Argenti, Fabrizio

Cuccoli, Fabrizio

Facheris, Luca

English

Florence Research

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 1407Normalized Differenzial Spectral Attenuation among Co-rotatingLEO Satellites: Performance Analysis for Estimating theTropospheric Water VaporF. Cuccoli1, L. Facheris2, and F. Argenti21U.O. CNIT RaSS, c/o Dipartimento di Elettronica e Telecomunicazioni, Universita` di FirenzVia di Santa Marta, 3, Firenze 50139, Italy2Dipartimento di Elettronica e Telecomunicazioni, Universita` di Firenzevia di Santa Marta, 3, Firenze 50139, ItalyAbstract— NDSA (Normalized Differential Spectral Absorption) is as a differential attenua-tion measurement method for the estimation of the total content of water vapor (IWV, IntegratedWater Vapor) along a tropospheric propagation path between two Low Earth Orbit (LEO) satel-lites. The NDSA has been deeply analyzed assuming two counter rotating satellites in Ku andK band for tangent altitudes up to 10 km. In this paper we present the main NDSA measure-ment characteristics of the CO-rotating configuration with one transmitting satellite and manyreceiving ones assuming the multi-band (K, Ku and M) approach on each radio link and plausiblereference propagation conditions for tangent altitudes up to 15 km.1. INTRODUCTIONThe Normalized Differential Spectral Attenuation (NDSA) approach is based on the conversion of aspectral parameter called “Spectral Sensitivity” (SS) into the total content of water vapor along thepropagation path between two LEO satellites. In [1] the potential of spectral sensitivity in providingdirect estimates of Integrated Water Vapor (IWV) along LEO-LEO tropospheric propagation pathsin the 15–25GHz range is shown while in [2] the accuracy with which the spectral sensitivityparameter can be measured has been analyzed.A basic result was that the spectral sensitivity measurement accuracy can be estimated throughthe theoretical approximation as long as the averaged Signal to Noise Ratio (SNR) keeps above20 dB, while below such SNR level measurements become not reliable. Moreover, this holds as longas the integration time used for measurements keeps smaller than the decorrelation time of thescintillation phenomenon.A complete simulation tool has been presented in [3] for the study of the NDSA measurementperformance in counter rotating LEO-LEO configuration (see Figure 1(a)) at global scale for singleand multiple global atmospheric profiles, assuming spherical symmetry for the atmospheric struc-ture, realistic LEO satellites’ orbit configuration, link budget, and disturbances sources. The mainresults of this performance analysis is that NDSA at 17.25GHz can be measured in case of mediumscintillation up to 6 km tangent altitude with NSED (the standard deviation of the absolute er-ror) smaller than 50% and at 20.20GHz can be estimated both in medium scintillation conditionsbetween 3 and 7 km with NSED smaller than 20% and also in strong scintillation conditions withNSED smaller than 60%.Some other studies about the NDSA in counter rotating LEO-LEO configuration in K/Kubands have been completed under the independent ESA (European Space Agency) study calledAlMetLEO [4] that provided a significant insight into the SS-IWV relationships up to 12 km altitudein the limb geometry based on an extended radiosonde data analysis, into the modelization of signalfluctuations due to tropospheric turbulence and into the SS accuracy achievable under given SNRand scintillation levels.In [5] for the first time the use of M band for NDSA measurements, always in counter rotatingLEO-LEO configuration, has been introduced. Specifically, 179 and 182GHz have been proposedas good candidates for water vapor estimates at tangent altitudes higher than 11 km, since M bandmeasurements are very robust to receiver noise and practically insensitive to scintillation effects.While the counter-rotating configuration provides valid SS measurements data only during therelative set/rise