Topic: ALL SI Models - Calculate GPS direction when not moving

[jimham82]

Model:	ALL SI Models
Suggested Upgrade:	Calculate GPS direction when not moving
Explanation:	Use two or more units with separate GPS pucks to determine the direction of the boat when not moving.    We can fish structure without marker buoys.
Added in Software Version:
Comments on Update:

[Bob B]

Yeah, I wish they would do the low speed direction also......I think they could do it with an electronic compass without much expense.....This would solve a lot of frustrations from slow speed trollers.

[Moose1am]

I'm wondering if a electronic compass would really help. I would think that it would.  But I have a Garmin eTrex Visa GPS which has a compass and the map screen will flip flop around when I'm moving too slow.  That tells me that it can't tell what direction I'm going and is mixed up.  This happens when I'm using the map view in Track Up mode.  I'll have to test it in North UP mode to see if it still does that.
Here is the problem they have to overcome. They must be able to get a much more accurate gps location fix. A fix that's more accurate than Plus or minus ten feet which is what we get now even with WAAS enabled.   One has to move farther than the accuracy error distance in a second to be able to actually calculate exactly where you are . And to figure out your speed you need to know two points, the distance between those two point and the travel time between those two points.  If you are not moving you can't do that.  Say you only move five feet in a second.  The accuracy is + or - ten feet.  If you didn't move your gps may tell you that you moved ten feet to the left or ten feet to the right when you didn't move at all.  Say your in a boat that's anchored at both ends and not moving.  Keep the gps going in this boat and record the lon and lat readings every five minutes for a 48 hour period of time.  I can garantee that the readings will vary over time and  show you boat moving even though it's not moving at all. This is because of changes in the timing of the signals traveling down to earth from the satellites though the changing earth's atmosphere.  All it takes if a fraction of a second change in the signals travel time and the position is changed.   This is why you must move father than 10 ft in a second or less to actually truely calculate your speed and travel direction.
 
Humminbird chose to show us the Circle to remind us that we can't really tell which way we are going at very low speeds. 
 
Now if the accuracy of the gps were in the Centimers or inches we could move very slowly and accurately calculate our position down to a centimer and  then accurately tell how far we moved and then more accurately tell which way we moved.
So basically what we are asking for is better accuracy!!!!  I'd love that too.

[ITGEEK]

I only use one unit with one GPS puck.
I don't want two units on my boat.

Your idea is not applicable to everybody.

[Bob B]

Moose1am,   What you are saying about GPS accuracy is absolutely correct....if the accuracy could be improved, the unit could show direction at a Lower speed utilizing GPS only.

I think your Garmin is not utilizing the electronic compass for your map orientation even though the unit has a compass.  With a handheld unit this would be very hard to do because you could have the unit pointed in various directions relative to your movement, or even in your pocket.

With a fixed mount unit, the software could utilize the electronic compass when it was unable to get a good GPS heading.  It would be able to determine the direction the unit is pointed relative to the boat when getting a good GPS heading and "calibrate" the compass.....hope this makes sense to somebody besides me.

[Humminbird_Greg]

Remember that the folks who run the GPS satellites and the GPS/WAAS system control the accuracy that we (civilians) are allowed to have.  Not sure what this is in theory but would guess that we are near that point now.

[xSilmarilSx]

Atmospheric disturbance will change the signal to what we have today in term of accuracy.
WAAS is used to offset the atmospheric disturbance but you will never have centimeter accuracy without using a DPGS setup which will cost you more than your boat to setup.

My brother have used sub-meter precision GPS from before year 2000. But you can only have that amount of precision with post-process, not in real-time in the unit.

[Moose1am]

Exactly: It takes some time to get the average readings over time before you can get the sub cm accuracy.

We are lucky that President Clinton gave us the better accuracy back when he was president. Before that the military degraded the signals to make it much less accurate.  That was stopped by Clinton's Order so that civilians could better accuracy on the civilian GPS units.

I think it use to be plus or minus ten meters before Clinton's order. Now it's plus or minus three meters or 12 ft 95% of the time

The Military GPS units are probably much more accurate and that allows them to put a GPS guided bomb right through the doorway of a Al Queda Safe House.  LOL>

Atmospheric disturbance will change the signal to what we have today in term of accuracy.
WAAS is used to offset the atmospheric disturbance but you will never have centimeter accuracy without using a DPGS setup which will cost you more than your boat to setup.

My brother have used sub-meter precision GPS from before year 2000. But you can only have that amount of precision with post-process, not in real-time in the unit.

[xSilmarilSx]

The old accuracy was ±300 feets..

