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New Garmin 60csx With Sirf Iii - More Details


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Following on from this thread about the new Garmin models with SiRF III, I contacted a knowledgeable contact at Garmin [name withheld].

 

I was also very enthusiastic about the new 60CSx but concerned about the settings for SiRF III "Static Navigation". This is a position filter in SiRF III which detects small movements and then freezes the position.

 

SiRF III is much more sensitive and this leads to "wander" because observations over a 24 hour period show that with good signals, 95% of the positions reported by the GPS will be within a 15 meter radius, while with weak signals, 95% of the positions will be within a radius of 50 meters.

 

The position filter locks the position for applications such as in-car use, which avoids the reported position wandering around and makes the user believe they have a more reliable position. Obviously a bad thing if you are geocaching under tree cover, walking slowly and expecting the position to update.

 

Here is the full text of the reply confirming that the Static Navigation will be off (good for geocaching!) and btw confirming the 60CSx exists.

 

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Static Navigation:

 

In Garmin's SiRF products, static navigation is OFF, for every product.

 

So static navigation is a kind of position pinning, which of course is very bad for a geocacher. In Garmin's integration of the SiRFstar III, position is never pinned (unless SiRF is doing even extra filtering I am not aware of). However, there are speed and track angle thresholds. This means that the speed will be displayed as zero and track angle will not update in cases where we estimate that speed and track are so noisy that they are not useful. For example, if you were to walk slowly in the middle of a cement parking garage with a 60CSx, you might have a fix on very weak signals. In this case, your position will update, but your speed might display as zero, and heading might not update, since in that situation the values coming from the SiRF are so noisy as to be not useful. Remember, one of our design rules is that it is better to display no guidance at all than bad or misleading guidance.

 

In summary, I believe that Garmin's SiRFstar III products will be very useful for geocachers. My personal testing in heavily treed areas here in XXXXXX shows the performance of the SiRFstar III to be very impressive.

 

A Comment on Accuracy:

 

The SiRFstar III will provide a fix with much weaker signals than a Garmin GPS. However, the accuracy of these fixes will be degraded, along with speed and track angle accuracy. This is an inevitable by-product of processing weaker and much noisier satellite signals. You don't get somethin' for nuthin', so they say.

 

It is my opinion that the best performance for a geocacher can be obtained by allowing the unit to get a fix and solid signal bars (ephemeris collected) on as many satellites in view as possible before going into the deep woods or other challenging signal environment. This "seeds" the navigation solution with a good initial position. Also, the SiRF is very good at maintaining lock on signals as they get attenuated. It is much less good at acquiring weak signals from scratch.

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Good to see more confirmation on the new products. Now it would be nice to at least get a ballpark date for first shipments.

 

In addition to the static navigation pinning, another concern with at least the SiRF I and SiRF II products was that they didn't provide the proper altitude data in the NMEA GGA sentence. In particular, altitude should be reported relative to the geoid not relative to the ellipsoid, and the geoid-ellipsoid separation value should be specified in the GGA sentence. Past SiRF products I've seen have either just reported the ellipsoid height with no separation value or have included the separation but still reported ellipsoid rather than geoid height.

 

Does anyone know for sure if this issue has been addressed in SiRF III? If not it would be good if Garmin's firmware could make sure a correct GGA sentence is generated.

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Past SiRF products I've seen have either just reported the ellipsoid height with no separation value or have included the separation but still reported ellipsoid rather than geoid height.

While I haven't tested it myself, gpspassion members indicated that the geoid altitude issue has been corrected to their satisfaction in sirf III. My only informal verification included hiking with a sirf III GPS and checking the reported altitudes against USGS markers. They were pretty darn close.

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