GPS position error...quick question/thoughts

[coldfront]

okay...I've been mulling this about for a bit...the discussions centering on GPS 'accuracy'...which go to how well a GPS unit can lock on and hold a spot with any accuracy.

As DougV points out, the HB antennas update at 4x second...Lowrance units update as near as I can tell at 1x sec...although when moving at TM speeds, not sure this is more critical than how many channels are actually 'locked on' at the moment...  and you can also get a 5x/second update rate if you add on an external antenna (LGC-4000)...


But I wonder:  how much of this just gets to be an exercise in 'mine is better than yours'?  Especially IF the antenna is located (as typical) in the sonar unit itself...and placed on the console?   then tied in to a transducer that's either hanging off the stern of the boat...or placed back where it's still able to read while the boat is 'on pad'...

Recognizing that in many cases, there's as much as 6-7 feet of horizontal distance between what you are 'reading' on the screen (transducer image) vs the actual GPS coordinates...

If GPS is 'accurate to about 1 meter'...and you are setting waypoints that are off by potentially 2 meters...is it any wonder it can be challenging to 'find' the bottom feature you want on a return trip?  And the more shallow the water, the harder it gets...

Now...if you put your GPS receiver (antenna) directly over your transducer (vertically aligned; 'zero' horizontal distance difference)...doesn't it seem more likely that the 'improvement' in unit accuracy will likely far outstrip what the manufacturers 'print' in the manual?



Fishton/DougV, anyone:  where am I wrong?

[Fishton]

I have read so many articles where the author talks about accurate casts within a couple of inches being paramount when targeting visible cover / structure.  So why would it be any different when targeting offshore structure / cover you cannot see with the naked eye?

It is really great to see more and more anglers like yourself realizing the importance of marking offshore structure accurately.  The primary factors that will influence your accuracy when capturing a waypoint are the following:-

Sonar :
1. Relationship between frequency properties and depth –
The deeper you go, the more difficult it is to get an accurate waypoint .  This is because a sound wave is not as focused as a laser for example.  The wide coverage area of the 83 and 50kHz cone shaped sound waves are great for finding fish, but not good for taking a waypoint of structure or cover.  The 200kHz frequency soundwave is much better for taking waypoints from, as it has a much smaller coverage area, thus improving your accuracy.   With the introduction of high frequency Side Scan, accuracy has been greatly improved.  This is because when you pass over an object of interest, and you see the object perfectly mirrored in the water column, you know that you are directly above the object and it is safe to take a waypoint from the 200kHz screen, or the 800kHz DownScan.  If the object appears slightly stronger either on the left or right, do NOT take a waypoint, but rather go back targeting either left or right whichever had the strongest return.

If you are using a Humminbird remember to set your DI mode to 'Narrow' and not 'Wide', as 'Wide' is merely an overlay of the left and right beams.  The HB DI units coming out later this year will have high frequency transducers facing down, thus offering the user True Down Imaging.

GPS :
1. EPE - Estimated Position Error
Create an overlay on your screen for EPE.  The reason for this is that at that the second you hit the waypoint button, your EPE might be way out just for a second or two, and if you didn't have an overlay showing your EPE you would never know.  Thus rendering this point useless the next time you return.
2. Position Update - Refresh rate in Hz
Most GPS / Chart Plotters Trail refresh rates are default Auto, this must be changed.  Make sure your Trail refresh rate is set to time, and set the value to the smallest possible option.  This will force the unit to update as fast and often as is possible for that unit.  The Lowrance LGC4000 receiver puck for example, updates 5 times per second.
3. Distance from transducer –
I prefer to use an external receiver that is positioned as close as possible to the transducer when taking waypoints from the console / helm.  Many users feel that bass boats are close enough due to the console being so far back.  But always keep in mind that if you approach the waypoint from the opposite direction, your 5-6' error, will now be a 10-12' error – rendering the waypoint useless once again.

