ference of the magnetic-field structure in the jets is complicated by the combined effects of Faraday rotation and time-variable structure. Previous studies [@Stirling2004; @Roberts2008; @Miller-Jones2008] have been hampered by lack of resolution and frequency coverage as well as the uncertain effects of spatial- and temporal-variations in Faraday rotation. The dependence of Faraday rotation on the square of the wavelength ($\lambda$), means that wide-band observations at mm wavelengths allow the projected field direction to be determined accurately in a single observation even close to the core, where Faraday rotation measures may be large [@Roberts2008]. We present our polarimetric 230GHz results in Sec.\[sec:polar\].

Optical spectroscopy and inference from Doppler shifts {#sec:SpeedsFromZeds}
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Spectra of SS433 spanning a wavelength range of approximately 5800 to 8500 Angstroms, and having a spectral resolution of $\sim4000$ were observed in the year prior to the ALMA observations whenever this target was a nighttime object. These were carried out with the multi-longitude Global Jet Watch telescopes each of which is equipped with an Aquila spectrograph; the design and testing of these high-throughput spectrographs are described by @Lee2018. The observatories, astronomical operations, processing and calibration of the spectroscopic data streams are described in @Blundell2018. Almost all of these spectra contain a pair of so-called “moving lines” arising from the most recently launched jet bolides in SS433. The wavelengths corresponding to the centroids of the blue-shifted and red-shifted H$\alpha$ emission were converted into redshift pairs with respect to H$\alpha$ in the rest frame of SS433 according to its systemic velocity with respect to Earth [@lockman2007]. From these redshift and blueshift pairs from a given spectrum were derived the launch speed of each pair of bolides [@Blundell05 equation 2]. This avoids the approximation of constant ejection