I describe and present the results of a newly developed fitting methodology optimized for very long time series. The development of this new methodology was motivated by the fact that we now have more than half a decade of nearly uninterrupted observations by the Global Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI), with fill factors as high as 82.2% and 89.8%, respectively. It was recently prompted by the availability of a 2088 day long time series of spherical harmonic coefficients produced by the MDI team. The fitting procedure uses an optimal sine multitaper spectral estimator, with the number of tapers based on the mode line width, the complete leakage matrix (i.e., horizontal as well as vertical components), and an asymmetric mode profile to fit simultaneously all the azimuthal orders with individually parameterized profiles. This method was applied to 2088 day long time series of MDI and GONG observations, as well as 728 day long subsets, and for spherical harmonic degrees between 1 and 25. The values resulting from these fits are intercompared (MDI vs. GONG) and compared to equivalent estimates from the MDI team and the GONG project. I also compare the results from fitting the 728 day subsets to the result of the 2088 day time series. This comparison shows the well-known change of frequencies with solar activity and how it scales with a nearly constant pattern in frequency and m/l. This comparison also shows some changes in the mode line width and the constancy of the mode asymmetry.
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