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There are only two globular clusters where a modest investment of HST time can yield a high-precision measurement of trigonometric parallaxdoublePoint NGC6397 and M4. The parallax to NGC6397 has been measured successfully with HST to 2% uncertainty. An important HST legacy would be the completion of this pair comma by measuring the M4 parallax while HST can still do so. In the context of the Galaxy.s globular cluster system comma these two clusters are complementary. NGC6397 is ancient (13 Gyr) and metal-poor (CORCHETE_OPENFe-HCORCHETE_CLOSE=-2.02) comma while M4 is younger (11.5 Gyr) and of intermediate metallicity (CORCHETE_OPENFe-HCORCHETE_CLOSE=-1.15). NGC6397 is a frequent fiducial for stellar population models comma while M4 hosts a rich set of RR Lyrae stars that can be used to better calibrate the period-luminosity relationship. The utility of each cluster as a population template and calibrator depends critically upon the distance uncertainties. The scarcity of population anchors in this regime is in contrast to the parameter space at younger ages (< 10 Gyr) and higher metallicities (CORCHETE_OPENFe-HCORCHETE_CLOSE > -1) comma where many open clusters have high-precision parallaxes. In a broader context comma these two clusters represent an essential independent check on the parallaxes from the Gaia mission comma which currently suffer from significant systematic errors on global comma intermediate (20 degrees) comma and small (1 degree) scales comma hampering distances to the Galaxy.s globular clusters. If these errors are eventually resolved comma HST parallaxes to globular clusters comma independent of Gaia with distinct methodology comma will provide an important validation. The observations we propose here will help anchor the distances to the entire globular cluster system of the Milky Way.