Topic: External GPS or not

[hookbender]

Just got another 798ci SI HD and the complete set up will be a 798ci SI HD bow/trolling motor, a 798ci SI HD in the console, a AS-GR50 with an Interlink to hook them all together. Since both units have internal GPS should I still leave the AS-GR50 on the Interlink and allow the Auto select feature to choose on both units?
Thanks,
Jed

[sonar2000]

You will get better satellite reception with the GR-50 than the internal... Chuck

[Humminbird_Greg]

That may be true but will be based more on the actual location of the units and GR50 as well as the noise floor level on your boat.  I would use the GR50 connected to the console mounted 798ci HD Si unit (mounted at the transom as close to the transducer as you can get it while still allowing for unblocked signals from the GPS satellites) and allow the bow mounted 798ci HD Si unit to use its own internal GPS Receiver.  In this configuration the GPS Receiver that the unit is using will be close to the transducer that it is using, thus allowing for more accurate marking of waypoints and more accuracy when you try and come back to them later on.

By default, the GPS auto select feature would pick the external GPS Receiver (GR50).

[hookbender]

What is "noise floor level on your boat" please?

[Humminbird_Greg]

The amount of background electrical noise in your boat's electrical system.  Like most receivers; the incoming signal has to be higher than this or the receiver basically ignores the signal as more noise.  For GPS Receivers that means that it has to be able to hear the incoming signals from the GPS satellites over the electrical noise present on your boat.  A larger ground plane, like that found in an external GPS Receiver, is better at lowering the background noise level.

[Brunswick]

I'm going to somewhat agree with Greg on this subject, and somewhat disagree. (This is one of those things that's kinda in my wheelhouse.)

When you are looking at an external receiver, as Greg said if you are able to keep that as near to  your transducer- I can understand that. And that reading will be good for +/- 15 min. Given basic WAAS signal, and baring in mind a decent Kahlman Filter, and baring in mind a good GPS azimuth range, and baring in mind a decent smoothing algorithm beyond the Kahlman Filter.  Our signal is only good for +/- a meter over 15 minutes. Over the term of a year it can move 2" or it could move 200 Feet, you don't know. So when you are looking at slant range, or horizontal distances from your reciever- if its in America that's what you're dealing with. If you go overseas- Other than Europe- it's probably worse.

A fella would also have to take into account his initial reading- the time of day, the amount of multi-path the system is catching which may or may not be included in your HDOP (Horizontal Dillution of Precision).  Then you probably aren't going to be 300feet in the air with perfect view of the sky, youre going to be stuck up against a bluff fishing a snag were you can see the sky if you look straight up and and over your back to about the 2-3 oclock position- allowing you visibility of about 6-7 satellites (Differential Satellites). So when you throw that waypoint down at 2pm on Sunday and come back the following Saturday at 8 am... that spot will only have moved a mere 15 feet or so (maybe).

The thing I would be more suggestive of is the internal receiver- purely from the standpoint of software updates. We may not have sub-inch accuracy for free yet. But the government is finally letting loose some bands where we could be using the same satellites they were using in the gulf war to fly missiles through windows. I've been with some testing crews on this and the best way I could tell a Layman is that right now we get two signals (L1, L2).... and we can stay within 3 feet over 15 min (governmentally assured ). We may one day have the ability to enhance that to 5 bands if the military allows. External receivers will be harder to update because of this- internal processing on receivers have better ability to acclimate change through simple software updates to the receiver hidden in your general display software update. Usually if a External receiver is processing it will be through routine serial connection- forcing extremely slow updates and reboot times with hardware that may or may not allow for new helix signals.

just my 2 cents

[newkid4si]

Brunswick

    That is a scary good explanation of what's going on in the background. Most users, myself included, operate on the "it works or it doesn't work" mentality.
   
 

           Mike

[Humminbird_Greg]

I hear ya Mike!

Good points there Brunswick but I was trying to keep it a bit more simplistic.

[Brunswick]

I'm sorry for be long winded, but it does affect alot. And When you add iPilot link to the setup and you read the scenarios of spotlock jumping around and the contours aren't driving true. Well, the problem probably isn't with the fundamentals of 'Bird's software, it's just the nature of the beast in most scenarios. There's only so much smoothing you can do.

And no ones able to do it better yet... not on the water- not even Garmin- the navigation experts.