The GIRO (Global Ionospheric Radio Observatory) comprises records of subpeak
ionospheric characteristics, including altitude profiles of electron
density and plasma drift velocity, in the altitude range 90 km to the
peak height of the F2 layer, measured by a global ground-based network
of active high frequency (HF) radiowave remote sensing instruments,
ionosondes. The ionosonde uses stepped- or fixed-frequency probing
radio signals transmitted from the ground for propagation into ionosphere,
total reflection from an area of matching plasma density, and return to
the same or different ionosonde location for detection. The detected
echoes are evaluated for their travel time and other characteristics
such as amplitude, phase, polarization, Doppler frequency shift, and
angle of arrival. The signal travel time is commonly translated to
the virtual range to the reflecting area in the ionosphere in the
assumption of the signal propagating at the speed of light.
Stepped-frequency measurements provide adequate information to compute
true range of reflections by accounting for refractive properties of plasma.

The ionosonde designs are commonly divided in two classes, (1) a pulsed sounder
with stepped frequency that transmits short pulses to detect the echo arrival
in the time domain, and (2) a chirp-sounder that transmits a continuous
swept-frequency signal to detect its reflections in the frequency domain.
The stepped- or swept-frequency sounders produce inherently 2-dimensional
data record (frequency x travel time), ionogram. Each element of the 2D
ionogram structure holds multiple signal characteristics measured in the
ionosonde's receiver channels. Thus acquired information is processed to
detect echoes and use their attributes to derive ionospheric characteristics
along the propagation path.

The fixed-frequency measurement, while using the same concept of radio
sounding by total reflection, targets specific plasma densities in the
ionosphere, and usually provides higher Doppler frequency resolution by
transmitting pulses at the same frequency for several seconds.
Simultaneously occurring reflection points in a structured ionosphere
are identified by their different echo Doppler frequencies allowing the
computation of Doppler skymaps and plasma drift velocity.