Barometric altimeter improvement

This article deals around the improvement of Niras CV's barometric altimeter (detailed in Elektor magazine, oktober 2009). The improvements are:

* firstly, at http://wiki.motionbased.com/mb/Barometric_Altimeter it is mentioned that GPS provides altitude information if 4 satellites or more are found, and even then, the accuracy will not be as exact as a barometric altimeter. However, as a gps-receiver does allow some automation (see below), I would definitively include a possible link-up with a USB-GPS receiver.

* the clock should be discarded, this is not useful for ISA-model calculation. Instead a digital calendar should be integrated, similar to Elektor's time switch calender as this is useful (see below)

* The EEPROM menu should be modified:
* Set altitude should be discarded and replaced with "Calc ISA-m (u. c. alt) this meaning "Calculate ISA-model using current altitude";
* Set QNH should be discarded and replaced with "Calc ISA-m (u. c. temp-sea) this meaning "Calculate ISA-model using current temperature at sealeavel";
* Auto-fetch ISA-m should be added; this meaning "Automatically fetch ISA-model"; this will be possible when adding the GPS (and when the added calender is integrated)

First of all, aldough there is only 1 ISA-model in the device (the one for 0m to 11000m where the temperature goes from 15°C at sealevel to -56,5¨°C at 11000 meters), one could actually consider several ISA-models (ie with the same temperature range, but with 1 degree more/less at sealevel. Since great temperature differences occur on the planet, a range should be possible of eg -50 to +50. The use of these models then allows automated fetching. The automatic fetching would be done by gathering the current GPS-location and figuring out what the ISA-model is for that location; this will depend not only of the location but also of the current time of the year (the range shifts during the year). determining the ISA-model would be done by letting the software compare the data to a certain range; if the GPS string falls into a certain predefined "rectangle" on the map, we know the ISA-model for a certain time of the year (eg January); then we look at the current time of the year (eg let's assume June), and then the ISA-model is changed to the right ISA-model for that location+time (eg ISA-model +19atOm,-52,5°Cat11000m)

Regarding the USB-GPS; I'm not sure whether any type of GPS can be used, but I assume so. Perhaps a problem could be the power supply (the USB is not only used for transmitting data but also to powers the device). If the problems are too great here, perhaps one of Elektor's older do-it-yourself GPS-receivers can be used.