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Old 14-02-2014, 07:09   #61
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Originally Posted by captain58sailin View Post
I don't know about the military, but us regular folks don't get DGPS unless we pay a subscription fee. It is supposed to be much more accurate, we use it on Dynamic Positioning vessels when we need to keep a very tight foot print due to operations.
Someone else has already commented on this. You're not exactly correct but you're not 100% wrong either. WAAS (Wide Area Augmentation System) signals are freely available now and they provide varying degrees of accuracy, down to sub meter accuracy in some applications. You may be referring to RTK (Real Time Kinematic) signals which can provide sub inch accuracy (and which require a subscription) or there may still be differential correction signals available for a subscription. DGPS using WAAS is however free.

The reality for most of us is that free GPS provides a higher level of accuracy than we can steer to. Thank you very much the US taxpayer.
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Old 14-02-2014, 07:37   #62
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Re: The Modern Navigation Trap

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Originally Posted by bobofthenorth View Post

The reality for most of us is that free GPS provides a higher level of accuracy than we can steer to. Thank you very much the US taxpayer.
Not only the US taxpayer, in Europe, we have EGNOS, based on the same system as WAAS, which all come under the umbrella of SBAS /Satellite Based Augmentation System. Similar system over Japan as well, and most likely other area's.


In addition, the differential corrections are also transmitted from radio beacons (typically a 100 mile radius). Provided a GPS receiver has the receiving circuits and the correct antenna, the radio signals can be used for diff corrections.
Furuno GP-150 is one such receiver. I dont know of any chartplotter GPS which has this feature.

The RTK signals that bobofthenorth mentions are way beyond our means. At work, the subscription is about $2400 per month per boat.

Nice discussion
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Old 14-02-2014, 07:51   #63
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Re: The Modern Navigation Trap

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Originally Posted by bobofthenorth View Post
Someone else has already commented on this. You're not exactly correct but you're not 100% wrong either. WAAS (Wide Area Augmentation System) signals are freely available now and they provide varying degrees of accuracy, down to sub meter accuracy in some applications. You may be referring to RTK (Real Time Kinematic) signals which can provide sub inch accuracy (and which require a subscription) or there may still be differential correction signals available for a subscription. DGPS using WAAS is however free.

The reality for most of us is that free GPS provides a higher level of accuracy than we can steer to. Thank you very much the US taxpayer.
You are closer, but not quite there, yet...

Some people call WAAS a type of DGPS. Kinda like intracoastal waterway and Intracoastal Waterway. WAAS is a type of dgps, but DGPS and WAAS are two different systems.

All modern (last 5-6 years?) GPSs sold have WAAS. It is free.

The old DGPS is different and requires a separate vhf receiver to get the signal. It is free.

There are some companies selling proprietary receivers, some companies providing additional ground transmitters, and some doing both. These companies charge for you using their enhanced capabilities. The Bering Sea is a place where the DGPS does not have great coverage, so companies have stepped in to provide better coverage for mining/drilling/fishing customers.

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Old 14-02-2014, 11:16   #64
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Re: The Modern Navigation Trap

Something not mentioned yet is the fact that the GPS system has always been capable of very fine accuracy. But the system was programmed to send a false location that was within a few hundred feet of the right position. And constantly "roaming around". So the "differential" receivers were getting the information as to exactly how far and in what direction the false signal was off. Therefor correcting the "error". So the perscription fee for a DGPS unit was a way of keeping track of who you were and that you had good reason to need that kind of accuracy. Preventing "unauthorized use" of the system.

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Old 14-02-2014, 11:50   #65
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Re: The Modern Navigation Trap

Our 1998 B&G GPS was a differential unit. It did not require a separate vhf receiver (other than whatever circuitry was in the GPS sensor itself), used a standard 1998 Garmin DGPS-capable sensor, and did not require any subscription fee. There was absolutely no way to track anyone using the system.

It always had a DGPS connection in the east coast US, but by Puerto Rico, there were no longer any stations in range and we have not encountered one that our receiver understands in the rest of the Caribe. I believe the US radio stations were setup and maintained by the US Coast Guard.

These units were very common before WAAS, and have been almost completely supplanted by WAAS now in the consumer marine market. DGPS still exists in the surveying and other technical markets (and is how the past America's Cup maintained 2cm accuracies). The USCG stations are still transmitting.

