GPS drifting ? At anchor

[a64pilot]

Survey mode is nothing but position averaging, it may not even have been called that, I Just remember the capability.
It was also one of the very few GPS’s that had MGRS as a database so that I could input Military grid coordinates.
Apollo was the company that Garmin bought that got them into Aviation Nav systems.
Garmin has a history of acquiring companies as a way to break into a new field.

[EveningTide]

My first GPS receiver lasted 10 years and then lost its ability to get good signals from satellites. One result was that it did exactly what you are describing. It would look like I was dragging at anchor when I was probably not. As the unit was dying it would only pick up 4 or 5 satellites. After replacing it I could again get 8 or more satellites and I never saw any bad "drifting" again.

[SURV69]

Before SA was turned off, we didn't need to tell software the direction and distance to the actual anchor position ... we never knew if we were in the middle or on one edge or another of a 320' radius(not diameter), invisible circle.

I wish I had the money(to buy a boat), that my last survey-grade GPS cost(after 10 years of continuous price dropping) .... that was just shy of $30,000 for a topcon base & rover HiperLite system .... accuracy is not cheap.

But, for a few hundred dollars, a person could buy a used ProMark GPS and add to it a rather precise antenna(like micropulse), and enjoy sub-meter ... actually quite a bit less than a meter accuracy, along with an RS232 feed to a computer and a set of accurate aerial photos, the capability of not only coming into a marina, but pretty much to the right dock.

A64pilot: Altitude is unreliable, I believe in part due to that the GPS software assumes a round Earth, and the Earth isn’t.
For sure, the earth is a perfect sphere as far as GPS is concerned, but models that indicate the elevation difference from that sphere to the actual surface are getting more and more accurate all the time. I think this is why OPUS invites anyone with adequate equipment to allow OPUS to compute final data ... just more and more data to refine the models. When I stopped working the elevations were very reliable ... and accurate ... I'm sure that all the parameters, including time, is getting ever more accurate.

Soon everyone will have ready access to submeter accuracy .... what are we all going to do with all that accuracy?

[SURV69]

Originally Posted by EveningTide

My first GPS receiver lasted 10 years and then lost its ability to get good signals from satellites. One result was that it did exactly what you are describing. It would look like I was dragging at anchor when I was probably not. As the unit was dying it would only pick up 4 or 5 satellites. After replacing it I could again get 8 or more satellites and I never saw any bad "drifting" again.

I think, in 2003 or so the Y2K of the GPS world occurred. It was when the GPS system came to the end of it's first 1,024 weeks.

No one was sure what was going to happen ... would the GPS(especially consumer-grade), no longer work, would the GPS be able to download almanacs, or would the GPS units get lost in endless cycles?

I thought that I heard some consumer-grade GPS in fact, did stop working.

Then older GPS were subject to problems revolving around the newer sats that were not backward compliant to older GPS.

I don't know the result of all this, but I do know the very, very old Promark 10cm GPS(the really big ones), similar to the ProMark 5's, are still working today

[B25Matt]

Originally Posted by hellosailor

"To set the record straight a GPS receiver requires at least four satellites to come up with a position solution."
True but deceptively misleading. As kas mentioned, you only need 3 satellites for a 2 dimensional fix, and "sea level" being somewhat all that sailors need, that's all our GPSes need. Unless you are using the GPS on a submarine or other vessel that might not be on the surface.

I told my self I wasn't going to go down this rabbit hole but you're the third person to insist that three satellites is sufficient for a horizontal 2D position fix. Can you please provide a reference this assertion? One that actually uses equations?

The inner workings a GPS are part of my daily experience. I know what an Extended Error State Kalman Filter is and how it is used inside a GPS to calculate a best position fix from twenty satellites. I sat at the feet of one original GPS architects. It takes a minimum of four satellites to come up with a position fix of any kind.

A GPS calculates ellipsoid height internally and then uses builtin terrain models to come up with an AGL height. A good GPS will allow you to select which height you want.

To the OP: sorry for hijacking the thread. signing out.

[a64pilot]

I have always been told a 2D fix from three and a 3D from four, that geometry of where the sats were is just as important as how many your receiving, but as a general rule, the more, the better.


The answer is I believe is that non Commercial GPS’s assume an altitude of zero and can calculate a 2D fix based on this assumption.
But to get a true fix does require four.

