How close do you expect your GPS to get to a point

[finga64]

Gidday all,
Just a couple of simple questions.
How close do you expect your GPS to get you to a point of interest??
How many satellites does your GPS use (as listed in specs.) and how close do you get to fixed points that you know don't move (like your driveway)??
And finally, would you expect more accuracy from your GPS if it tracked more satellites?? ie. should a GPS tracking 18 satellites be more accurate then one tracking 12 satellites and so on.
Cheers Scott

[MulletMan]

1. Plus or minus ten metres is exceptional but round the fifty metres is also OK

2. I get from five to ten satellites normally. There are about 25 useable sats up there for
recreational/commercial use but at any one time you can have a huge variance in what is "useable".
In excess of forty five degrees either side of directly overhead, the angle of intercept is somewhat
too low for accurate calculations of position. On the horizon is useless, best fix is overhead and in
between variable.

3. A GPS requires a minimum of TWO satellites to operate (2D mode) and THREE (3D) mode for high
accuracy. Anything in excess of this makes little or no difference as the three angles of intercept
from the satellites form what is know as a "cocked hat" in which you should be the middle.

As an example if you had three sats all close to being overhead, their fix would be far better than
twenty sats that are popping up or dropping below the horizon or around the forty five degree mark.

4. Worth remembering that the antenna is the whole guts of a GPS. You gotta believe that an inbuilt
antenna on a small hand held GPS worth $150.00 is not going to be as good as the stuff the
surveyors use or a $10,000 commercial Furuno!

The older GPs's (like mine :'( :'( ) have an accuracy scale on them that varfies from 1 to 100.
Watching it makes you seasick as it runs up and down the scale whenever you do a turn or simply
watch the effects that the moving sats have on the calculations.

5. The manufacturers know what is best and I think any modern day set will throw up a warning
when it has insufficient quality or useable sats to use. Invariably tells you that accurate navigation
is no longer possible.

[wayne_cook]

no point adding to that.
majority of the time mine are very close well within 20 meters

Scott on your post "another crook weekend"page3 @ the moment I made some comments I would be interested in your opinion.
Thanks bear

[Fishin_Dan]

I'd be a little upset if my GPS was anything more than 10m! My little GPS72 averages about 2-3m accuracy, but I have seen it at 6m once... And that's for a little handheld... For a much more expensive unit with a proper aerial, I would expect better to be honest!!!

[fishenmick]

Hello All,

Very close ThePinkPanther,

Three is good, but four is the number of satellites required to have an accurate position.

It comes down to Triangulation. Three satellites give the position and the fourth removes the errors (put simply)

The Position is calculated from distance measurements (ranges) to satellites.

Mathematically we need four satellite ranges to determine the exact position.

Three ranges are enough if we reject ridiculous answers or use other tricks.

If measuring the travel time of a radio signal is the key to GPS, then our stop watches had better be darn good, because if their timing is off by just a thousandth of a second, at the speed of light, that translates into almost 200 miles of error!

On the satellite side, timing is almost perfect because they have incredibly precise atomic clocks on board.

But what about our receivers here on the ground?

Both the satellite and the receiver need to be able to precisely synchronize their pseudo-random codes to make the system work.

If our receivers needed atomic clocks (which cost upwards of $50K to $100K) GPS would be a lame duck technology. Nobody could afford it.

Luckily the designers of GPS came up with a brilliant little trick that lets us get by with much less accurate clocks in our receivers. This trick is one of the key elements of GPS and as an added side benefit it means that every GPS receiver is essentially an atomic-accuracy clock.

The secret to perfect timing is to make an extra satellite measurement.

So if three perfect measurements can locate a point in 3-dimensional space, then four imperfect measurements can do the same thing.

If our receiver's clocks were perfect, then all our satellite ranges would intersect at a single point (which is our position). But with imperfect clocks, a fourth measurement, done as a cross-check, will NOT intersect with the first three.

So the receiver's computer says "Uh-oh! there is a discrepancy in my measurements. I must not be perfectly synced with universal time."

Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point.

That correction brings the receiver's clock back into sync with universal time, and bingo! - you've got atomic accuracy time right in the palm of your hand.

Once it has that correction it applies to all the rest of its measurements and now we've got precise positioning.

One consequence of this principle is that any decent GPS receiver will need to have at least four channels so that it can make the four measurements simultaneously.

I am no expert by any means on this, I am a military GPS instructor though so the data above I know is correct.

10-20 meters is the accuracy you can expect from a hand held reciever that can "see" four satellites. When selective availability was on prior to 2000, you needed DGPS to achieve these measurements. ANy better than 10-20 metres can still be achieved by DGPS. Those users include military, surveyors and other professionals.

