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UAV returning from long range photomapping mission from Krzysztof Bosak on Vimeo.

I have attached an Excel sheet that I made with the equations needed to design a barometric altitude sensor.

For lower altitudes and for small altitude deltas, one can assume that there is a linear relationship between pressure and altitude. Please see the attached Excel sheet for more details.

The SCP1000 pressure sensor seems like a very good candidate for a pressure sensor:
http://www.sparkfun.com/commerce/product_info.php?products_id=8161

The manufacturer gives these data:

Resolution: 3 Pascal (the fineness to which the sensor can be read - nothing to do do with the accuracy)
Relative accuracy (how close to the real pressure the sensor measurements are) over temp range +10C to +40C and 600 hPa to 1200 hPa: +/- 50 Pa. (It is not given in the datasheet if this is sensor to sensor or for the same sensor)

The manufacturer does not give noise data. However sources on the Internet may indicate that the sensor has an inherent noise of +/- 50 Pa.

Datasheet: http://www.sparkfun.com/datasheets/Components/SCP1000-D01.pdf

Sample code: http://www.sparkfun.com/datasheets/Components/SCP1000-D01.pdf

Another guy using the sensor for a variometer for paragliding: http://www.pixelproc.net/varios.html

The noise may be filtered out using an averaging filter however. Even a kalman filter and two sensors may be used.

baro_height_measurement_unit.xls

void switch_event(byte switchPosition){        switch(switchPosition)        {                case 1: // First position                set_mode(MANUAL);                // this is handy function to restart your mission:                reset_waypoint_index();                break;                case 2: // middle position                set_mode(AUTO);                break;                case 3: // last position                set_mode(STABILIZE);                break;        }}

void waypoint_event(byte event){        switch(event)        {                case EVENT_WILL_REACH_WAYPOINT:                        // called just before wp_index is incemented                        Serial.print("Reached WP:");                        Serial.println(wp_index,DEC);                        break;                case EVENT_SET_NEW_WAYPOINT_INDEX:                        // called just after wp_index is incemented                        Serial.print("Now going to WP:");                        Serial.println(wp_index,DEC);                        break;                case EVENT_LOADED_WAYPOINT:                        // called just after wp is loaded from memory                        Serial.print("Loaded WP index: ");                        Serial.println(wp_index,DEC);                        print_current_waypoint();                        // custom loitering code                        if (wp_index == 3){                                set_mode(LOITER);                                elapsedTime = 0;                        }                        break;                // called when the pattern to be flown is automatically restarted                case EVENT_LOOP:                         Serial.println("Looped WP Index");                        print_current_waypoint();                        break;                  }}

// called AFTER every control loopvoid mainLoop_event(void){        if (control_mode == LOITER){                if (wp_index == 2 && elapsedTime > 120000 ){ // 2 minutes                        elapsedTime = 0;                        // our waypoints index is not altered during LOITER                        // All we need to do is reload the waypoint                        load_waypoint();                        // and return to Autopilot mode!                        set_mode(AUTO);                }        }}

GAUI TG-609 from Michael Yang on Vimeo.