The performance and the ergonomic aspects of various PPU solutions

The choice of the right PPU depends on several factors. This approach of comparing PPU types was presented and discussed for the first time at the 2018 IMPA conference in Dakar, in occasion of a presentation held by Cadden on PPU technology. Its content was well perceived. The delegation of international pilots as well as all main PPU vendors attending agreed that this is a good summary of the main types of solutions available nowadays. The interesting thing about this table is that the key performance metrics have been mapped to give a general idea of what to expect from a type of PPU. Ergonomics is also taken into account, as well as the weight and dimension, finally the pricing. Five main types of PPU hardware are considered:

1. A PPU made of two wireless beacons with RTK type of precision
2. A PPU made of two beacons connected via one cable with RTK type of precision
3. A PPU made of two wireless beacons with SBAS type of precision
4. A pilot plug receiving navigation data from the AIS and having its own embedded gyro
5. A pilot plug receiving navigation data from the AIS without any other type of aiding sensor neither embedded nor connected

Let´s take a look at these one-by-one:

Two wireless beacons with RTK type of precision

• RTK precision means that a GNSS receiver delivers a precision of position of 1cm horizontally and 2cm vertically. This is most accurate precision one can have therefore POS (1cm) and SOG (1cm/sec) rank TOP.
• Since the beacons are not connected by cable, they can be placed far apart from each other and deliver an excellent performance in terms of Heading (see formula) and ROT. Heading and rotation is calculated using GNSS sensors, implementing a so called GPS compass.
• The separate beacons need individual means of power supply, they must have ways to communicate amongst each other and require connecting with a RTK base station based on land. This pose some constraints in terms of volume and weight. This type of PPU is not yet so light and small to be carried inside a pocket, therefore the ergonomics is not the best possible, however, the absence of cables make it simple to use and less prone to points of failure.
• On the other hand, the fact that the two beacons are isolated, means that the cost of the technology behind a multi-constellation RTK GNSS L1/L2 receiver is double making a PPU of this type cost as much as a nice German car.

Two beacons connected by cable with RTK type of precision

• RTK precision is TOP, like in the previous case, so no changes here.
• Since the beacons are connected by cable, they cannot be placed far apart from each other and deliver a good performance in terms of Heading but not the best one can have (see formula).
• The two beacons can be powered by a single battery. They communicate amongst each other via the cable connector which links the two GNSS antennas to the same receiver. This poses less constraints in terms of volume and weight but still makes this type of PPU not so light and small to be carried inside a pocket, therefore the ergonomics is not good at all, especially since cables make it cumbersome to use and set up but also prone to failure.
• The fact that the two beacons not isolated, means that the cost of the technology behind the RTK GNSS receiver is nearly half of the one behind a wireless system which needs two separate receivers, making a PPU of this type cost circa 40% less of an equivalent one wireless. The price is therefore comparable to a modern city car.

Two wireless beacons with SBAS type of precision

• RTK precision means that a GNSS receiver delivers a precision of position of 50-70 cm horizontally and circa 1-2m vertically. This is OK but still not the most accurate precision one can have therefore POS (50cm) and SOG (10cm/sec) rank OK.
• Since the beacons are not connected by cable, they can be placed far apart from each other and deliver an excellent performance in terms of Heading (see formula) and ROT. Please note that a Moving Base Line (MBL) can be implemented with a so called relativeRTK precision despite the level absolute precision of position delivered by SBAS technology.
• The separate beacons need individual means of power supply, they must have ways to communicate amongst each other. This pose some constraints in terms of volume and weight. This type of PPU is still not as light and as small to be carried inside a pocket, therefore the ergonomics is not the best possible, however, the absence of cables make it simple to use and less prone to points of failure.
• On the other hand, the fact that the two beacons are isolated, means that the cost of the technology behind the GNSS receivers, (which is less than RTK receivers) is still quite high since one needs to have two L1 SBAS GNSS receivers still making a PPU of this type cost as much as a city car.

