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Journal of Quantitative Spectroscopy &Radiative Transfer

Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 247–253

0022-40

doi:10.1

n Corr

E-m

bing.lin

journal homepage: www.elsevier.com/locate/jqsrt

Initial flight test results of differential absorption barometric radarfor remote sensing of sea surface air pressure

Roland Lawrence a,n, Bing Lin b, Steve Harrah b, Yongxiang Hu b, Patrica Hunt c, Carl Lipp d

a Old Dominion University, Norfolk, VA, USAb NASA Langley Research Center, Hampton, VA, USAc Locheed Martin Engineering Services, Hampton, VA, USAd ATK, Hampton, VA, USA

a r t i c l e i n f o

Keywords:

Atmospheric pressure

Differential absorption radar

Microwave remote sensing

Microwave differential absorption

73/$ - see front matter & 2010 Elsevier Ltd. A

016/j.jqsrt.2010.06.001

esponding author. Tel.: +1 757 683 3717; fax

ail addresses: rlawrenc@odu.edu (R. Lawrence

@nasa.gov (B. Lin), steven.harrah-1@nasa.gov

a b s t r a c t

The accuracy of numerical weather model predictions of the intensity and track of

tropical storms may be significantly improved by large spatial coverage and frequent

sampling of sea surface barometry. The availability of a radar operating at moderate-to-

strong O2 absorption bands in the frequency range 50�56 GHz to remotely measure

surface barometric pressure may provide such capability. At these frequencies, the

strength of radar echoes from water surfaces has a strong gradient with frequencies

owing to the absorption of atmospheric O2. Our recent research has developed a

technique based on the use of a dual-frequency, O2-band radar to estimate surface

barometric pressure from the measured attenuation due to O2. The ratio of reflected

radar signals at multiple wavelengths is used to minimize the effect of microwave

absorption by liquid water and water vapor in the atmosphere, and the influences of sea

surface reflection over the frequency of operation. A demonstration instrument has

been developed to verify the differential O2 absorption measurement approach. Recent

test flights to evaluate the in-flight performance of the demonstration instrument have

been completed. The measured radar return and differential O2 absorption show good

agreement with the modeled results. These flight test results are consistent with our

instrumentation goal of 75 mb uncertainty and indicate that our proposed differential

absorption measurement approach may provide a useful measurement of sea surface

pressure. Future test flights will provide higher altitude data and assess the precision of

the sea surface pressure measurement for the existing demonstration radar.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The surface barometric pressure is one of the mostimportant meteorological parameters in weather fore-casts, especially in the prediction and forecast of theintensity and track of tropical storms and hurricanes. Toimprove predictions and forecasts of the intensity and

ll rights reserved.

: +1 757 683 5666.

),

(S. Harrah).

track of tropical storms, large spatial coverage andfrequent sampling of sea surface barometry are criticallyneeded for use in numerical weather models, but, usually,cannot be realized by in-situ buoy and aircraft dropsondetechniques. However, remote sensing techniques includ-ing those on board manned aircraft, unmanned aerialvehicles, and satellite platforms may fulfill the largespatiotemporal requirement.

During the last two decades, the development of remotesensing methods, especially airborne and satellite techni-ques, for large and global scale sea surface pressuremeasurements significantly lagged methods for other

R. Lawrence et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 247–253248

important meteorological parameters, such as temperatureand humidity. There have been suggestions for using satelliteoxygen A-band methods, both passive and active, to measuresurface pressure [1–3]. The active instruments rely on theoperation of complicated, highly stable laser systems on aspace platform and are thus technically difficult. Passivemethods are restricted to daytime measurements and areasof no cloud cover [1]. These challenges have resulted inminimum progress and there are no operational remotesensing instruments to provide the needed sea surfacebarometry.

