Radar Fix
Using Marine Radar for Accurate Position Fixing in All Conditions
How Marine Radar Works
Marine radar operates by transmitting short pulses of microwave energy from a rotating antenna. When a pulse strikes a solid object — a coastline, an island, a vessel, a buoy — some of the energy is reflected back to the antenna as an echo. The radar measures the time delay between the transmitted pulse and the received echo, and since radio waves travel at the speed of light (approximately 162,000 nautical miles per second), the range to the target can be calculated with great precision.
The radar display presents the surrounding area as a plan view centered on the vessel. The antenna rotates continuously (typically 20-25 RPM), and each echo is painted on the screen at the correct range and bearing relative to the vessel. The result is a map-like picture of the surrounding coastline, islands, vessels, and navigation aids. Modern displays are high-resolution, color-coded, and can overlay electronic chart data, but the fundamental principle remains unchanged from the earliest marine radars.
Most pleasure craft and small commercial vessels use X-band radar (9 GHz, approximately 3 cm wavelength), which provides good resolution and target discrimination. Larger vessels may also carry S-band radar (3 GHz, approximately 10 cm wavelength), which penetrates rain better but has lower resolution. For coastal navigation and position fixing, X-band radar is generally preferred because its sharper beam produces more detailed echoes of the coastline and small targets like buoys.
Allow your radar at least 3-5 minutes to warm up and stabilize before relying on it for navigation. Modern solid-state radars are ready almost instantly, but older magnetron sets need warm-up time.
Radar Range and Bearing
The radar display provides two key measurements for each target: range and bearing. Range is measured using the fixed range rings (concentric circles at known intervals, such as 0.5 NM on the 3 NM scale) or the variable range marker (VRM), an adjustable ring that can be placed precisely on a target. The VRM provides a digital readout of range, typically accurate to within 1-3% of range or 30-50 meters.
Bearing is measured using the electronic bearing line (EBL), a radial line that can be rotated to pass through a target. The EBL readout gives the bearing of the target relative to the ship's head (in head-up mode) or relative to true or magnetic north (in north-up or course-up mode). Radar bearings are generally less accurate than radar ranges — a typical accuracy is plus or minus 1-3 degrees — because the radar beam has a finite width (often 2-6 degrees for small-craft antennas), causing target echoes to appear wider than they actually are.
Because radar range is more accurate than radar bearing, experienced navigators prefer to use ranges rather than bearings when taking radar fixes. A fix from two or three radar ranges to identified targets is typically more accurate than a fix from radar bearings alone. When only one target is available, a combination of radar range and radar bearing gives a useful fix, but be aware that the bearing component is the weaker element.
Radar bearings are less accurate than radar ranges due to beam width. Always prefer radar ranges for position fixing when possible.
Which radar measurement is typically more accurate for position fixing?
Taking a Radar Fix
The most accurate radar fix uses ranges to two or more identifiable targets. Each range produces a circle of position centered on the target. Two circles intersect at two points, and your DR position resolves the ambiguity. Three ranges produce three arcs that ideally meet at a single point, or form a small cocked hat similar to a three-bearing visual fix. The targets must be positively identified on both the radar display and the chart — misidentification is the most common source of radar fix error.
To take a radar fix: (1) Select two or three targets that are well separated in bearing (ideally 60-120 degrees apart) and that produce clear, sharp echoes on the display. Rocky headlands, steep islands, and prominent points are excellent radar targets. (2) Place the VRM on each target in turn and read the range. Record each range and the time. (3) On the chart, set your drawing compass to each range at the chart scale and swing a short arc centered on each target near your estimated position. (4) The intersection of the arcs is your radar fix. Label it with the time and 'Radar Fix.'
When only one target is available, use the radar range and bearing to that target. Place the VRM on the target for the range and the EBL for the bearing. On the chart, draw the bearing line through the target and swing the range arc. The intersection of the bearing line with the range arc is your fix. This is effectively a bearing-and-distance fix using radar. Remember that the bearing is the less accurate component, so treat the fix with appropriate caution and cross-check with other information such as depth soundings.
When taking ranges to multiple targets, measure them as quickly as possible to minimize the vessel's movement between readings. On a fast-moving vessel, take the ranges in rapid succession and record a single time for the fix.