An AIS pilot plug with gyro

• Using AIS data is a good idea butGNSS positioning is not precise (2.5m-4.0m CEP-Horizontal). Furthermore, SOG is only accurate when navigating offshore, not fit for navigation at slow speed or inside harbours. The AIS provides a low update rate which is not constant. Pilots use pilot plugs mainly for situational awareness. Another benefit is to have the choice of being independent from the ECDIS. Clearly, this solution is not fit for navigation.
• This type of solution must be complemented with other sensors, one of the typical solution is to embed a good gyro inside the pilot plug. This gives ROT which is good enough in some types of non-extreme situations. The heading information provided is not the best one can get but it is still OK. Fusing the data from the AIS with the embedded gyro is the key element which provides the right HDG during the interval between each AIS feeds. However, when the intervals are too long even a good gyro might lose a few degrees per minute.
• The great thing about this equipment is that it is light and small. The ergonomics is high, except that in some cases, the pilot plug on the ship is not wired correctly and this makes it a solution which comes with some risks, therefore it still does rank 5 stars in terms of ergonomics, but it is not the ideal solution.
• The price is mainly driven by the quality of the gyroscope embedded and is equivalent to a moto scooter.

An AIS pilot plug with no aiding

• Same as above, this solution is not fit for navigation but for situational awareness and to be independent from the ECDIS.
• This equipment is light and small. The ergonomics is high, same as above.
• The price of the hardware is mainly driven by the quality of the wireless features (Wi-Fi, or Bluetooth) and some software which helps the configuration. We can safely say that e pilot plug costs as much as a bicycle.

PPU are becoming increasingly important

When it comes to rules on navigation safety there is no official obligation for pilots to use a PPU. The IMO (International Maritime Organization) sole hint to its usage can be interpreted in the Convention on the International Regulations for Preventing Collisions at Sea, 1972 (COLREGs)both in Rule 5 (Look-out):

“Every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and or the risk of collision.”

And in Rule 7 (Risk of collision):

“(a) Every vessel shall use all available means appropriate  to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt such risk shall be deemed to exist.”

The term “by all available means appropriate” is where we can assume that a PPU is a good candidate to minimize risk.

These rules, last edited more than 10 years ago, clearly focus on making sure that conventional equipment is properly working such as radar technology.

“(b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observations of detected objects.

(c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information.”

As we witness a huge increase both in the size of the modern vessels and in their traffic in busy harbors and seaways. Pilots tend to minimize some risk factors by means of more modern tools providing insightful situation awareness information to make a full appraisal of the risk of collision. Proper look-out is definitely better off with additional modern navigation aid tools such as a PPU.

In this article, we will discuss 7 most important situations when a PPU is useful.

Limited Visibility

A PPU is a great piloting aid system in conditions of limited visibility. Traditional look out can be hard due to the huge size of modern vessels, furthermore, in circumstances when large structures block the sight (i.e. a high stack of containers) the job of the pilot can become quite stressful. The problem becomes more noticeable at night or when extreme meteorological conditions such as thick fog or heavy rain, reduce visibility. In extreme conditions, a PPU sensor can also be considered as an excellent navigation tool.

Up-to-date Charts

Bathymetry and cartography does change. As the size and the weight of modern vessels goes increasingly up, the need for accurate and up-to-date charts becomes a key factor. There can be situations when notably, the ECDIS or the printed charts cannot be trusted completely. Pilots can bring accurate charts provided to them by specialized local authorities, but they need to plot onto them, their vessel in real time. The most common method is to plug their tablet to the AIS plug, making this the most basic form of Portable Pilot Unit, in this way they can at least be detached by ECDIS. In situation when extreme accuracy is necessary, other types of navigation sensors can also be brought onboard, but, in any case, the main objective of using their own chart is achieved, and this is very important.

Precise SOG, POS, HDG and ROT

For most of the reasons discussed in this article, pilots are in demand of berthing aid tools that provide accurate vessel location and speed in real-time. Accurate heading information can be determined in several ways. The ROT is equally important. For a deeper analysis on this topic, it is strongly advised that you read the articles on how to determine vessel ROT and accurate heading as well as the article on vessel position and speed accuracy.

The main characteristics of a PPU is to deliver accurate navigation information to provide pilots with sufficient aid to do their job better, safer and with less stress. There are several ways to provide the accuracy required. For more information on these differences, please see the article: the choice of a PPU depends on several factors.

Navigating very heavy/large vessels

When the weight of a vessel is so big that she makes very little headway, the forces which must be applied are massive. This has an impact both on the fuel consumption but also on the attention which goes to applying thrust in a way that is optimal. Generally, this type of operations take a long time and are performed very slowly to reduce risks but also to control the huge amount of power applied. If a very heavy ship accelerates more than expected the predicted position must be calculated sufficiently ahead of time to control her motion. Detecting with accuracy these types of acceleration requires sensors which evaluate the situation very often, and amongst all, with extreme accuracy (1cm/sec), in this case a PPU is the answer.

