigh, that is at low elevations, with a maximum error inradiometric temperature of ± 5 K. The accuracy of thetemperature of the absorbent foam at liquid nitrogen tem-perature is important when the radiometric temperature islow. The foam was assumed to be at 77 K, and the accu-racy of this figure is difficult to estimate.

(b) The emissivity of the foam. Although the emissivityof the Plessey AFP foam could not be measured directly, itwas possible to measure the reflection coefficient of thefoam. From thermodynamic considerations these quan-tities should add up to unity. The reflectivity of the foamwas measured using a signal source, a directional couplerand a power meter to measure the power reflected fromthe foam. This experiment was repeated for both 35 and 89GHz, and for the two foam temperatures. The return lossof the foam was found to be greater than 23 dB in allcases, indicating that the emissivity of the foam is greaterthan 0.995; i.e. that it is 0.9975 ± 0.0025. The error causedby uncertainty in the emissivity of the foam is thus clearlynegligible.

(c) The effect of antenna sidelobes. These will receiveoff-boresight radiation which is not from the correct ele-vation. However, it is believed that this factor was negligi-ble due to the narrow beamwidth of the antennas and theslow variation of sky temperature with elevation, exceptpossibly at low elevations. (Most of the significant antennasidelobes are close to boresight.)

(d) Antenna loss. This was measured, as describedbelow.

3.1 Measurement of antenna lossesThe calibration technique outlined above does not takeinto account the loss in the double reflectors in the Casseg-rain antennas. A technique for measuring this was there-fore devised. This method relied on the fact that, at thezenith, the sky temperature does not vary much with ele-vation. Therefore, if the Cassegrain antenna feed is pointedat the zenith it should see the same radiometric tem-perature as the full antenna, at least to within + 5 K. (Thebeamwidth of the feed was about 20°.) Thus, any change inthe radiometer output voltage can be attributed to loss inthe two reflectors. Thus loss was found to be 6% + 2% at

89 GHz and 5% ± 2% at 35 GHz, and these figures weretaken into account as follows.

The ambient temperature calibration point will be accu-rate whatever the reflection coefficient, because theantennas themselves are at ambient temperature. However,any change in the radiometric temperature from ambientwill be multiplied by the loss factor before it reaches thefeed, and thus the gradient of the calibration line should bedivided by 0.95 (at 35 GHz) and 0.94 (at 89 GHz) to com-pensate for this effect.

4 Results

As mentioned in the introduction, measurements of radio-metric sky temperature were performed twice a day over aperiod of six months. This was done on most working daysbetween 26 July 1984 and 26 January 1985. 230 measure-ments were made, of which six examples are given in Figs.2-7. Fig. 2 shows a result obtained on a day when the skywas clear; i.e. the radiometric temperatures are entirelydue to gaseous emission. Fig. 3 gives a result when a

300 r

20 30 AO 50 60 70 80 90elevation , degree

. 300

o 250

| 2 0 °1*150N

o 100oo

50

0 10 20 30 A0 50 60 70 80 90elevation , degree

Fig. 2 RUN 117 12/10/84 am: clear day

medium amount of cloud was present (about 6 Okta),showing little change from the clear sky result. Fig. 4shows the effect of a varying thickness of cloud on theradiometer outputs. A large black cloud is present in thescan. Some interference is apparent on the 35 GHz radi-ometer. Fig. 5 is an example of a completely overcast day.

Figs. 6 and 7 are results obtained in different degrees ofrain. Fig. 6 shows the effect of medium rainfall, whereasFig. 7 shows that of heavy rainfall. It is clear that rain isthe weather condition that has the most striking effect onthe radiometer output.