Hope this helps,

Regards,

Mick

[CHRIS_aka_GWH]

my achieves 10m every time I reenter Horizon shores - if you were out on the trail you'd be on the rocks. Even though I sight it in visually I always watch the trail out of interest - for fine tracking like that I overzoom the navionics as far as i can

chris

[finga64]

Thanks all.
You probably wonder why I'm asking.
As Bear mentioned I haven't had a good run with electronics.
The GPS is about 15 months old and hasn't fired a successfull shot in combat yet.
It has ability to track 18 satellites and accuracy is quoted at about 10m.
The real accuracy is 100m :( and once you approach the point the gps locks up and you have to enter another point for anything to move on the screen. So drifting around a point or trying to find a channel marker at night is out.
The distributors have replaced the unit once and have had it since X-mas to sort out. They now say that 100m is as close as it gets and all their unit are the same til you spend over $3,500.
Their theory is if you accuracy you have to spend bucks. (these units cost me $399
My theory is my old $150 Garmin 45XL got me closer to points (Before I brought I asked if the new unit will do everything the Garmin did as well and the answer was yes....reality....NO!!
I'm not naming the people or brand concerned because I'm seeing what they want to do about it. I'm not happy because the boat is a long way away and I only use it occassionaly and to date the GPS has been pulled out 3 times and the last trip I had to install another old one I had lying around. I have also wasted a lot of fuel/time idling around the ocean looking for the reefs we fish off (they're all only very tiny..about 10-20m across).
Below are the quoted specs. What performance would you expect with these specs??

Specifications:
Display: 4.5-inch high contrast monochrome LCD
GPS Receiver: 18-channel parallel, 1575.42 MHz, C/A code, WAAS-ready (GP-200), DGPSready (GP-100)
Data Updating Rate: 1 second
Position Accuracy (2D RMS, 95%): approx. 10m (without correction), 5m (DGPS), 3m (WAAS)
Display Pages: NAV. data page, track plotter page, CDI page, highway page, speedometer page, satellite information
page and user-definable page
Track History Storage: 1000 fix points
Discrete Waypoints: 2000
Routes: 20, each with 300 waypoints
Alarms: arrival, anchorage, crosstrack error, DGPS, time, trip, odometer, battery
Power Requirements: 11 to 40 VDC, floating ground, approx. 3W
Interface: NMEA-0183 and RTCM-104
Ambient Temperature: -15 to 50°C (cabinet), -25 to 70°C (antenna)
Max. Dimensions: approx. 215/175 (W).80 (D).130/108 (H) with/without mounting bracket
Weight: approx. 650g with mounting bracket attached

Opinions will be really good to show these guys selling the units.

[wayne_cook]

Scott price shouldn't matter
100m is not acceptable.No one would purchase one if this was the case.
I use a narrow channel to navagate @ times and the track seldom varies much more than a boat width.
To say 100m is as good as it gets must piss u off I take a position mark it for anchoring purposes and adjust rope to the merter a slight wind change alters ur track and reanchor This would be impossible with there claims.
If it carn't be fixed get ur money back.Thers no need to spend the money there claiming.



Cheers bear

[icey]

i expect mine to be within 3 feet I'm running a lowance i finder pro..
it's got be right on the nuts if I'm out 10 miles from no where it's gotta be right 100% of the time..

I'm running 12 satellites all the time..

[Steve_Monckton]

G'day Fishenmick, Is it true what I hear about GPS readings fluctuating during times of global conflict??

[Heath]

We get our bait off a tiny reef approx 6m x 3m & my GPS puts me smack bang on it every time.

[SatNav]

1. Tracking 18 satellites more than tracking 12 is a mute point as a GPS will never see 18 GPS satellites anyway.
2. If you could track 18 satellites then there would be no apparent improvement in accuracy than tracking 12 or 6
3. Tracking more satellites (above minimum required) does not necessarily give you any better accuracy
4. Accuracy will be LESS THAN 10 metres most of the time but do not expect it be less than 10 metres all of the time
5. If a satellite is in view then it is useable
6. A GPS requires a minimum of THREE satellites for a 2D fix
7. A GPS requires FOUR satellites for a 3D fix
8. 3 sats directly above would give an extremely poor position
8. Accuracy is a primary result of the location of each satellite NOT the total number
9. With 4 satellites the best arrangement is 1 directly above and the other 3 equal spaced and 10 degrees above the horizon
10. Accuracy that the GPS displays is not accuracy and means very little
11. There is little difference in comparable GPS accuracy between a $150 handheld and a $30,000 survey unit
12. No handheld GPS will provide 2-3 metre accuracy, simply not possible
13. Those expecting 3 feet are disillusioned and 100% of the time even more disillusioned
14. GPS readings have never varied during times of any conflict to-date.
15. No GPS receiver will see 12 satellites ALL the time

One question? How do you know the real accuracy is 100 metres?

