Li, YunyangEimer, JosephAppel, JohnBennett, CharlesBrewer, MichaelBruno, Sarah MarieBustos, RicardoChan, CarolChuss, DavidCleary, JosephDahal, SumitDatta, RahulCouto, Jullianna DenesDenis, KevinDunner, RolandoEssinger-Hileman, ThomasHarrington, KathleenHelson, KyleHubmayr, JohannesIuliano, JeffreyKarakla, JohnMarriage, TobiasMiller, NathanPerez, CarolinaMoralesParker, LucasPetroff, MatthewReeves, RodrigoRostem, KarwanRyan, CaleighShi, RuiShukawa, KojiValle, DenizWatts, DuncanWeiland, J.Wollack, EdwardXu, ZhileiZeng, Lingzhen2025-03-112025-03-112025-01-21https://doi.org/10.48550/arXiv.2501.11904http://hdl.handle.net/11603/37731We present measurements of large-scale cosmic microwave background (CMB) E-mode polarization from the Cosmology Large Angular Scale Surveyor (CLASS) 90 GHz data. Using 115 det-yr of observations collected through 2024 with a variable-delay polarization modulator, we achieved a polarization sensitivity of 78 µK arcmin, comparable to Planck at similar frequencies (100 and 143 GHz). The analysis demonstrates effective mitigation of systematic errors and addresses challenges to large-angular-scale power recovery posed by time-domain filtering in maximum-likelihood map-making. A novel implementation of the pixel-space transfer matrix is introduced, which enables efficient filtering simulations and bias correction in the power spectrum using the quadratic cross-spectrum estimator. Overall, we achieved an unbiased time-domain filtering correction to recover the largest angular scale polarization, with the only power deficit, arising from map-making non-linearity, being characterized as less than 3%. Through cross-correlation with Planck, we detected the cosmic reionization at 99.4% significance and measured the reionization optical depth τ = 0.053⁺⁰˙⁰¹⁸₋₀.₀₁₉, marking the first ground-based attempt at such a measurement. At intermediate angular scales (ℓ>30), our results, both independently and in cross-correlation with Planck, remain fully consistent with Planck's measurements.24 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domainhttps://creativecommons.org/publicdomain/mark/1.0/Astrophysics - Cosmology and Nongalactic AstrophysicsText