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Application of FM/CW Techniques to Ionosondes
Hiraiso Solar Terrestrial Research Center,
Communications Research Laboratory
3601 Isozaki Nakaminato Ibaraki, 311-12 Japan
In this paper, advantages of FM/CW techniques over the pulse radar are presented as they apply to ionosondes. The data processing gain that comes from a matched filter enables an FM/CW ionosonde to make observations with very low transmitting power. The frequency and height resolution of an FM/CW ionosonde can also be changed after an observation has been made. These make the FM/CW ionosonde particularly useful for some specific observations, while it also has functions equivalent to those of the pulse ionosonde.
An FM/CW ionosonde (or a chirp sounder) has a high duty factor and narrow band width. So, compared to a pulse ionosonde, it has the advantages of:
(i) Low peak power, so, it creates less interference for other radio users.
(ii) Good noise and interference discrimination.
Doppler analysis, angle of arrival, polarisation, and amplitude measurements can be performed equally as well by a chirp sounder as by a pulse ionosonde [1,2,3].
The high cost and laborious task in frequency sweep synthesisers and spectrum analysers have prevented FM/CW radars from becoming popular for use in the field of the ionosphere observation. These obstacles to the use of chirp sounders, however, can now be easily overcome due to recent improvements in semiconductors and microprocessors.
The transmitter frequency increases (or decreases) linearly with time as shown in Fig. 1. Frequency differences between the echo and the transmitting signal are proportional to the distance. De-chirped signals from the receiver are sampled at a constant rate while a chirp sounder sweeps from the starting frequency to ending frequency. Fast Fourier Transform (FFT) is generally used to analyse the de-chirped signal. We get echo amplitude vs. range in every FFT.
Frequency analysis of the de-chirped signal is known to be identical to a matched filter in pulse compression radar. The pulse compression ratio G is given by
G = BT (1)
where B is the frequency sweep range and T is the time duration for one spectrum analysis. Range resolution &$r is given by
Dr = c / 2B (2)
where c is the velocity of light. Velocity resolution &$v is determined by
Dv = c/2fs (3)
similar to that of pulse radar in a fixed frequency mode. Here f is the observation frequency and s is the Doppler observation time.
Fig.1 Frequency-time diagram of a chirp sounder.
Applications to Observations
Low Power Operation
Using eqs. (1) and (2), and considering that B=(df/dt)T, we obtain
G increases with fine range resolution and a slow sweep rate. Let the frequency sweep rate df/dt=100 kHz/s and the range resolution 5 km, for example, then we obtain a pulse compression ratio of about 10,000. A chirp sounder with an output of only 1 W is equivalent to a typical 10 kW output pulse ionosonde. Figure 2a shows an oblique incidence ionogram by means of a chirp sounder of 1 W transmitting power while Fig.2b shows a vertical incidence ionogram obtained by means of a conventional mono-pulse ionosonde near the chirp sounder.
Variety in Observation Parameters
The equivalent power, range, frequency resolution, and time resolution (and velocity resolution in the fixed frequency mode) are closely related to each other in the chirp sounder. By decreasing the frequency sweep rate, one can increase equivalent output power without increasing the actual output power.
Fine range resolution is accompanied some difficulty in pulse ionosonde because a narrow pulse needs high output peak power and wide frequency band allocation for the same signal to noise ratio as wide pulse observation. However, figure 1 and eq. (2) indicate that the wide range of time domain (T) data provides wide frequency sweep range B, thus, fine range resolution. There needs no change in the transmitting parameters.
Post Observation Analysis
When using a pulse ionosonde, the range resolution and frequency resolutions are defined by the pulse width and frequency step which must be decided before the observation. On the contrary, these resolutions can be decided after an observation with a chirp sounder. A series of de-chirped signals can be stored in a memory and frequency analysis can be applied after the observing frequency scan has been completed.
Fine height resolution but coarse frequency resolution is required to detect the fine motion of a flat type Es, whereas coarse height resolution but fine frequency resolution is required to determine an accurate critical frequency of a normal layer. We can select any of these resolutions from a single observation according to the characteristics of the analysis after observation by recording the de-chirped signal in the time domain.
About 30 kBytes of sampling data is recorded by a chirp sounder with a frequency range of 1 - 30 MHz, a height range of 750 km, and 8 bit amplitude resolution. This is comparable to the amount of data in amplitude ionogram from a pulse ionosonde.
A chirp sounder has certain advantages of its own along with common functions with a pulse ionosonde. Actual transmitting power of a chirp sounder can be reduced while equivalent output power is kept as high as a pulse ionosonde. The ability to analyse the time domain data after observation makes the chirp sounder particularly useful in some specific observations.
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