Quote:
Originally Posted by transmitterdan
You were taught incorrectly as were apparently many others so don't feel bad about it. From day 1 the GPS system was designed so that speed was as easy or easier to obtain than position. For many applications speed is more important than position. And speed needs to be known near instantaneously for lots of applications. So the GPS system designers planned for that feature and they made it easy and fast to calculate speed without knowing anything about position. A GPS receiver can compute its speed at a single moment in time thus position is not required (nor would it even help).
Here is how it works:
The GPS receiver measures the frequency of each satellite it receives (more accurately it measures the change in frequency). The frequency will always be a little bit off from the transmitted frequency. This is known as frequency or doppler shift. When the receiver is moving this frequency shift changes. If stationery the frequency doesn't change much (it does a tiny amount because the satellites are moving but not enough to matter). Einstein and another guy named Lorenz figured out the equations. It's how Einstein proved that the speed of light is constant in a vacuum. Anyway, if the receiver knows how much each satellite has shifted off frequency it can compute it's speed relative to each the satellite it can hear. No position data is required to do this calculation. It only needs the amount of frequency shift for each satellite and a lookup table of constant numbers that is sent down by the satellites. Because frequency shift can be measured at any moment in time speed can be computed nearly instantly using only one measurement (frequency shift).
The same technique is used by doppler radar to measure how fast the wind and rain is blowing and in what direction.

Transmitterdam. The basic calculation of any onboard GPS is position of your vessel on the surface of the earth at a point in time.
All other calculations from your GPS and display ( generally a plotter) such as AVERAGE SPEED between consecutive points, HEADING, SOG, VMG, ETA, TTG etc are just simple calculations and extrapolations based on the average speed between two consecutive GPS positions and GPS distance to your waypoint.
Because the GPS and PLOTTER make calculations about once each second SPEED, HEADING, SOG, VMG, ETA !TTG etc etc can be considered REAL TIME values.
The magic of GPS is in the detail. The GPS position is updated many times a minute and thousands of times an hour depending on the number of satellites in range and the quality of your onboard GPS.
As an example if your vessel is travelling at 6Kn and the satellites provide the data to calculate your position each second your GPS will be calculating your position 3,600 times in the hour it takes you to travel 6 Nautical Miles. This means your GPS UPDATES your position on the earth and hence your speed about every 10ft that your vessel travels in a second.
The other calculations provided by your plotter such as HEADING, SOG, VMG, ETA, TTG etc are also updated every 10ft or so.
You need to get your
head around the speed and volume of the satellite signals and the GPS calculations. The GPS satellites are about 22,000Km above the earth and their signals travel at 186,000 Miles Per Second and your GPS may be receiving the signals from around 46 satellites simultaneously. The information received, calculations made and information provided per second, minute, hour by your GPS (and plotter calculations) is incredible compared to any information that can be provided by traditional
navigation techniques.
Imagine
Captain Cook getting his
head around satellite and GPS/PLOTTER
navigation and plotting his boats position on the earth accurately every second..........but that is another topic
Now returning to the speed
camera in the C&D
Canal. Assuming again that your vessel is at constant revs and your actual speed is a constant 6Kn (Actual) then the GPS would be calculating your position each second and calculating your AVERAGE SPEED every second. In each second at 6 Knots your vessel will travel just over 10 feet. If your vessel is 40 foot long then the GPS would calculate you average speed 4 times while your vessel passes a fixed point on the bank of the C&D
Canal.........for example this point may be a fixed speed camera
If you were setting your throttle based on a paddle log the maintain within the speed limit then considering the inaccuracy of the Log and the variable tide currents in the Canal..... All I can say I'd good luck with your actual speed
Now the comment is that a vessels speed calculated by GPS is the AVERAGE speed between two points. CORRECT.
However the average speed between two points is done each second therefore for our purposes it is an INSTANT speed calculation...... Calculated maybe 3,600 times an hour.
When I departed from
Galapagos heading to Hiva Oha, French
Marquesas my GPS and PLOTTER did two different calculations. The GPS calculated my average speed for the first 10 feet of the
passage as 6 Knots. The PLOTTER used this average speed information from the GPS to calculate my HEADING to the waypoint i.e.at Hiva Oha, my ETA, SOG, ETA, TTG. etc.......IF I maintained the
current average speed of 6 Knots. The GPS and PLOTTER redid/updated these calculations more than 1,000,000 times during the 20 day
passage .........automatically and accurately.
As I said previously the paddle log lets me know when my bottom needs a clean as it gets further and further away from showing my true speed.