[fishenmick]

G'day Fishenmick, Is it true what I hear about GPS readings fluctuating during times of global conflict??

Hello Rabbi,

Prior to 2000, the USA had an error (deliberate) in the timing of their GPS sattelites, this was called "Selective Availability". This was to deter threat forces from utilising GPS sattelites for their own gains, ie positioning, Indirect Fire Attacks etc. The only way to bypass the error was to have very expensive relay stations (for professionals, surveyors etc) or input codes each month into your military GPS (that is why military GPS cost so much).
In 2000 (from memory) the USA turned off "Selective Availability". This was such a monumental decision it was announced across the globe on international TV by none other than the President of the USA.

The option to turn back on "Selective Availability" is still there, although it is not envisaged that it will be. The error given was usually in the 200m mark, so 200m either side of your mark.

SatNav,

Good Post. Not sure where Ser 14. sits with the above info though.

Regards,

Mick 8-)

[finga64]

Real accuracy was measured by GPS mounted in sales rep.'s car and driving along the road and marking a fixed point such as a tree on the side of the road, then driving away and try to get back to the tree again. Tree's don't move too fast to get an inaccuratre reading of true accuracy ;)
This particular sales rep. (who has been a tower of helpfullness to me in trying to fix the problem) stated to me that's not good.

Don't think anybody is wanting a handheld or permanently mounted GPS to get within 3m of a point but would expect a lot closer then 100m (according to the survey at the top of the page). My old handheld was usually within 15m (on the outside) to my gatepost in my driveway and the shed at the holiday house.

Why do they make them 18 sat. trackers if they're no more accurate? Maybe the money would have been spent better developing the software to achieve better accuracy from 100m to the stated accuracy of approx. 10m or to make them more user friendly with this particular brand of GPS. Maybe there's better chance of getting the better sats. (as stated by satnav) to give you a more accurate reading. I don't know, I'm only an old broken down dumb sparky :'(

With this particular GPS the advertising blurb states "Equipped with new 18-channel GPS receiver for extremely short first-time-to-fix and enhanced accuracy for
position/heading/speed readouts."

I'd doubt if anybody would be happy if the STATED accuracy (in my case approx. 10m) actually was 100m with any GPS they purchased.

Insult in my case is also added be trying to get the thing fixed for over 12 months. The GPS has been in and out of my boat on 3 occassions. Lucky I'm a sparky and didn't have to pay somebody to do this for me. Insult in my case was also given by another sales rep stating that accuracy in this case is 100m because the purchase price of $399 warrants the inaccuracy no matter what the STATED accuracy is.
I'm also peeved because this GPS was fitted to a boat located a long way away from my home. Fishing trips with this boat are always pushing for time as we only ever have limited time away. We didn't need the hassles of idling around the ocean looking for our little reefs or the hassles of vertually fully installing and then uninstalling a GPS unit. Only the holes (yep, more holes to try to cover up in the boat :'() and the power cable remained in the boat. The unit and aerial were removed on each occassion.
At 100m accuracy we would have been better heading due north til the three power poles going up the hill line up and then head due east til the big tree lines up with the water tower on the ridge. :-?

[SatNav]

1. GPS signals don't like trees, number 1 rule, stay away from trees
2. That is not real GPS accuracy
3. 18 channel receivers at this point in time are nothing more than a marketing gimmick
4. More than 12 channels are designed for countries that have other systems that assist GPS accuracy
5. More than 12 channel receivers are futuristic designs, the future hasn't arrived as yet
6. If the GPS does not meet the advertised specifications then get your money back
7. To do this you need to measure (record) the accuracy properly
8. 100 metres is not acceptable and if this GPS can do no better than 100 metres (measured properly) then get your money back

[finding_time]

I can almost tell you to the minute when selective availability was turned off... ;) It sure made finding ,fishing and diving those small wrecks i like soooooo much a whole lot easier!!

Another thing to note with your marks and accuracy is that most sounders have a 15 degree signal so it doesn't take much water depth to put a show on the sounder even when you more than 10m from your small mark. Hope thaat made sense.

Ian

[SatNav]

Quote from Mick: Not sure where Ser 14. sits with the above info though.

Could you expand on why your not sure where it sits? Selective Availability has never been turned back on since it was toned down.

"Selective Availability" was always in the "less than 100 metres" range

[SatNav]

Quote by Finding-time: I can almost tell you to the minute when selective availability was turned off...