[W9GFO]

The old accuracy was ±300 feets..

It wasn't that bad before SA was turned off. I do not recall the exact figures but I remember Estimated Position Errors of as low as 30 feet. Now with SA off it is much better.

Two identical GPS units sitting side by side will not report the same position. Nor will their errors track each other. In order to determine an accurate heading on a stationary vessel using two GPS units, the separation between the two would need to be many times the position error. A fifty foot separation would be nearly useless. You would need to have a super yacht before it would work well. At that point you could probably afford something a little better.

[sonar2000]

The years past inaccuracies were due to the (military) putting in error factors so that exact location was not displayed. That has been lifted for a few years back now. Also with WAAS now available the error is less than 10 feet. We regularily see or can navigate to a position within 3 feet. Of course you have to slow down the final 50 feet to key in on the position..
Chuck

[Rickard]

Since I have some experience from GPS technique I must chime in and tell that position, direction and speed are calculated from a blend of doppler shift and position data with help of statistical and Kalman filtering methods in all GPS units and not from successive positions only. These methods can be tuned in many ways by the GPS engine manufacturer and the navigation software programmer. It's not that easy to tell what HB should do about any issues with direction at low speed since it's not known how stuck they are in the hardware/software preconditions. But I know that correct speed and direction can be estimated very accurately from only the doppler shift in the satellite signals, much more accurate than what's possible from the GPS positions.
Rickard

[xSilmarilSx]

Since I have some experience from GPS technique I must chime in and tell that position, direction and speed are calculated from a blend of doppler shift and position data with help of statistical and Kalman filtering methods in all GPS units and not from successive positions only. These methods can be tuned in many ways by the GPS engine manufacturer and the navigation software programmer. It's not that easy to tell what HB should do about any issues with direction at low speed since it's not known how stuck they are in the hardware/software preconditions. But I know that correct speed and direction can be estimated very accurately from only the doppler shift in the satellite signals, much more accurate than what's possible from the GPS positions.
Rickard

Exactly... the doppler shift and the filtering method are very important in GPS usability... Something HB should check more thoroughly with their wandering position and inaccurate speed display.

[Moose1am]

Statistical Analysis of data that's not accurate will give you inaccurate output.  The trouble with going slow is that the data available to analyze is limited.  Math can only do so much when the data set is limited and the accuracy of the numbers is not precise nor accurate 100% of the time.

Now if you were to sit in a spot for hours then you may be able to collect more data points but each data point is still subject to the inaccuracies of the GPS system. 

I've read that the more accurate GPS units take a while to collect the data before they can get the accuracy down to the centimeter range.

I would guess that the US military has this figured out though.  But they probably use different signals and equipment to figure out the position.

Doppler shift is nothing new.  Every time you hear a train going by or a siren screeching down the road you are experiencing the Doppler shift of the train's whistle or the  siren sound as it approaches and then passes by and goes away from you.

I wonder how they figure out which data points are the outliers.

Exactly... the doppler shift and the filtering method are very important in GPS usability... Something HB should check more thoroughly with their wandering position and inaccurate speed display.

[Moose1am]

I understand what you are saying.  But remember that the boat may be moving so a gps fixed to the boat will also be moving. 

When I use the eTrex I hold it horizontal to the water and steady while sitting on my front elevated seat. I hold it pointing to the front of the boat and then use my foot controlled trolling motor to go to the waypoint showing up on my map view.

You may be right in that the gps is not using the compass feature to navigate and show the map view.  I'll have to try using the map view with the compass feature turned on and then turned off to see if there is any difference in how the map view works.

Moose1am,   What you are saying about GPS accuracy is absolutely correct....if the accuracy could be improved, the unit could show direction at a Lower speed utilizing GPS only.

I think your Garmin is not utilizing the electronic compass for your map orientation even though the unit has a compass.  With a handheld unit this would be very hard to do because you could have the unit pointed in various directions relative to your movement, or even in your pocket.

With a fixed mount unit, the software could utilize the electronic compass when it was unable to get a good GPS heading.  It would be able to determine the direction the unit is pointed relative to the boat when getting a good GPS heading and "calibrate" the compass.....hope this makes sense to somebody besides me.

[xSilmarilSx]

You have to realise that Garmin have a way better GPS system than Humminbird.
The speed read-out don't jump around like HB.

The difference between my 798 HD and my Garmin is night and day!
HB should put the 3D magnetic compass that Garmin use on their devices!

[xSilmarilSx]

Here some info about Selective Availability and the 300ft error range:

http://en.wikipedia.org/wiki/Selective_Availability#Selective_availability