Navigation / Instrument Acronyms

February 22, 2011

Since users ask about these from time to time, here is the list of acronyms used to denote certain navigation related data items in PolarCOM instruments and elsewhere:

POS – position, that was easy
SOG – speed over ground. Speed of a vessel relative to stationary land.
COG – course over ground. Course of a vessel relative to stationary land.
VTW – speed (velocity) through water. Speed of a vessel relative to water.
HDG – heading. Compass direction into which the bow of a vessel is pointing.
DBK – depth below keel. Water depth measured from bottom of keel
DBS – depth below surface. Water depth measured from surface of water or vessel waterline.
DBT – depth below transducer. Depth as returned by transducer, unadjusted.
AWA – apparent wind angle. Angle of apparent wind as seen from a moving vessel, relative to vessel bow-stern axis.
TWA – true wind angle. Angle of true wind relative to vessel bow-stern axis.
TWD – true wind direction. Compass direction from which the true wind is coming.

VMG – speed (velocity) made good. Speed with which a vessel is moving towards its destination, as measured on a line between current vessel position and destination.
XTE – cross track error. In general, shortest distance from current vessel position to the nearby route leg.
BTN – bearing to next. Compass bearing to the next waypoint enroute.
DTN – distance to next. Distance to the next waypoint enroute.
ETEN – estimated time enroute to next.
ETAN – estimated time of arrival to next.
DTD – distance to destination. Distance to the final point of multi-leg route. Measured along the route.
ETED – estimated time enroute to destination.
ETAD- estimated time of arrival at destination.

ROT – rate of turn.
RDA – rudder angle
SDA – set and drift angle. Used in combined set & drift instrument.
SDD – set and drift direction. Used in combined set & drift instrument.

Any of the direction acronyms above may have a suffix of ‘M’ to denote that they are in degrees magnetic. Without this suffix, assume degrees true. I.e. HDGM vs. HDG.

PolarView NS 1.4 Update (Mark III)

September 14, 2010

A new maintenance release of PolarView NS 1.4.91 is now available. A long-running issue related to live vessel track handling has been (finally) fixed in this update of PolarView NS.

In short, long tracks with more than a certain number of points ran a risk of data corruption. By default PolarView uses advanced “adaptive track” algorithm, that provides precise tracking with significantly reduced number of track points. Because of that, the problem was rarely seen by most users. However, users that configured PolarView to use periodic tracking with a short track interval were more likely to experience problems.

This update is highly recommended to all users, and in particular those using PolarView NS for live GPS navigation.

As always, download PolarView NS here.

Fun with ENC charts

August 29, 2010

Looks like NOAA may be using automated conversion software to create ENC charts from satellite images. It works great most of the time, but sometimes things don’t go as planned. In this chart (US5NY43) Mid Hudson Suspension Bridge seems to be charted based on a shade of the bridge rather than bridge itself, moving it somewhat North. Stated accuracy of 1:40000 scale chart is about 140′ so this one is probably just about within the limit.

Good navigators always remember that chart is not a substitute for a proper lookout.

P.S. A bit more on this – the following is my guess, correct me if I am wrong.

So, NOAA may be trying to do the right thing. Note that the satellite image is taken from position slightly due North of the bridge, rather than vertically. From that point of view bridge as tall as this one (134 ft) would be visually shifted South relative to anything at “sea”/water level. On the other hand, if the sun was at high noon and directly overhead, shade from the bridge would be precisely under it at exactly the sea level, and would provide a perfect outline of the bridge on a chart. I think this method works “most of the time”, in particular for images taken during summer months in Southerly latitudes.

Here we have a relatively high Northern latitude, and photo likely not taken at noon, so the sun is low in the southern sky. Because of that, the shade is located North of the bridge. The “real” bridge position should probably be charted in the middle between the shade and the bridge on the satellite photo.

