Digital elevation models of glaciated terrain produced by the NASA/Jet Propulsion Laboratory (JPL) airborne interferometric synthetic-aperture radar (InSAR) instrument in Greenland and Alaska at the C- (5.6 cm wavelength) and L-band (24-cm) frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth, or penetration depth, Δp. On cold polar firn at Greenland summit, Δp=9¿2 m at C- and 14¿4 m at L-band. On the exposed ice surface of Jakobshavn Isbrae, west Greenland, Δp=1¿2 m at C- and 3¿3 m at L-band except on smooth, marginal ice where Δp=15¿5 m. On colder marginal ice of northeast Greenland, Δp reaches 60 to 120 m at L-band. On the temperature ice of Brady Glacier, Alaska, Δp is 4¿2 m at C- and 12¿6 m at L-band, with little dependence on snow/ice conditions. The implications of the results on the scientific use of InSAR data over snow/ice terrain is discussed. ¿ 2001 American Geophysical Union