That minute was?

[Owen]

From what little I understand, the best way to check the accuracy of a GPS is to stand on a known point at several different times and record the position.
Locally, our VMR base has such a point marked at the boat ramp.
If you go back at several differnt times and record the position given from your GPS, you will get an idea as to the accuracy.
I haven't done it yet, but at the weekend I made a point of watching the postion of channel markers and the quoted position of my boat. My GPS displays a circle around the quoted position which is said to be related to the accuracy. At the time, the GPS was saying 5m accuracy and the bluechart seemed to agree.
That's pretty good to me, but bear in mind that if (as in my case) these channel markers are in narrow channels and unlit, I wouldn't like to bet my left one that I could navigate them in the dark without head butting one of them ;D

cheers,
Owen

[MulletMan]

FishinMick, I stands before ya, a beaten and shamed man!
I am nothing but trash in your presence, I am unworthy of this topic.......
For I have sinned and got my threes ands fours mixed up on the number of satellites needed for a fix!

But, ahah, all is not lost as I struggles to redeem myself on a point of order!

The "error" the yanks used to inject was to fiddle with the atomic clock in the satellites.
I heard they could either bend the radio beam a milli-whiska of a second or mess about with the hourly update of it's Almanac.

Don't the defence guys operate on a different carrier wave or something that we civvies can't access or don't have the equipment for?

That is my sad and futile contribution to redeem myself to the readers! [smiley=hammer.gif] [smiley=hammer.gif]

I hangs me head in shame as I slinks off to the bush.................... :'( :'( :'( [smiley=dunce.gif] [smiley=dunce.gif] [smiley=stupid.gif] [smiley=thumbsdown.gif] [smiley=oops.gif]

[finding_time]

Quote by Finding-time: I can almost tell you to the minute when selective availability was turned off...


That minute was?





I remember the announcement and from memory the next day we went out for a few dives and as usual we had a bit of trouble findng this particular wreck but once we hit it and put in a new mark( a weekly event) and deleted the old ones we have never had to search for this thig again , it always there. Bloody brilliant :D

[fishingnottake]

Quote by Finding-time: I can almost tell you to the minute when selective availability was turned off...


That minute was?

11:15.01

[wayne_cook]

;D ;D ;D
whats next

[fishenmick]

Quote from Mick: Not sure where Ser 14. sits with the above info though.

Could you expand on why your not sure where it sits? Selective Availability has never been turned back on since it was toned down.

"Selective Availability" was always in the "less than 100 metres" range



Hi SatNav,

No worries mate, you are correct, since "Selective Availability" was turned off it has not been used in another conflict. Prior to that date, it was used in all of them since its introduction.

Regards,

Mick

[fishenmick]

ThePinkPanther,

Mate, your the goods. Always happy to read your posts.

The defence just input code into their recievers to read and correct the errors.

Just so readers know, we use Rockwell PLGRS 11 recievers which are the expensive defence model and the majority of other systems in use are Garmin 12XL.

The Rockwell are very versatile and do a magnitude of operations including data burst via radio broadcast, but for simplicity, accuracy and general robustness, it is hard to beat the Garmin.

Regards,

Mick

[SatNav]

Quote by Mick: since "Selective Availability" was turned off it has not been used in another conflict. Prior to that date, it was used in all of them since its introduction.

Mick, What conflicts was "Selective Availability" used in?

[SatNav]

Quote by Ian: I can almost tell you to the minute when selective availability was turned off...

Ian, That "minute" is something that is not mentioned in too many places, which is why I ask. Mick should be able to tell you the exact time Selective Availability was discontinued?

[subzero]

THE WHITE HOUSE


Office of the Press Secretary

For Immediate Release May 1, 2000

STATEMENT BY THE PRESIDENT REGARDING
THE UNITED STATES’ DECISION TO STOP DEGRADING
GLOBAL POSITIONING SYSTEM ACCURACY


Today, I am pleased to announce that the United States will stop the intentional degradation of the Global Positioning System (GPS) signals available to the public beginning at midnight tonight. We call this degradation feature Selective Availability (SA). This will mean that civilian users of GPS will be able to pinpoint locations up to ten times more accurately than they do now. GPS is a dual-use, satellite-based system that provides accurate location and timing data to users worldwide. My March 1996 Presidential Decision Directive included in the goals for GPS to: “encourage acceptance and integration of GPS into peaceful civil, commercial and scientific applications worldwide; and to encourage private sector investment in and use of U.S. GPS technologies and services.” To meet these goals, I committed the U.S. to discontinuing the use of SA by 2006 with an annual assessment of its continued use beginning this year.

