GPS

I have recently completed the Coastguard Boating Education GPS Operator course. ** Warning ** this is a long wordy post – mainly for my own benefit of storing my study notes online – so read on if you are interested in learning more about GPS – but if you are looking for gossip and photos – sorry you had better go and check out some of the other cool pages on my blog!

I have got a hand held GPS chart plotter which I have had for about five years. It is amazing how far technology has come in that short time, as I also have got Navionics installed on my iPad which is of course a much bigger screen, and loads a lot faster than the handheld GPS. Anyway I knew that GPS has some amazing capabilities and so I wanted to learn how to use my GPS more effectively and also get a better understanding as to how the whole thing works! I would highly recommend doing the course. It is by correspondence and is well laid out. There is an exam at the end and if you answer the questions correctly then you get a nice certificate to add to the ever growing collection of boating qualifications.

Here are some of the things that I learnt.

GPS was developed in the 1970’s and testing began in the 1980’s. There were two channels – one encrypted for military use and the other degraded for civilian use – this was known as selective availability. In May 2000 this was set to zero – which improved the accuracy from around 100m to 20m. The facility for them to reinstall selective availability still exists.

How does GPS work?

There are a series of satellites (between 24-31) about 20,000km above the earth. These are arranged in six different orbits and are inclined 55 degrees to one another. Each satellite transmits an individual time signal and information on its position – an almanac which predicts where the satellites are in space. They broadcast on individual microwave frequencies which travel at 300,000,000 metres per second! The GPS have two atomic clocks installed.

The GPS receiver knows the position of the satellite from its almanac (which contains the predicted positions in space of all the satellites) by recording the time it received the signal, and hence the time it took to travel, it can work out the distance to the satellite. This results in a position circle. At any point on that circle the distance/time from the satellite is equal.

To get a reliable fix, you need at least three satellites – the position in which all three circles overlap gives you your position.

The GPS receiver on your boat has only a digital clock, which is not as reliable as the atomic clock on the satellite. Any time errors can cause huge errors in your position. To get around this, your receiver uses Pseudo-Ranging – which is a series of calculations of adding or subtracting a certain amount of time to get the position circles to all intersect.

A satellite is considered to be in view when it is at least 7.5 degrees above the horizon. Anything under this suffers from distortion due to refraction as it passes through the atmosphere at a shallow angle.

Generally at any one time there will be at least 6 satellites above the horizon and at least 4 of these will be greater than 9.5 degrees above the horizon.

To get a longitude and latitude fix you need at least 3 satellites. If you want 3D – i.e. your height as well – then you need 4 satellites.

Most GPS units will have an alarm to tell you if it can’t find enough satellites to get a proper fix.

When you first turn on your GPS or if it has been switched off for some time, it will need to update its almanac with any correction signals – and it can take some time to do this.

GPS units usually have the following options:

  • Choice of language
  • Units of measurement – metres or feet
  • Units of distance/speed – miles, nautical miles or km
  • Bearings – in true or magnetic
  • Co-ordinates – Lat & Long
  • Time – UTC or local time
  • Survey Datum – WGS84

The GPS display will usually give your position in Latitude and Longitude in degrees, minutes and three decimal places of a minute – however the accuracy is probably not that great – The common level of accuracy is about 20 metres from your actual position.

The reason for any inaccuracy can be as follows:

  • refraction of signal – issues with the radio waves transmitting through the atmosphere – 5-12 metres
  • Clock error – 2m (any inaccuracy with your GPS clock)
  • Satellite position error – 4m (any discrepancy from its actual vs its stated position in the almanac)
  • Multipath error – 1-3m (signal reflected by your own vessel or other objects – high cliffs or marinas can cause the signal to bounce)
  • Satellite masking – variable (high cliffs or buildings can block the satellite signals)
  • Poor satellite geometry – variable (if the satellites are too close together)

HDOP – Horizontal Dilution of Precision

GDOP – Geometric Dilution of Position

EPE – Estimated Position of Error

The greater value of these means the greater error of GPS positions. So when HDOP is 1, the geometry is closer to ideal. Anything greater than 5 will result in unreliable fixes. You can check this number on the satellite page of the GPS. This page also shows how many satellites are visible, how many are being used and the theoretical fix accuracy.

So in other words – you don’t want to rely on your GPS being 100% accurate all the time, particularly when navigating close to dangerous objects!

There are a few things to remember when you are siting your GPS antenna:

  • It needs an uninterrupted view of the sky
  • It should be positioned as low as possible – the higher up it is the more inaccurate it will be when pitching or rolling
  • Take care when putting it near other electrical equipment

Glossary

  • WPT – waypoint – a position stored for navigational purposes
  • DIST – Distance or range to waypoint
  • DTD – Distance to destination
  • BRG – bearing (true or magnetic) – a bearing from present position to next waypoint
  • DTW – distance to waypoint
  • COG – course over ground – an average over a pre-selected time
  • CMG – course made good – an average from point of departure to current position
  • SOG – speed over ground – how fast you are going over the ground
  • SMG – speed made good – average from point of departure to present position
  • VMG – velocity made good – closing speed to waypoint
  • ETA – estimated time of arrival
  • TTG – time to go – to waypoint
  • XTE – cross track error – the perpendicular distance of the vessel from the course line – you can set an alarm to make sure you don’t go too far off course.
  • CTS – course to steer

Velocity smoothing – some units give you the option to average the SOG and COG over a selected time – use with caution if operating at high speeds!

