Research On Atmospheric Stellar Occultation Technology In Near Space

Near space refers to the range of 20-200 km altitude area,which has become the focus of scientific research because of its important use value.At present,the space environment is known to be extremely complex,including temperature changes,fluctuations,tides,turbulence and other phenomena.Existing space-based remote sensing detection technology,rocket in-situ detection technology and other detection data have conducted a lot of research on the above aspects.Atmospheric stellar occultation technology,as an important one of advanced space-based detection technologies,is an effective means to detect the middle and upper atmospheres of planets by using the spectral absorption and refraction characteristics of starlight of the earth's atmosphere.It can detect the density,temperature,and composition of the atmosphere,and other planetary atmospheric parameters,the relevant research of this technology has not been carried out in China.This technology was developed in the1960 s and has been successfully applied to atmospheric exploration of planets such as the Earth,Venus,Saturn,Mars,and a series of scientific researches.The principle of stellar occultation is that the ratio of the spectrum of stars obtained through the planetary atmosphere to the spectrum that has not passed through the planetary atmosphere is closely related to the atmospheric absorption component.Based on Beer Lambert's law,it can be used to retrieve the number density of the global related absorption component vertical profile.Stellar occultation technology has the advantages of simple light path,high detection accuracy,and global coverage.Based on the front-end scientific research of stellar occultation,this paper forms an end-to-end data system,that is,from principle analysis to occultation event simulation and prediction to data inversion and error analysis,laying the foundation for the subsequent formation of miniaturized instruments,and the following are carried out Four aspects of work.(1)In-depth analysis of the principle and system of stellar occultation detection,and using the MODTRAN model to establish an occultation working model,analyze the characteristic spectral absorption lines of ozone,nitrogen dioxide,nitrogen trioxide and oxygen,and calculate the atmospheric transmittance of the corresponding band.Based on this result,the signal-to-noise ratio and relative error of the absorption lines of the four components are further derived and calculated.Based on the calculated signal-to-noise ratio and relative error,the star sources in the whole sky are screened.Under the standards of high signal-to-noise ratio and low detection error,the range of the apparent magnitude of the target star source,absorption spectrum line that can be used and detected,and coordinate distribution and spectral type of the target star are given.The results show that the apparent magnitude range of the star source that can be used as a light source for detection is-1.45 to 3.55.At this time,the signal-to-noise ratio of the received spectral signal is greater than 100,and the relative error of detection can be as small as 1%.(2)Based on the purpose of providing theoretical guidance for satellite orbit design and detection load design,the observation event simulation is carried out,that is,the relative coordinate position of the LEO satellite and the star in the earth-fixed coordinate system is used to perform the star-LEO occultation orbit observation simulation.The specific process is: firstly,reading the coordinate position of the LEO satellite and the star,and setting the simulation observation time to 24 hours.Secondly,determining whether it is in the occultation state,when the occultation event starts,calculate and give the occurrence of the occultation event speed,latitude and longitude,etc.,until the end of the time.According to the simulation results,statistical calculations and analysis of the daily observations,duration,global distribution and drift speed of the occultation event,etc.,have the following results: 1)During the 24-hour orbit simulation period,5563 daily observations of the occultation event were obtained,including 2737 ascending occultations and 2826 descending occultations;2)From the perspective of global distribution,the events are mainly distributed in low latitudes,with the fewest poles.The number of mid-latitudes is equivalent,and the distribution in the longitude direction is relatively uniform;3)Distribution from azimuth,normal occultation accounts for 78.25% of the total,with an average duration of 1.5 minutes,and the horizontal drift range of the cut point is18km?600km;4)The occurrence rate of side occultation events is 21.75%,which is compared with normal occultation.Its duration is longer,the horizontal drift speed of the tangent point is greater,and the azimuth angle changes greater.(3)Observation simulation can realize the fine structure analysis of the spectral absorption,grasp the status of the observation,and the obtained atmospheric transmittance can be used to develop the inversion algorithm.First,the infrared spectrum of Sirius(755nm ? 774nm)is input as the original spectrum,and the route of infrared light transmission in the atmosphere from the ground to the height of110 km is simulated by three-dimensional ray tracing.Among them,the set frequency is 3.95e+14Hz,the shape of the earth is ellipsoid,the atmosphere model is selected as a neutral atmosphere,and the position data of the target star and the low-orbit satellite in the geo-fixed system are known,and each point on each ray is obtained.Secondly,use the HITRAN database to output the relevant parameters of the oxygen molecular absorption line,including the absorption line intensity and low-energy state energy,and use the line-by-line integral to calculate the molecular absorption cross section.Finally,calculate the atmospheric transmittance of the band.In order to facilitate the verification of the algorithm and realize the miniaturization of the instrument,the characteristic absorption lines of 760 nm and 762 nm,namely oxygen molecules,are selected to calculate the atmospheric transmittance of the two spectral lines with altitude and the signal-to-noise ratio to guide the design of the instrument.In addition,in order to have a clearer understanding of the actual observation status,we have calculated the transmittance change caused by the atmospheric refraction.Through the above simulations and calculations,the following calculation results are obtained:The atmospheric transmittance obtained at the three altitudes of 80 km,100km,and110 km in the near-infrared band from 755 nm to 774 nm tends to approach 1 as the altitude increases.Compared with the 0.2nm spectral resolution,the atmospheric transmittance varies from 0.28 to 1 at 0.1nm resolution,which is larger than the former.In addition,the transmittance is 0.987 at a height of 110 km,and the accuracy of detection can be one less.The part of the transmittance caused by refraction is 1above 60 km,so the effect of atmospheric refraction can be ignored at altitudes above60 km.The light intensity signal-to-noise ratio obtained at the characteristic absorption lines of 760 nm and 762 nm are both above 100.When the spectral resolution is 0.1nm,the value of the light intensity signal-to-noise ratio is smaller,indicating that oxygen has a stronger absorption effect on the spectrum.(4)The development of the inversion calculation is the core of the stellar occultation technology.This paper uses the onion peeling method to invert the simulation data,and obtains the vertical number density distribution of oxygen from 0to 120 km,and compares it with the MSISE00 data.The fit is very good.The relative error does not exceed 0.3%.,And did local arc correction work.In order to further verify the inversion algorithm,the measured data of GOLD and GOMOS are selected for inversion.Among them,comparing the 130-230 km oxygen obtained from the inversion with the data released by GOLD,the error is within 15%;comparing the ozone data obtained from the inversion with the results released by GOMOS,the maximum relative error below 63 km does not exceed 20%,63 km The above shall not exceed 10% at most.