occultation events the co-rotating one provide them in a time continuous manner.For this reason the authors decided to investigate the NDSA capabilities also in LEO-LEO co-rotating configuration (see Figure 1(b)).1408 PIERS Proceedings, Moscow, Russia, August 19–23, 2012(a) (b)Figure 1: (a) Counter rotating and (b) co-rotating LEO-LEO configuration.Figure 2: IWV at (top-down) 3, 6 and 9 km tangent altitude vs. SS at (left to right) 17, 19 and 21GHz.For each plot the correlation coefficient ρ and the relative error re after a linear least squares fit are shown.Red, green and blue colors are related to the latitude range: red for [0◦– 30◦], green for [30◦–60◦] and bluefor [60◦–90◦] both for the Northern and the Southern hemisphere.In this paper we present the main NDSA measurement characteristics of the CO-rotating con-figuration (see Figure 1(b)) with one transmitting satellite and many receiving ones. We discussthe result of a performance analysis of the sensitivity measurements assuming the multi-band (K,Ku and M) approach on each radio link and plausible reference propagation conditions.2. SENSITIVITY MEASUREMENTS VS INTEGRATED WATER VAPORIn order to recall the main features of the SS-IWV relationships, Figure 1 shows the SS vs. theIWV for the 17, 19 and 21GHz sensitivity channels at 3, 6 and 9 km tangent altitudes. Theplots are computed basing on atmospheric vertical profiles at global scale related to the ECMWF(European Centre for Medium-Range Weather Forecasts) model for the 12:00 UTC on 15 October2011 interpolated on a 5◦ latitude× 5◦ longitude grid. The SS are computed using the simulationtool developed in [3].Notice that the highest correlations are: at 3 km for 17GHz, at 6 km for 19GHz and at 9 km for21GHz.Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 1409Table 1: Values of the link power budget parameters. f0: sensitivity channel, f1/2 frequency tones.fo[GHz]f1[GHz]f2[GHz]Tx Power[dBW]Tx & Rx AntennaGain [dB]Rx System noisetemp. [dBK]17.00 16.90 17.10 3.0 26.9 25.319.00 18.90 19.10 3.0 27.8 25.821.00 20.90 21.10 3.0 28.5 26.3179.00 178.80 179.20 3.0 34.8 23.8182.00 181.80 182.20 3.0 34.9 23.8Figure 3: Received power vs. tangent altitude.3. CO-ROTATING OBSERVATION GEOMETRYAssuming an elliptical orbit with the semi-major axis of 6651 km, the spherical Earth with theradius of 6371 km, the eccentricity of 0.0005 and two satellites separated by an angle of 0.582398rad (this case corresponds to the 0 km tangent altitude for circular orbit), the tangent altitude variesbetween 3 and 10 km and the LEO-LEO link length vary between 3777 and 3780 km corresponding,respectively, to 10 and 3 km tangent altitude. The revolution period for this kind of orbit is 5061 sthat gives an average speed of about 8 km/s for the link projection point at ground. Assuming asampling time of 1 s, the link altitude can be considered constant during the integration interval.Therefore, a sensitivity measurement each 8 km is provided and this is related to the constanttangent altitude, given by orbital position, within the 3–10 km range.In order to sweep the troposphere between 3 and 15 km with the minimum number of tworeceiving satellites, the angularly separation must vary between 0.005232 and 0.005242 rad (thatcorrespond to a distance of about 34 km). In this way, when the tangent altitude of the fartherreceiving satellite link is 3 km, the tangent altitude of the closer one is 8 km and when the tangentaltitude of the farther receiving satellite link is 10 km the tangent altitude of the closer one is 15 km.4. PERFORMANCE OF THE SENSITIVITY MEASUREMENT IN CO-ROT GEOMETRYUsing the theoretical analysis proposed in [2] it is possible to compute the Normalized Root MeanSquare Error (NRMSE) of the spectral sensitivity as function of the SNRm (the average SNR ofthe two tones received on the corresponding channels), the differential attenuation ∆A (betweenthe two tones of each channel) and two other parameters for the scintillation