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WAAS Information...

Old 11-24-2002, 03:13 PM
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www.navcen.uscg.gov is the offical site of US Coast Guard Information. This is where the USCG stands on WAAS. On August 24, 2000, the Federal Aviation Administration (FAA) announced that their space-based, L-band Wide Area Augmentation System (WAAS) became available for use by "some aviation and all non-aviation" users. The FAA announcement has prompted numerous inquiries to the Coast Guard regarding the maritime use of WAAS and the status of the Coast Guard DGPS system. The following Questions and Answers are directed at helping to clarify the status of these two systems for the mariner.

1. Why did the Government design and build two different GPS augmentation systems?

The 1994 National Telecommunications and Information Administration (NTIA) Technical Report to DOT on a National Approach to Augmented GPS Services studied the necessity of expanded government efforts in providing DGPS services. Its goal was to recommend the optimum integrated system to meet aviation and terrestrial navigation needs. A variety of systems were being proposed at the time. The study concluded that a combination of two systems, the FAA’s Wide/Local Area Augmentation Systems (WAAS/LAAS) and the USCG’s DGPS system, was the optimum mix. This integrated system, consisting of the L-band line-of-sight WAAS for aviation users, and the terrain-following medium frequency DGPS for maritime and terrestrial users, meets the vast majority of the nation’s precise navigation and positioning needs.

2. Is WAAS currently certified for maritime navigation?

No. WAAS is not yet fully operational and is currently in a testing status, undergoing further development. It is not certified for use as a safety of life navigation system in the maritime navigation environment. WAAS may be used, with caution, in the maritime environment to improve overall situational awareness, but it should not be relied upon for safety-critical maritime navigation. The Maritime DGPS Service, on the other hand, is fully operational and meets all the standards for the harbor entrance and approach phases of navigation

3. After WAAS reaches initial operating capability (IOC) in a few years, will it be suitable for maritime navigation?

WAAS is not optimized for surface (maritime and terrestrial) use, rather, it was designed primarily for aviation use. It is intended to eventually support aeronautical enroute through precision approach air navigation. The current WAAS test signals are transmitted by two geo-stationary satellites on a line-of-sight, L-band radio frequency. This means that if anything obstructs the view of the portion of the sky where the satellite is, the WAAS signal will be blocked. Since geo-stationary satellites are positioned over the equator, the farther north users are, the lower the geo-stationary satellites are in the sky - increasing the likelihood of an obstruction. In contrast, the medium frequency (285-325 kHz) radiobeacon-based Maritime DGPS Service is optimized for surface (maritime and terrestrial) applications because it's ground wave signals "hug the earth" and wrap around objects. This means that the Coast Guard DGPS system is well suited for the marine environment (down in the "ground clutter") where a geo-stationary satellite can be blocked by terrain, harbor equipment and other man-made and natural objects.

4. Can the Coast Guard’s DGPS system be used by aviation?

That’s up to the FAA. However, the Coast Guard’s system was designed with the surface (maritime and terrestrial) user in mind. It was neither designed nor intended to meet aviation requirements. Although aviation users could potentially get some modest benefit from the Coast Guard’s DGPS for applications such as surface traffic management at airports or General Aviation, it could not attain the type and level of aeronautical service for which WAAS and LAAS are designed, without significant re-engineering.

5. Is the Coast Guard DGPS system a "transient technology" that is here today but will be gone tomorrow?

No. DGPS has already been adopted globally as an international maritime standard established by the 1994 International Telecommunications Union document ITU-R-M.823. It meets IMO Resolution A 815(19) standards for navigation in harbor entrances and approaches. Over 40 nations have fully embraced this robust technology and are implementing DGPS services identical to our own.

6. Which system is more accurate, WAAS or DGPS?

On the average, WAAS and DGPS accuracy are virtually the same, although DGPS accuracy is better when the user is near a DGPS transmitting site. The WAAS architecture is designed to provide uniform 7m accuracy (95%) regardless of the location of the receiver – within the WAAS service area. DGPS is designed to provide better than 10m navigation service (95%), but typically provides better than 1 meter horizontal positioning accuracy (95%) when the user is less than 100 nautical miles from the DGPS transmitting site. Accuracy then degrades at a rate of approximately 1 meter per hundred nautical miles as the user moves away from the transmitting site. A total of 56 maritime DGPS sites provide coastal coverage of the continental United States, the Great Lakes, Puerto Rico, portions of Alaska and Hawaii, and portions of the Mississippi River Basin..

Conclusions: Once WAAS becomes fully operational, the combination of Coast Guard and FAA systems is expected to provide a robust, complementary service to all modes of transportation. We look forward to the day that industry provides the public with a fully integrated receiver, one that uses all available radionavigation systems to provide unprecedented accuracy, integrity, and availability.

Despite the differences between DGPS and WAAS, it should always be kept in mind that both services ultimately rely upon a single navigation system – GPS – which is vulnerable to interruption at any time. This lends additional credence to the recommended practice of using all available means of navigation and not relying upon any single system. Remember, prudent mariners will always keep looking out the window!

