Demon woman with snake tongue (above) http://go.nasa.gov/2h0uwWT
Sakhalin & the Sea of Okhotsk (below & above) http://go.nasa.gov/2i8tDkl
Radio waves and Sounding the Ionosphere – Part 3
By Marcel H. De Canck,
Most of the information we have gathered worldwide concerning the properties of the ionosphere has come through the direct measurement of quantities by radio soundings. This technique employs the use of a radio transceiver that can transmit and receive vertically incident radio waves. There are several types of ionosondes currently in service of which I will examine some in this episode.
Techniques for Probing the Ionosphere
The technique of probing the ionosphere by using a transceiver was first used, as explained in the previous chapter, by G. Breit and M.A.Tuve. Also I already explained that radio waves are refracted or bent when they enter the region of increased electron density in the ionosphere. For each concentration of electron densities there is a plasma frequency below which all radio signals are refracted back to the earth, regardless of the angle of incidence used. For example: if the plasma frequency of the ionosphere above a transmitter is 5 MHz, then all radio signals transmitted vertically to the ionosphere that are less than 5 MHz will be returned back to the earth and all frequencies higher than 5 MHz will pass through the ionosphere into space.
It is this characteristic of the ionosphere that we exploit to probe the properties of the ionosphere. The device used to probe the ionosphere is known as an ionosonde. It consists of a combined radio transmitter and receiver capable to transmit pulses toward the above ionosphere and receiving the same signal pulse as it returns back to the receiver. Depending on the transmitter power and the wave attenuation during its travel, it is not rare to receive multiple echoes. The returned signal pulse can be reflected back towards the ionosphere by the earth surface and be refracted there again towards the ionosondes. In some cases this might happen a few times. It is self evident that each successive multiple echo pulse signal strength becomes weaker.
The pulsed Ionosonde
This type of ionosonde is the basis for most modern ionosondes. It works by transmitting a series of pulses vertically upwards into the ionosphere. A typical oscilloscope can be used to form a very simple view screen for ionosonde devices, although most modern ionosondes are more complex than this. The time-base of a cathode ray tube is used as a timing device to measure the delay time. When the echo or reflected radio pulse signal is received by the ionosondes, it is fed to the Y-plates of the tube in such a way that the electron beam deviates in direction, Fig. 65.1 (the A scan method).
The position of the pulse on the time base is a measure of the traveling time of the pulse and hence of the virtual height of reflection. The Y-deflection is related to the echo amplitude. It is also possible to apply the echo pulse to the grid of the oscilloscope to blank out the time base, Fig. 65.1 (the B-scan method). The conventional ionosonde for measuring the virtual height of the ionized layers and their respectively critical frequencies is a sweep frequency pulsed radar device. The frequency can range from about 0.1 MHz to 30 MHz with a sweep duration from a few seconds to a few minutes. As the frequency of the transmitted pulses is increased, the path of each pulse through the ionosphere varies. The time taken for the pulse to reach the ionosonde may and can change. Higher frequencies penetrate deeper into the ionosphere and there is also a retardation process (see later).
Slower ramp-up times can increase the resolution of the probed ionosphere and result in better signal to noise ratios, but slow ramp times prevent the ionosonde from following the sometimes rapid changes that might occur in the ionosphere. Rapid ramps in frequency provide a better instantaneous snapshot of the ionospheric layers and state, but suffer from lower signal to noise ratios. So often a compromise has to be taken between better resolution and instant picture.
An ionogram plotted by a computer program.
An operational ionosonde can cause considerable interference with other radio communications. Many radio amateurs have heard ionosondes, perhaps without realizing it. Many ionosondes produce an audible sound like a pecking or clicking noise with rapid pulses for certain periods of time and then abruptly cease as the transmission ends or the frequency of the pulses changes.
A highly sophisticated pulse amplitude sounder is the digisonde. It is capable of measuring a host of additional ionospheric parameters such as the amplitude of returned echoes, the travel time of the reflected echoes, the precise Doppler frequency, the angle of arrival and the separation of the ordinary and extraordinary waves, the wave polarization and the curvation of the wave front of the returned pulses. The obtained parameter values are digitally preprocessed and displayed on a computer monitor or printed on paper.
The Center for Atmospheric Research at the Universityof Massachusetts Lowell (UMLCAR) is in the lead for research and development of digisondes. The Lowell Digisondes are the most used worldwide, Fig. 65.a – 65.b. The receiver uses an array of crossed loop antennas to facilitate the measurements of the angle of arrival of the ordinary and extraordinary components of the received pulses. An array of four (DSP-1) or seven (DGS-256) of these antennas is used and by switching in the appropriated delay filters, the receiver beam is pointed vertically and in 14 directions arranged in two circles at two angles that are off from the vertical direction, Fig. 65.c. Each receiving antennas has a preamplifier (1 – 30 MHz) and is connected by low loss coax cable of identical length with the switching and receiver system. The direction and magnitude of the maximum signal can then be measured. The Doppler shift is also recorded as either positive or negative from normal.
The digisondes include stages of band pass filters capable of filtering out spurious transmissions and interference that could destroy the results. Band pass filters on the order of 400 KHz have been used in many digisonde devices. Digisondes are complex devices, but have been successfully used to uncover many features of the ionosphere. Digisondes can work totally independently and are mostly used remotely. A program and algorithmic activates the transmitter, receiver and antenna switching and computes all the necessary parameters and plots them as an ionogram. Nevertheless, in some cases the automatic interpreting of the data to compute the various ionospheric parameters is not correct (see later).
detail Dec.20, 2016 / This stuff is so weird…totally unnatural.
Above Antarctica & west of South America (four above) http://go.nasa.gov/2h0qNZJ
Old man in the chem-clouds / middle-upper right
Western Pacific south of Alaska (two above) / Dec. 20, 2016 http://go.nasa.gov/2h0ni5q
USA west coast California (two above) Dec.20, 2016 http://go.nasa.gov/2h0nFg6
New Zealand Dec.20, 2016 http://go.nasa.gov/2i8frrt