Sun outages
Twice a year the sun passes behind each geostationary satellite and blinds the dishes pointed at it for a few minutes a day. Find the windows for any place, satellite and dish.
Calculator
| Date | Starts | Peak | Ends | Minutes | Closest, ° | Peak, local | Peak, your time zone |
|---|
· Times are UTC unless marked. A shared link keeps these settings.
The whole arc: which latitudes are in outage, day by day
For a receiver looking at a satellite on its own meridian, the outage happens near local noon on the days when the sun's declination matches the satellite's apparent declination. That picks out one band of latitude per day, in each hemisphere. The band sweeps from the equator towards the poles and back each season, and the same band holds for every satellite on the arc, with times shifted by longitude. Satellites far east or west of a receiver shift its band slightly.
Shaded: latitudes in outage at some time of day for a satellite on the receiver's meridian, with the threshold set above. Vertical line: today (UTC).
Telling a sun outage from interference
| Sun outage | Uplink (feeder-link) interference | Ground jamming near receivers | |
|---|---|---|---|
| When | Same time each day, give or take two minutes, for about six days. Predictable months ahead. | Any time. Often hours or days at a stretch. | Any time. |
| Where | Only receivers in one band of latitude on a given day. The band moves north in autumn and south in spring. | The whole satellite footprint at once. | The area around the jammer. |
| What's lost | Every channel on every satellite at that slot, at that receiver. | Specific transponders or channels; content can be replaced. | Whatever that dish receives. |
| Length | Minutes. | Hours to weeks. | Hours, or continuous. |
| Who can confirm | Anyone, from geometry. | The operator, by geolocation; ITU filings. | Local reports; the operator may not see it. |
A short loss at the predicted time does not rule out interference at the same moment. It only means the loss itself is not evidence of interference.
How to read this calculator
Twice a year the sun passes behind each geostationary satellite as seen from a given place, and the dishes pointed at it lose the signal for a few minutes a day. These losses are natural and predictable months ahead. A loss inside a window is expected; a loss outside one deserves a second look.
- Geometry only. Sun position from a low-precision solar ephemeris (Meeus, accurate to about 0.01°). Each satellite is treated as exactly geostationary at its nominal longitude; the receiver sits on the WGS84 ellipsoid at sea level. An outage is the sun's centre within the threshold angle of the satellite.
- Threshold. Half the dish's 3 dB beamwidth (about 70 × wavelength ÷ diameter, in degrees) plus the sun's radius (0.27°). A 0.9 m dish at 12 GHz gives about 1.2°. Larger dishes get shorter, deeper fades; C-band dishes have wider beams and longer outages.
- Edges can shift. Real satellites move within a ±0.1° station-keeping box, so window edges can shift by a day and peak times by a minute or two.
- Checked. This page runs the same calculation as the project's Python tool (
tools/sun_transit.py); an automatic test compares the two on six receiver and satellite pairs in both hemispheres. The Python tool was itself checked against a separately written calculator. - Strength varies. How deep a fade gets depends on how bright the sun is at that frequency, which changes with solar activity. The home page shows NOAA's live solar radio measurements beside the predicted windows.
- Computed in your browser; nothing you enter is sent anywhere. "Use my location" asks your browser for permission and is not stored.