| Summary: | A total of 4991 ionograms recorded from April 1997 to December 2017 by the Millstone Hill Digisonde (42.6°N, 288.5°E) were considered, with simultaneous <i>N</i><sub>e</sub>(<i>h</i>)<sub>[ISR]</sub> profiles recorded by the co-located Incoherent Scatter Radar (ISR). The entire ionogram dataset was scaled with both the Autoscala and ARTIST programs. The reliability of the <i>h</i><sub>m</sub>F2 values obtained by ARTIST and Autoscala was assessed using the corresponding ISR values as a reference. Average errors <i>Δ</i> and the root mean square errors RMSE were computed for the whole dataset. Data analysis shows that both the Autoscala and ARTIST systems tend to underestimate <i>h</i><sub>m</sub>F2 values with |<i>Δ</i>| in all cases less than 10 km. For high magnetic activity ARTIST offers better accuracy than Autoscala, as evidenced by RMSE<sub>[ARTIST]</sub><RMSE<sub>[</sub><sub>Autoscala</sub><sub>]</sub>, under both daytime and nighttime conditions, and considering all hours of the day. Conversely, under low and medium magnetic activity Autoscala tends to estimate <i>h</i><sub>m</sub>F2 more accurately than the ARTIST system for both daytime and nighttime conditions, when RMSE<sub>[</sub><sub>Autoscala</sub><sub>]</sub><RMSE<sub>[ARTIST]</sub>. However, RMSE<sub>[</sub><sub>Autoscala</sub><sub>]</sub> slightly exceeds RMSE<sub>[ARTIST]</sub> for the day as a whole. RMSE values are generally substantial (RMSE>16 km in all cases), which places a limit on the results obtainable with real-time models that ingest ionosonde data.
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