WHAT HAPPENS WHEN YOU INADVERTENTLY ERASE ALL DATA STORED IN THE GARMIN Because of confusion and misinformation on the newsgroup, I have asked Garmin Engineering to give us a consensus view of how the accuracy and operation of the G-12XL (and other Garmin equipment can be affected by operating the factory test mode while in view of "real" satellites. In my view, the significance of the "problem" is minimal since calibration points are automatically restored when the unit is operated locked to SVs the next time(s). The Garmin answer follows: Joe Mehaffey ================================================================= The answer from Garmin Engineering Follows: Joe, Some one did talk to Jay on the phone. I guess the answer was not clear to him. What was said to Dr. Reid in the original email to him (who altered it and then posted it (Garmin never posted this information)) was that by operating the unit in test mode, AND locking on to a specific satellite, you would destroy the unit's oscillator compensation curve AT THE TEMPERATURE WHERE THE LOCK OCCURS. Perhaps destroy is a bad word, it does sound somewhat permanent. In fact, it WILL NOT damage the unit in any way, but will erase the oscillator temperature calibration point. It would appear that some have interpreted it as a permanent degradation of the units performance. For that we apologize. That is NOT correct. Accuracy was never an issue. The only issue at stake is the ability of the unit to lock on to satellites and calculate a position. As we know from GPS 101, A GPS receiver must be precisely synchronized with the satellites to operate. We must know the exact time the signal left the satellite, and the time the signal arrived at the receiver, to calculate the time difference and corresponding range to the satellite. By knowing the precise location of the satellite when the signal left, and the range (pseudorange), we know our exact distance from that satellite (at that point in time). By performing this calculation on multiple satellites, we can triangulate and calculate our position. Obviously, timimg is everything. We must have a precise timing source for this to work. We could install a rubidium or cesium beam oscillator in our GPS receivers, but this would be a little pricey, use a lot more battery power, and the unit would be a little bulky. Instead, we use a relatively cheap oscillator, and a lot of software finesse. Oscillator compensation data is stored in the unit as a table based upon temperature. When the unit locks on, it calculates the unit's oscillator error and enters a correction factor into this table based upon current internal temperature. In this fashion the unit is CONSTANTLY "learning" and fine tuning itself. We burn these units in when new to calculate and store calibration constants across the entire rated temperature range of the product. What Darrin and the previous Email tried to explain, perhaps in different tones, was that running the GPS 12XL, or other GARMIN units, in test mode, would mis-calibrate the units internal oscillator if the unit locked onto the same satellite it expects to see in the factory environment when connected to a signal generator. The amount of mis-calibration depends on where this particular satellite happens to be in the sky. If the satellite is low on the horizon and heading directly towards or away from you, there will be significant doppler shift on the signal which in turn will grossly mis-calibrate our oscillator. This would be worst-case and statistically rare. The net effect of this type of mis-calibration may range from long acquisition times to, on rare circumstances, failure to acquire at all. If the latter occurs, as Darrin pointed out, the user can put the unit into Autolocate mode, which ignores oscillator calibration and the unit will lock on and begin to repair the correction table for each temperature it experiences. Of course, some users might not know to put the unit into Autolocate mode, they may see the unit not locking onto satellites and believe it to be malfunctioning. As soon as the unit locks on, oscillator calibration is performed and peak accuracy is immediately restored. Of course, for complete repair, the unit will need to be locked on to real satellites for each temperature point that the unit was mis- calibrated to. Darrin and I do not disagree on any points. He was stating his perspecive from a designers point of view, and I am giving you mine from the standpoint that we don't want anyone to erase the calibration from his unit. GARMIN International 1200 E. 151st Street Olathe, KS 66062 USA