However, the signal-to-noise ratio is fairly low and advanced information processing is required for a correct and reliable detection of the echoes generated by the objects.
Signals that are not of interest might be echoes from the ground or rain, which can mask and interfere with the detection of the desired echo from the aircraft.
What if, for instance, researchers couldn't repeat the 1963 Bell Labs detection of the supposed echo from the big bang?
Passive acoustics relies on the detection of sounds produced by the target (animal), whereas active acoustics relies on the detection of the return echo of a transmitted acoustic signal that reflects off the target.
A sudden increase in h'F and hmF2 can also be noticed a few minutes prior to detection of the first echoes on July 27.
However, with the increase of the resolution of modern radars and other detection equipments, the echo signal of a target may be distributed in a different range resolution cell, so the measurement is no longer equivalent to a point, i.e., a single target may produce multiple measurements.
In the present study, we design and examine three types of ultrasound interface detection techniques: the echo intensity technique, the local Doppler technique, and the velocity-variance technique, and investigate and compare the merits and limitations of each.
We found that the airglow measurements (630 nm) made by a collocated all-sky camera show the occurrence of ionospheric bottomside F-region perturbations prior to the detection of ESF echoes in all three nights.
The airglow measurements (630 nm, red line) made by a collocated all-sky camera indicate the occurrence of ionospheric bottomside F-region perturbations as early as 1 hour prior to the detection of ESF echoes.
The detection of F-region echoes during the observation campaign is in good agreement with previous radar studies of June solstice F-region irregularities (Patra et al. 2009; Yokoyama et al. 2011; Guozhu et al. 2012; Otsuka et al. 2012).