Recently, researchers have considered a technique thatuses a dual-frequency, O2-band radar to measure thedifferential O2 absorption to estimate the total oxygen andinfer the surface air pressure [4]. The technique uses dualwavelength channels with similar water vapor and liquidwater absorption characteristics, as well as similarfootprints and sea surface reflectivities, over the range ofmeasurement frequencies. The microwave absorptioneffects due to liquid water and water vapor and theinfluences of sea surface reflection should be effectivelyremoved using the ratio of reflected radar signals of thetwo channels. Simulated results suggest that the accuracyof instantaneous surface air pressure estimations fromusing this technique could reach 4–7 millibars (mb) [4].The potential improvement of storm intensity and stormtrack predictions possible with such an instrument hasalso been studied. In fact, numerical simulations ofhurricanes Ivan and Fran indicate that adding surfacepressure with resolution and spatial sampling consistentwith the differential absorption radar concepts presentedhere could significantly improve 24 h storm intensitypredictions and landfall predictions [7].

In this paper we will present a description of the radardifferential absorption measurement concept that couldprovide the surface pressure measurements and enablethe improved severe storm predictions. In addition, wewill describe demonstration radar built to assess thetechnique. The results of laboratory and flight testing, andthe expected performance of this differential absorptionbarometric radar (DiBAR) demonstration instrument willbe described.

2. Measurement approach

The DiBAR instrument is based on the retrieval of thedifferential absorption near the O2 line complex (frequen-cies: 50–56 GHz). This selection of frequencies provideslarge changes in absorption for the reflected radar signalsas a function of the frequency of the radar due in part to thedifferent atmospheric O2 attenuation coefficients. In theatmosphere, O2 is generally uniformly mixed with othergases. The O2 in the column observed by the radar isproportional to the column air mass, the column air mass isproportional to the surface air pressure, and the reflectedpower measured by the radar can be approximated as [4]

Prðf Þ ¼PtGtGrl

2

ð4pÞ3

!s0ðf Þ

r2

� �exp �

2a0M0P0

g�2aLL�2aV V

� �

ð1Þ

where the first term in Eq. (1) includes frequencydependent characteristics of the radar, which must bedetermined by instrument calibration: Pt is the transmit-ter power and G represents the transmitter and receiverantenna gain. The second term includes changes in thesurface reflectivity, s0, over the radar frequency, and thelast term represents the atmospheric absorption, whereM0 is the mixing ratio of O2 to total air and P0 isthe surface pressure. Thus, if the frequency response ofthe radar is well characterized from 50 to 56 GHz and theabsorption characteristics due to liquid water and watervapor, and spatial resolution of the radar is similar overthis range of frequencies, then the ratio of the radarreceived powers from two frequencies is

Prðf1Þ

Prðf2Þ¼

Cðf1Þ

Cðf2Þ

� �exp �

2ða0ðf1Þ�a0ðf2ÞÞM0P0

g

� �ð2Þ

where C(f) is the frequency dependent radar character-istics. This ratio is dominantly decided by the surface airpressure. Further, if we define the differential absorptionindex, Ri(f1, f2), as the logarithm of the radar returnratio shown in Eq. (2), then the surface pressure can bewritten as

P0 ¼2ða0ðf1Þ�a0ðf2ÞÞM0

g

� ��1

lnCðf2Þ

Cðf1Þ

� �Prðf1Þ

Prðf2Þ

� �

P0 ¼2ða0ðf1Þ�a0ðf2ÞÞM0

g

� ��1

ðCiðf1, f2ÞþRiðf1, f2ÞÞ ð3Þ

or defining terms for a linear relationship between Ri

and P0:

P0 ¼ C0ðf1, f2ÞþC1ðf1, f2ÞRiðf1, f2ÞÞ ð4Þ

The term C0(f1,f2) includes the instrument residualcalibration error. The differential absorption index,Ri(f1,f2), is the logarithm of the ratio of the radar returnexclusive of the frequency response of the radar.

The linear relationship between Ri and the surfacepressure shown in Eq. (4) was firstly suggested by theresults of modeled differential absorption for severalfrequencies in the range of interest here [4]. Further,these simulated results suggest that the accuracy ofinstantaneous surface air pressure estimations from themeasured Ri could reach 4–7 mb [4]. However, the O2

absorption increases at higher frequencies and thereceiver signal-to-noise ratio (SNR) may limit theretrieval accuracy as this loss increases. For a fixedtransmit power the optimum frequencies for the surfacepressure measurement will depend on the receivedpower, which in turn depends on the atmospheric lossand surface reflectivity. The flight testing of the DiBARinstrument discussed in Section 4 is intended to measurethe atmospheric attenuation as a function of frequencyand the differential absorption index Ri(f1,f2). Thesemeasurements can then be compared to predicted valuesto assess the measurement approach and the affect ofreceiver noise on the measurement of barometricpressure.