When it comes to long and large vessels, brings yet another difficulty, to control the drift both on the stern and on the bow and to evaluate the ROT with extreme accuracy. Imagine a vessel which boasts a length of 350m whose ROT is not accurately monitored. It is unthinkable. The water space needed for maneuvering must be enormous, and unfortunately, in some cases, a pilot must assist the commander in tight water spaces.

In tight water spaces

Even in cases when ships are not over 300m in length, it is not uncommon to have vessels with beams over 30m. The size of vessels in the last decade has rocketed up. On the other hand, harbors, canals, rivers, locks have not been able to grow adequately. Despite intensive dredging, building new and larger harbors, today, it is not uncommon that a huge ship must enter a water space with a margin of a few meters. And if this was not a problem big enough, the traffic has also gone up, so the space available needs to be shared. The need to control is never been so crucial as never. This is also why the usage PPU has taken off in the last few years.

For real-time dynamic under keel clearance

So far in this article, we have not touched upon another huge issue, probably the biggest of all: UKC. Depth limits are above all a commercial handicap for shipping companies. On the section of the Elbe river before Hamburg, vessels with a combined beam of more than 90 meters cannot meet in the navigation channel.  Access to some ports is limited in small time windows and is dictated by the tide. In some cases, commanders must take the risk to navigate in circumstances where the UCK is at the limit. To achieve this, they must have and excellent control of the position of the ship, the bathymetry must be accurate, the meteocean information must be in real time, the areas with restricted access must be clearly marked. A PPU plugs itself in this scientific environment as a technological tool which acts as a precision sensor of position both on the horizontal plane but also on the vertical axis. Some have inertial sensors which measure the heave of the ship, and the pitch, which in some cases helps detecting the SQUAT effect.

For training and in case of emergency

Some software allows saving and replaying manoeuvres. This is an excellent way to train less experienced pilots. At the same time, more experienced pilots, can learn new ways, once they have been recorded by others.

In the unfortunate event when a complicated situation starts developing, pilots can ask for support to other peers who are more experienced with the problem. Some navigation software, which works in combination with PPU sensors, have a solution for broadcasting  in real time on the internet, what is seen on the screen of  the pilots on board. Any pilots, even off duty, can be alerted. They can connect to the server using their smartphone and they can see what is going on, in this way they can make a quick precise assessment of the situation and provide support remotely.

Four piloting softwares to use with a PPU sensor

When it comes to software for maritime pilots, there are numerous companies worldwide who have developed very useful applications. Some come in a bundle with the purchase of navigation sensors hardware; clearly a few PPU vendors have either developed their own software, or are in a strategic partnership with software developer companies who provide exclusive solutions. These will not be covered in this article because they are not considered as software companies, but rather as hardware companies. Other suites are open for use as a software licence and work well in conjunction with all modern PPU sensors.

If we restrict the list of available solutions to piloting software made for very demanding situations, four companies are leading the innovation wave. Let us look into these in alphabetical order:

QPS – Qastor

QPS is a leading software company with a rather large development team who is well acquainted to undertake diverse challenges around solutions for navigation, hydrography, ENC production, geo-spatial processing, real-time processing, 2D-3D- 4D visualization, data integration, etc. The company is part of SAAB (Marine Traffic Management) group, which is well exposed to a multitude of use cases related to the marine environment. Worldwide use of their software and large client base allows them to continuously grow their domain knowledge and gather extensive feedback. This ultimately reflects in high quality products with good standard in terms of support. Their strong development methodology allows them to swiftly adapt to new requirements. In fact, they can offer ad-hoc programs to provide custom-made modules or computations fit for a specific purpose. For example, they can integrate specific hydrological factors, which are very particular in a harbour and apply unique rules to create algorithms that implement distinctive navigation prediction techniques.

