Speed Through GPS Versus Old Fashioned Paddle Log

[transmitterdan]

Originally Posted by valhalla360

Just because a device doesn't output position doesn't mean it isn't using position to calculate speed. I could see where such a device might be able to get a "speed fix" faster on start up as it doesn't care about positional accuracy only accuracy relative to the last position (there might be a stumbling block in there though).

I find this interesting so I poked around a bit and could find nothing online to conflict with my undertanding. If you have a link to documentation that GPS can measure speed directly, I would be interested.

Believe whatever you like. Google:

How does GPS measure speed?

[gbgreen59]

Originally Posted by transmitterdan

Edit: I should have added the measurement the GPS chipset uses is the doppler shift frequency of the various satellites. This can be used to compute speed "instantly". I think 99.9% of all GPS chipsets use doppler shift to compute real time speed. But speed displayed on a recorded track is done as you say by taking distance between position fixes and dividing by time between the fixes. Not many track recorders record the real-time speed at each point probably because it would just confuse people.

This is a true statement. GPS receivers have two tracking loops running for each satellite being tracked. The loop tracking the carrier signal produces a Doppler shift that is then used to calculate velocity (accurate to <0.3 knots 1-sigma). The other loop tracks code that produces range measurements that then go into producing a position solution (horizontal accuracy at 20m 95% of the time). Many receiver manufacturers advertise better than these numbers. Also, this is without differential GPS. These are spec'd values that are tested before delivering product.

Note: good antenna siting is critical to meeting these numbers.

[valhalla360]

Originally Posted by transmitterdan

Believe whatever you like. Google:

How does GPS measure speed?

I wasn't trying to be snarky.

That is exactly what I did and found several articles from apparently reputable sources that say it works exactly like I think it does. I found none suggesting otherwise.

If I'm wrong, I would like to learn.

[rwidman]

As long as everybody understands that a GPS measures speed across the surface of the earth and the paddlewheel measures speed through water that may be moving with or against the direction of the boat, we are all fine.


If you anchor in a 2 knot current, your paddlewheel will display 2 knots. Your GPS will display 0 knots.

[transmitterdan]

Originally Posted by valhalla360

I wasn't trying to be snarky.

That is exactly what I did and found several articles from apparently reputable sources that say it works exactly like I think it does. I found none suggesting otherwise.

If I'm wrong, I would like to learn.

This is the first hit I got on Google:

Question:

 How does a GPS tracking unit calculate speed?

Best Answer:  For average speed over a long distance it takes the distance traveled and divides it by the time taken - exactly what the other answers said. 

For instantaneous speed, how fast you were going at one point in time, this approach doesn't work. Which is a pity because that's exactly what you want to know if you want to check your drivers aren't speeding. 
Each GPS position has an error of up to 5 meters, that means that if you take two positions a second apart take the distance between them and calculate the speed you could be off by up to 10 meters per second or about 22.5 mph. 

The way a GPS actually calculates speed at any given moment is by measuring the doppler shift in the signals from the satellites. Just as a car engine or police siren sounds higher pitch when it's heading towards you than when it's heading away the signals from the satellites change very slightly depending on how fast you are moving towards or away from them. 
If you know where you are (you have a GPS position) and you know where and how fast the satellites are moving (they broadcast that information) then by measuring the doppler on the signals you can calculate how fast the receiver is moving far more accurately than by taking the difference between two positions. 

How accurate is it? As with anything to do with GPS it depends on how many satellites are being tracked and how strong the signals are. It would be rare for it to be more than one or two mph wrong.

[transmitterdan]

Originally Posted by rwidman

If you anchor in a 2 knot current, your paddlewheel will display 2 knots. Your GPS will display 0 knots.

I have actually done this (anchored in a current). The paddlewheel struggled to measure anything and the GPS displayed 0-1.5 knots as the boat swung back and forth on the rode.

[sailorboy1]

Originally Posted by Sailor Doug

I will try again. Have you ever sat tied to a dock and left your gps track screen on. Usually you will see points 100 feet or more around your true position.

You need a new GPS unit!

[gbgreen59]

"100 feet of position variation"

This is evidence of antenna siting issues. Don't know how your GPS antenna is installed. Could be bad DOP due to blockage; could be due to noise from low signal level (antenna in cabin); or both. Most likely the major contributor is due to multipath with other two also contributing. A dock area is a forest for multipath with masts and standing rigging everywhere.

Go out and anchor away from the dock area. You will likely find that the variability goes away except for movement of your boat.

[Training Wheels]

My NKE uses a high speed GPS receiver to determine TWS and TWA. Guess that's pretty fast. My speed puck changes speed pretty much immediately if you pick it up and move it.

I believe if you really want to know what the TWS and TWA is, if relation to your boat, you need accurate speed through the water, not SOG.


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[Sailor Doug]

Doppler effect on a marine gps, what make it? I am a old guy. I was taught a gps measures a time differential between receiver and satellite. After it requires re than one satellite it can calculate position, using time between positions it calculates speed. The more satellites it tracks and the faster it process the more accurate the position. Since the earth is no actually round it has additional calculations to give you true position.
Doppler effect gives a frequency if you are moving closer or farther from a satellite. At 5kt it would require some pretty slick electronics.
When out sailing I watch speed log for changes in to sail trim.

PS I use a Garmin 76s if I sit it on a picnic table speed will be shown in 1/10 units.


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[lordgeoff]

Originally Posted by sailorboy1

You need a new GPS unit!

.....and replace or clean the paddle weel log...

[transmitterdan]

Originally Posted by Sailor Doug

Doppler effect on a marine gps, what make it? I am a old guy. I was taught a gps measures a time differential between receiver and satellite. After it requires re than one satellite it can calculate position, using time between positions it calculates speed.

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.

[Sailor Doug]

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.

What commercial marine gps dose this. I know how Doppler shift works. Yes it will tell you a speed change from a satellite. It still needs more than one satellite. Next it needs to calculate change in position in 3D space to get position change rate (speed). No electronic device gives information in real time.



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[transmitterdan]

So far as I know all GPS chipsets do this. It does not have to calculate change in position. It only has to know the frequency shift to determine speed. Position is no where in the formula for Doppler speed measurement. It is possible to measure frequency shift in real time. Frequency shift is an analog real time value. You can measure it as often and as fast as you like even a billion times per second. And only one measurement of frequency shift is needed to calculate speed.

Here is another example. The police radar used to measure speeding cars doesn't know anything about the position of the speeding car. It uses the same formulas and frequency shift measurement. It works in real time to capture the maximum speed of its victim.

[Sailor Doug]

Transmitterdan you need your money back.


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