Challenges of Radio Occultation Data Processing in the Neutral Atmosphere S. Sokolovskiy COSMIC Retreat, 13-14 October, 2011

Upper stratosphere and lower troposphere are the regions of maximum errors and uncertainty of the GPS RO inversions In the lower troposphere: the signal reduces below noise level in terms of the amplitude Additive noise - main error source In the upper stratosphere: the signal reduces below noise level in terms of the exc. phase (Doppler) Multiplicative noise - main error source

Main problem of RO in LT - water vapor: - strong mean defocusing - fluctuation of phase and amplitude - accurate connection of phase (a fundamental observable of RO) is not possible high latitudes, winter: - low water vapor, no problem - RO signal blocked by surface before defocusing to noise - phase stays connected - low sensitivity to noise low latitudes: - strong defocusing & fluctuation - RO signal gradually descends below noise level - connection of phase not possible - inversion results depend on cutoff height and noise level

Uncertainty of wave optics inversions in LT cutoff height: -100 km; -150 km solid - as is; dashed - added noise (17V/V) tropics polar winter

Transforming RO signals from time - frequency to bending angle - impact parameter representations polar tropics noise cutoff Non-spherically symmetric irregularities of N in LT broaden local spectrum of WO transformed RO signal and make BA sensitive to cutoff height and noise

Testing of deep OL tracking (down to HSL=-350km) on FM-1 on Oct 1) Down to what heights the LT RO signals can be observed? 2) What other signals (contaminant to RO) are observed? Interfering GPS signals were first found in Metop-GRAS data, Bonnedal et al., 2010 Interfering signal from GPS PRN 23 The height from which the deep signal arrives RO signal propagated through LT, observed down to -300 km.

Main sources of RO inversion errors (including biases) in LT No significant errors in high latitudes, in winter Largest errors in tropics; reason: water vapor (defocusing, fluctuation) - receiver tracking errors (resolved with open loop tracking) - superrefraction: --- bias in refractivity (bending angle not biased) --- constrained inversions of BA to refractivity --- direct assimilation of BA in LT (difficult, large representativeness errors) - spread asymmetric spectrum of RO signal in LT - effect of noise - dependence on the length of RO signal - horizontal gradients (negative bias?) - how deep the signals shall be used ? (can deep signals be caused by spherically symmetric N) What can be done to reduce inversion errors in LT? - reflected signals - accurate measurement of delays

Ionospheric correction: LC=Ca*La-Cb*Lb where Ca=fa^2/(fa^2-fb^2); Cb=fb^2/(fa^2-fb^2) For La=L1=1.57542GHz, Lb=L2=1.2276GHz, Ca=2.546, Cb=1.546 For La=L2=1.2276GHz, Lb=L5=1.17645GHz, Ca=12.252, Cb=11.252 Errors of the ionospheric correction: - ray separation at different frequencies: - higher order terms of dependence of refractivity on frequency - diffraction effects by small-scale irregularities GPS LEO irregularities L1 L2

eliminated) by combining L1 and L2 BA at the same impact parameter Large-scale error due to ray separation is substantially reduced (but not eliminated) by combining L1 and L2 BA at the same impact parameter Residual error can be estimated by ray-tracing, using a model of electron density, and applied for correction of ionosphere-free bending angle An alternative approach (more simple but less accurate): a non-linear regression on L4=L1-L2 bending angle

Bending angle bias correction for solar max. 2002 and solar min. 2007 For improved accuracy, direction of magnetic field must be taken into account (higher order terms)

L2 LEO GPS L1 irregularities Small-scale ionospheric irregularities introduce fluctuating error 100-1000 times larger than the large-scale (bias). Main error source at heights > 30 km. Reasons: - ray separation - diffraction effects GPS LEO irregularities L1 L2 Left occ: larger bias error Right occ: larger fluct. error

Different types of ionospheric scintillation A - no scintillation B,C,D - scintillation in F-layer E,F,G - effect of E(Es) layer, can results in large inversion errors at 30-40 km Scintillation from E(Es) layer is observed ~10 times more often than from F-layer Effect of Es layer on RO signals

Localization of ionospheric irregularities by back propagation observation trajectory (phase & amplitude) solving Helmholtz equation in a vacuum, given boundary condition Y X F-layer E-layer

Localization by BP requires elongated irregularities of a given orientation. Can BP reduce the errors of the ionospheric correction without the knowledge of orientation of irregularities? L1 bend. ang. L2 bend. ang. LC bend. ang.

In Gras-METOP data scintillation from F-layer is observed more frequently than from E(Es)-layer (due to uneven sampling in LT) An example of Gras- METOP occultation with localizable scintillation in F-layer Ionospheric correction see next slide

Ionospheric correction after BP propagation of RO signal to location of irregularities (+2200 km) allows to significantly reduce the residual Errors of the ionospheric correction Unfortunately, this improvement is not possible for all occultations

Can reduction of the difference in the two frequencies reduce the diffraction-related errors of ionospheric correction? NO L1=1.57542GHz, L2=1.2276GHz, Ca=2.546, Cb=1.546 L2=1.2276GHz, L5=1.17645GHz, Ca=12.252, Cb=11.252

Reduction of noise propagation in inversion of bending angle Current approaches: Optimization of BA 1st guess does not depend on obs. BA 1st guess depends on obs. BA 1st guess: climatology 1st guess: NWP model 1st guess: fitted exponent 1st guess: fitted climatology GFZ WegC JPL DMI, UCAR Abel Inversion Generalization: optimal estimation of the state vector (P,T) by including ancillary information or constraints in the stratosphere - parametrization of T between mesopause and stratopause (incl. stratopause height as a parameter); - including T from NWP or other satellite observations; - including (T,P) from NWP