DOCKING SYSTEMS FOR PILOTS
Benefit versus Cost
- A comparison of shore-based and portable docking systems
DGPS Portable Pilot Unit
Differential GPS applies a secondary source of correction data to raw GPS signals.
This improves the accuracy of the position from a typical PPU by about three to four metres.
Navicom Dynamics’ newest GPS receivers are accurate to 2.5m in raw GPS, and with differential
corrections applied they achieve an accuracy of 50-60cm and often better than this.
DGPS corrections are freely available through the WAAS and EGNOS systems (covering parts of
Northern Hemisphere only), or via free-to-air MF beacons operating in the300kHz band in many
countries. Where neither is available, it is possible to set up a dedicated DGPS base station at a port.
Level of accuracy:
- Less than 60cm, often better. (Higher accuracy is often not noticeable on PPU
software or laptop screens) - ± 2cm/sec (0.05 Kn) for velocity
Equipment needed:
- PPU (with dual antennas for heading calculation)
- DGPS reference station where free-to-air DGPS is not available (range 30 – 40km)
Disadvantages:
Free DGPS corrections are not available in some areas of the world
Advantages:
- A DGPS PPU can be used at every berth in the port
- Can be used for the entire pilotage
- Real-time heading, regardless of whether DGPS corrections are available or not
- Data is consistently available
- Lowest cost solution
Approximate cost:
Full-function PPU from approx USD 30,000
Where needed, a DGPS base station is approx USD 10,000
plus one receiver for each PPU(approx USD 4,000).
TOTAL SOLUTION PRICE: From USD 30,000
Summary:
DGPS-based PPUs have proved accurate enough for all pilotage situations, including docking.
In most cases, no shore-based infrastructure is required. The technology is consistent, stable
and can be operated anywhere there is an available chart and GPS satellite visibility. Where
DGPS is unavailable, the PPU is very often accurate enough working from raw GPS.
DGPS systems are the most flexible, as well as the cheapest, option and will capably
perform all pilotage requirements.
Nav-Dif Portable Pilot Unit
Nav-Dif is a Navicom Dynamics DGPS PPU with additional software and sensors able to
refine the GPS corrections in the docking area. The concept is similar to RTK Float mode.
Nav-Dif improves the accuracy by a further 30cm over normal DGPS, giving pilots a high accuracy
of 20cm. This is only slightly less accurate than an expensive RTK solution at its best, but is
more robust than RTK often is.
Nav-Dif gives the pilot far more security in a tight situation, because the unit will maintain its
accuracy with only a very slow and graceful degradation if there is any problem with the differential
correction. In contrast, if RTK loses Lock and Float, which is a relatively common occurrence,
the pilot is left with raw GPS only, and this is not sufficient ly accurate to complete a critical berthing
manoeuvre.
The reference station is installed in the vicinity of the berths. Accuracy is better than 20cm for about
3NM and then degrades gradually to 50cm accuracy, whish is more than enough for navigation.
As the Nav-Dif reference station is less than half the cost of an RTK one, and the PPU itself is only
marginally more expensive than a normal DGPS one, Nav-Dif makes high precision
accuracy both more reliable and very affordable.
Level of accuracy:
- ±20cm horizontally in vicinity of reference station, ±50 cm at long range
- ±2cm/sec for velocity
Equipment needed:
- PPU (with dual antennas for heading calculation) and UHF modem for receiving correction data
- Nav-Dif reference station with UHF transmitter
Disadvantages:
- One-off cost of base station and the additional software makes this a little more expensive than a DGPS unit, but still significantly cheaper than RTK.
Advantages:
- Very good accuracy
- Extremely reliable real-time heading
- Can be used on any berth in the port
- At longer ranges reverts to DGPS accuracy of ±50cm
- Reliable signal in busy port environments
- Lowest cost solution available for an accuracy of better than 50cm
- Can be incorporated into both lightweight and ruggedised units
Approximate cost:
Full-function PPU from approx USD 33,000
Nav-Dif refereence station approx USD 11,000 plus installation costs
TOTAL SOLUTION PRICE: From USD 44,000
Summary:
Nav-Dif gives you all the benefits of a DGPS PPU with greater accuracy and
smoother headings when you need them most. It reverts gracefully to DGPS levels
of accuracy at longer ranges. The base station is half the cost of an RTK base station.
For many ports where space is tight, Nav-Dif is the best of both the DGPS and RTK worlds.
RTK Portable Pilot Unit
PPUs based on RTK technology need an RTK base station, which has a reliable radius of 5-10km.
RTK systems give centimetre accuracy, making them the highest-precision PPUs. However this level of
accuracy is not achieved all the time - it is possible in 'Lock' mode only. In ‘Float’ mode, the level of
accuracy achieved is about 40cm. There are many factors that can make it difficult for a PPU to
achieve and/or maintain ‘Lock’ (eg distance from the base station), and the receivers are sensitive
to outside interference. Once Float mode is lost due to range from the reference station, accuracy
reverts to raw GPS levels
Level of accuracy:
- 1-40cmdepending on whether RTK Lock is achieved. 2.5m if 'Float' is also lost
- ±1cm/sec for velocity
Equipment needed:
- PPU (with dual antennas for heading calculation) and UHF receiver for correction signals
- At least one RTK base station. (Extra stations for use beyond approx 10km radius)
Advantages:
- Very accurate distances and velocities
- Can be used at every berth in the port and for channel navigation
- Real-time headnig
Disadvantages:
- RTK Lock status difficult to achieve/maintain
- Laptop resolution limits ability to view centimetre accuracy
- Use limited by range of base station
Approximate cost:
Full-function PPU from about USD 45,000
At least one RTK base station at approx USD 15,000
TOTAL SOLUTION PRICE: USD 60,000 for passages up to 10km.
USD 80,000+ for longer pilotages
Summary:
RTK is the most accurate PPU technology. However that extra accuracy comes at a significant
cost in terms of the PPU itself and the shore-based infrastructure. It is unlikely the enhanced
accuracy will even show on the pilot’s display computer, given that one pixel corresponds to
between 60-80cm on the typical pilot’s laptop at a range scale appropriate for docking..
Laser Docking System
This consists of two laser sensors, with monitoring units located in a control room ashore.
Vessel movement data can be displayed on a jetty-mounted monitor, and/or or transmitted
wirelessly to the ship’s display, or a laptop on board the ship.
Laser docking is the most accurate docking solution, but only works for the last 300 or so metres
of the docking procedure, and then only when the vessel is immediately opposite the lasers.
It is unable to measure speeds in any direction except at right angles to the berth. It does not
perform any other functions, such as channel navigation or swinging assistance, and it only
operates on a single berth.
Level of accuracy:
- 1cm
- ±1cm/sec for velocity
Equipment needed:
- Two laser sensors with monitoring units, a control PC and jetty interface
- A large jetty display and/or wireless link to on-board display
- Speed warning lights
Disadvantages:
- Does nothing but aid in final docking manoeuvre on a single berth
- May not work on dark-hulled, non-reflective ships
- Jetty display may be hard to see in some conditions
- Very expensive
Advantages:
- Very accurate and stable
- Berthing operation monitored from on shore
Approximate cost:
Upwards of USD 250,000
TOTAL SOLUTION PRICE: More than USD 0.25million
Summary:
Unless used in conjunction with other purposes such as mooring load monitoring, the benefits are
limited.. A major premium is being paid for a minor increase in accuracy. This extra accuracy is
unlikely to give any actual operational advantage over a PPU, and the system cannot be used for
navigation outside the immediate vicinity of a single berth.
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