  Many techniques for measuring neutron star radii rely on absolute flux
measurements in the X-rays. As a result, one of the fundamental uncertainties
in these spectroscopic measurements arises from the absolute flux calibrations
of the detectors being used. Using the stable X-ray burster, GS 1826-238, and
its simultaneous observations by Chandra HETG/ACIS-S and RXTE/PCA as well as by
XMM-Newton EPIC-pn and RXTE/PCA, we quantify the degree of uncertainty in the
flux calibration by assessing the differences between the measured fluxes
during bursts. We find that the RXTE/PCA and the Chandra gratings measurements
agree with each other within their formal uncertainties, increasing our
confidence in these flux measurements. In contrast, XMM-Newton EPIC-pn measures
14.0$\pm$0.3 % less flux than the RXTE/PCA. This is consistent with the
previously reported discrepancy with the flux measurements of EPIC-pn, compared
to EPIC-MOS1, MOS2 and ACIS-S detectors. We also show that any intrinsic time
dependent systematic uncertainty that may exist in the calibration of the
satellites has already been implicity taken into account in the neutron star
radius measurements.
