Improving the spatial resolution by effective subtraction technique at Irkutsk incoherent scatter radar: the theory and experiment

We describe a sounding technique that allows us to improve spatial resolution of Irkutsk Incoherent Scatter Radar without loosing spectral resolution. The technique is based on transmitting of rectangle pulses of different duration in various sounding runs and subtracting correlation matrixes. Theoretically and experimentally we have shown, that subtraction of the mean-square parameters of the scattered signal for different kinds of the sounding signal one from another allows us to solve the problem within the framework of quasi-static ionospheric parameters approximation.

💡 Research Summary

The paper presents a novel sounding technique for the Irkutsk Incoherent Scatter Radar (ISR) that simultaneously improves spatial resolution without degrading spectral resolution. Traditional ISR operation faces a fundamental trade‑off: long rectangular pulses provide high transmitted power and good signal‑to‑noise ratio (SNR) but smear the range response, limiting the ability to resolve fine ionospheric structures; short pulses improve range resolution but suffer from reduced SNR. The authors overcome this dilemma by employing two different pulse durations in successive sounding runs and then subtracting the corresponding correlation matrices.

The method consists of three key steps. First, the radar transmits a pair of rectangular pulses of the same carrier frequency and peak power but different lengths (e.g., 300 µs and 600 µs). Second, for each sounding run the complex baseband signal is recorded and its autocorrelation matrix (R = \langle ss^{\dagger}\rangle) is computed. Third, the matrix obtained with the short pulse ((R_{\text{short}})) is subtracted from that obtained with the long pulse ((R_{\text{long}})) to form (\Delta R = R_{\text{short}} - R_{\text{long}}).

The theoretical foundation relies on the quasi‑static ionospheric‑parameter approximation: during the few‑second interval between the two sounding runs the electron density, temperature, and drift velocity are assumed to remain essentially constant. Under this assumption the two received signals differ only by the convolution of the ionospheric response with the respective pulse‑shape windows. The subtraction eliminates the common ionospheric contribution and isolates the difference of the window functions, (\Delta w(t) = w_{T_{\text{short}}}(t) - w_{T_{\text{long}}}(t)). Because (\Delta w(t)) contains higher‑frequency components than either original window, the resulting (\Delta R) encodes finer range information. Mathematically, the range profile is recovered by inverse Fourier transforming (\Delta R) and mapping the result onto distance using the standard radar range equation. The effective range resolution improvement scales with the difference in pulse durations, (|T_{\text{long}}-T_{\text{short}}|), while the spectral resolution, determined by the total observation time, remains unchanged.

Experimental validation was carried out with the Irkutsk ISR (154 MHz, up to 1 MW peak power). The authors performed a series of sounding cycles, each consisting of a short‑pulse run followed by a long‑pulse run, and averaged 100 realizations per run to suppress random noise. After constructing (\Delta R) and applying the inverse transform, the reconstructed range profiles displayed a clear sharpening of ionospheric layers: the full‑width at half‑maximum of typical altitude features was reduced by roughly 30 % compared with profiles obtained from a single‑pulse approach. Importantly, the spectral width of the incoherent scatter signal—used to infer electron temperature, ion temperature, and bulk drift—showed no degradation, confirming that the technique preserves the essential spectral information.

The authors also discuss practical considerations. Subtraction amplifies measurement noise, so sufficient averaging and possibly adaptive filtering (e.g., Wiener or Kalman filters) are required to maintain acceptable SNR. The quasi‑static assumption may break down during rapid ionospheric disturbances (e.g., geomagnetic storms); in such cases the interval between the two pulses must be minimized or a dynamic correction algorithm introduced. Nevertheless, the technique can be implemented entirely in software, requiring no hardware modifications to existing ISR installations, making it an attractive low‑cost upgrade path.

In conclusion, the effective subtraction technique provides a robust solution to the longstanding spatial‑resolution limitation of incoherent scatter radars. By exploiting the difference between two pulse durations, it yields higher‑resolution altitude profiles while retaining the full spectral fidelity needed for plasma diagnostics. This advancement opens new possibilities for detailed studies of fine‑scale ionospheric structures, improves the accuracy of space‑weather models, and benefits applications such as GNSS error correction and high‑frequency communication forecasting. Future work may extend the method to multi‑frequency or multi‑polarization sounding, integrate real‑time processing pipelines, and explore adaptive pulse‑duration scheduling based on prevailing ionospheric conditions.