Mid-ocean there is no transit to check the compass against: you use the Sun and the stars. When the satellites fail, that checked compass, the log and the sextant bring you in.
How do I work out compass deviation from a true bearing?
Find the compass error, true minus compass, then subtract the variation for your position: what is left is the deviation. Count east as plus and west as minus.
The compass error is the difference between a true bearing and the same bearing by compass. It is the sum of variation, which belongs to the place, and deviation, which belongs to the boat on that heading. Count east as plus, west as minus.
263°C + 2°E − 16°W = 249°T.
Take the variation for your DR position from the pilot chart's isogonic lines or the chart's rose, corrected for the annual change; it alters by degrees across an ocean. Subtract it from the error: what is left is the deviation.
Error = true − compass. Compass least, error east; compass best, error west. Deviation = error − variation.
How do I check my compass with the rising or setting Sun?
Compare its compass bearing with the true bearing from its amplitude, the angle from due east or west: sin Amp = sin Dec ÷ cos Lat, no time needed. Take it with the Sun's centre on the celestial horizon.
The amplitude is the angle between the rising or setting Sun and due east or west. It needs only latitude and declination, not the time: sin Amp = sin Dec ÷ cos Lat. Name it E at rising or W at setting, then N or S like the declination: in either hemisphere the Sun rises and sets towards the name of its declination.
Sunrise at 40°N with declination N10°: the Sun bears 077°T.
Take the bearing with the Sun's centre on the celestial horizon: its lower limb then stands about half a diameter above the visible horizon. Near the equator the amplitude equals the declination; it grows with latitude, and in high latitudes the slanting Sun makes a late bearing degrees out. At the equinoxes it rises due east everywhere.
How do I check the compass by the Sun at any time of day?
Compare the compass bearing with the Sun's azimuth, Zn, worked from latitude, declination and LHA in the PZX triangle, so you need the time to the minute. Best take it low, under about 30°.
At any other moment the body's true bearing is its azimuth, Zn: the angle at your zenith in the PZX triangle, worked from latitude, declination and local hour angle (LHA = GHA − west longitude, or + east). So you need the time to the minute. LHA over 180° puts the body east of you.
| Tables give Z | LHA over 180° | LHA under 180° |
|---|---|---|
| North latitude | Zn = Z | Zn = 360° − Z |
| South latitude | Zn = 180° − Z | Zn = 180° + Z |
Choose a low body, under about 30°: a high one is hard to sight across the compass, and near the zenith its bearing swings fast. Stars and planets serve at twilight; for their amplitude take the bearing a Sun's diameter above the horizon. Polaris bears within about 1° of north in low and middle latitudes, but is out of sight south of the equator; there the Southern Cross bears 180°T when its long axis is vertical.
Does compass deviation change when I change tack or heading?
Yes: a deviation holds only for the heading and heel it was found on, so record it by heading and tack in a deviation log. After a gybe it is unknown until checked again.
A deviation holds only for the heading it was found on, and for that heel: log it by heading and tack. After a gybe it is unknown until checked again. A sudden jump means steel or electronics stowed near the compass.
| Ship's head | Deviation |
|---|---|
| 000°C | 2°E |
| 090°C | 5°W |
| 180°C | 2°W |
| 270°C | 5°E |
To swing the compass at sea, steer each heading in turn and bear the Sun on each, with a fresh azimuth for every bearing, since the Sun moves degrees in half an hour. Between headings interpolate: on 045°C this card gives about 1½°W. Steer by the card; leave the corrector magnets to an adjuster in port.
A fluxgate compass also feels the boat's magnetism: run its calibration turns, then compare it with the steering compass. A satellite compass has no deviation but dies with the GNSS signal; only the magnetic compass needs no power.
How can I tell if my GPS is being jammed or spoofed at sea?
Jamming loses the fix on every receiver at once, since GPS, Galileo, GLONASS and BeiDou share one band. Spoofing gives a plausible position that the log and radar disagree with.
A receiver finds its range from each satellite by the signal's travel time; three ranges and its own clock error need four satellites. Bunched satellites give a high HDOP, a poor fix. GPS, Galileo, GLONASS and BeiDou together improve the geometry, but share one band: a jammer blocks them all.
| What you see | What it means |
|---|---|
| All receivers lose the fix at once | Jamming |
| A plausible position, but log and radar disagree | Spoofing |
| Plotter lost, handheld fine | The shared antenna or its cable |
| Two sets disagree | A third decides: DR or a sight |
The fix may be right and the chart wrong: an old survey can put a reef a mile out. Log position, course and distance every hour, take a daily sight, and keep a handheld GPS with sealed spare batteries; in lightning, into the oven, unplugged.
AIS repeats the other ship's own GPS. A steady compass bearing is a risk of collision, whatever the AIS says.
How long should I leave my EPIRB switched on after activating it?
Until you are picked up: a 406 MHz EPIRB alerts an MRCC through the Cospas-Sarsat satellites, but rescuers home on its 121.5 MHz signal. The Return Link light only confirms the alert was received.
A 406 MHz EPIRB sends its identity through the Cospas-Sarsat satellites to an MRCC, so register it to you and this boat. With built-in GNSS it sends your position within minutes. Rescuers home on its 121.5 MHz signal, so leave it on until you are picked up. The Return Link light only confirms receipt of the alert.
| System | Satellites | Limits |
|---|---|---|
| Iridium | Low polar orbits, whole globe | Needs open sky |
| Inmarsat | Geostationary over the equator | Lost beyond about 75°; the rig can mask it |
Without satcom only HF reaches mid-ocean: high-seas bulletins and weatherfax. A passing ship will relay a routine message.
Questions from this topic, as the app asks them. Open one to see the answer.
Making landfall in the Azores, you steer along this transit. What deviation do you log for this heading?
A. 3°E
A transit needs no almanac: error = 248° − 252° = 4°W; deviation = error − variation = 4°W − 7°W = 3°E.
You plan a compass check by amplitude at sunrise on the ARC route. At which moment do you take the Sun's bearing?
A. Lower limb half a diameter above the visible horizon
The amplitude is for the Sun's centre on the celestial horizon: with refraction and dip, the lower limb then shows about half a diameter up.
You take the Sun's bearing mid-morning for a compass check. Which true azimuth do you compare it with?
D. 127°T
Zn = atan2(−cos Dec sin LHA, cos Lat sin Dec − sin Lat cos Dec cos LHA) = 127°; LHA over 180° puts the Sun in the east.
In the trades you gybe onto a new heading. What do you now take as the deviation?
D. Unknown until you check it on 200°C
Deviation changes with the ship's head: a value found on 260°C does not hold on 200°C — check again or use the card for that heading.
You switch on the backup GPS mid-ocean and see this. Why is there no position yet?
A. It needs a fourth satellite to solve its clock error
Each satellite gives a range from the signal's travel time; with the receiver's clock unknown, three ranges and the clock need four.
Mid-Atlantic, your family sees your AIS position online. How does it reach them?
B. Satellites pick up your AIS signal and relay it
Satellites in orbit receive AIS transmissions and pass them ashore; ships still see you only within VHF range, some 20 M.
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.