Wave Optics Based LEO-LEO Radio Occultation Retrieval

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Abstract

This paper describes the theory for performing retrieval of radio occultations that use probing frequencies in the XK and KM band. Normally radio occultations use frequencies in the L band and GPS satellites are used as the transmitting source and the occultation signals are received by a GPS receiver on board a Low Earth Orbit (LEO) satellite. The technique is based on the Doppler shift imposed, by the atmosphere, on the signal emitted from the GPS satellite. Two LEO satellites are assumed in the occultations discussed in this paper and the retrieval is also dependent on the decrease in the signal amplitude caused by atmospheric absorption. The radio wave transmitter is placed on one of these satellites while the receiver is placed on the other LEO satellite. One of the drawbacks of normal GPS based radio occultations is that external information is needed to calculate some of the atmospheric products such as the correct water vapor content in the atmosphere. These limitations can be overcome when a proper selected range of high frequency waves are used to probe the atmosphere. Probing frequencies close to the absorption line of water vapor have been included, thus allowing the retrieval of the water vapor content. Selecting the correct probing frequencies would make it possible to retrieve other information such as the content of ozone. The retrieval is performed through a number of processing steps which are based on the Full Spectrum Inversion (FSI) technique. The retrieval chain is therefore a wave optics based retrieval chain and it is therefore possible to process measurements that include multipath. In this paper simulated LEO to LEO radio occultations based on 5 different frequencies are used. The 5 frequencies are placed in the XK or KM frequency band. This new wave optics based
retrieval chain is used on a number of examples and the retrieved atmospheric parameters are compared to the parameters from a global ECMWF analysis model. This model is used in a forward propagator that simulates the electromagnetic field amplitudes and phases at the receiver on board the LEO satellite.
LEO-LEO cross-links radio ooccultations using high frequencies are a relative new technique and the possibilities and advantages of the technique still needs to be investigated. The retrieval of this type of radio occultations is considerable more complicated than standard GPS to LEO radio occultations, because the attenuation of the probing radio waves is used in the retrieval and the atmospheric parameters are found using a least square solver. The best algorithms and the number of probing frequencies that is economical viable must also be determined. This paper intends to answer some of these questions using end to end simulations.
Original languageEnglish
JournalRadio Science
Volume51
Issue number6
Pages (from-to)589–602
ISSN0048-6604
DOIs
Publication statusPublished - 2016

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