Early In-Orbit Test and Initial Observations of IAMMAP, the Ionospheric Plasma Measuring Instruments on CAS500-3

AMMAP on CAS500-3, launched in November 2025 into a ∼600 km Sun-synchronous orbit, combines a Langmuir probe (LP), biased with respect to the spacecraft body, and an impedance probe (IP), which derives the density from the upper-hybrid resonance regardless of the spacecraft potential. We report the operation from December 2025 to June 2026 and initial observations on two reference days in February and June. Before the orbital noon, the sun-pointing attitude turns the solar arrays toward the direction of flight and the spacecraft charges negatively beyond the LP sweep range, so that the LP collects only ions in 50–70% of the dayside sweeps. The IP is lost mainly in the low-density winter hemisphere, where its resonance is too weak. Where both are valid, the densities agree within a factor of ∼1.7 in the median, the IP typically 20–40% lower; both lie above the IRI model, the IP by a factor of 1.4–2.1 and the LP by 2.1–3.2. Even in the ion-only sweeps, the slope of the ion current increases with the density and, calibrated against the IP for each day, gives the density with a scatter of 20–35%. Taking the IP as the reference and filling its gaps with the calibrated LP data yields a continuous profile over more than 90% of the two orbits examined.