If you do not wish to make this investment then I suggest you drop your trolling motor in the water that is connected to your bow sonar / GPS (built-in receiver), and network GPS and Sonar data from the front unit to the console unit.  This will improve your accuracy dramatically.

[coldfront]

3. Distance from transducer –
I prefer to use an external receiver that is positioned as close as possible to the transducer when taking waypoints from the console / helm.  Many users feel that bass boats are close enough due to the console being so far back. But always keep in mind that if you approach the waypoint from the opposite direction, your 5-6' error, will now be a 10-12' error – rendering the waypoint useless once again.

If you do not wish to make this investment then I suggest you drop your trolling motor in the water that is connected to your bow sonar / GPS (built-in receiver), and network GPS and Sonar data from the front unit to the console unit.  This will improve your accuracy dramatically.

Thanks Fishton...you helped me clear up some more stuff...

the technology is frankly amazing...what we as anglers have access to anymore...and as I think about it, about how accuracy is awful important to me...I also start seeing where 'what I do' has in many ways, as much/more to do with the accuracy as the actual technology...

For me, it's primarily about figuring out the capabilities of the products I HAVE and working to maximize their usefulness to me...

Plug-n-play is nice...but there are some pretty important things going on that every angler can benefit from knowing/understanding...

what I know is this:  I'm just starting to scratch the surface...

[DougV]

This is why I mount dual receivers and dual SI Transducer. One on the trolling motor and one on the transom. I mount the GPS receivers close to the transducer to reduce adding any additional offset. So when you mark a waypoint with the bow unit it uses the bow mounted receiver and transducer readings. When you mark a waypoint at the console it is using the transom mounted receiver and transducer.

My Bow Mounted Set-up:



Here's the Transom with SI on Jackplate, GPS Receiver and XMNavWeather Receiver

[coldfront]

This is why I mount dual receivers and dual SI Transducer. One on the trolling motor and one on the transom. I mount the GPS receivers close to the transducer to reduce adding any additional offset. So when you mark a waypoint with the bow unit it uses the bow mounted receiver and transducer readings. When you mark a waypoint at the console it is using the transom mounted receiver and transducer.

My Bow Mounted Set-up:



Here's the Transom with SI on Jackplate, GPS Receiver and XMNavWeather Receiver

way I do it as well...with exception of using internal antenna on the bowmount...  and of course, a different manufacturer...


the key is how to maximize the accuracy of the data the equipment generates...if we really want accuracy to the degree that surveyors use...we'd have to look at Trimble-type antennaes...and spend a whole lot more $$$

[screwballl]

GPS is GPS, regardless if it is a small handleld unit, a car GPS or a boat GPS. What makes the difference is the extra Error correction abilities such as WAAS. Also the number of satellites it reads can affect the accuracy as well. Thick cloud cover, fishing behind trees and such can reduce the number of satellites it can read accurately. My handheld shows a screen with the satellites and their numbers, as well as signal strength. Usually in the woods it reads 4 or 5 strong, and 2-5 more weak. Out on the water when I had my canoe, out in the middle of the small lake, it usually had at least 6-8 strong signals from satellites plus 1-4 more weak. For accuracy, it is good to have at least 4 good signals.

With WAAS error correction signal available, the signal is accurate within 10 feet (3 meters).
Without WAAS, the signal is less accurate, within 35-45 feet (approx 10 meters).

You will need to look up the specs of your specific models to see if it has WAAS or not.

Refresh rate is just how often it reads the satellite signals and refreshes them on your screen. It has little to do with accuracy except for the chance of picking up an extra satellite or two as you travel around.

WAAS reference stations for the USA.  WAAS coverage is approximately 200nm (230 miles) around these stations.


Also I found out that there are now 2 WAAS specific satellites above North America (came online fall 2010) which should help any WAAS enabled GPS unit to get a more accurate read if the land WAAS stations are out of range.

