Well, I have been noticing that my GPS seems to read about .5 to 1.5 knots faster than my ST30 (depending on wind and point of sail) and this is after SA was discontinued. This was the same with my ST50 on my old boat when compared to the GPS (before SA was discontinued). The question I have is how accurate is the GPS (Hand held Garmin 48)for speed. As much as I would like to believe the speed...... I notice that the speed is very accurate in the car (ok, matches what the speedometer says) but at the lower speeds on the boat, I am unsure of the accuracy. Any one care to shed some light on this. I understand how to calibrate the ST30, and can do this pretty quickly, but I am just wondering how accurate the speed shown on a GPS is. Thanks. Bruce. Neon Moon
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Just do it
Bruce: Why don't you get on a deserted road (probably hard to find one in Grapevine) and drive at 10 mph in the car and see how close it is. This would at least give you some evaluation of the accuracy. I would just go ahead and re-calibrate it. What you are looking at is more a relative speed anyway. *How fast am I going, How fast was I going.* You know what I mean? Steve
Bruce: Why don't you get on a deserted road (probably hard to find one in Grapevine) and drive at 10 mph in the car and see how close it is. This would at least give you some evaluation of the accuracy. I would just go ahead and re-calibrate it. What you are looking at is more a relative speed anyway. *How fast am I going, How fast was I going.* You know what I mean? Steve
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Bruce Grant
Makes sense
I know what you mean, and will do as you suggest. I still wonder what the overall accuracy of GPS is relating to overall speed. Thanks for the input. Bruce. Neon Moon
I know what you mean, and will do as you suggest. I still wonder what the overall accuracy of GPS is relating to overall speed. Thanks for the input. Bruce. Neon Moon
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Jim Ewing
Only calibration that works
Is speed=distance / time. GPS, as mentioned is an average of how fast you were going over the last several position samples. This is basically the GPS calculating the distance and dividing the time. Now that the positions are much more accurate on the GPS your speed will be much more accurate. But the slower you go the greater the percentage of total speed the inaccuracy will represent. In the SA days you could see the odd speed reading of 20+kts when your position suddenly shifted. Currently my GPS is reporting estimated position errors of from 0 to 3 feet so my speed is much more accurate. Your ST30 does the same thing but the distance (the circumference of your paddle-wheel) and sampling times are much smaller so you get a more "instantaneous" speed. However you have to remember that you are comparing apples and oranges (sort of). The ST30 is reading speed through the water while the GPS is reading speed across a mathematical representation of the earth. So to find out which is correct you have to run a measured course in both directions (to average out wind and current effects) and compare your calculated value to the readings on both your GPS and ST30. I will hazard a guess that your GPS will be spot-on and your ST30 will report the speed through the water within 1 or 2 tenths of a knot. Knowing this you can then calibrate your ST30. When you now compare the two you can believe the difference as the measurement of the wind and current effects on your boat. Jim "Prospect"
Is speed=distance / time. GPS, as mentioned is an average of how fast you were going over the last several position samples. This is basically the GPS calculating the distance and dividing the time. Now that the positions are much more accurate on the GPS your speed will be much more accurate. But the slower you go the greater the percentage of total speed the inaccuracy will represent. In the SA days you could see the odd speed reading of 20+kts when your position suddenly shifted. Currently my GPS is reporting estimated position errors of from 0 to 3 feet so my speed is much more accurate. Your ST30 does the same thing but the distance (the circumference of your paddle-wheel) and sampling times are much smaller so you get a more "instantaneous" speed. However you have to remember that you are comparing apples and oranges (sort of). The ST30 is reading speed through the water while the GPS is reading speed across a mathematical representation of the earth. So to find out which is correct you have to run a measured course in both directions (to average out wind and current effects) and compare your calculated value to the readings on both your GPS and ST30. I will hazard a guess that your GPS will be spot-on and your ST30 will report the speed through the water within 1 or 2 tenths of a knot. Knowing this you can then calibrate your ST30. When you now compare the two you can believe the difference as the measurement of the wind and current effects on your boat. Jim "Prospect"
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Richard Gottlieb
The speedo and GPS measure different things
Actually, the GPS is measuring your "speed over ground" while the speedo measures your "speed through water". The difference is attributable to the effect of current and other factors that affect overall speed from point "A" to point "B"
Actually, the GPS is measuring your "speed over ground" while the speedo measures your "speed through water". The difference is attributable to the effect of current and other factors that affect overall speed from point "A" to point "B"
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SteveC
Carrying this one step further....
if you motor at a constant RPM, with no wind and at either high tide or low tide with no current, you should see the same numbers in theory.. Right???????
if you motor at a constant RPM, with no wind and at either high tide or low tide with no current, you should see the same numbers in theory.. Right???????
