# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 3 State of the Art of Sky Wave Sounding
### Giro  
Global Ionospheric Radio Observatory (GIRO) is a project that provides access to a global network of digital ionosondes (_digisonde_), which most of them fed the GIRO database in real time. GIRO consists of three main parts: (i) a network of 64 digisonde stations providing online (42 out of 64) and offline data (see map on Figure 2), (ii) two databases, the Digital Ionogram Data Base (DIDBase) and the DriftBase for skymap/drift measurements and (iii) an associated software capable of automatic and interactive data and the derivation of higher order products for end user applications [33]. A digisonde is a radar that uses high-frequency waves for remote sensing of the ionosphere. Lowel Digisonde International develops and manufactures these equipment in collaboration with UMass Lowell Center for Atmospheric Research. Since 1969 they have developed up to 4 different versions of this equipment (see picture in Figure 3) and the current version is the DPS4D, which monitors the effects of space weather on Earth's ionosphere, supports communication and navigation satellite operations as well as HF and VHF radiowave communications. Its main technical characteristics can be summarized as follows. Regarding the transmitter device:  
* Frequency scan range from 0.5 up to 30 MHz with a size step selectable to 1 kHz;
* Ionogram scan time from 2 up to 200 s;
* Fully digital frequency synthesis (frequency switching time <1 \(\upmu\)s);
* Pulse repetition rate of 100 and 200 pps;
* 533 \(\upmu\)s (16 chips of 33 \(\upmu\)s) waveform with 30 kHz signal bandwidth;
* 2 transmit antenna connectors for turnstile transmit antennas;* Dual RF MOSFET amplifiers for polarized transmission;
* A peak pulse power at each of the channels of 150 W.  
Regarding the receiver device:
* Frequency range from 0.5 up to 30 MHz;
* Bandwidth of 34 kHz;
* Noise figure of 11 dB at the receiver preamplifier;
* Receiver sensitivity down to \(-130\) dBm;
* Instantaneous dynamic range of 90 dB;
* 4 receive antenna connectors for crossed magnetic dipole antennas  
Regarding the signal processing capacity:  
* Two Embedded Intel Core 2 Duo Dual Core processor SBCs;
* Selectable range of bins (256 or 512);
* Height range from 0 up to 1200 km;
* Height resolution of 2.5 km (500 m sample spacing if using differential phase technique);
* Processing pulse compression waveform of 16 chips provides 15 dB of processing gain;
* Integration of 4 to 128 waveform provides a processing gain up to 21 dB;
* Doppler range of \(\pm 3\) Hz to \(\pm 50\) Hz;
* Doppler resolution of 0.0125 Hz to 12.5 Hz;
* Amplitude resolution of \(<\)0.01 dB;
* Alternating transmission with O and X, synchronized receive antenna polarizations. Among the records provided by a digisonde, the ionogram is the most classical. The system performs a frequency sweep and provides a representation of the virtual height (according to time of flight) as a function of frequency. The virtual height is greater than the real height since the ionosphere behaves as a dispersive medium and radio pulses travel slower than light. However, by applying inversion ionogram algorithms it is possible to compute a real height profile of the electronic density. Besides, the Total Electron Content (TEC) could be also estimated through the integration of the electron density profile. Figure 2: Map of digisondes contributing data to the Global Ionospheric Radio Observatory (GIRO). Red stars identify online digisondes providing data to GIRO in near-real-time, orange stars denote digisondes providing data to GIRO in no real time, yellow stars denote other type of ionosondes that also contribute to GIRO archives, and blue stars correspond to digisonde installations planned for the next future. The skymap is another type of record provided by a digisonde. It gathers, for both a certain band of frequencies and a certain rank of reflection heights, the incoming angle of reflected pulses in the ionosphere (azimuth and elevation), and their Doppler shift.