R. Lawrence et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 247–253 249

3. DiBAR demonstration instrument

Although ground testing of the DiBAR instrument canbe used for functional testing and performance evalua-tion, the demonstration of the differential absorptiontechnique described above using ground based measure-ments is quite challenging. Large propagation distancesare needed to provide sufficient O2 absorption so that theprecision measurements of the differential loss from 50 to56 GHz can be obtained. The scattering from objects otherthan the desired target may also limit the fidelity ofground testing. Ground testing also provides no assess-ment of the assumption of similar surface reflectivity atthe measurement frequencies required to use the mea-surement approach discussed in Section 2. Thus, flighttesting is necessary to provide the best demonstration ofthe differential O2 measurement concept and validation ofthe model results presented in [4].

The goal in developing the DiBAR demonstrationinstrument was to use commercial-off-the-shelf hardwarewherever possible to develop the capability to collectdifferential absorption data that would verify the simu-lated differential absorption and to allow various mea-surement approaches to be assessed. An importantoperational characteristic for the radar and determiningfactor in most design tradeoffs for the DiBAR system is theSNR of the radar. The optimum channel to be used in theO2 absorption band is a function of the radar SNR, whichdepends on the surface reflectivity, range, and totalatmospheric absorption. Thus, rather than selecting a setof frequency based on simulations, the demonstrationinstrument will have the flexibility to vary the measure-ment frequencies and to measure the differential absorp-tion from 50 to 56 GHz, and allow multiple processing anddata analysis strategies in the evaluation of the same dataset.

The basic instrument concept utilizes a vector networkanalyzer and a millimeter wave Up/Down Convertersubsystem to enable operation from 50 to 56 GHz. Themillimeter wave Up/Down Converter will translate thesignals to the O2 absorption band and provide veryflexible signal processing options. As shown in Fig. 1, the

Agilent8362B

Options 014,UNL, 010, H11,

080, 081 15 X 5

Port 1

Port 2

10 MHz Ref

5-

5-

HD14100

HD14100

5-11GHz BP

5-11GHz BP

5-11GHz BP

5-11GHz BP

Fig. 1. DiBAR demonstration in

Up/Down Converter provides a millimeter wave poweramplifier for the transmitter and a low noise amplifier forthe receiver. The transmitted power is selectable but themaximum is limited by the output amplifier to +14 dBm.The maximum transmitted power and the receiver noisefigure, 5.3 dB, will establish the SNR for our selected flightaltitude. Our analysis indicates that for altitudes belowapproximately 1000 m the SNR will be sufficient to verifythe differential absorption across the O2 absorption band.However, the path length and total O2 amount in themeasured air column will be marginal at these lowaltitudes. The transmitted power can also be reducedduring the flight to assess the impact of SNR on variousdata analysis approaches. Finally, to maximize isolationand eliminate the need for a Q-band transmit/receiveswitch, the demonstration instrument transmitter andreceiver are each fitted with an antenna. A more detaileddiscussion of the DiBAR demonstration instrument andperformance testing can be found in Ref. [5].

The DiBAR demonstration instrument is extremelyversatile and can be operated in several modes to emulatea wide range of radar modes and processing concepts. Forthese test flights, the instrument will operate in a steppedcontinuous wave (CW) mode and use software gating,using Fourier transform and windowing in the timedomain, to minimize the effect of radar returns otherthan from the sea surface, or leakage between thetransmitter and receiver. This stepped CW technique willbe used for the test flights. The data will be time gated,using a rectangular window, to include only the seasurface reflection and transformed back to the frequencydomain. The frequency domain data will then be averagedin 1 GHz bands. These 11 bands will provide the radarmeasured return over the radar operating frequencyrange.