The decision to discontinue SA is the latest measure in an on-going effort to make GPS more responsive to civil and commercial users worldwide. Last year, Vice President Gore announced our plans to modernize GPS by adding two new civilian signals to enhance the civil and commercial service. This initiative is on-track and the budget further advances modernization by incorporating some of the new features on up to 18 additional satellites that are already awaiting launch or are in production. We will continue to provide all of these capabilities to worldwide users free of charge.

My decision to discontinue SA was based upon a recommendation by the Secretary of Defense in coordination with the Departments of State, Transportation, Commerce, the Director of Central Intelligence, and other Executive Branch Departments and Agencies. They realized that worldwide transportation safety, scientific, and commercial interests could best be served by discontinuation of SA. Along with our commitment to enhance GPS for peaceful applications, my administration is committed to preserving fully the military utility of GPS. The decision to discontinue SA is coupled with our continuing efforts to upgrade the military utility of our systems that use GPS, and is supported by threat assessments which conclude that setting SA to zero at this time would have minimal impact on national security. Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened. This regional approach to denying navigation services is consistent with the 1996 plan to discontinue the degradation of civil and commercial GPS service globally through the SA technique.

Originally developed by the Department of Defense as a military system, GPS has become a global utility. It benefits users around the world in many different applications, including air, road, marine, and rail navigation, telecommunications, emergency response, oil exploration, mining, and many more. Civilian users will realize a dramatic improvement in GPS accuracy with the discontinuation of SA. For example, emergency teams responding to a cry for help can now determine what side of the highway they must respond to, thereby saving precious minutes. This increase in accuracy will allow new GPS applications to emerge and continue to enhance the lives of people around the world.

[subzero]

Frequently Asked Questions About SA Termination

At what time was SA turned off?
Selective Availability ended a few minutes past midnight EDT after the end of May 1, 2000. The change occurred simultaneously across the entire satellite constellation. (Note: Previously, we incorrectly reported that SA ended at midnight GMT. We regret the error.)

Will SA ever be turned back on?
The United States has no intent to ever use SA again. To ensure that potential adversaries do not use GPS, the military is dedicated to the development and deployment of regional denial capabilities in lieu of global degradation.

Do I need to replace my receiver to get the higher accuracy?
No. Existing GPS receivers around the world should be getting the higher accuracy right now without any modifications.

[subzero]

The attached pdf doc shows the improvement the night Selective availability was turned off. (4 hours either side)

[subzero]

oops, that should have been 5 hours either side

[SatNav]

Now someone has been doing some homework? But one thing to keep in mind is that SA has not been turned off, it is in fact still on just that the affect is zero, SA capability is fitted to every satellite that is (and will be) in orbit including those that are still to be launched.

That minute was infact 5 past the hour. SA started to come down at 14:03:34 AEST 2/5/2000, was 90% complete at 14:04:44 AEST and at 14:05:00 AEST was completely stable.

1. There has never been a conflict (war) where selective Availability was used.
2. The clock in a GPS is nothing more than a clock, tells the time but serves no part in calculating the position.
3. A GPS receiver is essentially NOT an atomic-accuracy "clock".
4. A GPS receiver works on GPS time not current universal time
5. "Military" GPS are not the only units that have the capability to use military systems

[MulletMan]

Yeh, right on SatNav I reckon. Spot on with the atomic clock too, they can fiddle with it any time they want!

I was sitting in (uninvited) on a GPS conference in Singapore many yonks ago and the actual guys who built 'em said the same as you, SA was NEVER used in Desert Storm or any other conflict.

They had much better stuff gear available to the US forces stuffing about with GPS!

And what better way to outfox the bad guys than to spread the word the satellites were no longer spot on!

[Lobster48]

I have used all sorts of GPS units from hand held Garmin through to 'top of the line' gear and to tell you the truth most are about the same. The big issue comes with clear line of sight to the sky, particularly the Horizon and the signal strength at the time.

I can save a waypoint (a wreck of reef) and come straight back to it months later. In Darwin we have a wreck called the Bottlewasher which is no more than 15m X 15 m. I got right on top of it about three years a go with a Magellan hand held and have transferred that onto a NAVMAN Chartplotter and have driven straight onto the wreck. I guess that it all that matters.

You can loo at 18 channels, differential systems with marginal averaging and all the rest of the promotional garbage you want...does it get me to my fishing spot...that is the key.

From my knowledge and experience...they all do when used correctly. It all comes down to user friendliness and what you feel comfortable with to use.

Lobster

[Mr__Bean]

I reckon the best way to satisfy yourself about accuracy is to mark a waypoint at home.

I used to check my handheld by the location of my front letterbox, turns out my letterbox never moved around much at all.

