Marine electronics explained: GPS

15th April 2016

What is GPS?

GPS or Global Positioning System is a satellite based navigation system comprised of 31 satellites that orbit the Earth.

As well as marine navigation GPS has many uses in both civilian and military operations, including (but not limited to):

  • Aircraft tracking
  • Search and rescue
  • Tectonics
  • Recreational pastimes such as geo-caching and way marking
  • Clock synchronicity
  • Astronomy
  • Surveying
  • Telematics
  • Robotics

The satellites that make up the GPS network travel at speeds of approximately 7000 mph and use solar powered rocket boosters to maintain their paths at around 12,000 miles above the Earth.

How does GPS work?

Each GPS satellite orbits the earth on an exact path circling the planet twice each day. GPS receivers work by locking on to the signal of at least three of these satellites and triangulating the information to calculate the users location. The calculations used are a combination of both time and distance with one of the satellites being used to calculate the time and a further two to determine the latitude and longitude. A receiver must be locked on to a minimum of three satellites to provide the basic 2D location but a fourth satellite will also establish altitude.

Once the user has locked a signal with the orbiting satellite further tracking information can be determined such as speed and bearing. Depending upon the intended application of the technology being used additional user information can be calculated such as routes, time to destination, sunrise and sunset times, trip distance and much more besides.

How accurate is GPS?

The inception of GPS began in 1973 with the US military recognizing the need for a more accurate navigation system but it took until 1995 before the project was fully operational. In the last two decades the technology has continued to be developed at an exponential rate and, thanks to the current parallel multi-channel design, most GPS receivers are super-fast and super-accurate.

The first affordable civilian GPS devices were extremely slow to lock onto satellites and were usually badly affected by the prevailing weather conditions and environments. Dense foliage, urban settings or stormy weather would usually cause one or more of the signals to drop out leaving the user without a back-up.

Devices on the market today are generally accurate within 5-15m of their users precise location.

In addition, DGPS (Differential Global Positioning System) can provide a greater degree of accuracy (within 3m) and was specifically developed with nautical navigation in mind to combat the ‘black-spots’ where GPS was known to become unreliable (e.g. archipelago between Sweden and Finland).

Features of a GPS satellite system – Choosing your device

Whilst the primary function of a GPS system will remain satellite navigation, there are many side benefits and premium features that are included with some models of GPS; the importance and desirability of these will depend on the user but here are seven points to remember when considering your next GPS purchase:

1) Portability – GPS devices for marine use are available in a wide range of sizes and can be removed from your vessel for both security and flexible use. If you choose portability over a fixed unit then consider that the screen size may be reduced, as may be the integration capabilities of the device with other systems aboard your boat.

2) Connectivity – GPS receivers are capable of integrating with a range of other systems and software so be aware of how you are able to connect your GPS with other devices. Some are equipped with SD cards or USB cables whilst others may be hardwired into existing systems. Being able to download system updates and software upgrades is an important part of routine maintenance.

3) Antenna range – at sea it is imperative that your signal reception is strong to ensure that your GPS device can lock onto a sufficient number of satellites to maintain an input signal. Many devices come with an in-built aerial but at open sea these are often not as reliable as having an external antenna. Check that your device has an antenna port and choose a suitable external antenna that will provide a strong reception, whatever the weather or conditions.

4) Safety features – devices that are specifically designed for marine use often include several safety functions which offer greater protection whilst at sea. These include: a. Man Over Board Alarm – this feature allows the user to quickly drop a ‘pin’ in the event of a man over board situation which will allow the vessel to return to the precise location of the incident; b. Collision Alarm – a function which provides the user with an audible (and/or visual) alert in the event of an imminent collision; c. Arrival Alarm – A basic setting providing users with proximity alerts when nearing the destination heading; d. 12 channel receiver – ensuring that you maintain a strong signal is imperative when navigating far from shore and a multi-channel device is essential to secure satellite connection; 12 channels is recommended; e. Low Battery Alarm – if GPS is your primary (or only) navigation tool then knowing if the batteries are low (where not hardwired) is a critical feature to have.

5) Bells & Whistles – some GPS systems offer additional functionality which, whilst not essential, may be of interest to some users such as music, radio and audiobook entertainment as well as depth/echo sounders.

6) Voice Activation – manning a vessel can sometimes require more hands than seems possible and some GPS devices offer a hands-free function allowing the user to provide voice commands. Whilst this is not an essential component of the navigation system it does offer options to some skippers.

7) Suitability for Marine Use – lastly, you should ensure that the device you choose is waterproof. Whilst all marine devices will state this is the case the degree to which they are actually resistant to moisture does vary (much like a divers watch) so check that the model you are considering is suitable for the intended application of use.

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