The Qastor product is primarily designed as an ECS for piloting operations. The software provides features such as guard zone, overview, path prediction, CPA and meeting points. It also proved to be a useful navigation tool in several other fields including ship trials, oil rig positioning, inland river barges, SPM approaches, ferry operations, oil and gas tanker approaches and docking, patrol vessels and tugboat operations.  QPS developed some specific modules that can be purchased separately pending on your needs. An example is the Docking and Lock approach module, which is used during mission critical mooring and approach operations. Amongst other things, this module, coupled with a high precision PPU provides effective algorithms for real-time calculation of COG, SOG, ROT to accurately establish and predict bow and stern movements.

The Connect client/server integrated data system may be used in conjunction with Qastor to exchange chart updates, meteo live data and GRIB support (weather prediction along the route), tide, customizable alarms and SMS notifications. It is not just mariners on ships using Qastor, a number of harbour masters and most recently fleet operation managers use Qastor and the Connect Server for round the clock monitoring and automated alerting

SEAiq Pilot

SEAiq Pilot is the only multi-platform piloting solution, with support for all major operating systems, all pilot plugs and PPU devices. It combines a complete set of piloting features with unparalleled ease of use.  The company offers a product which is very focused around the needs of maritime pilots and offers a comprehensive set of hundreds of navigation features: record/playback, CPA, BCD, Meeting Point, overlays, tides, internet AIS, chart markup, overlays, and many more.

Besides the fact that it supports all tablet/laptop platforms (iPad/iPhone, Windows, MacOS, Android), SEAiq works very closely with all PPU and Pilot plug vendors to assure compatibility. Pilots are offered a wide choice of features which are fully configurable through a central web console. This allows to personalize the user interface, select the relevant features, edit user menus, lock settings, set common routes and chart mark-up. It can automatically download charts from PRIMAR, ChartWorld, and NOAA. It supports charts in all standard formats: S-57, S-63, iENC, BSB/KAP.

The company´s vertical focus in this field has allowed to propose a very flexible product which is fully customizable at unbeatable price. Consider that the enterprise monthly subscription for one pilot goes at USD 30 with no up-front license fee.  Combine all of this and you can easily understand why SEAiq Pilot is used by 25-33% of pilots world-wide. Customers include: Miami, London, Singapore. The company is not large but is indeed a leader in this field. It is in partnership with the Maritime Simulation and Resource Centre (MSRC) and the Maritime Pilots Institute (MPI) where training in SEAiq Pilot is offered.

SevenCs – ORCA Pilot G2

SevenCs IS A Maritime Software House that has been working with Pilots and developing Pilot software for 25 years. Alongside our extensive experience in the Pilot field, we also offer packages combined with bespoke chart production, software customization, and support.  The current PPU software is:

This modern navigation software, specially developed for pilots, with pilots, it optimised for touchscreen operation, can be integrated with any high-precision navigation sensor or any pilot plug and it can make use of AIS data via 3G/4G. It currently runs only on Windows OS, but 2019 sees the launch of the IOS version OPX.

ORCA Pilot G2 is available as professional standalone navigation software that can be integrated with additional services to improve the safety and efficiency of your pilot organisation.

Orca Pilot G2 highlights are:

• A docking mode providing automatic minimum distance calculation towards selected chart objects, berthed or slowly-moving AIS targets, ship motion predictions with adjustable time.
• It provides rather sophisticated and quick chart handling features including automated distribution within a pilot organisation. It supports ENC, bENC, Inland ENC.
• Trip data logging for playback analysis including the AIS targets (range of 10 nm distance to the own ship)
• Online water level correction (requires a web feature service providing the real time data)
• Display of AIS targets (and the possibility to select AIS targe
  ts as an own ship to get full overview and virtually board the target)
• Advanced route planning in graphical and tabular form
• Advanced docking mode facilities including Predictor, Simulator, and Distance measuring operator
• Anti-grounding functionality and advanced pre-warning system
• CPA calculation (open sea calculation), ERP (Estimated Rendezvous Position) calculation for river navigation along the planned route
• Navigational functions for distance / bearing calculations
• True scale and fixed size own ship symbol
• North-Up and Head-Up mode
• Display adjustable to various day/night light conditions
• Detailed query of chart object features (Pick Report)
• Split screen for docking and manoeuvring

Transas – Pilot PRO

Transas offers best-in-class navigation systems and integrated bridge solutions, recognized training and simulation solutions, well-known VTMS and coastal surveillance systems, shipping company and port management systems, onboard and individual decision support systems for professional crew and pilots.