impairment (i.e., ρthan accounts for the frequency tone correlation and σχ that accounts for the scintillation power).For the computation of SNRm and ∆A we simulated the power at the receiver assuming thelink power budget parameters listed in Table 1, the MLS Atmospheric model [1] and a sphericalsymmetry for the atmospheric structure. The propagation effects have been simulated using thecore of the software tool developed in [3] that is based on the MPM93 propagation model [5].Notice that the MLS model does not account for liquid and ice particles, therefore the simulatedpropagation scenario is related to a so called “dry atmosphere”.Figure 3 shows the received power for each tone and Figure 4 shows ∆A and SNRm for each1410 PIERS Proceedings, Moscow, Russia, August 19–23, 2012Figure 4: Differential attenuation and SNRm vs. tangent altitude.sensitivity channel. Notice that SNRm is greater than 40 dB above 1 km at 17GHz channel, above2 km at 19GHz, above 3 km at 21GHz and above 10 km for the M band.Considering ∆A greater than 1 dB and SNRm greater than 40 dB (that are surely optimalconditions for converting the SS in IWV) the NRMSE of the spectral sensitivity computed using thetheoretical analysis proposed in [2] is lower than 10% for medium scintillation cases (σχ = 0.3 dB)and lower than 5% for low scintillation cases (σχ = 0.03 dB).5. CONCLUSIONSThe performance of the SS measurements in CO-RO has been evaluated in a reference atmosphericscenario. Assuming a plausible power link budget for a CO-RO orbital geometry covering thetangent altitudes between 0 and 15 km, the received power has been simulated for the frequencytones related to the 17, 19, 21, 179 and 182GHz sensitivity channels. The NRMSE of SS hasbeen computed for ∆A greater than 1 dB and SNRm greater than 40 dB. In these conditions,the NRMSE is lower than 10% and 5% for hypothesis of medium and low scintillation effects,respectively. Even if the scintillation effects in CO-RO is still not fully developed it is envisagedthat the CO-RO configuration should be characterized by low scintillation impairments. Furtherstudies are in progress in order to develop a scintillation model for the CO-RO geometry followingthe same approach used in [3] for the COUNT-RO case.REFERENCES1. Cuccoli, F. and L. Facheris, “Normalized differential spectral attenuation (NDSA): A novelapproach to estimate atmospheric water vapor along a LEO — LEO satellite link in the Ku/Kbands,” IEEE Transactions on Geoscience and Remote Sensing, Vol. 44, No. 6, 1493–1503,2006.2. Cuccoli, F., L. Facheris, and F. Argenti, “Normalized differential spectral attenuation (NDSA)measurements between two LEO satellites: Performance analysis in the Ku/K bands,” IEEETransactions on Geoscience and Remote Sensing, Vol. 46, 2345–2356, 2008.3. Cuccoli, F. and L. Facheris, “NDSA measurements between two LEO satellites in Ku andK bands for the tropospheric water vapor estimate: Performance evaluation at global scale,”IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2009, Vol. 5, V-296–V-299, 2009.4. Facheris, L., et al., “Alternative measurements techniques for LEO-LEO radio occultation(AlMeTLEO),” ESA-ESTEC Contract, No. 17831/03/NL/FF Final Report, Jul. 2004.5. Liebe, H. J., G. A. Hufford, and M. G. Cotton, “Propagation modeling of moist air and sus-pended water/ice particles at frequencies below 1000GHz,” AGARD, 52nd Specialists Meetingof the Electromagnetic Wave Propagation Panel on “Atmospheric Propagation Effects throughNatural and Man-Made Obscurants for Visible to MMWave Radiation”, Palma de Mallorca,Spain, May 1993.

Normalized Differenzial Spectral Attenuation among co-rotating LEO satellites: Performance analysis for estimating the tropospheric water vapor

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Normalized Differenzial Spectral Attenuation among co-rotating LEO satellites: Performance analysis for estimating the tropospheric water vapor

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