Navigating the radar
A navigator tasked with setting up the radar prior to arrival or departure will need to make a number of decisions about which of the radar’s features and modes of display to use.
This article addresses some of the issues a navigator might consider when using radar for navigational purposes, and is designed to complement the information about using radar for collision avoidance on pages 4 and 5.
There are three orientation modes the navigator may choose from when setting up the radar: North-up, head-up and courseup. A North-up display is often preferred, as the orientation of the radar picture will match that of a paper chart and so also matches most peoples’ mind-set of an area. In head-up mode the ‘up’ direction of the display represents the vessel’s heading; in course-up mode it represents the direction which has been input as the vessel’s desired course. In normal use all three modes make use of the ship’s gyro-compass to reference the display, which is known as azimuth stabilisation. However, if the gyro-compass becomes inaccurate or unavailable the radar will have to be set to the head-up unstabilised mode. This could cause the smearing of land and other echoes with alterations of course.
Two motion modes are provided – True and Relative. In True Motion the displayed position of own ship moves at a scaled speed across the display that corresponds to the vessel’s actual motion. In Relative Motion mode the displayed position of own ship is static. When off-centred, this provides maximum lookout ahead, as well as possible early warning of rain showers/ squalls, landmarks, wheel over points and, of course, traffic.
There are two stabilisation modes – ground and sea. Ground stabilisation means that the display is referenced to the seabed by GNSS or through a twin-axis Doppler log. For sea stabilisation the display is referenced to the sea current that own ship is experiencing, typically based on SDME measurements. It is generally accepted that the ground stabilised mode is more useful for normal navigation, especially when combined with setting tracked targets to show their True vectors. However, since set and drift affect heading and aspect, ground stabilised displays should be used with caution where target tracking is concerned. Having a ‘predicted’ ground track in confined waters is especially useful, particularly where visual information and cues are limited or unavailable, such as in fog.
With ground stabilised radar, all stopped targets will show the set and drift, which is ideal for navigation in channelled waters with significant tidal streams. However, the effects of current will vary across the observed area.
Target trails can be set to be either True or Relative, whatever motion mode the display is set to. In coastal and confined waters, navigators often select Relative Motion True Trails (RMTT), as it provides the illusion of true motion while retaining the advantages of relative motion.
With ground stabilised radar, all stopped targets will show the set and drift, which is ideal for navigation in channelled waters with significant tidal streams. However, the effects of current will vary across the observed area. Since set and drift affect heading and aspect, ground stabilised displays should be used with caution where target tracking is concerned.
The variable range marker (VRM) and electronic bearing line (EBL) facilitate a quick and easy check of the vessel’s position in coastal waters.
For decades, radar has been a tremendously reliable tool for ascertaining the range and bearing of fixed and floating objects. The variable range marker (VRM) and electronic bearing line (EBL) facilitate a quick and easy check of the vessel’s position in coastal waters. By simply extending the VRM and rotating the EBL to a number of conspicuous points on the radar picture, and laying off those range and bearing lines to the corresponding charted objects, the navigator can quickly establish the ship’s position. This applies to both paper and electronic charts.
However, it is important to note that any position plotted is historical – a record of where the vessel once was. It provides no indication of where the vessel is likely to be. Only by creating an estimated position (EP) using the ship’s course and speed will a future position become clear. On ground stabilised settings, the true vector or ‘predictor’ provides a real-time EP. The vector length can be adjusted from a few seconds, typically up to 60 minutes or more. When sailing in shallow waters or close to land, this ‘predictor’ can provide early warning of deviation and prompt early corrective action. At anchor, a true vector set at 60 minutes or more could provide an early indication of dragging.
OFFSET VRMS AND EBLS
In addition to providing a range and bearing from the vessel’s own position, EBLs and VRMs can be offset (carried or dropped elsewhere onscreen) to provide a course to make good and a distance to go to a pre-determined position. This is extremely useful when approaching an anchorage or waypoint, or determining a course to steer when, for example, the vessel makes a planned departure from track. The carried EBL also serves as a parallel index line if required and can provide an indication of an object’s bearing further along the ship’s course, which can then be visually confirmed.
The use of parallel indexing to monitor cross-track distance is key to navigation. On modern radar sets, four or more index lines are typically available, often in different colours, enabling the navigator to index present and future courses. Parallel indexing allows an almost instant reaction to any unplanned deviation from course, as well as continuous monitoring of the vessel’s progress in relation to its track. The great advantage of parallel indexing is that it requires nothing more than the relative track of a fixed object parallel and reciprocal to the vessel’s own track. Remember the saying: “to safely get by, use your PI.”
Radar offers the navigator many useful functions for pilotage and coastal navigation:
- Parallel indexing and relative trails – for track monitoring and control
- True vectors – to indicate the vessel’s ground track in steady state conditions
- ARPA (also known as target tracking) – to acquire fixed objects to ground stabilise (echo referencing)
- Offset EBL and VRM – to provide a leading line, course to steer, distance to go etc.