Identifying Radar Targets
Correct target identification is critical for a reliable radar fix. The echo on the screen must be matched to the correct feature on the chart. Rocky headlands and steep-sided islands produce strong, well-defined echoes that closely match the chart outline. Low, sandy beaches may produce weak echoes or appear at a shorter range than their actual position (the radar beam reflects best from vertical or near-vertical surfaces). Built-up areas and harbors produce complex, bright echoes that can be distinctive but require careful comparison with the chart.
Buoys and small navigation aids produce weak echoes unless equipped with radar reflectors (or racons — radar beacons that transmit a coded response). On the chart, aids with radar reflectors are usually indicated. A racon produces a distinctive dash on the display extending radially outward from the target, making it easy to identify. However, small buoys in rough seas may be lost in sea clutter — the mass of small echoes from waves near the vessel — and should not be relied upon unless clearly visible.
Rain clutter appears as a bright area on the display and can obscure targets within or behind the rain. Most radars have a rain clutter (or FTC — Fast Time Constant) control that helps suppress these echoes. Sea clutter (STC — Sensitivity Time Control) suppresses echoes from waves near the vessel. Use these controls judiciously — excessive clutter suppression can also suppress genuine small targets. Practice interpreting your particular radar display in various conditions to develop the experience needed for reliable target identification.
Never assume a radar echo is a particular charted feature without carefully comparing the radar picture to the chart. Misidentification is the single most common cause of radar fix errors.
Radar in Restricted Visibility
Radar is most valuable when visibility is restricted — in fog, heavy rain, snow, or darkness. When visual bearings to landmarks are impossible, radar may be the only means of fixing your position (aside from GPS). In fog, the radar display becomes your primary window to the surrounding waters, showing the coastline, other vessels, and navigation aids that are invisible to the eye. This makes radar an essential tool for safe navigation in reduced visibility.
When navigating in fog, set the radar to a range scale appropriate for your situation. Use a short range (1.5-3 NM) for detailed pilotage in confined waters, and a longer range (6-12 NM) for open coastal navigation and early detection of other vessels. Alternate between scales regularly to maintain both situational awareness and detail. Compare the radar picture systematically to the chart: identify the coastline shape, prominent headlands, islands, and buoys. Build a mental model of your surroundings based on the radar image.
Take radar fixes frequently in restricted visibility — every 10-15 minutes, or more often in confined waters. Plot each fix on the chart and advance your DR from the latest fix. Pay close attention to radar targets that appear to be on a collision course (constant bearing, decreasing range). Remember that in restricted visibility, the International Regulations for Preventing Collisions at Sea (COLREGs) Rule 6 requires every vessel to proceed at a safe speed adapted to the prevailing conditions, and Rule 19 prescribes specific actions for vessels not in sight of one another. Radar is a tool, not a guarantee of safety — maintain a proper lookout by all means available, including listening for sound signals.
In restricted visibility, how often should you take radar fixes in coastal waters?
Summary
Marine radar works by transmitting microwave pulses and measuring the time delay of echoes from surrounding objects.
Radar range (VRM) is more accurate than radar bearing (EBL) — prefer range-based fixes whenever possible.
The most accurate radar fix uses ranges to two or three well-separated, positively identified targets.
Correct target identification is critical — compare the radar picture to the chart carefully and beware of clutter.
In restricted visibility, radar is essential: take fixes frequently, compare the radar picture to the chart, and maintain safe speed per COLREGs.
Use clutter controls (STC for sea clutter, FTC for rain clutter) judiciously to reveal targets without suppressing genuine echoes.
Key Terms
- Variable Range Marker (VRM)
- An adjustable range ring on the radar display that can be placed on a target to read its range precisely, typically accurate to 1-3% of range.
- Electronic Bearing Line (EBL)
- A rotatable radial line on the radar display used to measure the bearing of a target, less accurate than range due to beam width.
- Racon
- A radar beacon that receives the vessel's radar pulse and transmits a coded response, appearing as a distinctive dash on the display for easy identification.
- Sea Clutter (STC)
- Radar echoes from waves near the vessel that can obscure genuine targets. The STC control reduces these echoes but must be used carefully.
- Rain Clutter (FTC)
- Radar echoes from precipitation that can obscure targets within or behind the rain. The FTC (Fast Time Constant) control helps suppress these returns.
- Beam Width
- The angular width of the radar's transmitted pulse, typically 2-6 degrees for small-craft antennas. Wider beams reduce bearing accuracy and make targets appear broader than they are.