Ionospheric plasma frequencies at the altitude of the CHAMP satellite have been deduced from ionosonde true-height profiles for Jicamarca, Peru, and have been compared with the in situ measurements made by CHAMP. The differences between the plasma frequencies have been found to be well within the uncertainties associated with the ionosonde profiles, confirming the validity of the CHAMP measurements. For satellite-ionosonde separations of less than 250 km and for satellite altitudes below the peak of the F2 layer, the average discrepancy between the two plasma frequencies is 0.25 MHz or 4%. For the most reliable ionosonde measurements, the average discrepancies reduce to 0.18 MHz (or 1.7%), with a standard deviation of 0.16 MHz (or 1.5%). Given the validity of the CHAMP plasma frequencies, corresponding ionosonde and CHAMP observations have been used to support the practice of extending the ionosonde profile above hmF2 by assuming a Chapman layer with a constant scale height equal to that of the lower side of the F2 layer peak. The average discrepancy for CHAMP passing above the peak of the F2 layer is 0.22 MHz (or 2.6%), and the standard deviation is 0.8 MHz (or 13.3%).