This paper presents validation of ionospheric Global Positioning System (GPS) radio occultation (RO) measurements of the GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP-O). The primary source of uncertainty impacting GAP-O data products is the receiver differential code bias (rDCB). A minimization of standard deviations (MSD) technique for rDCB estimate has shown the most promise, and resulted in estimates ranging from -39 to -29 TECU, including a steady, long term decrease in rDCB magnitude. MSD estimates agree well with the 'assumption of zero topside TEC' method at satellite apogee in the polar cap. Bias-corrected topside TEC of GAP-O was validated by statistical comparison with topside TEC obtained from ground-based GPS TEC and ionosonde measurements. GAP-O and ground-based topside TEC had similar variability, however GAP-O consistently underestimated the ground-derived topside TEC by up to 8 TECU. Ionospheric electron density profiles obtained from Abel inversion of GAP-O occultation TEC showed consistently good agreement with F-region densities of incoherent scatter radar measurements, however RO-derived E-region densities were not as reliable at high latitudes.