Accuracy, Jumps, And Waas

[+EraSeek]

Q: So what improvement can I expect?

A: A beacon based DGPS system is somewhat dependent on how close you are to the beacon, but can achieve 1 to 5 meters overall accuracy, while a WAAS system can achieve an accuracy of under 3 meters if you are located in the area where ionospheric correction data is available. A LAAS is even more accurate and can get an accuracy in the neighborhood of 1.5 meters. These are still not as accurate as survey grade receivers that receive dual frequencies, have a dgps reference station located very close to the survey site, and use post processing techniques to reduce errors even further.

 

Q: What difference does it make if the DGPS correction is broadcast from a beacon tower or from a satellite?

A: The low frequency signals broadcast from the ground-based beacon stations are more susceptible to radio frequency and weather interference, and they have a limited range. The WAAS signal, since it is high frequency and satellite-based, is less susceptible to this kind of interference and will provide 3 meter Lat/Lon position accuracy over a large contiguous geographic area.

Edited January 15, 2004 by EraSeek

[+NightPilot]

The only real need for WAAS in aviation is to permit precision approaches, meaning vertical guidance as well as lateral guidance. The vertical accuracy of GPS is poor, and WAAS is supposed to improve it. It does relatively little for horizontal accuracy. GPS is perfectly adequate in horizontal accuracy without WAAS, and I have flown many approaches down to 200' using GPS and radar. The holy grail of WAAS for the FAA is to permit precision approaches, with a glideslope, using GPS. The FAA has investigated both WAAS and LAAS (Local area augmentation system, basically DGPS), and AFAIK hasn't given up on either. I'm not aware of any IFR-certified GPS units that utilize WAAS, but they may be on the way, and there are published GPS approaches with vertical guidance. We just can't fly them yet.

[+fizzymagic]

The only real need for WAAS in aviation is to permit precision approaches, meaning vertical guidance as well as lateral guidance.  The vertical accuracy of GPS is poor, and WAAS is supposed to improve it.  It does relatively little for horizontal accuracy.

That is just wrong.

 

Yes, WAAS was mainly designed to improve vertical accuracy for aviation.

 

However, because of the way GPS works, any improvement in vertical accuracy will inevitably result in an improvement in horizontal accuracy. They are inextricably linked. Thus, while a horizontal accuracy of 3 m is not needed to land a plane, it's not possible to have the required vertical accuracy without getting it as part of the deal.

Edited January 15, 2004 by fizzymagic

[+aka Monkey]

 

This discussion is, at times, interesting, but only one thing is going to prove that WAAS is effective for handheld GPSr used by Geocachers: Scientific Proof. This does NOT consist of saying "I tried turning WAAS on and seemed to have more difficulty finding the cache."

 

Get some software that does tracking. Hook it up to your GPS. Have it track you for an hour with WAAS off while your GPS is stationary. Save a screenshot. Do it again with WAAS on. Save a screenshot. Post them here.

 

I want visual, measurable PROOF. Not theories, not anecdotal evidence, not hyperbole.

 

This really shouldn't be this hard, and if I had a way to hook my GPS to my Mac right now, I'd do it myself. Any takers?

Edited January 15, 2004 by Indiana Cojones

[+EraSeek]

Well here' one test I know of:

http://www.gpsinformation.net/waas/vista-waas.html

 

When you look at this remember there is much less error to correct at night.

The test I've used is simply to stand on a horizontal (GPS) grade survey marker and compare with WAAS on and off. It did make a difference!, though not so much to make it a must have for geocaching.

[+NightPilot]

Yes, WAAS does improve lateral accuracy, but not that much. 3 meters versus 5 or so just doesn't make that much difference, but improving vertical accuracy by an order of magnitude or more does. To a geocacher, the lateral improvement is great. To a pilot, or a surveyor, or most other users, it isn't that significant - it's either too little or overkill. Different applications have different requirements, obviously.

[+EraSeek]

" The WAAS broadcast message improves GPS signal accuracy from 20 meters to approximately 1.5 - 2 meters in both the horizontal and vertical dimensions, allowing more efficient arrival, enroute, and departure operations at low cost to an increased number of airports throughout the U.S. WAAS also provides savings from simplified cockpit avionics for all classes of aircraft.

 

Additionally, WAAS provides indications to GPS/WAAS receivers of where the GPS system is unusable due to system errors or other effects. Further, the WAAS system was designed to the strictest of safety standards – users are notified within six seconds of any issuance of hazardously misleading information that would cause an error in the GPS position estimate.

 

The FAA commissioned WAAS at 12:01AM on July 10, 2003. This step moves the FAA directly to Lateral Navigation/Vertical Navigation (LNAV/VNAV) and LPV capabilities using WAAS. Both LNAV/VNAV and LPV approaches use the accuracy of the WAAS signal to include vertical (glide path) guidance capability. This capability facilitates improved instrument approaches to include vertical (glide path) guidance to an expanded number of airports throughout the U.S.

 

There are nearly 600 LNAV/VNAV approaches available today at nearly 300 airports throughout the NAS. The FAA is continuing to develop these procedures, adding many more each year."

 

 

Sound beneficial to me.

[Kerry.]

As with any GPS/WAAS type accuracy figure simply stating so many metres really doesn't mean anything at all. Also what might be currently being achieved has really nothing to do with the actual accuracy specification of the system.

 

3 metres appears to be a favourite quote but where did that figure actually originate from and exactly what sort of 3 metres are they.

 

Cheers, Kerry.

[+NightPilot]

It's easy enough to publish approaches. It's something else to actually be able to fly them. I just read of the first WAAS-capable avionics system, and it's not in production. The FAA is still wafflling about LAAS, and the new director is in a snit because of it. The FAA has been touting WAAS, LAAS, MLS, and God knows what else for years, and none of it works yet. The Europeans are still angry because the US changed the MLS specs and then abandoned the entire system. And the FAA still hasn't approved GPS as a sole-source navigation means. The aircraft I fly have IFR-certified GPS receivers installed, but we can't take off without 2 operable VOR receivers. Maybe someday, but for now WAAS is just hype in the aviation arena.

Edited January 17, 2004 by NightPilot