What exactly do the little D's mean?

[atszyman]

I've heard them mentioned with respect to WAAS being enabled but what exactly do the little D's in the signal strength bars mean?

I thought you could only get them once you locked to a WAAS satelite but I've gotten them without locking on to a WAAS bird. A typical satelite list is below. I know 35 is WAAS enabled, but the D's sometimes show up before 35 has a solid bar, sometimes when there is no bar at all.

03 09 14 15 18 21 22 23 30 35

To further confuse matters one of my co-workers was getting a 3D differential position on his Garmin eTrex Legend with WAAS disabled.

Just wondering.

[Moose Man]

You should read the FAQ's that you can find by linking from Jonasolof's sig line - but the short version is that they are receiving WAAS signals from the sats. It is apparently not necessary to have the D on 35 or 47 (North America) in order to receive WAAS.

I wouldn't worry too much about it - if you'd been reading since August some of the posters will note that sometimes the accuracy actually appears to be better without WAAS enabled. The most accurate reading in feet has been around 6 or 7 but Garmin only states that 3 to 5 meters is expected. :)

[atszyman]

The FAQ starts to explain but falls a bit short.

From the earlier posts that I read I understood that the D's/differential location required WAAS mode to be enabled. I also understand that a differential signal is inherently more accurate than a non-differential signal due to the effects of signals bouncing in the atmosphere that differential accounts for. I was just wondering what the requirements were for it.

The whole question arose when my co-workers eTrex was getting a differential position without WAAS enabled. So I was wondering if anyone here would be able to shed some light on the quandry.

I'll have to disable WAAS tomorrow and see if they still show up.

`Tis purely a curiosity.

[apersson850]

Differential data is transmitted by WAAS satellites, #35 and #47 in North America.
These data are then applied to the ordinary sats ("D" symbol is shown), to improve accuracy.
Differential data is transmitted from WAAS satellites even if your unit can't lock on to them. A hollow signal bar is sufficient.
Differential data lasts for a short while after loosing the WAAS reception.
If you get a differential position with WAAS disabled, something went wrong in the software.

[jonasolof]

To underline what Anders said. Differential correction is only obtained by having an extrenal source informingt the GPS receiver of the ionospehric diffraction error so that it can correct for it.

Sourcs can be i.a. :

WAAS- geostationary satellites

Coast guard long wave beacons used for marine purposes. Here input will be via serial connector into the GPS

FM broadcast for land purposes. Expensive, for professional use. USed for construction etc.

To come: Corrections via cell network systems. Cell base stations can be used to broadcast area correctrion data.

Without input, you can't have differential correction. So a "D" with waas disabled and without other input is a false statement

[atszyman]

Thank you, combined with my drive in to work (WAAS disabled) it all seems to make sense. Except for the whole eTrex phenomenon.

My co-worker swears that he doesn't have WAAS enabled, unless it automatically does WAAS when getting external power. We had it hooked to a laptop via the serial port and it was definitely giving 2D and 3D differential positions. The whole eTrex question would probably best be served on a different board.

Thanks for all of the info.

[apersson850]

Then I think I know what it is, even if this indeed is the wrong forum.

On the iQue, if you enable WAAS, then try to change into battery save mode, you'll get a message that these two settings aren't compatible.

On an eTrex, you can very well set both WAAS and battery save mode at the same time. Battery save has priority, so WAAS doesn't kick in. But when you feed it external power, it automatically changes from battery save to normal mode. Which means that WAAS kicks in, if that was enabled too.

[atszyman]

He doesn't seem to think WAAS is (or was) enabled, but it's possible it was.