Correcting Signal Drift

[+SpongeRob]

I received my units from garmin yesterday and the repair order had listed on it "ajusted GPS signal drift". Curious about what there was to adjust I went out to the web to do a search. The only entry I could find was where the WAAS and DGPS send messages back to the satalites to correct signal drift by adjusting the clock on the offending satellite. That made sense.

 

So my question is, what did garmin adjust?

 

- Rob

 

--

SpongeRob

rwmech@keenpeople.com

www.keenpeople.com

WPWU826

 

Cache'n Retrievers

[+mrp]

The little quartz oscilators they use in these GPS units are pretty good, but tend to have tempurature dependencies because higher tempuratures cause the little chunks of quartz to phycially expand, which changes their vibrational modes. (At least that's my understanding.) The signal in this case is the signal from the oscilator(s), not from the satelites (though those oscilators are used in the reciever section of unit.)

 

To combat this, GPS manufacturers use an internal table of correction values for each tempurature. This table is calibrated at the factory for each unit, and is constantly being refined while the unit is in use. However, it's possible for bad values to creep into that table, making is difficult to get lock, or making the position fixes less accurate. At that point, you have to dump the values in that table, and recalibrate the unit. I think that's what they did, but I don't really know.

 

-- Pneumatic

 

[This message was edited by Pneumatic on August 20, 2003 at 12:32 PM.]

[dave and jaime]

if i were to make a guess they reinstalled the firmware and tested the unit. there is very little adjustment available to be made other than firmware. if your unit had a particularly troublesome firmware version they may have replaced it.

 

'Get to the point---speak English!!!!'

[Kerry.]

quote:

---

Originally posted by SpongeRob:

.... The only entry I could find was where the WAAS and DGPS send messages back to the satalites to correct signal drift by adjusting the clock on the offending satellite. That made sense.

---

 

WAAS and DGPS have no capability in communicating back with any of the satellites.

 

That description could mean one of many things but one could assume it had something to do with the sequence of signal generated internally in the receiver that is similar to and matched against the incoming signal, which allows the receiver to determine time and hence the pseudo-ranges.

 

Cheers, Kerry.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Tahoe Skier5000]

Are we talking about XO drift here?? If so, my GPS III+ says it has a XO drift of "-20.62"

 

Is that good? or bad?

 

-----------------------------------------------------------

Garmin V

Garmin III+

[+SpongeRob]

quote:

---

WAAS and DGPS have no capability in communicating back with any of the satellites.

 

That description could mean one of many things but one could assume it had something to do with the sequence of signal generated internally in the receiver that is similar to and matched against the incoming signal, which allows the receiver to determine time and hence the pseudo-ranges.

---

 

I know the UNIT does not have that ability, but from what I have read the WAAS base stations DO have the ability to transmit to the satellite. Does anyone have any articles to back this up?

 

--

SpongeRob

rwmech@keenpeople.com

www.keenpeople.com

WPWU826

 

Cache'n Retrievers

[Kerry.]

quote:

---

Originally posted by SpongeRob:

I know the UNIT does not have that ability, but from what I have read the WAAS base stations DO have the ability to transmit to the satellite. Does anyone have any articles to back this up?

---

 

The WAAS ground stations do communicate with/to the WAAS geo satellites (onlt 2 of them) from which the corrections are re-broadcast to the users. The WAAS geo's also have other capabilities in that they can act as another look-a-like reference satellite however corrections and manipulation of the GPS satellites for things like timing, ephermeris, maintainence etc are the sole responsibility of the GPS Master Control Station at Schriver and the remote monitor stations, which has nothing to do with WAAS or DGPS or any other of the augmentation systems that rely on GPS to function.

 

WAAS and DGPS services, ground stations have no access to the GPS satellites (apart from using the signals like all users), which are steered time wise from Schriver in conjunction with US Naval Observatery (USNO).

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go