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Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.

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Meteorology for Navigators
Pressure Gradient
How quickly pressure changes over distance, which sets wind strength
The rate of pressure change per unit distance, seen on a chart as isobar spacing. Tightly packed isobars mean a steep gradient and strong winds; widely spaced isobars mean light winds. Wind speed is roughly proportional to the gradient, but for a given isobar spacing the geostrophic wind is stronger at low latitudes because the Coriolis parameter is smaller, so identical spacing implies very different strengths at 20 and 60 degrees latitude.
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Speed, Log & Distance
Propeller Slip (Apparent Slip)
Difference between the distance the propeller should drive you and distance actually covered
Engine distance equals propeller pitch multiplied by the revolutions turned; apparent slip is that figure minus the distance actually run, expressed as a percentage. A 0.60 m pitch propeller at 100 rpm for one hour turns 6000 revolutions, giving 3600 m or 1.94 M; if 1.80 M is run, apparent slip is about seven per cent. Values of a few per cent are normal, rising with fouling, heavy loading or a head sea, and going negative with a following stream.
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GNSS & Satellite Navigation

Pseudorange

The measured distance to a satellite before the clock error is removed

A receiver measures signal travel time against its own inexpensive clock, so each raw range contains an unknown common clock bias and is called a pseudorange. Because light travels about 300 metres per microsecond, a one microsecond clock error is a 300 metre range error. Solving simultaneously for latitude, longitude, height and clock bias therefore needs a minimum of four satellites; a fifth or more allows redundancy and error checking.

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Radar Navigation

Pulse Length

How long each transmitted burst lasts, trading detail against range

Pulse length is measured in microseconds and directly governs range discrimination and detection range. A short pulse of about 0.05 to 0.1 microseconds separates close targets and is selected for pilotage, while a long pulse of around 1 microsecond puts far more energy on distant targets for long-range detection at the cost of a coarser picture. Most sets change pulse length automatically with the range scale selected, but manual override is often available.

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Radar Navigation

Pulse Repetition Frequency (PRF)

How many pulses the radar sends each second

PRF is the number of pulses transmitted per second, typically several hundred to a few thousand hertz. It must be low enough that an echo from the maximum displayed range returns before the next pulse is sent, so sets use a high PRF on short range scales, where more pulses per target improve painting, and a lower PRF on long ranges. An unduly low PRF on short ranges gives a thin, poorly filled picture.

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Sailing reference

The YouSail sailing glossary

Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.