4. Test results

The performance of the DiBAR instrument will beevaluated in three phases. First, laboratory tests wereperformed to verify functional performance [5]. Nextan initial set of flight tests to verify the differential

Spacek WR-19 Tranciever

GHx PLO = 45GHz

11 GHz IF

11 GHz IF 50-56 GHz

strument block diagram.

Rec

eive

d P

ower

Rat

io (d

B)

50 100 150 200 250 300 350 4000

53 GHz to 56 GHz

Range (m)

-100

-150

-200

-250

-300

-350

-400

Fig. 3. Radar return from water tower vs. range.

R. Lawrence et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 247–253250

absorption measurement technique and evaluate theDiBAR instrument in-flight performance was conducted.Finally, we are planning an additional set of flight teststhat will provide higher altitude measurements andenable the evaluation of the high altitude and satellitemeasurement capability of sea level barometric pressure.

Laboratory testing of the demonstration radar instrumentshown in Figs. 1 and 2 was performed and verified theexpected ‘‘as designed’’ performance of the instrument [5].Laboratory functional testing of the systems, such ascharacterization of system linearity, noise figure, antennagain, and isolation between antennas, was completed. Resultsof these tests were nominal with two minor exceptions. Thefrequency response of the Up/Down Converter varied morethan 12 dB between 50 and 56 GHz. This variation was largerthan expected. However, it has been found very stable andassumed that low altitude DiBAR data would be used tocharacterize the frequency response of the instrument duringthe flight tests. Therefore, as long as frequency response isstable, this should not affect the DiBAR demonstration flighttests. The leakage from the transmitter to the receiver withinthe Up/Down Converter enclosure was larger than mutualcoupling between antennas. The impact of this leakage isminor since the software ‘‘time gating’’ used to isolatethe radar return from the sea surface will suppress theleakage term as long as the range to the target is more thanabout 50 m.

The DiBAR demonstration radar front-end is shown inFig. 2 during a quick test using a water tower as a target toverify the operation of the radar. The DiBAR instrumentcollected 16,001 stepped CW measurements for frequen-cies from 53 to 56 GHz. The Fourier transform of thesedata then results in a time domain representation ofthe radar return as a function of range. The resulting timedomain data are shown in Fig. 3 and the large return fromthe water tower and the internal leakage term can clearlybe seen in the figure. A discussion of these results can befound in Ref. [5]. A total path length of approximately1000 m, or target range of 500 m, is needed to obtainsufficient differential loss Ri(f1,f2), to be measured by the

Fig. 2. DiBAR demonstration radar.

demonstration radar. These measurements proveddifficult to obtain via ground based testing due tolimited range and oxygen absorption. Further, the radarcross section (RCS) of any target used for ground testingmust be well characterized over our frequency range.Thus, several flight tests were planned to providedifferential loss measurements over the Chesapeake Bayto assess the instrument performance.

The initial flight testing to verify the differential losswas accomplished utilizing a helicopter that providedseveral test flights over water in varying atmospheric andsea conditions. Several modifications to the DiBARinstrument were required for these tests. The integrationof the DiBAR instrument on board the helicopter requiredthe high gain antennas to be replaced with smaller hornantennas as shown in Fig. 4. The reduction in antenna gainresults in reduced system dynamic range and limits themaximum altitude where sufficient signal-to-noise ratiois available for useful pressure measurements. Tominimize the impact of the antenna modification, thefrequency sweep was increased from 53–56 to 50–60 GHzfor these flights. While the spectral response of the DiBARinstrument decreases above 56 GHz, the increased O2

attenuation at these frequencies may be useful for thelower altitude operations. Analysis using an instrumentmodel developed from laboratory testing and themicrowave absorption model described in [4,6] suggeststhat this configuration of the instrument will provide anestimate of the differential O2 absorption for an altitude ofapproximately 3000 feet (ft). Note that within the USaviation industry aircraft altitude is reported in feet. Sincethis is the value recorded by the flight crew, altitude willbe reported in feet in this report.