Now that I have a fixed mounted GPS in the boat with an external aerial etc, I marked a spot at the front driveway where I always flush the outboard. That spot never moves much more than a few metres either.

Try it at home, it's easy to do......

- Darren

[SatNav]

1. They could fiddle the system timing anytime they like but they HAVE NOT and they WILL NOT
2. Accuracy problems are caused by system malfunctions, the system is not perfect all the time
3. Acucracy problems are more often the result of the user
4. Being able to return to the letter box only means the letterbox hasn't moved, doesn't mean it's accurate
4. SA was not used in the first Gulf War as they didn't have enough military spec receivers to go around and were using civvie handhelds
5. GPS was not even fully operational during the first gulf war, was barely even partially operational
6. SA was fully implemented by November 1991
7. SA was not used in the second Gulf War, it had been discontinued by this

[finga64]

I don't think many people won't argue about the 'supposed' accuracy (or inaccuracy as the case may be) to a positional point (ie grid reference) but most people would like their GPS to get them back to a 'marked' spot (ie gate post or tiny reef) reasonably accurately. Which according to the poll is within 15m of a marked point.
How many people use their GPS's for navigation???
How many people use their GPS's purely to get them to a marked point??
More food for thought.

[Lobster48]

Finga,

I use my GPS for Navigation over long distance and then to get me onto a point once I am there. Accuracy is important but as per my last post most modern GPS units are accurate enough for me to get onto a spot in the middle of the blue and catch some fish. Especially considering all of the other variables such as drift, wind, ability to accurately anchor etc etc. 15m sounds god enough to me.

[barradise]

I test my GPS at an accurate mark at my local boat ramp before departing, I usually get within 10m, sometimes better.
Altitude above sea level is a different matter, this is all over the place.

[bidkev]

Finga,

I use my GPS for Navigation over long distance and then to get me onto a point once I am there. <snip>



OK guys. This is the spec of mine. I'd be interested to hear any comments regarding accuracy power usage etc. To be honest, I haven't really had the time to familiarise myself with it as I should do.

What features do you guys use when plotting a course in daylight? I never use Highway or Steering but simply point her nose in the rough direction I intend to travel and then steer so as the directional line doesn't cross any obstructions. If there's an obstruction (ie bank) in the path I just steer so that the directional line skirts that obstruction and then correct course when round it. Rather like plotting waypoints but not actually punching 'em in. Seems to work for me but I appreciate that for night navigation I will need me to be more familar with my unit, although I don't see why this method wouldn't work just as well in all but confined channels

I need to get out there on my own so that I can focus on it. Trips are that few and far between that what I learn by reading the manual at home, has gone out the window when I actually hit the water due to lack of practical sessions.

One thing that I have noticed.......and I'll give Brennans and Hutchies Shoal as an example. On the C map it is shown as a triangle with an exclamation mark (correct me if I'm wrong) When I zoom in to this triangle and position my rig within it......zilch..featureless bottom. I actually can find reef outside of that triangle ::) What am I doing wrong or is the map wrong? I have to assume the unit is accurate because it does put me on a few of my own marks but seems out on marks inbuilt into the map?

I'll appreciate any advice or comments that you may make.

kev

SPECIFICATIONS OF GP-1650W/1650WD/1650WF/1650WDF

GPS RECEIVER CHARACTERISTICS
1. Receiver Type Twelve discrete channels, C/A code, all-in-view
integral WAAS processor
2. Receive Frequency L1 (1575.42 MHz)
3. Accuracy GPS: 10 m (95%)
DGPS: 5 m (95%)
WAAS: 3 m (95%)
4. Time to First Fix 12 seconds typical (Warm start)
5. Tracking velocity 999 kt
6. Geodetic System WGS-84, NAD-27, and others
7. DGPS Capability
GP-1650WD/1650WDF: DGPS beacon receiver built in
GP-1650W/1650WF: External DGPS beacon receiver transmitting
data in RTCM SC104 v.2.1 format through
RS-232C interface or optional internal DGPS
beacon receiver
PLOTTER CHARACTERISTICS
1. Display 6 inch color LCD, 320 x 234 pixels
2. Map Scale 0.125 to 2,048 nm
3. Latitude Limits Between 85°N and 85°S
4. Plot Interval 1 s to 99 min 59 s or 0.01 to 9.99 nm
5. Display Modes Course plot, Nav Data, Steering Display,
Highway
6. Presentation Modes TM/RM North-up, Course-up
7. Memory Capacity Up to 5,000 points for ship's track and marks
800 waypoints and 200 planned routes
(Max. 35 waypoints/route)
8. Voyage Planning Waypoint navigation or route navigation
9. Alarms Arrival/anchor watch, XTE, proximity alert,
ship speed, depth*, water temperature*, fish*
*For GP-1650WF/1650WDF—Temperature sensor required for water
10. Nav Data Inputs/Outputs (NMEA 0183 ver. 1.5/2.0)
Outputs:
AAM, APB, BOD, BWC, GGA, GLL, RMA, RMB, RMC, VTG, WPL,
XTE, ZDA, DBT*, DPT*, MTW*, MSK
Inputs:
DBT*, DPT*, MTW*, TLL, YMWPL (YEOMAN wpt data)
*GP-1650WF/1650WDF
11. Electronic Chart FURUNO MiniChart or
NAVIONICS® Nav-Chart[ch63195] and
C-MAPNT Chart