4.1 Cumulative distribution graphA computer program was written to process data from themeasurements to a cumulative distribution graph which is

234 IEE PROCEEDINGS, Vol. 133, Pt. H, No. 3, JUNE 1986

shown in Fig. 8. It should be noted that this graph con-tains information from two measurements per day only(one in the morning and one in the afternoon). The graphshows the number of occasions on which the sky tem-perature at the zenith (90° degrees elevation) was below agiven value, plotted against this value. This information

300

* 2503

1 200aE2 150

50

0 10 20 30 40 50 60 70 80 90elevation , degree

was plotted for both radiometer frequencies. At 35 GHzthe zenith sky temperature was below 30 K for 90% of themeasurements and below 15 K for 50% of the measure-ments. Similarly, at 89 GHz, zenith temperature was below150 K for 90% of the measurements and 60 K for 50% ofthe measurements.

300r

25 0

* 200a.E- 150>N(A

100

50

10 20 30 40 50 60 70 80 90elevation. degree

300 r

ft 250

| 200

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100

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0 10 20 30 40 50 60 70 80 90elevation , degree

Fig. 3 RUN 104 3/10/84 pm: medium amount of cloud

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300

250

200

150

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0 10 20 30 40 50 60 70 80 90elevation, degree

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Fig. 5 RUN 55 30/8/84 am: overcast day

300

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200

150

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Fig. 4 RUN 27 13/8/84 pm: very large black cloud in scan

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250

200

150

100

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Fig. 6 RUN 108 6/10/84 pm: medium rain

1EE PROCEEDINGS, Vol. 133, Pt. H, No. 3, JUNE 1986 235

4.2 Conversion to one-way attenuationOne-way attenuation through the atmosphere is a param-eter of great interest in the fields of atmospheric science

300 r

; 250

10 20 30 40 50 60 70 80 90elevation, degree

300

; 2 5 0

200

| 150

100NXo 5.0

6 10 20 30 40 50 60 70 80 90elevation, degree

Fig. 7 RUN 107 5/10/84 am: heavy rain

50 100 150 200sky temperature, K

250 300

Fig. 8 Cumulative zenith data

and communications. It is related to sky temperature bythe following equation:

Ta = <xTq + (1 - <x)T0 (1)

where a = lOIn the above equation Ta is the sky temperature, To is

the ambient temperature and A is the one-way attenuationthrough the atmosphere in dB. Tg is the galatic noise tem-perature, which is negligible at millimetre-wave fre-quencies.

The sky temperature results can therefore be convertedto one-way attenuation, given an assumption about theambient temperature. In the work that follows, To is takenas 278 K, a reasonable average for the atmosphere as awhole.

A cumulative distribution graph for one-way attenu-ation is given in Fig. 9. As can be seen from the graph, at236

35 GHz zenith attenuation was below 0.5 dB for 90% ofthe measurements, and below around 0.25 dB for 50% of

35 GHz

2 3 4 5 6 7 8 9 10 11 12 13 U 15one-way attenuation, dB

0 1

Fig. 9 One-way attenuation data

the time. At 89 GHz, the corresponding figures are 3 dBand 1 dB.

4.3 Dependence on elevationIt can be shown [6] that in a horizontally stratified atmo-sphere, the dependence of sky temperature on elevation isgiven by the equation

In this equation, 0 is elevation and /? is a constant whichmay be calculated from the zenith temperature. Similarly

a(6) = cosec 0 (3)

It should be noted that the above equations can only beapplied in a horizontally stratified atmosphere, and are notvalid when there are large amounts of irregular cloud, as inFig. 4. However, it was found that the above equationsgave accurate predictions of variation with elevation formost of the measurements taken (roughly 90%), except atelevations below around 10 degrees. At these low ele-vations, the above equations are invalid because they arebased on a model that does not take into account the cur-vature of the earth.

4.4 Dependence of results on weather conditionsCorrelation of the sky temperature and one-way attenu-ation results with weather conditions proved to be difficult.This was thought to be at least partly due to the lack of aparameter in the weather data that represented cloudthickness or density. Previously, good correlations of skytemperature with meteorological parameters have beenreported in the literature [7], but these refer to clear skyconditions. Out of the 230 measurements reported here,only 21 were performed in such conditions, and it was feltthat this was too small a sample with which to perform acorrelation against meteorological parameters. Correla-tions of all 230 measurements with weather data proved tobe weak. Nevertheless, some general remarks on thedependence of the results on weather conditions will nowbe made.