Chart Legend

March 2, 2010

An item that’s been missing from our documentation so far is a chart legend for our vector chart rendering. Turns out – it’s not quite as easy to assemble, as I hoped. With all the effort we put into chart appearance and readability, there was little time left to organize and catalog our icons.

So, finally, here are essential symbols for our charts – IALA regions A and B (in day-time palette colors).

A few symbols in this list are new or updated based on user feedback, to improve visibility as well as to better reflect buoyage of IALA A region. They will be available beginning with the next version of PolarView.

PolarCOM “True” Wind Calculation

February 5, 2010

When used with “relative” (apparent) wind data source, PolarCOM provides two modes of “true” wind calculation. These modes are labeled “STW” (Speed Through Water) and SOG in the configuration. STW is a “traditional” method, used by most hardware wind instruments. It needs no GPS input and requires STW and HDG (your vessel heading) to calculate “true” wind. It also results in an approximation of “true” wind.

SOG mode provdes more precise “true” wind calculation, but it also requres COG as well as HDG. That means you still need a heading sensor to calculate “true” wind – having only GPS input without heading is not sufficient.

If you are interested in explanation as to why that is, read on:

Let’s say you have the following –
1. Relative apparent wind (from VWR/MWV NMEA sentence) as delivered by windvane
2. GPS information including COG/SOG.
3. nothing else

Consider this – your vessel is heading in some direction which is most likely NOT aligned with COG. Relative wind is a vector that you can calculate based on a coordinate system that has boat heading as one axis (and another axis perpendicular to it).

On the other hand your SOG (and SOG induced “wind”) is a vector that by itself can only be considered in a system with one axis along the COG (and another perpendicular).

Without heading there is nothing to reconcile these two systems and no way to do any reasonable operations on these vectors. You need heading to convert one of these vectors to the coordinate system of the other.

Here is a practical example using PolarCOM. The first set of dials (img 1) shows COG and heading that differ by 20 degrees. The true wind is calculated here based on SOG (and using both COG and HDG to adjust vectors). You can see true wind both relative to the boat (second dial from the left) and as an absolute direction (last dial on right). There is no STW (as the heading dial shows).

Img 1

On the second image (img 2) I adjusted heading to match COG. This is what would effectively happen if you used SOG for true wind calculation but did not have an adjustment angle (and assumed direction of travel matching heading).

It is quite clear that true wind resulting from such calculation is not the same as the one that results from properly accounting for COG/HDG difference. And the difference is not insignificant – 30 degrees and about 40% stronger.

Img 2

Incidentally, third picture (img 3) shows calculations using STW and heading. I set STW to be equal to SOG, which is not necessarily true, but it helps make an example simpler. As you can see it is also incorrect, but the result is essentially the same as when using SOG alone without proper heading. I.e. there is no gain in calculation precision.

Img 3

PolarView 1.0/PolarCOM 1.0/PolarIS 1.0

October 23, 2009

– Analog instruments (wind/course/heading)
– Depth/Wind/Speed through water etc.
– Navigation items – VMG, BTN, distance/time to next/destination, XTE
– Alarms – arrival/XTE/depth/anchor drag

– Initial release of our integrated navigation application
– “Live ship” navigation, routes, tracks etc.
– Fully integrated with PolarCOM

New Product – “Real Soon Now”

October 22, 2009

I generally avoid talking about things that are not quite finished. However – people have been asking about “live GPS” navigation product for a while. Until now the reply was – “it is being worked on”.

So now, to show that it is not entirely the “vaporware” it may appear to be, here is a screen shot of what the standard navigation view looks like. It shows a ship on a chart (in “North up” orientation), “radar rings”, and a simple set of dials including POS, SOG, COG and analog apparent wind.

1000 software simians are currently banging on 1000 keyboards to thoroughly test this software, and document every aspect of it, before it becomes publicly available.

New Polar Navy navigation product.

New Polar Navy navigation product.

Let me know what you think!