[DougV]

Global Positioning Systems are not all created equal and in today's modern world. GPS was founded to help Armed Services locate position and use for protection and guidance. Well today GPS is being used more and more by a wide variety of industries. Look at farming GPS Position can guide or steer machinery with Centimeter accuracy but these systems can cost approximately $50,000 and require Base Station Correction signal or Annual Subscription Fees to get this level of Accuracy.

Humminbird with the GR-50 Receiver we will see offers outstanding accuracy for standard Equipment Receiver on all Internal and External Receiver in the 700, 800, 900 and 1100 Series Units. A 50 Channel Receiver can maintain a lock on 50 different satellites at a single time which allows for faster acquisition of location and maintaining the lock on the satellites as they begin to leave the horizon and pick up new satellites as they start rising in the horizon. The more satellites the higher the accuracy level to calculate your exact location time after time, year after year. The GR-50 when hooked direct into the unit will provide 4 Times per Second Refresh Rate of your location which allows for faster direction changes and tracking your exact location.

Here is a Screen Capture of the GR-50 the other day tracking 17 Satellites at one time:



Here is the Current Satellites Planned and Orbiting From the 3 Northern Hemisphere Constellations.

US Constellation: 32 Satellites with 28 Useable
GLONASS (USSR): 24 Satellites Planned with 20 Useable
Galileo (European Union): 30 Satellites Planned and 2 Useable

This data was the most recent I have been supplied, may be more satellites available.

On the GPS Diagnostics Screen one thing I like to look is HDOP Calculation. This is the Horizontal Dilution of Precision which is the measurement of the contribution of satellite geometry to the uncertainty in a position fix. The lower the number the better. Here's a refence chart to valves and relevance of the GPS Data:

   1: Ideal - This is the highest possible confidence level to be used for applications demanding the highest possible precision at all times.

1-2: Excellent - At this confidence level, positional measurements are considered accurate enough to meet all but the most sensitive applications.

2-5: Good - Represents a level that marks the minimum appropriate for making business decisions. Positional measurements could be used to make reliable in-route navigation suggestions to the user.

5-10: Moderate - Positional measurements could be used for calculations, but the fix quality could still be improved. A more open view of the sky is recommended.

10-20: Fair - Represents a low confidence level. Positional measurements should be discarded or used only to indicate a very rough estimate of the current location.

  >20: Poor - At this level, measurements are inaccurate by as much as 300 meters with a 6 meter accurate device (50 DOP × 6 meters) and should be discarded.


How to Find the Accuracy of your Humminbird
The GPS Diagnostic View can tell you alot about the performance of your GPS Receiver and the Reception you are receiving. To get to the GPS Diagnostic View you can access it through the Start-up Menu or the easiest way is to go into the Master Menu (Hit Menu Twice) then scroll with 4 Way cursor over to View Tab. Hit Up Arrow on the cursor and towards the bottom you will see the GPS Diagnostic View and status will be Hidden. Turn it on to Visible. (See Pic Below)



Now page through the VIEWS to get to the GPS Diagnostic VIEW. Hitting VIEW will page forward. EXIT will page backwards through the VIEWS. It will look like the Image Below.



So what does all this mean and how do I use it? We will go through each letter and explain what it is and what it does.

A: The "Bullseye" tracks the satellites and tells you signal strength of each one. Depending on the Channel of GPS Receiver you have you may see more or less. GR-4 tracks up to 4 Satellites, GR-16 up to 16 Satellites and the new Standard Equipment on many models the GR-50 can track up to 50 Satellites. The Bar Graph next to each number tells you signal strength. THe darker the number the stronger the reception. This correlates to the distance the satellite is from the center or your location. The more satellites near the center or represented in black the better the postion or accuracy to mark your location.

B: Is the type of fix you are getting. No Fix, 2D Fix, 3D Fix or Enhanced. Enhanced is the highest level. Enhance uses correction data from WAAS, EGNOS or MSAS to help increase position accuracy. A 3D or Enhanced Fix is need for Navigation.

