  Observing exoplanets through transmission spectroscopy supplies detailed
information on their atmospheric composition, physics, and chemistry. Prior to
JWST, these observations were limited to a narrow wavelength range across the
near-ultraviolet to near-infrared, alongside broadband photometry at longer
wavelengths. To understand more complex properties of exoplanet atmospheres,
improved wavelength coverage and resolution are necessary to robustly quantify
the influence of a broader range of absorbing molecular species. Here we
present a combined analysis of JWST transmission spectroscopy across four
different instrumental modes spanning 0.5-5.2 micron using Early Release
Science observations of the Saturn-mass exoplanet WASP-39b. Our uniform
analysis constrains the orbital and stellar parameters within sub-percent
precision, including matching the precision obtained by the most precise
asteroseismology measurements of stellar density to-date, and further confirms
the presence of Na, K, H$_2$O, CO, CO$_2$, and SO$_2$ atmospheric absorbers.
Through this process, we also improve the agreement between the transmission
spectra of all modes, except for the NIRSpec PRISM, which is affected by
partial saturation of the detector. This work provides strong evidence that
uniform light curve analysis is an important aspect to ensuring reliability
when comparing the high-precision transmission spectra provided by JWST.
