Author Topic: 798ci si with GPS position inaccuracies, Is that Normal? (Read 18212 times)

revilo304 · « **on:** April 07, 2012, 08:23:59 PM »

Hey,
Im new to the forum and appreciate the feedback Ill get.

I just bought new 798ci si (not HD). I notice in my driveway which is clear from trees, etc after 10 minutes from bootup the location which is stationary bounces around up to 40 feet on my unit when my unit is plugged in alone without other connections (also shows speed from 0.1 to 0.7mph). This is with 3d fix and up to 15 satellites. I have downgraded to 4.960 from the latest 5.xxx version so I can interlink with Matrix 97 to link SI waypoints from 798 to Matrix for navigation. However, I noticed the bouncing even before downgrade.

My question is, IS this normal for an internal antenna OR is it a hardware/software problem. My concern is accuracy especially marking structure (wrecks) on SI so I can actually be near the structure without a doubt. IF THIS IS normal, should i hookup my external antenna to my 798 with a y-link or what?

My external GPS antenna when hooked to my matrix 97 also showed me stationary without the bouncing.

Thanks

Ollie

Bob B · « **Reply #1 on:** April 07, 2012, 10:06:49 PM »

You are going to have to see how it works on the water. There is never a good view of all the satellites in a driveway.

revilo304 · « **Reply #2 on:** April 07, 2012, 10:20:34 PM »

Im not sure if it matters if its a driveway or the water, if there is nothing blocking the signal, it should always be accurate. Again, my old Matrix 97 with external is fine in driveway.

Bob B · « **Reply #3 on:** April 08, 2012, 12:03:14 AM »

Can you post a picture of the mounting?

Bebson · « **Reply #4 on:** April 08, 2012, 06:01:38 AM »

My 798SI (also internal) have ~100ft accuracy when on land, and 10-15ft on open water. If screen show you 15 sattelites, it doesnt meen it is locked on 15 sattelites. Unit is connected (locked) only on sattelites with black square, and is triyng to connect on sattelites with gray square.

This is example:

-Accuracy 90ft
-12 sattelites, but locked only on sattelite no 1, 10, 23 (with black square)...

Dont worry, you should get better accuracy when on water !

sonar2000 · « **Reply #5 on:** April 08, 2012, 04:27:42 PM »

I am guessing it may take 4 satelites to get a the better accuracy. Some call it 3d. I think three will show connected as 2d..

Chuck

revilo304 · « **Reply #6 on:** April 08, 2012, 07:22:03 PM »

I did not have it mounted yet. I was getting those GPS coordinates when unit was on ground in driveway. I did purchase a y-link cable to hook 798 to external gps receiver I already have mounted on boat but I guess I may just need to get out on the water to check accuracy.

I just wanted to make sure the internal receiver is not malfunctioning.

sonar2000 · « **Reply #7 on:** April 09, 2012, 09:32:55 AM »

It does not sound like the internal is acting up. Location and the internal do make for less of a signal reception especially if in trees or around buildings..

A 2d status will not be as accurate as a 3d status. The more accquired the better the accuracy..

Chuck

wolfs4evr · « **Reply #8 on:** April 09, 2012, 09:44:15 AM »

There has to be a known issue with the 7xx series that HB isn't acknowledging publically. If one jumps around all the different forums out there there is a TON of posts about 7xx GPS issues (You never see one about 8xx, 9xx, etc). Me included. My 798ci HD SI GPS speed and position accuracy is junk. If I want to know how fast I am going I have to either look at my I-Pilots remote or console 898 since the 798 jumps all over the place. Last weekend I trolled a bit for deep Stripers. Console and I-Pilot remote are dead on to one another and nice and "smooth". Say 1.7mph. My 798 is jumping from .3 to 5mph or so so fast you can’t even try to break out a estimate in your head. Plus the track history on the 798 makes it look like I am driving drunk. Lol Verse the nice straight line my 898 is showing. If something is wrong acknowledge it… Kills me. Maybe someday I will get around to calling about it and eventually sending my unit back. But I would like to hear HB actually admit there is a known problem and what they are doing to correct it. It would not make me happy to send my nearly brand new unit in to get a “refurbished” unit back like I have read has happened to others.

