Speed Through GPS Versus Old Fashioned Paddle Log

[goboatingnow]

Originally Posted by fryewe

At the link below is a good (a bit dated) description of the parameters used in calculations by your gps...with both pseudo-range (phase difference...which is time difference...key for position determination) and pseudo-range-rate (Doppler) used for speed calculations...from multiple satellites.



So, the answer is "both."



GPS Determination of Course and Speed

No it's not both.

Modern systems use Kalman filters to improve position accuracy estimation. In that regard they use both doppler information and previous data to help reduce anomalies. ( Kalman is a form of linear quadratic estimation )

If you look at the equations in the filter instantaneous speed is an important input.

The fact remains that all modern chips sets do both code phase and carrier phase correlation and carrier phase correlation gives very accurate velocity measurements as an aside

The code phase correlation process isn't good enough to give good speed information in itself.

Dave


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[goboatingnow]

Originally Posted by Sailor Doug

Wotnot I see you are a professional, have you read all the posts. The question is do marine gps determine position and speed by time differential or by Doppler shift. I understand that Doppler shift helps identify satellites in a constant orbit relative.


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In order to do accurate range measurements on the satellite signals modern GPS systems use both code phase correlation , on the pseudo random code signal and also carrier phase correlation . As part of this you are measuring the difference in signal flight time of not only the pseudo range but also the much higher frequency carrier. If you remove the effects of satellite velocity ( known data ) and earths rotation ( also known data) what's left is your speed.

In a standard one antenna system, instantaneous speed is determined by Doppler shift of the carrier. Position is determined by determining range ( ie flight time of the signal ) and performing a triangulation process ( more correctly called Trilateration)

In two or more antenna systems phase correlation, can be used to determine instantaneous heading and altitude without needing to decode position

Dave



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[lordgeoff]

Originally Posted by transmitterdan

Lord Geoff,

Technically, GPS is a PNT (positioning, navigation and timing) system. GPS is sort of a trademark like GLONAS and others. The US GPS system provides many features beyond positioning alone. It is used as a time and frequency standard by millions of devices. They will be distressed to learn that it only provides position capability.

Technically, speed is not distance divided by time. That would be the "average speed". Speed is defined as an instantaneous value. You do not have to wait until you reach another place to know speed. The simplest example of that is an old fashioned cable driven car speedometer. They do not measure the distance traveled and divide by time. Neither does a GPS receiver have to do that. Mathematically speed is the first derivative of displacement with respect to time and is an instantaneous value. Speed does not just exist at discrete points along the journey. Interestingly, speed is also the integral of acceleration over time. Just as distance traveled is the integral of speed over time. So it is possible to (and many inertial navigation systems can) measure the distance traveled without ever knowing a single position during the entire trip. Again we can look to the example of an old fashioned car odometer. It measures the distance traveled but it never knew where it was.

Doppler shift is caused when a transmitter and a receiver move in relation to one another. The amount of shift is a linear indication of that relative velocity. The GPS receiver measures enough of these doppler shifts from multiple satellites so that it is possible to compute speed very quickly and accurately. For low speed applications such as walking, running, boating and the like Doppler speed is much more accurate than distance/time and can be updated as fast as desired.

Much of what you wrote is not accurate (e.g. GPS satellites know nothing about NMEA) but it doesn't matter for this thread. If you are interested in learning more about this subject you can PM me for an offline discussion.

Sorry guys. You have it wrong.
Satellites only send simple signals which basically say, this is my position in space, this is my position in space. The satellites repeat this information very quickly. If you have ever watched the strings arriving at your GPS receiver. You would see the strings are very simple and repetitive.
Don't get too hung up on the Doppler effect. It merely means moving toward or moving away from an object.
The string from the satellites is received by the GPS and through calculations from CONSECUTIVE strings from the Satellite can calculate how far it has moved toward or away from the satellite. Hence the GPS RECEIVER can then make further calculations such as the average speed between these two points.. I.e. Average speed which in effect is instantaneous speed as it is calculated each second or less.