[Cavalier]

Originally Posted by a64pilot

I have always been told a 2D fix from three and a 3D from four, that geometry of where the sats were is just as important as how many your receiving, but as a general rule, the more, the better.





The answer is I believe is that non Commercial GPS’s assume an altitude of zero and can calculate a 2D fix based on this assumption.

But to get a true fix does require four.

^^^ this

[Viking Sailor]

Originally Posted by B25Matt

I told my self I wasn't going to go down this rabbit hole but you're the third person to insist that three satellites is sufficient for a horizontal 2D position fix. Can you please provide a reference this assertion? One that actually uses equations?

The inner workings a GPS are part of my daily experience. I know what an Extended Error State Kalman Filter is and how it is used inside a GPS to calculate a best position fix from twenty satellites. I sat at the feet of one original GPS architects. It takes a minimum of four satellites to come up with a position fix of any kind.

A GPS calculates ellipsoid height internally and then uses builtin terrain models to come up with an AGL height. A good GPS will allow you to select which height you want.

To the OP: sorry for hijacking the thread. signing out.

A GPS fix is calculated by first measuring the psuedo-range between the satellites and the receiving antenna. Then an algorithm is used to determine the position where these measurements intersect.

If the antenna is at an unknown height, and with only three satellite measurements to work with, it is possible that the receiver will calculate a vertical arc of possible positions instead of an single position fix. However, if the antenna height of the receiver is known then the vertical position is resolved and a position fix is obtained. It is also possible to resolve the height of the antenna by taking psuedo-range measurements from additional satellites. These measurements assist the algorithm in determining intersection of the measurements, and thus the antenna height.

It is also possible that even if the antenna height is know and the three satellite geometry is so poor that a position fix becomes impossible. Under these conditions the addition of one or more satellites will improve the geometry to the point that a position fix becomes possible.

[SURV69]

"GPS", is such a mis-used term. It should mean ONLY the American sat system, but it seems that any positioning sat in the sky is now "GPS".

Anyway, I'm still wondering if the accuracy in the southern hemisphere is compromised. since so many sats are seen close to the horizon(but not for me), which should mean weaker positions.

In North America, we get the full rang of sats. Right now I think there are 27, maybe more sats in orbit(some turned off), and when I used to do sats locations, years ago, to determine obstructions, all of our sats came over the horizon at about 45 or so degrees latitude ... some further north, some further south, but I don't think any of them didn't cross over the Continental United States ... at least not generally. I've never seen a sat coming from the south, although some do come(or go) from the SE & SW, and some actually come from the north ... I did all my work at/near the 41st and 42nd parallel.

If the sats are 12-13 thousand miles above the earth, that would mean a lot of "GPS" sats in sight to me would be close to the horizon in the south latitudes ... not good.

I've heard of consumer-grade "GPS" that would/could use GLONASS, but I've never owned one. Are "GPS", units in France or Australia different than "American", GPS units? Are they called something different?/

[a64pilot]

I think GLONASS enabled units are very common, I Believe my Garmin Aviation handhelds are all capable and I think my B&G plotter is as well.
Although I have never tried using GLONASS as primary.

[ZULU40]

iphone have had GLONASS since iphone 4S

https://beebom.com/what-is-glonass-a...rent-from-gps/

[David M]

In addition to dragging anchors, satellite visibility etc, the GPS system is not perfect. Sometimes the DoD has its internal errors, intentional and I think unintentional errors occasionally that provide for a very inaccurate position....well outside of the 90 percent of the time averages.

[StuM]

Originally Posted by SURV69

"GPS", is such a mis-used term. It should mean ONLY the American sat system, but it seems that any positioning sat in the sky is now "GPS".

You are correct, but that's the fate of many names that have become generic (from aspirin to zipper ) Good luck with getting people to call them GNSS receivers in everyday usage

I've heard of consumer-grade "GPS" that would/could use GLONASS, but I've never owned one. Are "GPS", units in France or Australia different than "American", GPS units?

They're all the same brands - most of them nowadays use chips that can work with GPS, Galileo, Glonass and Beidou.

(Quote from Garmin: Most Garmin receivers track both GLONASS and GPS, and some even track BeiDou.
Just about any modern smartphone/tablet with "GPS" will also do this. )

Are they called something different?/

Nope, as you've identified "GPS units" has become the generic term.