Authors: Jia Huang (SSL/UCB), Davin E. Larson (SSL/UCB), Orlando Romeo (SSL/UCB), Mingzhe Liu (SSL/UCB), Roberto Livi (SSL/UCB), Ali Rahmati (SSL/UCB), Phyllis Whittlesey (SSL/UCB), Samer T. Alnussirat (SSL/UCB), Sam Badman (CfA), Rohit Chhiber (U. Delaware), Mike Stevens (CfA), Michael Terres (CfA), Srijan Bharati Das (CfA), The SWEAP/FIELDS team

A key science goal for Parker Solar Probe (PSP) is to measure the angular momentum loss rate of the Sun due to non-radial outflow of the solar wind. Inside the subsonic region, the corona is expected to approximately co-rotate. This results in a significant transverse velocity component in the solar wind that is most significant in the near Sun region. As of orbital Encounters 22 to 27, PSP reached within ~9.8 solar radii of the Sun, and additionally the extremely large transverse spacecraft velocity brings the aberrated solar wind ion distribution almost fully in the field of view (FOV) of the SPAN-Ion instrument, resulting in a much better determination of the solar wind velocity. Accurate determination of the transverse velocity is biased by instrumental issues. We have re-evaluated relative geometric values for the SPAN-Ion anodes to improve systematic uncertainties in the transverse velocity of both proton and alpha particles that have not been accounted for in previous studies. We also apply forward modeling techniques to account for the unmeasured portion of the SPAN-Ion FOV. In addition, we also examine alternative methods to derive the transverse solar wind velocity with the best FOV coverage to date. With the calibrated measurements, we analyzed the solar wind angular momentum flux during the latest six encounters, and we discussed its variations in different solar wind environments.