New GPS issues - does the boat needs to move to in order to get the first fix?

[continuouswave]

Originally Posted by Dr. D

continuouswave: Thanks for encouraging me. Now I am certain all GPS chipsets use Doppler for velocity measurements. I did find one reference that said old (maybe before 2000) chipsets did not have the computing power, but current ones do.

I don’t agree.

To find the relative motion between a satellite and a receiver is very simple: the speed is directly proportional to the change in frequency. The direction of the frequency shift indicates the relative motion direction, either toward or away from the receiver. These simple-to-deduce parameters help the receiver know which direction to shift the receiver tuning and how much to shift.

But to figure out the exact speed of motion of the receiver from the Doppler frequency shift is a much more difficult problem.

[Dr. D]

Originally Posted by continuouswave

But to figure out the exact speed of motion of the receiver from the Doppler frequency shift is a much more difficult problem.

OK, so that is the basis of our difference. The problem of solving for velocity from Doppler measurements is more difficult, but it is a solved problem. The attached document does a better job than I could do explaining it. The document also points out the change to Doppler velocity measurements when the computing capability of the receivers were finally up to the solution.

[kevinnem]

Originally Posted by continuouswave

Compare at:

https://insidegnss.com/wp-content/up...-SOLUTIONS.pdf

Read the opening two paragraphs. They give a good summary of the two fundamental methods of speed calculation in a GPS receiver. Note that one method "is simple to implement", which I suspect is why it is used in most marine GPS receivers.

The article proposes several other methods for obtaining speed, but the methods are more complex. The notion that a more complex method is possible is not a basis to assume that the $3 GPS chip in a marine GPS is going to use that method.

I am sure that aboard world-class racing sailboats, there are devices for speed determination from GPS that are more accurate than the usual simple methods.

That is so cool, the fellow shown in that articial, Mark Petovello, was in school with me, I think he was 1 or 2 years ahead, cause I was in my last year, and he was a new TA.

I am not sure what gps receivers use Doppler shift vs position change, I do know that you can get rid of almost all the GPS errors by doing differences between time 1 and time 2 ( as well as between satellites) this is why receivers get not great positions , but really great direction and velocity.

[Brian.D]

[/sarcasm on]WOW, everyone is now a GPS Engineer. Cool! This is getting better by the minute. [/sarcasm off]

[Cadence]

Originally Posted by Brian.D

[/sarcasm on]WOW, everyone is now a GPS Engineer. Cool! This is getting better by the minute. [/sarcasm off]

Their going to cause another flight 19 into the triangle.

[continuouswave]

Originally Posted by Brian.D

[/sarcasm on]WOW, everyone is now a GPS Engineer. Cool! This is getting better by the minute. [/sarcasm off]

I don't quite understand the nature of your remarks.

I get the impression that you would prefer this discussion to stop.

I'd say that generally if YOU are finished with reading the discussion, the best thing for YOU to do is to stop reading the discussion, and the worst thing for YOU to do is to keep adding to the discussion by insisting it should not go forward.

But perhaps I have misunderstood your sarcastic remark. I hope my response is clearer.

[continuouswave]

Originally Posted by Dr. D

The attached document does a better job than I could do explaining it.

Thanks for the link to the document. The author is quite expert on his subject.

ASIDE on using Doppler Shift to deduce speed:

When the Soviets launched SPUTNIK in 1957, it was quite a sensational event. The radio signal that was transmitted was around 20-MHz. My father was a radio amateur and had a general coverage receiver. I remember sitting in the basement with him as he was tuning for the signal (on a Hammerlund HQ-129x receiver). I was seven-years-old at that time. I don't recall if we ever heard it, but it was quite exciting to just be trying to hear it--a signal from space in 1957 was literally out of this world.

There is a very interesting account of others who were also listening to the signal from SPUTNIK given at

https://www.jhuapl.edu/Content/techd...2-01-Guier.pdf

These scientists and engineers utilized the Doppler Shift of the satellite's signal to deduce it orbit's parameters and many other useful data.

As the article describes, their insight into what information could be derived from Doppler Shift on a signal from an orbiting satellite was quite a landmark in radio navigation.

I hope you might find it an enjoyable read.

Also, the information derived from SPUTNIK's signal was from a receiver that was not in motion; one might be able to draw an inference from that to the original topic of this thread.

[sgel]

Hi,
Just the latest update to conclude this discussion.
After sending screenshot and the nmea 2000 network traffic debug information to Garmin I got a following response:

Hi,
Our engineering found the following issues with the output sentences of the GS25:
“The COG in PGN 129026 being sent is invalid. Also the fix information in the PGN 126539 being sent is invalid. For our MFDs to recognize a GPS source that source must be providing valid Fix, Time, Position, Track and Velocity information.”
This will need to be fixed on the GS25 side of things. Or look at installing a different GPS Antenna.

After this email, I tried to get some clarifications such as what does it mean invalid, and if 126539 is the correct number since it doesn't appear anywhere.. (perhaps they meant 129029) but they didn't answer anymore emails.

I reached out Simrad and their suggestion was to buy a new GPS.

My main take away from this experience is that although nmea 2000 is standard, I shouldn't assume all devices are compatible (maybe I was just naïve...)

Anyway,
Thanks everyone for participating in this discussion and for you good insights,

Fair winds,
Sgel

[tanglewood]

Originally Posted by sgel

Hi,
Just the latest update to conclude this discussion.
After sending screenshot and the nmea 2000 network traffic debug information to Garmin I got a following response:



After this email, I tried to get some clarifications such as what does it mean invalid, and if 126539 is the correct number since it doesn't appear anywhere.. (perhaps they meant 129029) but they didn't answer anymore emails.

I reached out Simrad and their suggestion was to buy a new GPS.

My main take away from this experience is that although nmea 2000 is standard, I shouldn't assume all devices are compatible (maybe I was just naïve...)

Anyway,
Thanks everyone for participating in this discussion and for you good insights,

Fair winds,
Sgel

Congrats on getting to the bottom on it, sort of.


I have run into this sort of thing time and time again, and it's damning of N2K as a standard. Here you have two prominent vendors, and they can't agree on the interpretation of the GPS PGN, which is probably the most fundamental thing in the standard. And "NMEA 2000 Certification" is laughable because it doesn't evaluation PGN content. Not at all. It doesn't look at functionality at all above the data link layer.


There's a reason I still build all navigation systems using 0183, and there is a reason why IMO still doesn't accept N2K for things like IMO radars and Class A AIS. It's just a freaking joke.