[This message was edited by Mumblerone on 11-25-02 at 08:01 AM.]
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Old 11-24-2002, 04:37 PM
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Are you related to Trouty? [img]/infopop/emoticons/icon_biggrin.gif[/img]
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Old 11-24-2002, 04:48 PM
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The thought went through my mind as I hit the copy and paste button! NO...is the answer. [img]/infopop/emoticons/icon_eek.gif[/img] Hmmmmmm...now that I think about it, my mom did spend a little time in Australia back in the 40's. [img]/infopop/emoticons/icon_redface.gif[/img] Trouty, how old are you? [img]/infopop/emoticons/icon_razz.gif[/img]
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Old 11-25-2002, 09:33 PM
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Personally, I haven't found any advantage to WAAS. I normally get 15 ft. accuracy with my Garmin 162 here in Northern California. When I enable WAAS, the receiver indicates reception of WAAS (small D's on the signal bars), but the positional accuracy doesn't improve. It takes noticeably longer to "lock in" as well. Anyone experience improved accuracy with WAAS?
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Old 11-26-2002, 02:04 AM
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The thing with positional accuracy is that it's only an estimate as the unit doesn't know where it really is in the first place to compare things to anyway.

Noticed the following comment recently as part of revision changes made in the software updates of one particular unit.

quote:Improved the accuracy of the estimated accuracy value

What! that's like saying that up until this update it wasn't accurate [img]/infopop/emoticons/icon_eek.gif[/img] [img]/infopop/emoticons/icon_rolleyes.gif[/img]

Cheers, Kerry.

I never get lost, everybody tells me where to go!
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Old 11-26-2002, 04:26 AM
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Great to hear the conversations about accuracy...when they got the sextant working, with a good clock, they were quite sure they were still on the planet. Evan then, they thought it was flat. Now if Magellan had a GPS the whole world process would be a little farther ahead. I guess it's all a matter of perspective! [img]/infopop/emoticons/icon_eek.gif[/img] [img]/infopop/emoticons/icon_rolleyes.gif[/img]
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Old 11-26-2002, 02:06 PM
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That "good" clock was a product of one man's life, John Harrison.

http://www.surveyhistory.org/john_harrison's_timepiece.htm

http://www.nmm.ac.uk/server.php?requ...d=005001000002

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Old 11-26-2002, 03:18 PM
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The short answer is that DGPS is a pretty expensive system requiring a separate integrated radio to receive the USCG beacon. It also requires that the USCG constantly monitor and broadcast position corrections and that you be able to receive those broadcasts. WAAS will be integral to the satellite constellation and GPS receivers will be (are) built to receive the signal without extra equipment. I cannot vouch for the positional accuracy of WAAS but our DGPS mapping and surveying equipment provides a real time position error of roughly +/- 3'.

My question is, for most navigation applications, why would anyone care? All of the single frequency GPS receivers that I have tested, from $100 handhelds to $5000 surveying systems, all provided a real time positional accuracy of roughly 15' to 20'. This is not much more than the round-off error built into the display of most marine receivers and fairly equivalent to the mapping accuracy of NOAA charts. Since this is also the size of most boats, unless you are trying to position a particular part of your boat (as in pile driving, hydrographic surveying, etc.), it is plenty accurate to get you exactly back over a wreck that you found, put you into an entrance channel or almost any other normal boating activity.

Of course, this is just my opinion based on some real world experience on land and water.
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Old 11-26-2002, 03:59 PM
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Greatdog, welcome.

That is generically correct with respect of SPS accuracy since Selective Availability was set to zero. This thinking by some that they really need sub whatever GPS accuracy to find whatever is when generally their GPS antenna and transducer are several times distant than the accuracy they think they require.

It's not so much the accuracy difference that's important with any differential as much as the integrity of the position or in other words one has much more confidence in the position as possible errors are monitored.

As for WAAS, it is not integral to the GPS constellation (apart from being integrated along the same frequency spec) as such being provided by a third party, the FAA. Similar with DGPS it's provided by a third party and both rely entirely on the GPS constellation. WAAS has an extremely weak link in those 2 satellites from which the WAAS corrections are broadcast.

WAAS requires just as much, actually even more monitoring and broadcasts than dGPS and compared to the GPS system which covers the entire world is one extremely extremely expensive system considering the area of the world it supports.

GPS all up to date has cost around the US10-11 billion where as WAAS is not yet even completed or fully operational and cost in the vicinity of US5-6 billion [img]/infopop/emoticons/icon_rolleyes.gif[/img].

Cheers, Kerry.

I never get lost, everybody tells me where to go!
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Old 11-27-2002, 10:01 AM
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Thanks, I was trying to limit my answer to very lay terms. Just as a side note on accuracy, I have been using single frequency GPS units pretty much continuously since SA was turned off and I have yet to see a raw position (with unobstructed sky) more than 6 meters out. This is based on comparing the raw position to the actual geodetic position determined by post-processing using multiple receivers and direct observations on NGS HARN stations or downloaded CORS station data.

After all those years of SA skewing the position, this is pretty impressive stuff to me. I have often taken our DGPS GIS mapping system (plain USCG beacon driven DGPS not RTK) out to recon for control monuments and set it down within a few cm's of the monument that I am searching for! To say that it has revolutionized my work is quite an understatement.

To me, the bottom line for boating is that any GPS system is now providing positions that are more accurate than the mapping techniques used to generate the NOAA charts we are using them with.
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Old 11-27-2002, 09:22 PM
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[img]/infopop/emoticons/icon_cool.gif[/img] I'll go along with all of that (even understand that language [img]/infopop/emoticons/icon_biggrin.gif[/img])

It's interesting to put things in Perspective. Australia is consistantly "moving" several inches per year (generally NE) and also adopted a new geo-centric datum in 1994 and by the time civil dual freq is fully operational the accuracy will basically be less then the total distance this country will have moved in the time since the datum was first fixed [img]/infopop/emoticons/icon_eek.gif[/img].

Cheers, Kerry.

I never get lost, everybody tells me where to go!
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