Apologies to Lewis Carroll for the mis-quote above, but we are so accustomed to knowing exactly where we are – or at least having smartphones and chart plotters telling us.
I have noticed that some power boat skippers and even sailors (hrmphh) plot routes perilously close to hazards or marker buoys. As if the GPS is so accurate that it can help you miss a hazard by metres; that the hazard marker hasn’t drifted since the chart in the plotter was last updated; or that the hazard itself hasn't moved – a sandbank for example.
Chart plotter and smartphone manufacturers like to re-enforce the myth by claiming accuracy “less than 2m” for example…hmmm.
Lets just remember that GPS isn't a collective noun it is only one system. So where is all this data coming from? There are two live systems and several planned ones to consider:
1. United States NAVSTAR Global Positioning System (GPS). 24 satellites operational.
2. Global Navigation Satellite System (GLONASS) operated by the Russian Space Forces. 31 satellites operational.
3. The Chinese BeiDou Navigation Satellite System is a regional system being expanded into the global Compass navigation system by 2020. 15 satellites operational, 20 additional satellites planned
4. Galileo positioning system of the European Union planned to go live by 2020. 4 satellites operational, 22 additional satellites planned
5. Indian Regional Navigational Satellite System (IRNSS) is an autonomous regional satellite navigation system being developed by Indian Space Research Organisation. 4 satellites operational, 3 additional satellites planned
6. Quasi-Zenith Satellite System (QZSS), is a proposed three-satellite regional time transfer system and enhancement for GPS covering Japan. 1 satellite operational.
7. The Doris system designed by Cnes, the French Space Agency.
To all intents and purposes its only the GPS and the GLONASS that we need to worry about at the moment. You may have spotted that the forces at play behind these networks are on opposite sides of a significant political divide but lets not worry about that…for now…
In order to get a fix on your location, your chart plotter or smart device needs an unobstructed view to at least four satellites. Four out of the GPS system’s 24 sounds like a lot, but the satellites are spread around the world and can’t always be accessed where you are. Access to a larger blanket of satellites supplies your devices with more accurate location data. If your chart plotter or smart device can access the additional 24 satellites of GLONASS (not all 31 are operational at all times), then it will acquire satellites up to 20% faster than devices that rely on GPS alone and allow your location to be pinpointed to as close as 2 meters. Not so with GPS alone.
When it comes to Raymarine the situation is more complicated – what did you expect! The ‘a’ series units (a95, a97, a98, a125, a127 & a128) have the GPS/GLONASS acquisition built in. On top of that you can enhance this with an external GA150 antenna. If you stupidly bought the other hybrid chart plotter, the e series (I bought the e125), then they have GPS only….even if you add the recommended Raymarine RS130 external antenna…..go figure.
Search & Rescue
One last word in support of Galileo. Galileo is planned to provide a unique global search and rescue (SAR) function. Satellites will be equipped with a transponder which will relay distress signals from the user's transmitter to the Rescue Co-ordination Centre, which will then initiate a rescue operation. At the same time, the system is projected to provide a signal to the users, informing them that their situation has been detected and help is on the way. This latter feature is new and is considered a major upgrade compared to the existing GPS and GLONASS navigation systems, which do not provide feedback to the user. Tests in February 2014 found that for Galileo's search and rescue function, operating as part of the existing International Cospas-Sarsat Programme, 77% of simulated distress locations can be pinpointed within 2 km, and 95% within 5 km.
So check out the spec for your chart plotter; iPad or other smart device – and give those hazzards and marker buoys a wide berth !