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Old 14-02-2014, 18:07   #66
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Re: The Modern Navigation Trap

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Our 1998 B&G GPS was a differential unit. It did not require a separate vhf receiver (other than whatever circuitry was in the GPS sensor itself), used a standard 1998 Garmin DGPS-capable sensor, and did not require any subscription fee. There was absolutely no way to track anyone using the system.

It always had a DGPS connection in the east coast US, but by Puerto Rico, there were no longer any stations in range and we have not encountered one that our receiver understands in the rest of the Caribe. I believe the US radio stations were setup and maintained by the US Coast Guard.

These units were very common before WAAS, and have been almost completely supplanted by WAAS now in the consumer marine market. DGPS still exists in the surveying and other technical markets (and is how the past America's Cup maintained 2cm accuracies). The USCG stations are still transmitting.

Mark
I was hoping someone with actual experience with the free version would speak up. That fits with what I have read. But it was before I was into larger boats, so I was not aware of how common they were, and especially not aware it was common to have DGPS built into the GPS itself.

Thanks.

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Old 14-02-2014, 18:20   #67
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Re: The Modern Navigation Trap

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A cold front being in error by two miles is one thing, Skeeter and Booger reporting a "new" rock in the middle of a shipping channel is another.
The Office of Coast Survey will take reports of discrepancies on charts from Skeeter or Booger or Jammer Six or anyone else. Their website has a portal for doing so. The reports are evaluated so you really aren't relying on us belly-scratchers and rum sponges...the big-headed guys and girls look the reports over carefully before making permanent changes. You might however see Booger's report in a Notice to Mariners if the report warrants that mariners take caution.
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Old 14-02-2014, 18:54   #68
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Re: The Modern Navigation Trap

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The location of a rock, on the other hand, is precise, deadly and unforgiving.

While what Skeeter and Booger think about whether or not it's about to rain might be amusing, delivered in the charming, broken version of English that they both speak, I'm not interested in what either one of them think about the coordinates of Klas Rock.
To a certain extent I agree, but I also think that with modern instruments you almost don't have to have Skeeter and Booger involved. With a GPS you can get accurate location and time information. NMEA can tie that together with depth sounding information seamlessly. Given depth, time, and location you can correct for MLLW. It would take a certain amount of commitment from the boat owner, which probably rules out Booger. Either way you can collect a data stream automatically, with pretty much the only variables being the accuracy of the depth instrument and GPS location.

Maybe then you require two or three or five submissions from the same general area that are consistent? And with vector electronic charts it's really easy to put this kind of information on a separate layer that can be turned off (and maybe is by default?).

After all, in many of the areas we are discussing your are relying on 200 year old data that was collected by Booger's great-great-great granddad. You don't think it was Cook himself that was swinging the lead? It was Booger's ancestor, and he (because there were almost no women involved at the time) probably couldn't read and maybe had a hard time counting. That crew passed through once, and because their information made it to a hydrographic office chart we take it as Gospel. If 10 people passed through the same area with modern instruments, and 8 of those 10 all agreed that the bay was 10 feet shallower, I'd believe the crowd sourced data.
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Old 14-02-2014, 19:29   #69
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Re: The Modern Navigation Trap

Sidetrack Warning
How did Haida art make it onto a Cross 40 in Oakland? The Haida never raided that far south. Not from ability-- there was nothing there they wanted that couldn't be taken closer, with less risk. There were tribes and bands that weren't fighters between where they were and Oakland.
End Sidetrack

I think I'd agree on a shallower bay, but if you got six or seven reports that a rock has suddenly moved, (or whatever number is a reasonable threshold to correctly conclude that the charts don't reflect reality) I'd like a fanatical, academy trained guy with twenty years in command behind him, a 21st century vessel purpose built and dedicated to his mission and a crew under military discipline to go out there and first, count the rocks, and second, use the latest gee-whiz methods that my son wouldn't even understand to locate and sound all the rocks he finds, complete with all the professional checks and balances.