ECHO SOUNDER
1. Display Modes Normal (single- or dual-frequency),
Bottom-lock, Bottom Zoom, Marker Zoom,
A-scope
2. Frequency 50 and 200 kHz (selectable on menu)
3. Output Power 600 W (rms)
4. Basic Ranges 8 basic ranges customized to max 800 m.
(2500 ft, 400 fa)
5. Range Phasing Up to 1600 m (5000 ft, 800 fa)

[SatNav]

1. Chart accuracy / GPS accuracy? Two different issues
2. Charts are manufacturer specific but generally all based on a similar data source
3. Chartplotters are made to suit a specific chart type/s not the other way round
4. GPS is more accurate than the accuracy of the chart
5. An explanation mark in a triangle is a precautionary area, in a shipping/navigation sense
6. Turn WAAS off
7. Check your GPS matches the chart datum

[SatNav]

1. Does appear the majority (51.7% @ 7/6/06) expect too much from their GPS

[lenm]

Gota love this techtalk :)
As SatNav said - chart inaccuracy is often the weakest link in the whole system which many people overlook.

One other thing about GPS - they will never be good for determining 'elevation'. This is due to earths surface not being a uniform sphere and hence makes accurate 'height' determination difficult.
Fixed basestations are a must for sub-metre GPS accuracy.

As long as you GPS unit gets you back/close to your saved marks consistently - who cares how accurate they are when compared/checked against true geodetic surveyed control points.

[Owen]

6. Turn WAAS off


Why is that SatNav?

cheers,
Owen

[SatNav]

1. WAAS does not work in this country
2. Elevation / height have a direct connection to the horizontal accuracy
3. A bad height will also reflect in the horizontal but not to the same extent
4. The main point with accuracy is still being missed
5. Being able to return to a saved point is not what accuracy is

[Owen]

So will turning off WAAS (not sure if I can, but I'll look) improve performance?
Either through increased accuracy or lower CPU usage?

cheers,
Owen

[SatNav]

1. WAAS is a waste of processing power
2. WAAS utilizes several GPS channels that could be used for GPS signals
3. WAAS generally decreases accuracy in this country
4. WASS does not improve accuracy in this country
5. WAAS in this country is a waste of time
6. WAAS must be turned off in this country
7. Many WAAS ready units are now programmed to ingore WAAS even if turned on in this country

[SatNav]

Question was: "How close do you expect your GPS to get to a point"
Question should have been: "How close should you expect your GPS to get to a point'

There is a difference

The other question is what "point"?

[Owen]

Thanks for that SatNav
You seem to know a bit about GPS and I'm glad you don't mind sharing your knowledge.

You might want to consider that the written word cannot convey the intentions of the person posting.
Personally I burr up at a bunch of numbered statements as they come across as condescending. i.e. You can't believe we didn't already know this stuff.

cheers,
Owen

[bidkev]

BUMP!

Bumping 'cause I'm still interested to know how you guys use yours. Also, whether the c-map marks can be found accurately, and if not, is it the maps or the GPs that is out? From the thread earlier:

"What features do you guys use when plotting a course in daylight? I never use Highway or Steering but simply point her nose in the rough direction I intend to travel and then steer so as the directional line doesn't cross any obstructions. If there's an obstruction (ie bank) in the path I just steer so that the directional line skirts that obstruction and then correct course when round it. Rather like plotting waypoints but not actually punching 'em in. Seems to work for me but I appreciate that for night navigation I will need me to be more familar with my unit, although I don't see why this method wouldn't work just as well in all but confined channels

I need to get out there on my own so that I can focus on it. Trips are that few and far between that what I learn by reading the manual at home, has gone out the window when I actually hit the water due to lack of practical sessions.

