East Siberian Center for the Earth's Ionosphere Research (SDC ESCEIR)Institute of Solar-Terrestial Physics SB RAS
Upper Atmosphere Physics and Radiowave Propagation Department
The main instrument of our investigation is chirp ionosonde, which has the vertical (VS), oblique (OS) and back-scattering (BSS) sounding modes.
Chirp ionosonde ionosphere sounding principles
Linear frequency modulated continuous signals sounding method is relatively simple and has wide application in various special-purpose systems.
A typical functional diagram of the ionosonde with a continuous chirp-signal is presented in the figure :
Let the transmitted signal have the form:
0 -- signal amplitude;
Then in the case of mirror reflection the signal at the receiver input has a time delay t with respect to the transmitted signal (to start heterodyne time) and has the form:
where R0 -- coefficient of signal relaxation due to the propagation and reflection process.
Let us assume that the heterodyne receiver chirp-signal characteristics are strictly identical to the transmitted signal characteristics. If the receiver synthesizer is started simultaneously with the transmitter synthesizer by the synchronization system, then the low frequency filter output signal has the form:
-- difference frequency;
So the difference frequency W=bt is uniquely determined by the propagation time on the run.
Thus the received signal heterodyning process due to the chirp-signal generator transforms difference delay information into the difference of frequency information.
The received signal has a multymodal structure. Each run of propagation (propagation mode) has his own difference frequency.This peculiarity imposes some restriction on equipment, which must supply all expecting signal delays registration.
The sounding chirp signal parameters must usually meet the following requirements:
The working frequency range is mainly determined by the sounding mode, run of investigation and also by seasonal factors, time, and solar activity. A high frequency is usually chosen well above the expected critical frequencies, but the working range is very often restricted by sounding equipment technical characteristics and by its transmitting and receiving antennas. A typical ionosonde freuency range varies from 1 to 30 MHz.
The chirp speed (b) is the parameter which mainly defined the noise stability and resolution power or sounding equipment. Chirp signals which are applying for ionosphere channel diagnostics usually have10KHz/s -- 1MHz/s. We must point out that under modulation speed increasing there is an increase in noise suppression and resolution power.
Main technical characteristics
Along with basic set there is a mobile receiving chirp ionosonde set which is situated at Magadan (IKIR SB RAS). Its main parameters are analogous to the basic set's parameters with transmitting power of 50 -- 200 Wt.
Experimental investigations have shown the high efficiency of this set, although it has some shortcomings.
We now discuss a complete new chirp ionosonde variant on the basis of frequency synthesizer (USA), special processor SP-30 with kernel TMS320C31PQL40 (made in the USA)], and IBM - 486. In this variant the rate of frequency chang is 10 - 1000 KHz/s, a maximum run and frequency points an 800, and as a receiver we can use practically any of existing receivers (Katran, Flash, Panorama, Lily and so on). It is possible to take recording without IBM but then functional capabilities are restricted.