As has been stated above, the radiometer output wasmost affected by rainfall. Zenith sky temperature at89 GHz was always below 150 K when there was no rain,corresponding to a one-way attenuation of 3 dB. Similarly,at 35 GHz the corresponding figures were 40 K and0.7 dB. In very heavy rain, zenith temperatures as high as270 K (corresponding to around 15 dB attenuation) wererecorded at 89 GHz, and 200 K (6 dB) at 35 GHz.

IEE PROCEEDINGS, Vol. 133, Pt. H, No. 3, JUNE 1986

Thin cloud did not appear to affect radiometer outputmuch, whereas thicker, possibly rain bearing, cloud had agreater effect.

In all cases the weather was found to affect sky tem-perature far more at 89 GHz than at 35 GHz. This can beseen from the cumulative distribution graph, Fig. 8, wherethe 35 GHz output always varies over a much smallerrange than at 89 GHz. This result is to be expectedbecause of the larger attenuation of water at 89 GHz.

5 Conclusions

This paper has described a set of millimetre-wave sky tem-perature measurements carried out over a period of sixmonths. Sky temperature was measured in both W-bandand Ka-band against elevation. About 230 such measure-ments were taken, in a wide variety of weather conditionsranging from clear days to very heavy rain. The resultswere converted to one-way attenuation through the atmo-sphere using a reasonable estimate for absorber tem-perature.

89 GHz zenith temperature was below 150 K(corresponding to 3 dB one-way attenuation) for 90% ofthe measurements and below 60 K (1 dB) for 50% of themeasurements. At 35 GHz, the corresponding figures were30 K (0.5 dB) and 15 K (0.25 dB). Rainfall was the mostsignificant weather condition affecting sky temperatures.The next most important factor was cloud thickness, par-ticularly at 89 GHz.

6 Acknowledgments

The work described in this paper was carried out with thesupport of the Procurement Executive, Ministry ofDefence.

The author is grateful to D.L. Atkins who wasresponsible for the mechanical work on the project, and toP.M. Ballard and D.H.J. Body, who helped with the mea-surements. He is also grateful to R.P. Vincent, Dr. A.R.Cusdin and Dr. R.N. Bates, who helped with the organis-ation of the project.

The author would like to thank the IEE referees, whoprovided useful and extensive comments on an earlierversion of the paper.

References

1 PRICE, R.M.: 'Radiometers' in 'Methods of experimental physics, Vol.12' (Academic Press, 1976), Chap. 3.1

2 SKOLNIK, M.I.: 'Radar handbook' (McGraw Hill, 1970) Chap. 393 WATERS, J.W.: 'Absorption and emission by atmospheric gases', in

'Methods of experimental physics, Vol. 12' (Academic Press, 1976)Chap. 2.3

4 SMITH, E.K.: 'Centimetre and millimetre wave attenuation andbrightness temperature due to atmospheric oxygen and water vapour',Radio ScL, 982,17, pp. 1455-1464

5 BATES, R.N, NIGHTINGALE, S.J., and BALLARD, P.M.: 'Milli-metre wave E-plane components and subsystems', Radio & Electron.Eng., 1982, 52, pp. 506-512

6 CCIR Report 720-1 'Radio emission from natural sources above about50 MHz' Recommendations and Reports of the CCIR, XVth PlenaryAssembly, Geneva, 1982, Volume V

7 CLARKE, W.W., MILLER, J.E., and RICHARDSON, P.H.: 'Skybrightness temperature measurements at 135 GHz and 215 GHz',IEEE Trans., 1984, AP-32, (9), pp. 928-932

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