The HAARP Digisonde
What is a Digisonde?
A Digisonde is a modern digital device used to determine ionospheric characteristics above the vicinity of the instrument. A Digisonde is comprised of a radio wave transmitter, receiver, and associated transmit and receive antennas. A Digisonde generally operates within the 1 to 20 MHz band with a physically large transmit antenna and four smaller, spaced receive antennas deployed in an equilateral triangle (the 4th antenna being placed at the triangle's centroid). Received radio wave parameters measured directly are time of flight, polarization, amplitude and phase spectrum, and angles of arrival. Computer processing of the received radio waves is employed to develop ionograms which are used to determine the characteristics of the various ionospheric layers outlined in the section About the Ionosphere.
How does a Digisonde Work?
One may liken a Digisonde to a radar for the ionosphere. A radar transmits a radio wave and then listens with a matched receiver for radio waves reflected from targets within the radar beam. Common radars, such as those used for air traffic control, are designed to detect and track "hard targets," i.e., aircraft. The ionosphere presents a volumetric and seemingly tenuous target; so it is logical to ask what ionospheric characteristic provides the "target" that reflects radio waves back to the receiver? The answer is the free electrons as discussed in the section About the Ionosphere. The number density of free electrons may be expressed as an ionospheric plasma frequency; when a Digisonde transmitted frequency matches the ionospheric plasma frequency, the transmitted radio waves are reflected and returned to the Digisonde's receiver. Thus, the ionosphere's free electrons are the Digisonde's "target" in the radar analogy.
The time it takes for the transmitted radio wave to travel to the ionosphere and return to the receiver upon reflection (the "time of flight") is twice the virtual range to the ionospheric target and the returned radio wave frequency is related to the ionosphere's number density. As the transmitter sweeps in frequency, the combinations of time of flight (virtual range) and reflected frequencies (electron number density) characterize the ionospheric layers as discussed in the section About the Ionosphere. Recall that light travels at meters per second in a vacuum; but the atmosphere and ionosphere aren't vacuums. Corrections must be made to the measured virtual ranges to determine the true heights of the corresponding electron number densities. At frequencies higher than the maximum ionospheric plasma frequency (maximum electron number density), the ionosphere is no longer an effective "target" for Digisonde transmitted radio waves. These higher Digisonde transmitted radio waves are not reflected to the receiver and continue to travel (or "propagate") in a line-of-sight manner into the far reaches of space.
The propagation of a radio wave through a magnetic field results in the splitting of the transmitted radio wave into an "ordinary" wave with the original frequency and an "extraordinary" wave with a frequency different from the original. The frequency difference is dependent upon the magnetic field and the mass and charge of the electron. The frequency difference between the ordinary and extraordinary wave is approximately one-half the orbital frequency of the electron about the Earth's magnetic field line. At an altitude of several hundred kilometers above Gakona, this frequency difference is approximately 700 kilohertz.
The digital nature of the Digisonde enables modern radar processing techniques to be used to improve signal gain, form numerous receive beams within the larger transmit beam to determine direction of arrival, assess ionospheric motions (Doppler processing), avoid interference. and to enable near real time, automatic scaling of multi-parameter ionospheric characteristics and calculation of true-height electron density profiles.
The HAARP Digisonde
The HAARP Digisonde, which was developed and provided by the University of Massachusetts at Lowell, was one of the first instruments installed at the HAARP Research Station. The purposes of this instrument are to support frequency management of the 3600 kilowatt Ionospheric Research Instrument transmitter and to provide real time ionospheric characteristics for guiding experiment selection. The Digisonde data are processed, archived, and are available for post-analysis of experiment results. Various formats of the information derived from the Digisonde are made available on the HAARP web site in chart form in near real time and can be accessed from the Data Index.
An example, annotated ionogram is shown in the figure to the left. Annotations in this figure are in blue lettering. (Click on the figure to see a full size image.)
This ionogram shows a well-formed F-Layer with a peak ionization at around 220 km. There is also a less obvious E-Layer at an altitude of about 110 km. The ionogram clearly shows how the transmitted digisonde signal splits into reflected ordinary and extraordinary waves. The ionogram chart and the parameter FoF2 in the list on the left side of the chart, show that the highest frequency that will be reflected from the ionosphere for vertical incidence (ie., for a radio wave traveling straight up from the ground) is 4.8 MHz.
The ionogram also shows what appear to be faint reflections from an altitude of 425 km. In reality, these
chart points are the result of the sounder's radar signal being reflected by the ionosphere, returning to the ground where they are reflected back to the ionosphere for a second reflection before being detected by the Digisonde receiver. The time-of-flight is, therefore twice that of the normal reflection and the computed virtual
height is twice the normal virtual height. This type of presentation is often seen under conditions of low ionospheric absorption.