Waypoints are a position that can be entered in to your GPS. It can give you the course and distance between waypoints. You can link them together to form a route. You can name the waypoints and routes. You can tap to add them, enter them in when you are underway, or enter the co-ordinates of Lat & Long.

MOB – a Man Overboard button enters an active waypoint from the vessels present position.

If you are on a route – the GPS will ‘realise’ the waypoint once you are abeam and it will then calculate the track to the next waypoint.

Issues with GPS

  • Human error – entering the wrong waypoints
  • Forming a route without checking for hazards on the track
  • Entering the exact position of a buoy, beacon or headland as the waypoint – you might hit it!
  • Not noticing that you have drifted off course, and that possible danger now lays between you and your next waypoint
  • An assumption that the chart and GPS is 100% accurate
  • Electronic charts not updated

Alarms

Most GPS units have got an anchor watch function which will sound an alarm if you drift too far from a set position. There are also arrival at waypoint alarms, distance to waypoint alarm and a XTE alarm – if you go too far off your intended course.

Chart Plotters

Chart plotters use two types of charts – Raster and Vector

A Raster chart is a scanned copy of an existing paper chart. They are cheap to make but take up lots of memory space. They look exactly the same as paper charts and contain the same information. When you zoom in, you are effectively looking through a magnifying glass. They are quality assured as they are copies of existing charts.

A Vector chart is layers of digital information stored in a database. They use less memory but they need more processing power. Vector charts can be zoomed in or out to a far greater extent with the symbols remaining the same size on the screen. The display can be customised – to hide light characteristics during the day, or removing depth sounding figures over a certain depth. Certain features can be ‘interrogated’ with the cursor to give lots more detail.

To be reliable, electronic charts must be maintained and kept updated. Vector charts can be corrected online – or sometimes via a new memory card. Raster charts are a bit more complicated, with a new tile of information needing to be pasted in to the programme.

There are different modes – you can have the screen set to head up, course up or North up.

  • Head up – the top of the screen is displayed so that the top of the screen corresponds with the vessels direction. This means that the corresponding view is the same orientation as the navigators view. The disadvantage to this is that any alterations will require the chart to be redrawn. This can cause disorientation
  • Course up – the chart is displayed so that the top of the screen is aligned with the active route. The chart is only redrawn when the vessel changed course more than a certain angle  – so virtually the same orientation as the navigators view.
  • North up – the chart is displayed so that the top of the screen is North and the bottom is South (true or magnetic) the advantage of this is that it is the same as the paper chart. The disadvantage is that when the boat is heading south it can be a little confusing figuring out which way to turn the helm to correspond with the GPS.
  • Relative motion – the vessel stays in the centre of the screen and the chart moves past the vessel. You can offset the position of the vessel so it shows more ahead than behind the track.
  • True motion – the chart remains stationary and the vessel moves over the screen – when it reaches the end then the chart is redrawn.
  • You can split the screen to show extra information – the chart plus the course, speed, radar, etc.
  • You can set a range ring around your vessel to easily show how far charted objects are from the screen

Route Planning Tips

  • Look at the route as a whole and identify any hazards
  • Decide on a safe distance from the hazards and any turning points
  • Plot the intended route on the chart screen using the cursor
  • Cross check with paper charts to ensure the safe distances are ok
  • Note the minimum XTE on individual legs
  • Document your route, waypoints, distances and course to steer on your passage plan

While underway:

  • Monitor your position constantly with the GPS and by using your eyes!
  • Depth soundings, light sectors and radar ranges can be used to confirm your position with what the GPS is saying
  • Monitor your XTE
  • Monitor your time/speed/distance
  • Document in your log

Always check the survey datum of the paper chart. The standard datum is WGS84. There are lots of charts in use which were surveyed before GPS came along. All NZ charts are set to WGS84. If the chart has been surveyed to a different datum then you need to make adjustments.

Remember that GPS units can fail, perhaps a lightening storm might take out all your electronics, or the satellites can be affected be meteors, wars or solar storms. So it is a good idea to also have a back up. A main GPS on the boat, a handheld chart plotter and perhaps an iPad with Navionics too. Also paper charts or like me, you can do a celestial navigation course!

So that is about it! The best way to learn I think is to use your GPS to plot a route – add a few waypoints, and go out and use it! Here is how I went about it with my Navionics app and the iPad on a recent coastal trip.

Happy navigating!

6 thoughts on “GPS

  1. Pingback: Ocean Yachtmaster | Astrolabe Sailing

  2. Pingback: Coastal Skipper | Astrolabe Sailing

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