GPS digital navigation map data

Yes. I use a handheld to double check position. I do not navigate with it [see below].

Aviation units compute location much faster. AFAIK there aren't any consumer oriented units that can reliably locate themselves while moving at 150 knots ... many turbocharged and/or multi engine aircraft can cruise faster than that.

Also aviation units with navigation plot great circle courses rather than straight lines.

I think it's more because GPS is controlled by the U.S.

George

-- for email reply remove "/" from address

The question is whether he would have ended up on the rocks due to inaccuracy or to his failure to set the correct reference point (the "datum"). If the GPS is using one datum and the charts another then even if the GPS is giving accuracy to the nanometer its reading will differ from that on the chart. This is something that most GPS users don't even seem to be aware of.

Also, take a look on comp.risks: there's an item in the current Risks bulletin on in-car GPS issues: as noted on this thread, they all derive from a single database, which is *not* error-free. So using two different makes of GPS won't help either.

Would you do an approach without much assurance of the GPS satellite signal quality ? Assuming that the SA would be suddenly switched on during the approach (or a satellite would experience some sudden problem), this could have quite serious consequences. For any critical navigation, you really need some assurance about the signal quality.

A differential-GPS station can constantly verify the signal quality from all satellites and broadcast this signal quality information to nearby users (in addition to the differential correction). Without a DGPS transmitter at every airport, I don't see how you could make a safe approach with only a GPS receiver.

Paul

That is irrelevant as long as the system contains the option of SA. How can you be sure that the SA is not activated tomorrow ?

Paul

Dear Mark,

Thank you for your informative reply.

(I think your posting was the only relevant reply to the original question.)

Ray

Does anyone know if there is an equivalent to "US Census Bureau's Tiger/Line database" in Australia?

Google "WAAS".

Not at all irrelevant to Anton's anecdote, which you snipped.

I cannot remember but believe it does not make much of a difference. As long as SA can be activated, then for safety critical applications one has to be able to operate as if it is switched on. The biggest problem was that during a storm, when the navigational data accuracy is at it most critical, one can have problems aquireing one or more of the GPS satelites. With a lower signal to noise ratio, it takes longer to get an acurate position. If one is moving, then one can up with position data of which the accuracy is severly down graded. The author of the aricle I was refering to was greatly concerned that this degrading of accuracy was not made clear in the documentation of the GPS devices, never mind actually having a clear indication on the unit that the current data has degraded accuracy. Naturally the GPS device manufacturers has a dentancy to indicate the accuracy one gets under ideal weather conditions. For any safety critical application one must never rely on best or even typical performace specs, but on worst case performace specs.

That is also why I mentioned the accuracy versus precision thing. Most people have a tendancy to equate precision with accuracy. This can have disastrous consequences, hence my opinion that GPS devices should display the data in a format which clearly indicates the CURRENT accuracy when displaying the data. Even if the map database data was 100% accurate, one should still be careful to rely on the data, since the accuracy of any GPS device can vary greatly depending on what the current local conditions are. From "No data" up to the resolution limit of the GPS system as a whole. Because of filtering, the user might not even know that the current displayed position is based on "No Data" values for the last number of readings.

Regards Anton Erasmus

Hummm...are you talking about pilots trusting their lives to personal GPS?

Handheld aviation GPS units are remarkably accurate, and are great at facilitating navigation. But that is as far as the 'hand held' variety goes, I know of no pilots who actually trust their lives to the hand held units. think of it as a way to verify and cross check other nav aids.

My own flying, I use it to supplement the regular aviaonics...and the hand held is much more accurate than the standard VOR type units.

That is too verbose. May I propose a notation from my physics experiments 143K2m accurate to 0k05m (Conventional, half of last dec.) A bar under the 2 accurate to 0k1m A twiggle under the 2 accurate to 0k2m

This should do the job and is unobtrusive. Come to think about it, it should be a standard feature on digital volt meters. At last those display only significant digits, even if that is a cost issue more than anything else.

That sounds sound.

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You're a bunch of naive optimists ;) I remember a sophomore EE lab ~1962. Goal - design 1 transistor amplifier Classmates regularly requested bias resistors spec'd to less than 1% Value depended on beta which had < order of magnitude precision ;/

I'll odds that typical user would assume you were trying to identify center of circle as *precise* location.

Then again, a circular blob might work where a circle outline might not. Then again, I've written test procedures which intelligent persons have followed "literally" getting some strange results ;{

Hey, at least they weren't looking for ten farad 5000 volt capacitors to filter the power supply.