# Garmin Astro DC 30 - Altitude

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#### Arnold

##### New Member
Anyone know if the Astro DC 30 reports altitude? And if it does, would you know the ceiling?

#### Stymye

##### Well-Known Member
it's claims to , it has a barometric altimeter just like many rocket electronics boards.

interesting !

#### new2hpr

##### Well-Known Member
The Astro main unit has the baro altimeter, but the remote (dog) units do not. I believe the dog units store z-axis data along with x and y in their log, but do not transmit the z data live.

-Ken

#### bobkrech

##### Well-Known Member
The GPS altitude obtained from the dog collar will not be as accurate as your lat/lon fix because of the satellite locations.

The probability of satellites being near the horizon is much greater than the probability of a satellite being overhead, so the lat-lon fixes are derived by multiple satellites whereas quite a few times where there's no overhead satellite so the error in altitude can be an order of magnitude higher for altitude than lat-lon. The lat-lon accuracy of a GPS should put you with 10 M of your actual location and give your altitude with 100 M.

WAAS can reduce the error by an order of magnitude if the signals are strong. With WAAS and good signal your likely to be within 2 M of your true lat-lon location and with 20 M of your true altitude and frequently with half that distance.

He's what Garmin has in the ASTRO on-line Q&A section.

Question: </SPAN> How accurate is the GPS elevation reading?

Answer: GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, they are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.

Question: </SPAN> How accurate are the calculations when using a GPS with a pressure altimeter?

Answer: Using the autocalibration feature on the device, the accuracy is +/- 50-125ft (same as GPS elevation). When manually calibrating (autocalibration off) to a known elevation it is approximately +/- 10ft for the first 15 minutes. The unit should be calibrated hourly to maintain accuracy when using manual calibration because of possible preassure changes.
If using a locally reported pressure reading to calibrate the device, the accuracy of the altimeter would depend on the time and location of the reading.
With WAAS and good reception you can obtain an accuracy of +/- 25-50ft.

Bob

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