It would be best if we got a Captain who was minor league obsessive-compulsive, and believed that the accurate location and documentation of rocks and shoals is ninety percent science, ten percent art and one hundred percent religious duty, and could only make nook-nook after correcting errors in existing charts, but that's probably a pretty high bar.
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Old 14-02-2014, 20:52   #70
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Re: The Modern Navigation Trap

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Built by hippies in the '70s. Lots of trimarans started out life that way Wasco has kept us safe for a long time (or was it seamanship?).
End Sidetrack

The reason I posted that thought in the first place was in response to the observation that we can't seem to get those anal-obsessive academy types interested in anything other than money-making shipping channels. So, if they're not interested, and we can't swing the political clout to get their attention, then maybe we have to start looking out for ourselves. In many ways that's how a lot of us feel about cruising anyway. I'd love to see some way to upload my track along with depth information to OpenCpn. Wouldn't even mind spending a few $ to get my depthsounder certified or calibrated or something along those lines that allows people to place confidence in my data. To me that's part of the community and a way to give back.
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Old 14-02-2014, 21:04   #71
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Re: The Modern Navigation Trap

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Something not mentioned yet is the fact that the GPS system has always been capable of very fine accuracy. But the system was programmed to send a false location that was within a few hundred feet of the right position. And constantly "roaming around". So the "differential" receivers were getting the information as to exactly how far and in what direction the false signal was off. Therefor correcting the "error". So the perscription fee for a DGPS unit was a way of keeping track of who you were and that you had good reason to need that kind of accuracy. Preventing "unauthorized use" of the system.

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It has been many years since the USAF "dithered" the GPS signal - I believe it was Clinton who signed the order to end that. The "differential" signal did not correct the timing error - it simply took a GPS receiver in a fixed, known position, read what the satellites said, compared that location to its true location and broadcast the difference to receiving DGPSs. Ingeniously simple. The fact that this very simple work-around could be employed by anyone to overcome the Selective Availability error, made SA ineffective, so there was no reason to keep dithering the signal. No one has ever required a subscription to DGPS.
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Old 14-02-2014, 21:54   #72
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Re: The Modern Navigation Trap

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Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons.

In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.

The United States has no intent to ever use Selective Availability again.

In September 2007, the U.S. government announced its decision to procure the future generation of GPS satellites, known as GPS III, without the SA feature. Doing this will make the policy decision of 2000 permanent and eliminate a source of uncertainty in GPS performance that had been of concern to civil GPS users worldwide.

GPS.gov: Selective Availability
It always amused me that one branch of the government put in place systems (DGPS) to undo what another branch of government was doing (SA).

For a pretty good discussion of all the augmentation systems:

GPS.gov: Augmentation Systems
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Old 15-02-2014, 05:25   #73
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Re: The Modern Navigation Trap

Unless you have a particular survey grade requirement , paid DGPS is extinct for normal users. Note that SBAS is DGPs, just delivered via satellite as opposed to ground.

Currently we have location precision in excess of anything we need for navigation

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Old 15-02-2014, 06:07   #74
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Re: The Modern Navigation Trap

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Currently we have location precision in excess of anything we need for navigation
Map accuracy rather than the GPS signal are generally the limitation when plotting our position, but an improved GPS position does have real benefits.

Accurate SOG makes a significant difference to current, and ground wind calculations. An accurate GPS position also helps in setting an anchor alarm that is free of false positives. Finally being able to retrace your track can be useful such as escaping from a tight anchorage in poor visibility.

Crusing sailors often seem quite happy with a relatively poor GPS position signal, but I think there are real practical benefits in striving for the best possible position information. At least make sure you have a good antennae signal and enable SDGPS (WAAS and EGNOS) where available. Especially EGNOS in Europe. Many boats do not have this correctly set up.

The GPS system has got more accurate in recently years with more satellites and a spread of the SDGPS coverage. GPS receivers have got significantly better, with innovations like the new 10Hz units. Hopefully we will see further developments in GPS accuracy and while we are at the stage of diminishing returns improvements will provide some real practical benefits. As well the development of the alternative Russian (and other) GPS system which promises some welcome redundancy.
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Old 15-02-2014, 06:18   #75
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Re: The Modern Navigation Trap

On a tangent - back in the bad old days of SA, the military users had systems that got the dithered C/A signal and an encrypted P/Y signal with the clock correction embedded in the nav message. This came on a different frequency, so the unit was then able to calculate actual atmospheric refraction, rather than relying on tabulated info. That said, the new GPS constellation now delivers two "civilian" frequencies, so that more info can be passed in the nav message (including WAAS, I assume). I was wondering if any new civilian GPS receivers use the two frequencies to calculate more accurate atmospheric corrections?
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