The leading market positions and their achievements in the sphere of research and development have allowed them to launch, amongst other things, a solution called Pilot PRO which is used by a lot of pilots worldwide. We cannot tell you how many, but rest assured that if you stick around a pilot while on duty, chances are they are using something made by Transas.

Transas experience and good reputation is a guarantee for pilots. Besides, several pilots love Pilot PRO because it is very simple to configure, it is intuitive and it is reliable.

Transas Pilot PRO is the best-in-class iPad-based chart plotter for professional pilots and it comes at a very affordable price. In addition to monthly updated TX-97 format marine vector charts, Pilot PRO features a number of functions requested by pilots specifically to improve their daily efficiency: Docking Mode, Ship Maneuvering Predictor, operation with AIS Class A transponder and independent third party pilot sensors over Wi-Fi, Weather Service, Internet AIS targets, Tide and Currents, AIS Meeting Points, Advanced Data Logging, Playback and many more.

At the core of all Transas products and solutions lie innovative technologies which are based on extensive experience in IT. Although Transas core business is not to solely make navigation software, the company has made an excellent solution fit for use with high precision PPU. Their reasoning goes behind the need to provide standard piloting tools. In fact, the Pilot PRO offers several e-Navigation functions, such as multi-connection to different data sources, secured AIS data feed from Transas VTS and AIS meteo data from local onshore sensors. This turns the Pilot PRO into an important link of the composite infrastructure of pilot, vessel, shore, shipping company, training centre, with a range of integration possibilities between its components which Transas clearly masters.

Secondary factors which make a good PPU

In previous articles, some key facts which make the basis of a performant PPU were discussed. There are several other factors which should be studied. In this article we discuss only three of them:

Motion Sensors

As far as a PPU technology is concerned, the roll and pitch of a vessel can easily be determined with gyroscopes. On the other hand, speed and position, as well as accurate heading, are largely measured with a GNSS system. As already discussed in the article about orientation, sophisticated inertial sensors, at least within the context of piloting inside harbours, provide the added value to measure the vertical heave of a vessel, only if they are equipped of accelerometers, in which case, they are generally referred as: Inertial Motion Unit (IMU). Some MEMS-based IMU are a good option only when a vessel is moving quickly through shallow water.

An IMU can flag the imminent SQUAT effect created by an area of lowered pressure that causes the ship to be closer to the seabed than would otherwise be expected. Of course, this is only possible if inertial data is aided by a high-precision GNSS system and if a SQUAT software algorithm is smart enough to give suitable alert. The amount of squat will depend upon several factors but in certain conditions it may be as much as two meters and a good inertial navigation system can feel a heave change of a few centimeters even if it occurs slowly. The asymmetric flow of water around the stern frame reduces the power of the rudder, making steering more difficult and losing stability. The vessel might experience yaw and roll. This is how pilots are alerted of SQUAT already happening. So, a sophisticated PPU as such, can be of good aid to give pilots an early warning. A minor heave change is otherwise impossible to feel without auxiliary sensors, unless the SQUAT is already occurring, and that can be dangerous.

Hyper connectivity

As technology innovation evolves, solutions around VTS become sophisticated. Reliable monitoring of all maritime traffic brings enhanced situational awareness. The picture provided is getting increasingly accurate. Modernization, is giving the ability to integrate data from multiple sensors such as AIS, Radar, CCTV, live weather streaming, real-time feeds from hydrological monitoring systems, drones and much more. Integration is taking place, so is real-time digital display technology in 2D and 3D, which gives an amazing picture of ports and waterways. Nobody knows what the future holds but the idea is becoming quite clear. As far as modern PPU technology is concerned, more situational awareness can be given if a PPU is communicating with the VTS.

If a PPU is equipped with sufficient telemetry equipment such as modems and antennas to work on 3G/4G cellular and UHF/VHF radio and AIS transceivers. Suddenly the PPU becomes another integrated sensor. Some PPUs have a slot where to insert a SIM card, others can even handle two SIM cards and concurrent mobile internet connections.

High precision PPU are meant to be in constant radio connection to UHF/3G base stations to acquire RTK differential corrections. Pilots use Wi-Fi or Bluetooth connections to dialogue with the device. Long-range Wi-Fi allows radio communication to a range of a couple of hundred meters. These are all very important features to be considered when investing in a PPU.