The demonstration DiBAR instrument was installed ona helicopter (Fig. 5) for several test flights. Data werecollected with in-situ estimated barometric pressureranging from 1007 to 1028 mb. At each measurementsite, the DiBAR instrument made three to fivemeasurements of radar return for frequencies from 50 to60 GHz. These measurements were performed while the

High Gain antennas replaced by Standard Gain Horn antennas for aircraft integration

Fig. 4. The DiBAR instrument fitted with smaller ‘‘standard gain’’.

Fig. 5. DiBAR instrument installed in vehicle for initial flight tests.

50 52 54 56 58 60-90

-85

-80

-75

-70

-65

-60

-55

-50

-45

-40

Frequency (GHz)

Rad

ar R

etur

n (d

B)

2000 ft

3000 ft

5000 ft

Fig. 6. Comparison of DiBAR measured return and model predictions.

R. Lawrence et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 247–253 251

helicopter was in a hover and each measurement setincluded altitudes from 500 to 5000 ft. Thesemeasurements were performed at each altitude with thehelicopter at nominally the same location. The 500 ftaltitude measurements for each measurement set wereused to provide correction for sea surface reflectivityvariations and spectral calibration of the instrument.

The results for a data set performed on a day with anin-situ estimated barometric pressure at the measure-ment location of approximately 1018 mb are shown inFig. 6. DiBAR data for 2000, 3000, and 5000 ft altitudes areshown, as well as the modeled radar return. Three DiBARmeasurements were performed at each altitude, and areindicated by the three different symbols in Fig. 6. Thepredicted radar return (solid curve) is estimated using theradar equation for an extended target (sea surface) andthe microwave absorption model adapted from Refs. [4]and [6]. The measured transfer function of the DiBARinstrument was then combined with these models toestimate the expected radar return, shown in Fig. 6 as the

solid curve. The DiBAR measurements for each altitudeare very repeatable, suggesting that the DiBAR instrumentand the sea surface scattering characteristics weresufficiently stable. The reduction in radar return as themeasurement frequency increases can clearly be seen inFig. 6. This reduction is partially due to the increased O2

attenuation discussed in Section 2.The measured results agree very well with the model

for 2000 ft altitude measurements. The results for 3000 ftalso agree well with the model for frequencies from 50 to58 GHz. The difference between the measured andpredicted values above 58 GHz is likely due to the noisefloor of the modified DiBAR instrument. That is, due to thereduced antenna gain the signal-to-noise ratio of theDiBAR is insufficient at frequencies above 58 GHz at3000 ft altitude and above 56 GHz at 5000 ft altitude.The deviation between the DiBAR measurements and the

500 1000 1500 2000 2500 3000 3500 4000 45000

5

10

15

20

25

30

Altitude (ft)

Ri(5

3GH

z,58

GH

z) (d

B)

Measured Ri 1028 mb Measured Ri 1018 mb Measured Ri 1007 mb Predicted 1028 mb Predicted 1018 mb Predicted 1007 mb

Fig. 8. DiBAR derived and predicted differential absorption coefficients.

R. Lawrence et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 247–253252

model results for 5000 ft is likely due to sampling processwithin the vector network analyzer, which is notoptimized for total path lengths this large. It appears thatthe optimum tradeoff between sufficient O2 absorption(path length) and the noise floor of the DiBAR instrumentfor these flights occurs at an altitude of approximately3000 ft. Future flights with the high gain antennas will nothave this limitation.

DiBAR data for 3000 ft from three different days areshown in Fig. 7. Three measurements are indicated foreach day (symbols) as well as the predicted values (solidline). The increase in attenuation with increase infrequency can be seen in the data for all the three days.Further, the attenuation appears to increase with increasein barometer pressure as would be expected. Thedifference between barometric pressures for each day isapproximately 10 mb. While no statistical analysis wasperformed, the variation in the measured attenuationabove 57 GHz appeared to be on the order of the variationbetween each day. That is, the measurement-to-measurement variation was on the order of 75 mb forthe 3000 ft altitude data. The stability of thesemeasurements over several minutes indicates that seasurface scattering can be assumed constant for theseconditions. As discussed in Section 2 this increase inattenuation is expected to result in a linear change indifferential absorption, Ri(f1,f2) defined in Eq. (4).

