GPS Boffins please

[martinwoolwich]

Difference between;

DGPS & SDGPS? which is better/more accurate

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[DavidP]

This is straight from the manual of the Chartplotter i bought back in April/May this year
This handbook describesthe Raychart 425 Chartplotter and its associated Raystar 120 GPS Receiver. The Rastar120 utilizes Satelite Differential (SD) signals for enhanced navigational accuracy over conventional shore based differential GPS systems (dGPS)

The Raystar 120GPS Reciever utilizes a new satelite differential correction system to improve the accuracy and integrity of the basic GPS signals. Three seperate compatiable systems currently exist or are being developed.
1,,,, Wide Area Augmentation System (WAAS) USA
2,,,, European geostationary Navigation Overlay System (EGNOS) European
3,,,, The MTSAT Satalite-Based Augmentation System (MSAS) Japan

The Raychart 425 Chartplotter is EGONS compatiable. However, at the time of going to print the EGNOS was still under test.
This manual was printed in March 2002,

For more info on WASS and EGNOS links can be found at www.raymarine.com
Hope this is of some help,

Dave



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[martinwoolwich]

Thanks Dave. Problem is that I've been told there's a difference between dGPS and DGPS. Also I've searched the web and almost all of the sites talk about Raymarine equipment and to find out more go to raymarine.com. I've done this and can't find any specific information about the systems. Just product blurb that says it's comaptible with WAAS or EGNOS or/and SDGPS etc etc. No real help on understanding the differences. Thank you for taking the time to reply though

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[ccscott49]

One thing, DGPS is only operational, within range of an (MF) shore station and comes into the GPS via a seperate DGPS receiver, the EGNOS or WAAS is worldwide and I believe just needs enabling in your SDGPS set. the sattelite system is to replace the shore based differential, which was really only a stopgap system, whilst selective availability was in force, it will probably dissapear once SDGPS (WAAS, EGNOS) is fully operational.

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[hlb]

But has that not all been done away with now that the Mericans have switched off the variation thingy. Any sort of GPS is now accurate to a few feet. Well it is on my boat anyway.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[ccscott49]

Selective availability has been done away with by the mercans, but could be truned back on, (not that I can think of a reason to do so), but DGPS is still more accurate, but not that we would notice, much. I'm not sure of the reason/validity of this new system, but I'm sure somebody will tell this village idiot.

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[hlb]

Dont they use it for drilling holes and building things like tunnels or maybe bridges. So they meet up at the other end and the bolts go in exactly. Anyway my boat parks itself right alongside the pontoon on the chart. So thats good enough for me.

<hr width=100% size=1> <font color=blue>No one can force me to come here.<font color=red> I'm a volunteer!!.<font color=blue>

Haydn

 

[ccscott49]

A couple of meters is good enough for rig positioning, they seem to get it fairly right. Most of the charts were drawn before GPS anyway, so a lot of the old chart surveys, with islands in the wrong (GPS) place, doesn't bother me too much, I only use GPS to get me roughly where I want to be, then it's down to pilotage anyway, with Mk1 eyeball/radar etc.

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[h4nym]

Tunnel boring machines... GPS guided... what happens if it decides it's off course? Can't exactly swing the stern a metre or so to port can it?

H



<hr width=100% size=1>Life balance?

 

[ccscott49]

You're right, it's a laser that guides the tunnel borer, which is set with GPS, but the tunnel borer can deviate, up/down and left/right not a big dogleg, but it can steer.

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[duncan]

My unit suggests an accuracy of 30ft 'ish when the differential unit is disconnected and around 7ft when it's on. Leaving the unit on when attached to the pontoon produces plots that support those figures on both machines.
Either way it's good enough for navigation, and with the differential on 'you are where it says you are'!
oh - and I have the old style land based differential.

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[jimg]

Absolutely right, any form of GPS id goods enough and more accurate than your charts. Although they have vbeen reissued to WGS84, they are still dependant on survey data that could have collected 50 years ago! Although you can navigate by GPS to within a few feet, you are fooling yourself if you think you are that sure of where you are relative to an obstacle close to you. It could be out by far more than the acuuracy of your GPS.

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