There is no communication from the GPS receiver on your boat to the Satellite as it is only a receiver. Imagine the billions of GPS receivers, walking Apps, hand heads, motor vehicles, trucks, having a continuous two way chat with the satellites. It would be Comms meltdown.
From your link...

"GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed."

The calculations or algorithms of both the satellite and GPS receiver are complex. The calculations such as HEADING, SOG, etc are simply calculated using the distance and time between the two CONSECUTIVE positions. VMG, ETA, TTG etc are again simple calculations once a waypoint is introduced into the calculations.
Your link confirms what I have been saying the SPEED is the average between two positions. To all intents and purposes an average speed calculated each second or less is as good as an instantaneous.
Your link also confirms the range of speeds the GPS receiver is capable of from zip to hundreds of mph/kph.
NMEA is simple the Comms standard for most GPS receivers. I think the new version is NMEA2000 or something like that. Raymarine still has it proprietary code SEATALK?

[transmitterdan]

Misah give up...

[Sailor Doug]

It must be true GPS suffers from latent speed error and factual passage measurement, long live the knot log.


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[Training Wheels]

Think I'll go back to LORAN.


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[Robin3]

Originally Posted by Training Wheels

Think I'll go back to LORAN.


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Loran was never really a small boat option where I learned/sailed. We had Decca and I confess to buying a set at the eariest opportunity when they became available to mere leisure users as previously they were only on a rental licencing deal and IIRC a very lucrative one to the system owners. Purchasers of the first leisure sets paid a once off 'licence waiver fee' or somesuch. I remember my first trip using it was back to Poole , UK from the Channel islands in very thick fog, in company with another boat also equipped with the new device. We passesd several boats headed the other way and my friends in the other boat took great delight in furnishing them with precise position data to help them on the last bit of their trip in the thick fog in a place strewn with rocks and approach channels with vicious tidal currents, even cross currents running in places up to 9kts. Decca could have serious errors at times but was repeatable. When GPS became available I was at the front of the queue for a set too, but Decca remained as a backup. Prior to acquiring these modern devices we navigated the same waters in all conditions the old way by DR and EP although we did have basic RDF sets in those days too. This is the background to my wanting a proper 'log', to provide distance travelled through the water data rather than speed as such, which though nice to know was not needed to work up a DR and EP on the charts. Hence when we installed all new electronics on our current boat here in the USA, we insisted on a paddlewheel log transducer to provide distance and water speed data to the network as well as having full GPS data. Old fashioned it may be but for me essential information that I do not want to miss out on.

[klmmicro]

I use both as they tell me different things. Comparing the information between the two give me a good indication of what sort of current I am dealing with. My primary speed indicator though is the GPS.

[goboatingnow]

Originally Posted by lordgeoff

Sorry guys. You have it wrong.

Satellites only send simple signals which basically say, this is my position in space, this is my position in space. The satellites repeat this information very quickly. If you have ever watched the strings arriving at your GPS receiver. You would see the strings are very simple and repetitive.

Don't get too hung up on the Doppler effect. It merely means moving toward or moving away from an object.

The string from the satellites is received by the GPS and through calculations from CONSECUTIVE strings from the Satellite can calculate how far it has moved toward or away from the satellite. Hence the GPS RECEIVER can then make further calculations such as the average speed between these two points.. I.e. Average speed which in effect is instantaneous speed as it is calculated each second or less.



There is no communication from the GPS receiver on your boat to the Satellite as it is only a receiver. Imagine the billions of GPS receivers, walking Apps, hand heads, motor vehicles, trucks, having a continuous two way chat with the satellites. It would be Comms meltdown.

From your link...



"GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed."



The calculations or algorithms of both the satellite and GPS receiver are complex. The calculations such as HEADING, SOG, etc are simply calculated using the distance and time between the two CONSECUTIVE positions. VMG, ETA, TTG etc are again simple calculations once a waypoint is introduced into the calculations.

Your link confirms what I have been saying the SPEED is the average between two positions. To all intents and purposes an average speed calculated each second or less is as good as an instantaneous.

Your link also confirms the range of speeds the GPS receiver is capable of from zip to hundreds of mph/kph.

NMEA is simple the Comms standard for most GPS receivers. I think the new version is NMEA2000 or something like that. Raymarine still has it proprietary code SEATALK?

