# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 3 State of the Art of Sky Wave Sounding
### Recommendation ITU-R P.845-3
#### 3.1.2 HF Field-Strength Measurement Technique  
In order to minimize uncertainties that may impair an HF field-strength measure campaign and to provide high quality data ITU suggests a number of arrangements in both transmitter and receiver setups. The transmitters are recommended to be frequency agile, radiating in up to five frequency bands, preferably in fixed frequency bands (e.g., 5.5, 8, 11, 15 and 20 MHz), sequentially with steady signal periods combined with coded sequences to allow computer identification of the source as well as computer evaluation of the intensities of both signal of interest and background noise or interference. With the goal to gather data representative of HF propagation characteristics ITU proposes to locate at least nine transmitters spread over the three world regions (i.e., Europe/Africa, America and Asia/Australasia) on both Hemispheres, at middle latitudes and in the tropics. Broadband antennas, omnidirectional in azimuth, with broad elevation pattern, such as broadband monopoles or conical structures, are recommended. A single antenna could be used to cover the whole HF frequency range (i.e, from 2 to 30 MHz) to avoid switching high RF powers (i.e., between 1 and 5 kW) along with a frequency selective matching circuit to operate on the lowest operating frequency band. Transmitted signal should be of type F1B (i.e., FSK modulated telegraph signal) modulated with Frequency Shift Keying (FSK) with a frequency shift of 850 Hz. The transmitted sequence should have a duration of 12 s, be repeated for 4 min and be composed by the following parts:  
* FSK preamble at 100 bit/s for 1 s, comprising signal reversals commencing on the mark frequency;
* A pause of 50 ms;
* An identification signal in Morse code on the higher frequency contained within a period not exceeding 3.3 s;
* A pause of 50 ms;
* A 256-bit complementary sequence transmitted at 1200 bit/s as follows: Sequence 1 = 11100100001010000010101101100000110101001011011101110000001101111010101100110010101010010110001100011000110001100011000111111010101001001110100110001  
* A series of FSK reversals, comprising 273 bits at 100 bit/s, commencing on the lower frequency;
* A 127 bit Gold code identifier sequence [32], at 100 bit/s, commencing on the lower frequency;
* a constant signal on the higher frequency for a duration of at least 3 s, which should continue until the total time for the sequence is 12 s.  
To receive the signals coming from the nine proposed transmitters there should be as many receivers as possible placed not nearer than 500 km to any transmitter due to the null in the elevation pattern of the omnidirectional transmitting antenna. Short vertical active antennas placed on flat ground within an area of homogeneous and good ground conductivity, with no obstacles within 25 m radius and away from any metallic structure are recommended. The field-strength receiver should comply with the following minimum performance requirements:  
* synthesizer control (10 Hz step);
* external bus available for computer control;
* frequency accuracy \(\pm\)1 part in 10\({}^{6}\);
* synthesizer noise sidebands: reciprocal mixing performance better than 70 dB in a bandwidth of 3 kHz at 20 kHz offset;
* sensitivity SSB 1.0 mV terminated for at least 10 dB (S + N) / N for a bandwidth of 3 kHz;
* unwanted spurious responses (e.g., image, IF) better than 70 dB;
* selectivity: approximately 3 kHz bandwidth, shape factor (\(-60\) dB to \(-6\) dB) 2:1;
* linearity: 3 rd-order intercept point, 20 kHz spacing, +10 dBm;
* true average measuring capability within 4 s;
* timing accuracy within the receiver system should be maintained within 1 s.  
For each combination of transmitter, receiver and frequency the receiver should gather sufficient samples within each hour to allow statistically meaningful estimates of the hourly medians of both the signal and noise intensities, which is determined in relation to the likely within-an-hour fading.