Humminbird_Greg · « **Reply #9 on:** April 09, 2012, 11:01:22 AM »

Quote from: revilo304 on April 08, 2012, 07:22:03 PM

I did not have it mounted yet. I was getting those GPS coordinates when unit was on ground in driveway. I did purchase a y-link cable to hook 798 to external gps receiver I already have mounted on boat but I guess I may just need to get out on the water to check accuracy.

I just wanted to make sure the internal receiver is not malfunctioning.

Revilo304, In N.C. you should be getting an Enhanced Fix type (tracking WAAS satellites). This will increase the accuracy of the GPS Position and eliminate the speed and position from jumping around as much. I don’t have any experience in the approximate accuracy or speed/position jumping that you should experience with a 3D Fix type as we automatically track and use the WAAS satellite data here in Alabama.

Check the Fix Type and Estimated Position Error on your Matrix 97 unit to compare to your 798ci Si unit.

sonar2000 · « **Reply #10 on:** April 09, 2012, 11:20:45 AM »

The screen picture shows a 2d fix not a 3d fix.
He will probably get inconsistencies in accuracy..

Normal for blocked view of satelites. Trees, buildings, where positioned...etc

Chuck

Humminbird_Greg · « **Reply #11 on:** April 09, 2012, 11:49:37 AM »

Re-read his first post there Chuck: “This is with 3d fix and up to 15 satellites”.
If we knew what his Matrix 97 was showing for a Fix Type we could make more of a direct comparison. This comparison would be of more value though if it were done away from buildings, trees and mountains (anything that may block the satellite signals) because what may be blocking one GPS Receiver may not be blocking the other.

sonar2000 · « **Reply #12 on:** April 09, 2012, 11:56:31 AM »

I see the first post but the picture on down shows 12 satelites and only 3 accquired. this will be a 2d at this time.
I dont know if I ever have seen 15 on the horizon..

The garmin shows 3 accquired also.

So for this picture and time it is 2d.

It appears with internal and a handheld he does not always lock on enough satelites all depending on location and line of sight.....

Chuck

revilo304 · « **Reply #13 on:** April 09, 2012, 01:10:32 PM »

Quote from: sonar2000 on April 09, 2012, 11:56:31 AM

I see the first post but the picture on down shows 12 satelites and only 3 accquired.

That picture is of another posters not mine.

sonar2000 · « **Reply #14 on:** April 09, 2012, 02:28:43 PM »

I see that Ollie....
So lets reset...

On your 798 (new) your fix bounces around even with a 3d fix.

If you connect to a shared antenna it works ok..

It does seem like an issue....a 3d should not bounce that far off.
Is this stationary and/or what happens if you are moving around..? does it still "bounce"..

The post from wolfs4evr seems to point to the 7xx series. Maybe we can hear from other 7xx users..
Chuck

revilo304 · « **Reply #15 on:** April 09, 2012, 04:57:06 PM »

It bounces when stationary, i did not walk around with it. I have not hooked it up to external gps b/c my boat is being worked on but my Matrix 97 with external receiver in the same location never bounced around.

sonar2000 · « **Reply #16 on:** April 09, 2012, 05:19:30 PM »

I have seen some other postings on the 7xx series and the internal antenna where it does not capture the signal very well.

Confirm again that you have a 3d fix with more than 12 satelites.

Thanks, Chuck

Humminbird_Greg · « **Reply #17 on:** April 09, 2012, 05:27:29 PM »

He should have a 3D Fix type if the unit is tracking AND USING the signals from four or more GPS satellites. He will not get the Enhanced Fix type unless he is tracking the WAAS satellites.

revilo304 · « **Reply #18 on:** April 09, 2012, 05:31:01 PM »

I had a 3D fix BUT i cant confirm how many satellites were locked bc as a previous poster mentioned just b/c 15 sats were on screen, does not mean I was locked on all of them.