The differential absorption index is also provided byDiBAR measurements. The DiBAR demonstration instru-ment measures the radar return over the entire frequencyband from 50 to 60 GHz. However, the differentialattenuation index can be extracted from the data wherethe radar signals are sufficiently above the noise floor. Forexample, the differential absorption for f1=53 GHz andf2=58 GHz, or Ri(53,58), can be found from Fig. 7 bysubtracting the radar return for 58 GHz from that for53 GHz. Fig. 8 shows Ri(53,58) measured at altitudes of1000, 2000, 3000, and 4000 ft. The measured data for thethree pressure days are shown in the figure as well as thepredicted Ri(53,58) using the instrument model and

50 51 52 53 54 55 56 57 58 59 60-90

-85

-80

-75

-70

-65

-60

-55

-50

Frequency (GHz)

Rad

ar R

etur

n (d

B)

3000 ft

1007 mb

1028 mb

1018 mb

DiBAR Measurement 1 DiBAR Measurement 2 DiBAR Measurement 3

Fig. 7. DiBAR measured radar return and model predictions for several

sea surface pressures.

microwave atmospheric attenuation model discussedabove. The figure illustrates the affect of increase inaltitude. As the altitude increases the increased pathlength increases the proportionality constant between Ri

and P0 in Eq. (4). Thus, ignoring the receiver SNR, a lessprecise estimate of Ri is required for the same surfacepressure precision at higher altitudes. Conversely, at1000 ft larger changes in barometric pressure would berequired to produce a detectable change in Ri. Thisdemonstrates the impact of the reduction in antennagain and limiting the useful measurement altitude to3000 ft. However, the differential absorption index shownin Fig. 8 agrees well with the predicted values for Ri,through 3000 ft altitude.

5. Summary

The goal of the initial flight testing was to demonstratedifferential radar measurement approach. The DiBARmeasurements for the Chesapeake Bay at multiplealtitudes demonstrated very good agreement betweenmeasured and predicted results for altitudes belowapproximately 3000 ft and for frequencies below56 GHz. In addition, multiple measurements at thesealtitudes indicate little change over several minutes. Thissuggests that changes on the surface RCS over these timescales can be ignored for these surface conditions andspatial resolution. As expected, above 3000 ft the reducedantenna gain resulted in insufficient signal-to-noise ratio.However, the measured differential absorption index wasin general agreement with the modeled values. Further,although beyond the scope of these initial flight tests,variations in the DiBAR measurements for 3000 ft mea-surements appear to be in the range 75 mb. These resultsare encouraging and consistent with our accuracy goal.The next flight testing will include an assessment of thebarometric pressure measurement for high altitude andfuture satellite operations.

R. Lawrence et al. / Journal of Quantitative Spectroscopy & Radiative Transfer 112 (2011) 247–253 253

References

[1] Barton IJ, Scott JC. Remote measurement of surface pressureusing absorption in the oxygen A-band. Appl Opt 1986;25:3502–7.

[2] Korb CL, Weng CY. A theoretical study of a two-wavelength lidartechnique for the measurement of atmospheric temperature profiles.J Appl Meteorol 1982;21:1346–55.

[3] Wu M-L. Remote sensing of cloud top pressure using reflectedsolar radiation in the oxygen A-band. J Clim Appl Meteor 1985;24:539–46.

[4] Lin B, Hu Y. Numerical simulations of radar surface air pressuremeasurements at O2 bands. IEEE Geosci Rem Sens Lett 2005;2:324–8.

[5] Lawrence R, et al. Differential absorption microwave radar measure-ments for remote sensing of atmospheric pressure. In: Proceedingsof the IEEE international geoscience and remote sensing symposium.July 2007.

[6] Liebe H. MPM—an atmospheric millimeter-wave propagation mod-el. Int J Infrared Millimeter Waves 1989;10:631–50.

[7] Lin B, Harrah S, Neece R, Lawrence R, Fralick D. In: The feasibility ofradar-based remote sensing of barometric pressure, final report.NASA Earth Science Technology Office; August 10, 2006.