Colours and reliability
Each coloured cell shows the metric you choose, by default reliability: the share of the time the signal stays above what the chosen mode needs, taking into account the variability of the path. A green area above 90% is good almost all the time; yellow and orange are marginal; below the last class the map is left clear, meaning no useful signal. You can also colour the map by signal strength in S-units or by the SNR margin over what the mode requires; the hover readout gives the level in dBm.
S-units and dBm
An S-unit is 6 dB. On HF the usual reference is S9 at -73 dBm; above 30 MHz it is S9 at -93 dBm. The mode sets the threshold: voice needs a few tens of dB more than a digital mode such as WSPR or LoRa at its slowest speed. The noise around the receiver, set in the panel from quiet rural to city, changes it a lot on HF.
Profile, hover and limits
Move the pointer on the map to read the signal at that point. On VHF, UHF and SHF the terrain profile below the map shows the real ground between the stations, the curvature of the Earth, the first Fresnel zone and the main obstruction. The model is an estimate: it has no buildings or trees (only an environment loss), no sporadic E, ducting, scatter or rain fade, and the HF part is a parametric ionosphere, not a forecast. Use it to compare options and find weak spots, then check on site. More in how the engine works and the FAQ.
