Understanding GPS

Redcar FishTechnical

Courtesy of Redcar Fish

The way a GPS set finds its position is very much the same as the way you might take a fix using radar ranges. Suppose the radar shows that we are four miles from a lightship. We must be somewhere on a circle, with a four-mile radius centred on the lightship, and we can draw that on a chart.

If, at the same time, we are five miles from a conspicuous headland we could draw a second circle with a radius of five miles centred on the headland. Our position must be at one of the two points where the two circles cross, and we can decide which by taking the range of a third feature and plotting that in the same way. GPS also measures ranges and combines several ranges to produce a precise fix, but instead of using terrestrial landmarks, it uses satellites – 24 of them in orbit some 11,000 miles above the earth. The way it measures the range of a satellite is rather like the way radar measures distance.

Radar uses high-frequency radio waves (called microwaves), and it directs the microwave pulses horizontally instead of vertically, but the principle is exactly the same. GPS also uses microwaves, but instead of having to transmit pulses and listen for echoes, each satellite transmits a coded signal saying, in effect: 

“I am here …”    “I am moving this way …”   “And the time is now …”

The GPS receiver compares the time at which the signal was sent with the time at which it is received. From that, it calculates how long the signal took over its journey, hence how far it has travelled. The altitude of the satellites means that the GPS system is working in three dimensions rather than two, so the range measured from each satellite yields a position sphere instead of a position circle. But the principle is the same. The intersection of three or four position spheres represents a fix. The situation is made slightly more complicated by the fact that the satellites are moving, but this problem is overcome by the GPS control centre in Colorado which monitors the movement of satellites and tells each satellite where it is and where it will be in the future. The really big snag is that microwaves travel very fast indeed – 186,000 miles per second. So if the receiver’s clock were as little as a millisecond fast, it would overstate the range of each satellite by nearly 200 miles.

If you could plot the result on a chart it would form a huge triangle that traditional navigators would recognize as a “cocked hat”. But even that can be overcome by the receivers internal computer, which recognizes that it can’t be in several places at once and sets about adjusting its clock until the “cocked hat” reduces to a pinpoint fix.

Choosing the Right Equipment

The choice of GPS sets available is huge, so to make sense of it, it helps to divide them into several groups. Externally there are two obvious distinctions:  Hand-Held and Fixed.

Hand-Held sets are compact, self-contained units ranging in size from about as big as a mobile phone and the weight of a 35mm camera. They have their own aerials, either inside or protruding from the unit, they’re own internal power supplies, either in the form of dry cell batteries or rechargeable power packs. Some are waterproof and/or shockproof, and most can be supplied with brackets, carry cases, external aerials and power supplies.

Fixed sets need not be much bigger than the hand-helds, but they are designed for removable installation operating from an external power supply and with an external aerial. By and large, their displays tend to be bigger, giving them the opportunity either of presenting more information at once or presenting it more legibly. The larger ones also benefit from the lager control panels, making them either easier to operate or able to offer more functions.

The main way of classifying GPS sets is by the way they deal with the task of handling signals from up to twelve satellites at once. The most obvious option is called a parallel receiver, in which a single set includes up to twelve separate receivers each listening full-time to one satellite. A cheaper alternative is to have one receiver listening to one satellite for a millisecond or so before switching to the next. These multiplexing receivers can produce reasonably good results in most vessels but usually take longer to “find themselves” when first switched on, and are less able to give steady position fixes “on drift” and may have difficulty coping with any interference.

Apart from these technical differences, there are other points to consider that are, in some respect at least, more important. Bearing in mind that one of the main reasons any piece of electronic equipment fails to work is because the operator has done something wrong. Therefore user-friendliness is vitally important. No matter how hard the designers try, there’s no getting around the fact that lots of functions either mean that you need lots of controls, or each one has to do several jobs. This is why most professional navigation equipment is designed for ease of use.

Points to Consider before Purchase

1. Do I just want to navigate only from coordinates? 
There are plenty of sets available, which display only coordinates, these sets are usually the cheaper sector of the GPS market.

2. Would I prefer “steer to” and “plotter” displays?
It is of particular ease to navigate with the “steer to” display. Your viewing the navigation screen, it is displaying a runway and your position is the arrow in the centre of the screen. You steer to the bearing shown at the top of the screen, and a circle depicting your destination appears on the screen as you approach your waypoint. Yes, it’s that simple.

The “plotter display” can be particularly useful to any fisherman, on the screen you are a flashing diamond with a cross, a solid line represents your route travelled. A fisherman can repeatedly run over good fishing grounds by simply following his previous plotter line. You can also construct your own chart in the plotter memory of your local coastline and harbour. 

3. What range of plotter do I need?
Look for maximum range. You should be looking for 1/10 to 100 nautical miles for superior accuracy. If your navigator has a 200-pixel screen and you select the 1/10 Nm plotter range you are looking at 200 yards on the display this means that every yard you move, the display will recognize this with a pixel movement. Now that’s accurate!

4. What type of receiver should I buy? 
For the best accuracy and dependability of information received, you should look for a parallel receiver.

5. What about the ease of use?
The easier the set is to use the better especially when in fog, the last thing you want is to be struggling to use a complicated set or having to thumb through a manual.

6. Do I need a waterproof set?
The deciding factor may be that electronics and water don’t mix! Even with a dry cabin, there is always the freak wave that breaks the windshield, or bursts through the door. WATERPROOF is WATERPROOF! Water-resistant IS NOT WATERPROOF !!

7. What about WARRANTY?
Think not only about the period but will the manufacturer HONOUR your warranty. Look for the longest period possible on a waterproof set, as you are less likely to be quibbling over defects. BUY QUALITY!