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Ron Dague
Speed [+ or - (Turbulence) + or - (Current)] = GPS
I have noticed that my ST50 knotmeter will read different from the GPS on a port tack. Almost identical to GPS on starboard tack. I have calculated current speed and direction, making that correction, first. I credit the port/starboard variation to the location of my "paddle wheel", being in the sump in the head, located on the starboard side. I'm guessing that some turbulence or "shodowing" off of the keel affects readings. But the harder I work on it, the more I'm convinced that the GPS is very accurate.
I have noticed that my ST50 knotmeter will read different from the GPS on a port tack. Almost identical to GPS on starboard tack. I have calculated current speed and direction, making that correction, first. I credit the port/starboard variation to the location of my "paddle wheel", being in the sump in the head, located on the starboard side. I'm guessing that some turbulence or "shodowing" off of the keel affects readings. But the harder I work on it, the more I'm convinced that the GPS is very accurate.
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Alex
It puzzle me too..
..as when turning on my Lowrance GPS/plotter, in the marina , with boat not moving , it always shows me some speed , between .5 and 1 knot for quite a long time..( incl recently , after degradation ceased). Never understood it, but I read somewhere there is some reason , related to averaging software on low speed ..!? Anyone has some info on that.
..as when turning on my Lowrance GPS/plotter, in the marina , with boat not moving , it always shows me some speed , between .5 and 1 knot for quite a long time..( incl recently , after degradation ceased). Never understood it, but I read somewhere there is some reason , related to averaging software on low speed ..!? Anyone has some info on that.
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Jon Bastien
Averaging software
Alex, Here's how the GPS determines your speed: The GPS satellite system doesn't actually track your movement across the surface of the planet; The unit on your boat determines its own location using satellite data and 'remembers' where it is. After a specified time interval, it takes another reading and compares it to the last reading; By comparing the distance between the two points and dividing that distance by the specified time interval, it can determine your speed. Now, most commercial GPS units can adjust the specified time interval- Simply stated, how frequently the unit determines its position ('Takes a fix') from the satellites. The more frequently you take a fix, the more likely you are to get an accurate 'right-now' speed; The drawback is that it will not maintain as much 'history' (i.e., unit holds 2 miles worth of track data instead of 20 miles worth). The less frequently you take a fix, the more likely it is that you are looking at your 'average' speed than your 'right-now' speed. The primary advantages to this are longer battery life (in hand-held units) and larger "back-tracking" range (unit holds 20 miles worth of data instead of 2). As for your on-board unit registering a speed while tied off at the dock, This may be due to your time interval being set a touch long- like 2 minutes- and it takes at least that long for your unit to obtain an accurate fix. After the first two fixes, your unit should realize it isn't moving (much), and register zero (or close to it). --Jon Bastien H23 '2 Sheets to the Wind' Naviguessed by a handheld Garmin...
Alex, Here's how the GPS determines your speed: The GPS satellite system doesn't actually track your movement across the surface of the planet; The unit on your boat determines its own location using satellite data and 'remembers' where it is. After a specified time interval, it takes another reading and compares it to the last reading; By comparing the distance between the two points and dividing that distance by the specified time interval, it can determine your speed. Now, most commercial GPS units can adjust the specified time interval- Simply stated, how frequently the unit determines its position ('Takes a fix') from the satellites. The more frequently you take a fix, the more likely you are to get an accurate 'right-now' speed; The drawback is that it will not maintain as much 'history' (i.e., unit holds 2 miles worth of track data instead of 20 miles worth). The less frequently you take a fix, the more likely it is that you are looking at your 'average' speed than your 'right-now' speed. The primary advantages to this are longer battery life (in hand-held units) and larger "back-tracking" range (unit holds 20 miles worth of data instead of 2). As for your on-board unit registering a speed while tied off at the dock, This may be due to your time interval being set a touch long- like 2 minutes- and it takes at least that long for your unit to obtain an accurate fix. After the first two fixes, your unit should realize it isn't moving (much), and register zero (or close to it). --Jon Bastien H23 '2 Sheets to the Wind' Naviguessed by a handheld Garmin...
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