On an expedition ship the TV complaints start at the same latitudes every season: Norway and Svalbard in the north, the Drake Passage and the Antarctic Peninsula in the south. The antenna is not broken. The geometry is against it.
The satellite gets lower as you go towards the pole
Geostationary TV satellites sit above the equator at about 35,786 km. From a ship, the elevation angle to the satellite depends on the latitude and on the difference in longitude between the ship and the satellite:
c = cos(dLon) x cos(Lat)
El = atan( (c - 0.1513) / sqrt(1 - c^2) )0.1513 is the Earth radius divided by the radius of the geostationary orbit. Some values:
| Latitude | Satellite on your meridian | Satellite 30° of longitude away |
|---|---|---|
| 30° | 55.0° | 42.2° |
| 50° | 32.7° | 26.0° |
| 60° | 21.9° | 17.4° |
| 65° | 16.7° | 13.0° |
| 70° | 11.5° | 8.6° |
| 75° | 6.4° | 4.3° |
| 78° | 3.3° | 1.7° |
Above about 81.3° the geostationary arc is below the horizon altogether. Long before that, at 5° to 10° of elevation, reception becomes difficult.
What a low elevation does to reception
Blockage. At 50° of elevation the antenna looks over the superstructure. At 10° it looks almost horizontally, straight through the mast, the funnel, a crane or the second radome. Every course change moves the blocked sector. Ships with two antennas can use an arbitrator that switches to the one with a clear view; the SeaTel DAC 2202, for example, supports a dual-antenna configuration with an arbitrator.
Weaker signal. High latitudes are usually at the edge of the satellite's footprint, where the transmitted power is lower. The path through the atmosphere is also longer at a low angle. The margin above the tracking threshold gets thin, and a small roll or a bit of spray on the radome is enough to lose the lock.
Polarisation. For linear polarisation the skew angle changes with the ship's position relative to the satellite. The ACU normally computes it, but a wrong polarisation offset costs signal, and at low elevation there is little margin left to lose.
Why the TV stays off after the obstruction clears
A stabilised TVRO antenna tracks by watching its signal level (AGC). When the level falls below the threshold, the ACU waits a short delay and starts a search pattern. Searching moves the antenna away from where the satellite really is. When the mast is no longer in the way, the antenna may be pointing somewhere else entirely, and the channels stay off until the search comes round again.
There is a second trap. If satellite identification (NID) is not set or not unique, the ACU can settle on a different satellite that happens to give a signal above the threshold. The antenna reports "tracking", the signal bar looks good, and the receivers show nothing.
What helps
- Check the heading input. The ACU uses the gyro heading to point the antenna; a wrong or frozen heading turns every search into guesswork.
- Set the NID for every satellite you use, so a wrong-satellite lock is rejected.
- Before a high latitude voyage, check the footprint maps of your TV provider and plan the satellite changes with the bridge's route.
- Keep a log of where and when the signal dropped. It shows the blocked sectors and saves time when the service engineer comes on board.
Doing it with AntennaPilot
AntennaPilot calculates every two seconds where the selected satellite is from the ship's position and heading, compares it with where the SeaTel ACU says the antenna points, and sends the antenna back to the calculated position when the difference passes your threshold. When the obstruction clears, the antenna is already on the right satellite. It also shows target and current NID side by side, so a wrong-satellite lock is visible at once. Try the AZ/EL calculator for your own position.
All shipboard IT guides · Razvan Malciu, shipboard IT officer