C: HDOP or Horizontal Dilution of Precision. Is a value for the current position of the satellites used to calculate accuracy. The lower the number the better the accuracy. Humminbird usually shots for 0.6 to 3 for best accuracy levels. Here is some good reading on Wilkipedia: http://en.wikipedia.org/wiki/D...PS%29

D: Estimated Position Error is the estimated distance the current position is off from being exact. Humminbirds average is less than 2.5 Meters or 8.2 Feet.

E: Altitude is the calculated height of the GPS Receiver above sea level.

F: Speed: Is the GPS Calculated speed

G: Course: Is the direction the boat is heading. 0/360 Degrees is straight North, 90 Degrees is East, 180 is South and 270 is west.

H: Left Card Reader Slot. In this example I have a HyCapcity SD Card with 3.63 GB of 3.8 GB left for storage.

I: Right Card Reader Slot: In this example I have a Navionics Premium East Card installed and the unit recognizes it.

J: Current GPS Position

K: Time and Date from the GPS Receiver

L: Type of GPS Receiver the Unit is Using

M: Selected Readouts. These are the user defined readouts that are displayed in all VIEWS. I have Depth, Position, Speed. Course and Volts selected.

FYI: GPS Satellites transmit the time and date and the receiver uses the time and date information to calculate the distance from the satellite. This is how GPS position can be determined by receiving multiple Time and Date Transmissions and calculating the position on earth compared to satellite location and time and date.

Hope this helps you understand the relationship and performance differences between the different GPS Systems available for recreational use Plus how to know the accuracy of saving waypoints and returning precisely back to them.

---

I just got off the phone with some GPS Specialist that I know. One being the Resident GPS Expert at Humminbird to get the answers which you requested and end without a shadow of any doubt why I can track 17 Satellites with my Humminbird GR-50 Receiver.

All 3 Experts I talked with explained the US GPS Constellation is not symmetrical or spaced equally. Some satellites are in clusters or small groups, as you can see in the image below but still all help in locating position. They also stated that the advertised constellation is 24 satellites but right now there are from 31-32 active satellites in orbit that can be used to calculate your position. So Yes you can track over 12 satellites at certain periods of time in the current consumer US Constellation.

The reason for clusters or groups of satellites is because older satellites are being upgraded with new satellites and both the old and new satellites are both functioning at present time with accurate data to help in locating your position. They also stated that there are actually 3 WAAS Satellites orbiting right now. 2 (like advertised) are maintaining the intended track and a 3rd is in and out of service but can be displayed and provide position location as it becomes in service.

With consumer electronics the maximum allowable displayed trackable satellites are 18 but more can be tracked that are coming above the horizon or falling off the horizon but not displayed. If you look at the image below the light gray satellites are being tracked but the data is not being used at the present time to help establish position because they are outside the useable threshold Humminbird uses to help locate position. Other words until they reach a threshold that will help improve the accuracy that data is not figured in to calculate location but still being represented and tracked. When they reach that level they can add benefit to locating position then they will be represented in black.

The benefit to being able to track additional satellites is to increases the response to acquire position and it allows the GPS to maintain a higher level of accuracy as it can start tracking a satellite earlier as it rises above the horizon and hold on to it longer before it leaves the horizon. Having open channels allows for improved GPS performance.

I also was informed the Current Galileo Project by the European Union is not being used and only 2 Satellites are currently orbiting earth and the accurate track of these satellites are not valuable yet to improve GPS in locating your position. Glonass Project maintained by Russia is orbiting and can become available for civilian use in the near future but as of date none of the Glonass Constellation is being used in position location for Humminbird.

The Standard GPS Receiver from Humminbird is not only providing more as standard equipment but is prepared for expansion when new opportunities arise as more and more Position Satellites are added to the current Constellations and the use of other Constellations become developed.

Hope this helps you learn more about how your Humminbird GPS Performs.