One thing that I have noticed.......and I'll give Brennans and Hutchies Shoal as an example. On the C map it is shown as a triangle with an exclamation mark (correct me if I'm wrong) When I zoom in to this triangle and position my rig within it......zilch..featureless bottom. I actually can find reef outside of that triangle. What am I doing wrong or is the map wrong? I have to assume the unit is accurate because it does put me on a few of my own marks but seems out on marks inbuilt into the map?

I'll appreciate any advice or comments that you may make."

kev

[SatNav]

Owen, what I do see is that 53.8% (@ 15/6/06) have some problems with what they expect and dot points make a clear and concise statement without a lot of useless wording.

Kingtin, Cmap comments chart accuracy, GPS accuracy previously stated.

[leograyson]

Been ignoring this thread because it was a Poll and they seem to follow the old 80/20 rule (i.e. they are 80% B/S)

Stumbled into it today by accident and read it. Didn't know much about GPS before but now I'm really confused. Maybe answers to a few questions it might at least reduce my confusion.

SatNav, you obviously know your stuff, but it's mainly your posts that I'm having trouble understanding.

The following statements are just a few from your posts, but they summarise whats puzzling me.

1. "They could fiddle the system timing anytime they like but they HAVE NOT and they WILL NOT."

Are you saying that they had selective availability for about a decade, but NEVER used it.
If so, why all the HooHa when they turned it off ??. And was the document by Subzero just another part of some gigantic scam ??.

2. "There is little difference in accuracy btween a $150 handheld and a $30K unit"

So, why in the name of allah would anyone pay 30 Grand for one ??

3. "Being able to return to the letter box only means the letterbox hasn't moved, doesn't mean it's accurate" .

Sounds like nitpicking to me!!. I'm sure that the BIG majority of fisho's on this site want to be able to find a previously marked spot & this is how they measure accuracy.

4. "GPS readings have never varied during any conflict"

Seems to me that there has been some conflict going on between the USA and the Middle East well before 1991 and stlll is today.
Are you really saying it was never used during a Major Conflict or that it was never used at all ??.

Have mercy on this poor confused fisho, and end my confusion.

Leo.

[Spaniard_King]

Hey Guys,

don't suppose anyone can remember back to 15/01/1991 ::)

The USA turned Sa to its upper most limit for 7 days as they were worried that Sadam could lock onto them using GPS to guide his weapons During the first week of Desert Storm.

Garry

[IcemanII]

Interesting GPS link and some history.

http://en.wikipedia.org/wiki/Global_Positioning_System

[SatNav]

Just for you leo and based on the poll there is very few who really know what they should expect from their GPS.


1. "They could fiddle the system timing anytime they like but they HAVE NOT and they WILL NOT."
Since Selective Availability was stopped (May 2000) there has been this assumption by many that during conflicts since, that Selective Availability was re-started. This has not been the case and will not be the case. In the period that Selective Availability was running there wasn’t any conflicts anyway so No, it was never used for the real purpose it was designed for. In reality during the first Gulf war Selective Availability was specifically and deliberately turned off for the war.
The May 2000 toning down of Selective Availability has very little to do with Military purposes/reasoning and more to do with commerce as the future growth of GPS is with the civilian sector not the military.


2. "There is little difference in accuracy between a $150 handheld and a $30K unit"
Using the Standard Positioning Service and as a navigation device there is little to no difference in accuracy between the two. With two (2) 30 odd grand receivers running together then the whole scenario changes and so does the principles and methods involved, accuracy in the millimetre range over hundred’s of kilometres but only in hindsight and requiring many hours/days of recorded observations. With the same two (2) receivers and the addition of some quite expensive radio gear (12 grand plus), accuracy in the 20-50mm range in real-time but very limited by distance.

3. "Being able to return to the letter box only means the letterbox hasn't moved, doesn't mean it's accurate".
I will say this again, this is not an indication of GPS accuracy, This is NOT an indication of GPS accuracy, yes might locate a previous spot but this is NOT how accuracy is measured or an indication of accuracy.

4. "GPS readings have never varied during any conflict"
GPS was a long long way from being fully operational during the first gulf war and Selective Availability had been toned down by the second gulf war. During the periods Selective Availability was running it only annoyed civilian users. Selective Availability was fully de-activated between the period August 1990 and July 1991 (for very good reasons) and apart from the rumor mill and media beat up there has never been any evidence of or data to support Selective Availability use during any conflict, when Selective Availability could have been used. If Selective Availability had ever been used it would have immediately flashed up on hundreds of monitoring stations right around the world, nobody that I am aware of noticed even a twitch in the system. The one time there was any evidence of specific system changes was within 15 minutes after September 11 when the whole system became incredibly stable, which has led to speculation that Selective Availability is not completely off, not that it is actually off anyway.