I havent had a chance to mess with it since to find out how many sats were locked.

sonar2000 · « **Reply #19 on:** April 09, 2012, 05:32:16 PM »

Greg, I understand that.
What I am asking is if he has 3d why does he get the discrepancy?
which was his original question.
If the unit is 3d the accuracy should not be 100 feet.

Chuck

Humminbird_Greg · « **Reply #20 on:** April 10, 2012, 10:41:21 AM »

Chuck,
The Fix Type does not always directly translate into a certain accuracy level. It should but does not all of the time. Example: the GPS Receiver could be tracking every satellite in the sky but due to not being able to receive a direct signal from some of the satellites (multi-path error) its accuracy will suffer. The best indicator of position accuracy is the HDOP number which we translate (as does every manufacturer using their own formulas) to an estimated position accuracy number. All the Fix Type tells you is how many or what type of satellites the unit is tracking.

sonar2000 · « **Reply #21 on:** April 10, 2012, 07:42:46 PM »

OK, so why is he getting the lasrge offset error then?
He should be seeing around 10 feet and not much more..
Or is this inherent to the CI antenna?
Chuck

Humminbird_Greg · « **Reply #22 on:** April 11, 2012, 10:45:56 AM »

Well Chuck my guess is that he either has some of the signals from the satellites blocked from direct reception or that there is a problem with the internal GPS Receiver in his unit. Until he can get his unit out in the open to test we won’t know which it is.

sonar2000 · « **Reply #23 on:** April 11, 2012, 02:00:02 PM »

You are losing me on this Greg, If he has a 3d fix he should have enought to give better accuracy.
That is four satelites connected.
On our 1197 with 4 we have about 10 foot difference.

It must be the code for the 7zz series.
As a previous poster said, we dont see this problem for 8xx/9xx series..

Chuck

xSilmarilSx · « **Reply #24 on:** April 12, 2012, 02:59:45 AM »

Checked my brand new 798ci SI HD, never seen water yet.. Brand new update 6.250
And with a perfect satellite fix of 2ft on accuracy, the speed keep jumping between 0 and 1 mph.
It's not even funny.

I will try to connect my Garmin receiver to the NMEA port and hope it can override the internal GPS.
You should use a better filtering algorithm for position/speed. The 4x/s update rate is quite useless. The arrow keep jumping around.

In a week, I will have the chance to put the unit on the water. I wonder how it will react!

sonar2000 · « **Reply #25 on:** April 12, 2012, 09:36:57 AM »

If you are sitting still the speed and fix will jump a bit.
Kind of like wandering.

If you are moving it should be accurate..

Maybe a faster update of satelite signal would help..

Chuck

Humminbird_Greg · « **Reply #26 on:** April 12, 2012, 01:13:32 PM »

Quote from: sonar2000 on April 11, 2012, 02:00:02 PM

You are losing me on this Greg, If he has a 3d fix he should have enought to give better accuracy.
That is four satelites connected.
On our 1197 with 4 we have about 10 foot difference.

It must be the code for the 7zz series.
As a previous poster said, we dont see this problem for 8xx/9xx series..

Chuck

You are missing my point here Chuck: on your unit you have no blockage of the satellite signals. We do not know this for sure for him. His GPS Receiver could still be receiving the signals from these satellites but not directly. They may be getting reflected off of some surface before they are received by the GPS Receiver. Its called multi-path reception. This adds to the inaccuracy of the position but the GPS Receiver does not know that and so calculates the HDOP and estimated position errors the same as if it was receiving unblocked signals from the satellites.

sonar2000 · « **Reply #27 on:** April 12, 2012, 01:18:35 PM »

I give up. How do we really know where we are.......