Last but not least, the update rate with which the PPU sensor sends data to the navigation software, is a key factor which falls within the category of hyper connectivity. Generally, the software used by pilots reads data which comes in a format so called NMEA. The string contains navigation data and ought to be sent quite often as input to the pilot software so that it can make the necessary computations to show position, orientation, predicted position and so on. The throughput of a radio link can be strongly affected depending on the amount of information sent and depending on the frequency of these updates. A PPU should send data anywhere between 2 and 5 times per second. Anything above is unnecessary but anything below 2Hz is really not recommended.

Hot-swappable safe battery

Battery autonomy is always a worry for pilots, especially in operations lasting several hours. From the PPU manufacturers´ standpoint, it is more an affliction because the way to provide very long autonomy is by placing very large Lithium-Ion rechargeable battery which poses two big problems.

1. Extra weight, limiting the ergonomics of a portable system.
2. If overheated or overcharged, Li-ion batteries may suffer thermal runaway and cell rupture. This can lead to leakage or explosion. A faulty battery can cause a serious fire. Faulty chargers can affect the safety of the battery because they can destroy the battery’s protection circuit. Often a device which contains this type of batteries is not allowed on board of vessels carrying explosive goods and they are not allowed on planes and helicopters.

The only solution to these issues is to provide smaller batteries which are safe.

A nickel–metal hydride battery, abbreviated NiMH is the safest approach, they cost more but that is nothing compared to the threat they represent. Size reduction to assure a lighter PPU with NiMH batteries is tricky, that is, without ultimately affecting the autonomy of the device.  Fortunately, the technology exists today to provide hot-swappable NiMH batteries. A safe battery of average small size and low weight can last 6 to 10 hours, it can be recharged in 3 hours and, if necessary, it can be replaced without causing power loss during operations. This means, endless autonomy and good equipment safety.

HDG & ROT

Accurate heading information can be determined in several ways. The AIS plug does not provide heading information with an update rate which is frequent enough to allow any navigation software to resolve the ROT in real time, while a PPU like BANANAS can measure a ROT < 0.1°/min. To understand why this is so, we need to understand the following 3 main viable solutions:

GNSS Sensors – GPS compass

On water, a single GNSS antenna computing a course from point to point will always provide inaccurate results, unlike terrestrial applications, when it comes to marine navigation, to figure the direction of a vessel, one needs to have two precise GNSS antennas placed as far apart as possible. This is commonly referred with the term: GNSS compass (or GPS compass) and it works according to a very simple concept: by pinpointing the exact positions of both antennas, it is possible to work out the orientation of the imaginary straight line across the two.

(The topic of GNSS precision of position was discussed in another article on PPU technology, if you are not familiar with the terminology contained in the next paragraph, we strongly suggest that you read it. the link is here.)

In RTK Terminology, this  is a Moving Baseline RTK, useful for GPS applications that require vessel orientation. It is a positioning technique in which both reference and rover receivers can move about.  With Moving Baseline RTK, the reference receiver broadcasts data, while the rover receiver performs a synchronized baseline solution. The resultant baseline solution has centimeter-level accuracy. To increase the accuracy of the absolute location of the two antennas, the Moving Reference receiver can use differential corrections from a static source, such as a shore-based reference station.

The formula is: 0.2 degree of accuracy for every meter apart, to have a heading accuracy of 0.01 degree the antennas should be placed 20m apart. The limitations come from the fact that traditional (and old fashioned) PPU have cables between the 2 beacons, so one can gain in accuracy at the price of ergonomics. More recent versions of high precision dual antenna PPU, i.e. made of two wireless beacons, have come to the market to provide the best navigation aid. To understand the technology behind PPU one must also consider that the best systems are a mix of GNSS and Inertial sensors, let´s take a quick look at why this is so.

Inertial sensors

A 3-axis gyroscope is a very good way to determine heading information, some very accurate gyroscopes are made of bulky mechanical parts and cannot be brought onboard. By default, each vessel is equipped with a very good gyroscope, but as already discussed, data from the AIS is far from being in real-time. Some cheaper and lighter electronic equipment, disconnected from the GNSS positioning system, can be brought on board, this does not represent a real PPU, but it is a step forward to help pilots. Some PPU manufacturers have developed some small devices to address the problem of determining an accurate ROT. Manufacturers have tried some techniques to improve its performance, unfortunately, despite attempts, aiding inertial navigation data with magnetometers it is of little help; when it comes to high precision sensors made of compact electronics, a magnetometer is affected by electromagnetic interference and, last but not least, a magnetometer delivers magnetic heading not the true heading, making magnetometers obsolete. To achieve better performance is to primarily rely on fusing inertial navigation data with the position and heading given by the satellites.