Lordgeoff. I cannot understand why you continue peddling this nonsense. The fact is people that understand this technology at an engineering level have explained to you how this works. Equally the internet is full of technical explanations that explain this technology. The trouble is GPS is a conceptually simple system that's actually very very complex in reality , hence the incorrect explanations that abound,

GPS systems derive instantaneous speed from the Doppler effect which appears in the carrier wave from the sat. This speed for example is fed into the kalman filter to achieve better position accuracy.

All modern chipsets compute Doppler speed

Indeed as you say heading ( on single antenna ) systems are derived from position on RTK systems with two antennas heading is derived using inferometry and not position computation

Note . What appears on the NMEA output is often different. Garmin for example had traditionally not stored instantaneous speed in its track log for example in some products

Also. GPS satellites do not repeatedly transmit position. The pseudo code that is continuously broadcast merely serves to indentify the particular satellite , its primary purpose is to allow clock recovery / sync in the receiver.


Remember the position of the satellite is a far smaller contribution to positional error then clock error. The complexity in GPS is to regenerate the accuracy of the onboard atomic clock in a cheap receiver powered by a simple crystal oscillator. In fact if you just used a crystal oscillator uncorrected , after a week the position error would put you on the far side of the moon.

Hence the complexity in clock recovery. The position of the satellites is much less of a concern and can be transmitted much slower them anything else.


Overlaid on the c/A pseudo code is a data transmission, that contains the satellites almanac and more importantly ephemeris. This data is transmitted at a rate of 60 bits per second , very slow.

This is because the satellites orbit is tightly constrained and known in advance. The online data ( navigation message ) is primarily ephemeris corrections.

The navigation. Message information can actually come by lots of comms methods like in the case of assisted GPS.

PS. No one hear has said the receiver transmits to the sat

Note that VMG have nothing to do with GPS per se


It's all here http://www.gps.gov/technical/ps/1995...cification.pdf


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[Wotname]

Dave

[Sailor Doug]

Goboatnow / wotname. As a retired engineering professional (not directly in electronics) I have gone back and read a lot more about GPS. There are several papers written about determining speed and position by gps using Doppler shift. By the date they were published only very recent gps models could be using this method. All models of gps use Doppler shift as part of their algorithm to determine speed and position but only as it helps data set. Not withstand that there are other non gps devices that very accurately measure speed using Doppler effect.
Knot log has a lower speed latency and does not suffer from fractal distanced travelled than a GPS.

Ps I sail with 3 GPS units and use that for position and checking knot log calibration amount other things.


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[goboatingnow]

Originally Posted by Sailor Doug

Goboatnow / wotname. As a retired engineering professional (not directly in electronics) I have gone back and read a lot more about GPS. There are several papers written about determining speed and position by gps using Doppler shift. By the date they were published only very recent gps models could be using this method. All models of gps use Doppler shift as part of their algorithm to determine speed and position but only as it helps data set. Not withstand that there are other non gps devices that very accurately measure speed using Doppler effect.
Knot log has a lower speed latency and does not suffer from fractal distanced travelled than a GPS.

Ps I sail with 3 GPS units and use that for position and checking knot log calibration amount other things.


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Well Garmin post 12xl have been displaying instantaneous speed via Doppler calculations.

It makes no sense to use position data, Doppler speed is always available and using position data subjects the speed to probability positional errors.

Remember advanced filters like kalman are a product of the processing power of modern GPS units all right


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[Sailor Doug]

Wow goboatingnow are you a sailboater and have you ever raced? No instance speed with that gps, do you new know the scan time? If you sailed a lot would notice false gps speed when the boat is not moving and latent acceleration. What credentials do you have to cling to your sifi statements.


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[sailorboy1]

I don't really see why I need to understand how the GPS gives me my speed (even though I do). It does and that is enough for me!!!!!!!!!!!! Why do people insist on making it so hard?

[Wotname]

FWIW, I use a paddle wheel type transducer and a GPS for speed. Clearly one is STW and the other SOG. I ike both.
For back up I have a Walker trailing log and a handheld GPS