The RPR is a free reference tool for amateur radio operators, scientists, and anyone interested in High Frequency (HF) radio propagation. It calculates the grey-line — the twilight band that sweeps across the Earth at sunrise and sunset — and identifies windows where that band simultaneously crosses two selected cities.
During grey-line passages, the ionospheric D-layer is in rapid transition. This can create brief windows of unusually low absorption and enhanced long-distance HF propagation.
Select a Continent and City for Station Alpha (your location) and Station Bravo (the target). Pick a Date and press ⚡ CALCULATE.
Results are shown in two places:
(1) Mercator map
The map shows the short-path and long-path between two selected cities. The map also shows the grey-line for a selected date and time — initially for Civil twilight definition (Sun altitude −6°). You can change the twilight definition by pressing any of the three buttons placed below the map:
[ -6° Civil ] [ -12° Nautical ] [ -18° Astronomical ]
Pressing any of these buttons will automatically recalculate.
(2) Time line
The time line is placed below the map. Results show dawn, sunrise, sunset and dusk times in UTC for both cities, plus all four possible overlap scenarios (Dawn↔Dawn, Dusk↔Dusk, and the two cross-event combinations). The 24-hour timeline displays grey-line bars for both stations. A glowing green bar marks any overlap.
If you prefer a simpler view, untick Show all overlap rows to hide the Dawn↔Dawn and Dusk↔Dusk tracks, leaving the two cross-over tracks. Remember to recalculate.
Press 🔍 Scan All Cities to rank every city in the database by overlap duration against your Station Alpha — useful for finding the best DX targets on a given date.
You can narrow the search by using the MIN OVERLAP lever which sets the minimum duration of overlap. Press the scan button again to close the scan report.
Press 📅 Best Dates to see which months of the year offer overlap for the selected city pair — helpful for planning ahead. Press it again to close the report.
After changing any option that affects the map and timeline bars, remember to press ⚡ CALCULATE.