A IMU (Inertial Motion Unit) made of 3-axis accelerometer/gyroscope provides better performance. Some of these sensors are very sophisticated. The most advanced kind works on fiber-optic technology, providing very accurate heading, roll, pitch, speed, heave and position. They can be considered as reliable navigation devices even without aiding from a GNSS positioning system and are often used to navigate submarines or objects into space. Unfortunately, they cost a fortune, they are very large and extremely heavy to carry. Some lighter and smaller versions are quite precise and could work very well as a tool for maritime pilots, but they are still expensive, not so small (probably the size of a bottle) and are accurately reliable without GNSS position aiding for a limited time (a matter of seconds). These types of IMU are commonly used in survey applications which primarily rely on satellite navigation. These devices come in hand in case of absence of satellite reception (dead reckoning) which generally occurs inside tunnels, under bridges and in urban areas. Clearly, not an issue when navigating a 300m cargo vessel.

Therefore, the optimal solution comes from another type of inertial navigation sensors. Since the introduction of Micro-Electro-Mechanical Systems (MEMS), the size of a decent IMU has been enormously reduced a matter of centimeters, their weight to a few hundred grams and their price to around 2,000 dollars. They are less precise but still excellent since they are constantly aided by satellite navigation.

A combination of both GNSS & Inertial sensors (Kalman filter)

A good dual-antenna system (i.e. a GNSS compass) can work without a IMU, the concept of using accelerometers and RTK positioning is rigorously necessary to measure the heave a vessel, which is primarily useful to gage the SQUAT effect. When using a IMU in combination with GNSS sensors, we say that we fuse the GNSS navigation data with inertial navigation data. If accelerometers are not used, the sole use of gyroscopes (even not the most accurate ones) is still sufficient to determine accurate heading, roll, pitch, speed and position. In this case, we do not say that we fuse the data. However, a good PPU still needs to elaborate accurate navigation data from a series of measurements observed over time, containing internal state estimates of a linear dynamic system from a series of noisy measurements and other inaccuracies. For overcoming this difficulty, a good PPU must have an algorithm also known as Kalman filter, which produces estimates of unknown variables that tend to be more accurate than those based on a single measurement alone, by estimating a joint probability distribution over the variables for each timeframe. It sounds complicated, and in fact it is indeed. A good IMU will even have an advanced Kalman filter.

Bottom line: to determine accurate heading and a precise ROT, a pilot ought to have two wireless high-precision GNSS antennas/receivers arranged to implement a GNSS compass with Moving Baseline RTK technique, a good PPU must implements at least a Kalman Filter. If a good MEMS based IMU is embedded inside the PPU, better performance is guaranteed.

Position & SOG

Pilots are in demand of berthing aid tools that provide accurate vessel location in real-time. There are several ways of providing the necessary positioning/speed accuracy.

A PPU will facilitate efficient and safe manoeuvring within the entire navigation harbour zone enhancing vessel trajectory. To better understand what technological solutions are available to assure accurately positioning of a vessel, the following alternatives must be considered

AIS

The Automatic Identification System (AIS) is a shipboard broadcast system that acts like a transponder, operating in the VHF maritime band and transmitting real time information of the vessel. It relies on GNSS as primary positioning source. The onboard system uses a transponder system that operates in the VHF maritime band and continuously transmits vessels identity, geographic position, speed, course, vessel type, and cargo information along with other relevant information to all other AIS equipped vessels within range, in a real-time automated manner. The AIS uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet a high broadcast rate and ensure reliable ship-to-ship operation. The AIS transponders send data every 2 to 10 seconds depending on a vessel’s speed while underway.

Often, pilots use a very light electronic equipment called AIS plug, or Pilot Plug, or AIS Pilot Plug. This genius tool, which can be considered as the precursor of a PPU, is literally plugged into the AIS plug on board and, acting as a sensor, it sends AIS data to the pilot either via Bluetooth or Wi-Fi.  Besides the fact that a pilot plug costs only a few hundred dollars, it can be put inside the pilot´s pocket and it has a very long battery autonomy. As of today, it is the most widely used navigation aid tool by maritime pilots.

