  Time-distance helioseismology is one of the primary tools for studying the
solar meridional circulation. However, travel-time measurements of the
subsurface meridional flow suffer from a variety of systematic errors, such as
a center-to-limb variation and an offset due to the P-angle uncertainty of
solar images. Here we apply the time-distance technique to contemporaneous
medium-degree Dopplergrams produced by SOHO/MDI and SDO/HMI to obtain the
travel-time difference caused by meridional circulation throughout the solar
convection zone. The P-angle offset in MDI images is measured by
cross-correlating MDI and HMI images. The travel-time measurements in the
south-north and east-west directions are averaged over the same observation
period for the two data sets and then compared to examine the consistency of
MDI and HMI travel times after correcting the systematic errors.
  The offsets in the south-north travel-time difference from MDI data induced
by the P-angle error gradually diminish with increasing travel distance.
However, these offsets become noisy for travel distances corresponding to waves
that reach the base of the convection zone. This suggests that a careful
treatment of the P-angle problem is required when studying a deep meridional
flow. After correcting the P-angle and the removal of the center-to-limb
effect, the travel-time measurements from MDI and HMI are consistent within the
error bars for meridional circulation covering the entire convection zone. The
fluctuations observed in both data sets are highly correlated and thus indicate
their solar origin rather than an instrumental origin. Although our results
demonstrate that the ad hoc correction is capable of reducing the wide
discrepancy in the travel-time measurements from MDI and HMI, we cannot exclude
the possibility that there exist other systematic effects acting on the two
data sets in the same way.