Chuck

ITGEEK · « **Reply #28 on:** April 12, 2012, 01:35:15 PM »

Chuck,
I would assume that if you are in an area that is open, and has nothing
to block the satellites, you can probably trust your GPS.
The more open the better.

sonar2000 · « **Reply #29 on:** April 12, 2012, 01:40:13 PM »

What we hove found over the years is to run several different units and compare.

ITG you are right...it is the open area that produces best.

Which is a real headache for SAR in the woods, especially when you have assigned search grids with icon boundaries..

However since HB does not make a handheld this may not be appropiate to them..

It does come into consideration when using sonar on a deep creek or narrow river where the banks are lined with trees..
Chuck

xSilmarilSx · « **Reply #30 on:** April 12, 2012, 02:27:21 PM »

Quote from: sonar2000 on April 12, 2012, 09:36:57 AM

If you are sitting still the speed and fix will jump a bit.
Kind of like wandering.

If you are moving it should be accurate..

Maybe a faster update of satelite signal would help..

Chuck

The position wandering is one thing.. And perfectly understandable.
Speed jumping is not. Since speed is not derived from position, but from Doppler information.
You can have accurate speed with only a minimal GPS signal. In the 0.1mph range.

In my Garmin receiver, I can turn-off a filtering algorithm and the unit start to freak-out like the HB unit.
With the filtering on, the unit become steady and the speed is what you should have.

What I think is happening here, is that HB, in their way to provide us with fast update rate and absolute position accuracy, have skimped on a filtering algorithm to smooth thing out and provide a better user experience.

What is good to have 4x update rate, when the speed is not usable in the 0-2 mph range, which is where I fish. When my Garmin receiver is rock stable.

It was pointed last year I think that this problem is somewhat linked with the doughnut problem. Which can be solved with a couple lines of code inside each units.

sonar2000 · « **Reply #31 on:** April 12, 2012, 02:48:08 PM »

Makes sense to me. We probably dont see it as most of our sonar is done at 1.5 to 3 mph..

All of this makes us glad we are just sonar users.

but at times i do wonder about accuracy of position..
generally speaking we dont like our divers to have to do more than a 25 foot circle search..

chuck

xSilmarilSx · « **Reply #32 on:** April 12, 2012, 06:49:39 PM »

GPS accuracy, with the WAAS satellite in view and used is quite good.

You can say for sure, for 95% of the time to be in a circle of 15ft if you have a perfect fix (HB fix of 2-3ft on the unit).

If you want to know the real precision of your unit, just set it outdoor on your porch for 12 to 24h logging the track log and measure the "blob" that the unit produced. It should be within 25ft of diameter.

sonar2000 · « **Reply #33 on:** April 13, 2012, 09:06:42 AM »

I think that for our purposes the accuracy is fine. I dont know about fishing and how close that needs to be..

We use the 25 foot as the max for the specific search and generally if we saw the target the divers are within 5 feet (most of the time..)

chuck

FuzzyGrub · « **Reply #34 on:** April 13, 2012, 09:36:55 AM »

While I have a 798ci, I can't say I have used the internal receiver much. A few portable float trips, but mostly when I want to run the unit inside for a s/w update, or view recordings, etc. I have seen the jumping around (speed,direcection), but it was in the house. I didn't spend enough time on the sats screen to say what kind of lock it had.

I have used the standard 50 channel ext receiver for plotting some property lines. One of the tests I did was get a fix on a known location that I could then referance to a sat map. I used the corner of a bridge abutment. The HB indicated a 2-3' error, with an enhanced lock (WAAS). Clicking on the particular pixel on the sat map, and doing the math, idicated about 2'. I was impressed. This was in a generally open area (valley). Using it for the same purpose in the Adirondaks, through pine trees, the errror will climb to 6-7' or so.

bobxxx · « **Reply #35 on:** April 16, 2012, 01:21:43 PM »

I have followed this post with interest,as I have been questioning the erratic behavior(can't spell) of my 798cisihd in open water.Since your knowledge is beyond mine,I will try to get some better facts to back up my observations.You guys are great.
Bob

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