The problem is that the GNSS position Accuracy from AIS is generally 2.5m-4.0m CEP-Horizontal so a pilot lacks control of precision, he cannot always trust to receive the necessary berthing aid in terms of precision of position, so a pilot gets what he gets…. Data accuracy is limited to a degree of precision which might not be sufficiently high, causing a poor evaluation of speed.  Besides, if one considers that data is provided with a very low update rate, the SOG is not calculated with the accuracy required in difficult situations, furthermore, a low update rate, makes any sophisticated feature of the pilot software giving a predicted position of the vessel, obsolete.

SBAS

When it comes to improving position, a Satellite-based Augmentation System (SBAS) is a good alternative. Using geostationary satellites which broadcast the augmentation information, it is possible to greatly improve the position accuracy to 0.5m-1m CEP-Horizontal.

A relatively important limitation is that the its network coverage is not global, WAAS operates in North America, EGNOS in Europe, SDCM in Russia, GAGAN in India, MSAS in Japan, BDSBAS in China, so it is mainly in the northern hemisphere.

Finally, although SBAS is indeed widely used and rather precise in most applications, it does not deliver the highest precision needed in extreme cases where SOG must be precise. The precision or position on the vertical axis is circa double the horizontal accuracy, limiting in some case to navigate in shallow water, where UKC must be strictly calculated.

Altogether, when it comes to speed accuracy, it is a good GNSS technology, although in some cases it is necessary to bring their own receiver on board, hence you need a PPU. In fact, the most sophisticated receivers are generally not available on board.  SBAS receivers operate mainly in single point position (SPP) mode and estimate velocity either by differencing two consecutive positions (i.e., approximating the derivative of user position) or by using Doppler measurements related to user-satellite motion.

The former approach is the simplest to implement, but it has a meter per second–level of accuracy due to the dependence on pseudorange-based position accuracy. In contrast, Doppler frequency shifts of the received signal produced by user-satellite relative motion enables velocity accuracy of a few centimeters per second.

L-BAND

Another viable alternative is to have a receiver so called L-Band (RTX) which provides an accuracy of position in the range of 1 decimeter: 4cm -10 cm CEP-Horizontal.

Besides the fact that its initiation time can take several minutes, a PPU based on this technology is great. Likewise SBAS, it is a correction service which can be delivered via satellites, i.e no need of a land base station, receiver modem, antenna, clear line of sight etc…. Its coverage is global and it is a paid service (a couple of thousand dollars per year). This is a new technology delivered by Trimble (see Trimble Centre Point RTX) and not all GNSS receivers are equipped with this sort of advanced algorithm. Pilot must have a dedicated antenna/receiver, i.e. a PPU

RTK

When it comes to high precision navigation, the best approach is Real-Time Kinematics (RTK) which provides an accuracy of position or 1cm-2cm CEP-Horizontal. RTK delivers outstanding performances in terms of precision and velocity can be calculated with an accuracy of 1cm/sec. Furthermore, the technology can be used as relative positioning system (a.k.a. relative RTK), meaning that it is possible to evaluate with extreme accuracy the distance between a RTK base and RTK rover, this has enormous applications in off-shore operations and for a implementing the so called Moving Baseline RTK, that is, a technique useful for GPS applications that require vessel orientation.

When used as an RTK rover, i.e. inside a harbor, it requires a base station on land to deliver the corrections: a receiver requires integrating a radio modem, receivers and having either a good line of sight or mobile data coverage (when over IP). It must be pointed out that these are not real issues. Differential correction services nowadays is a challenge of the past, several operators worldwide have equipped harbours with networks of base stations,  mobile network technology works. The issue comes for the need of having such smart electronics available on board. In most cases, this type of precision equipment must be brought on board by pilots. Such tools are typically used by land surveyors and they generally are, heavy, bulky and full of cables. Not so ergonomic for a pilot to bring on board. That is why PPUs are adapted around the needs of pilots who must bring onboard light and easy to carry equipment with little amount of cables (or none at all, if possible).

Precision and sophistication has a price tag. A good RTK receiver which works on all GNSS constellations (GPS, Glonass, Beidou, Galileo), with UHF/GSM modem, and an output at 20Khz + RTK correction service can costs USD 15,000. Most PPU consist of a dual-antenna system, so the price range is far from a pilot plug, but so is its performance.