Mike as written a plain text file describing this process. I have formatted it for the web and placed it HERE, at the bottom of the page and to the left. Included in that page, a baro vs. gps altitude graph and links to the original plain text file and a link to the full log of EOSS-49.

by Nick Hanks, N�LP

EOSS uses one of two shuttles on it flights. One is the cross-band repeater and the other is the APRS shuttle. Both transmit their GPS latitude, longitude and altitude. But the APRS shuttle also sends down five telemetry values. The values are: battery voltage, reference voltage, barometric pressure, inside temperature, and outside temperature. Figure 1 shows the temperature and barometric pressure (shown as altitude) data from EOSS-51.

Figure 1. Barometric Pressure and Temperature Data from EOSS-51

These values are transmitted on the same downlink as the GPS data. The data is transmitted roughly once every minute in a packet similar to the APRS packet used for the position data, i.e., the AX-25 format. The exact format of the telemetry packet is:

W5VSI-11>BEACON <UI>:T#034,087,126,149,147,146,00111110

The components of the packet are:

- Call sign, in this case W5VSI-11
- Mode, in this case BEACON
- Packet type is an unnumbered information packet, <UI>
- Telemetry packet number, in this case number 034 (the numbers are sequential)
- The five telemetry words which range in value from 0 to 255
- The final eight-character value, 00111110, is a digital word and is currently not used.

The values of the telemetry words can be converted to engineering units as follows:

Divide the telemetry word by 0.1 and the answer is in volts. For the example packet, the battery voltage is 8.7 volts.

Divide 256 by the telemetry word and then multiply by 2.46.

For the example packet:

(256/126) * 2.46 = 4.99.

The value is in volts. This should be a constant 5 volts throughout the flight. (4.99 is close enough.) The next telemetry values use this reference voltage.

This value can be converted into the balloon�s altitude. However, this takes a bit of figuring. You will need a calculator that can do X to the Y power. The calculator in Windows can do this as can an Excel spreadsheet. First, divide the reference voltage value (the second telemetry value) by 256 and then multiply by the barometric pressure word.

For the example packet:

(4.99/256) * 149 = 2.904.

This value is in volts and is the output of the barometric sensor. Now to convert this value to altitude in feet, use the following formula:

If the voltage is greater than 1.4v then

altitude = 3620 * BP

^{2}� 32829 * BP + 73431

If the voltage is less than 1.4v: then

altitude = 55560 *BP

^{-1.444334}

where BP is the sensor output in volts.

For the example packet, the altitude is 8,264 feet.

Fourth word: inside temperature

Divide the reference voltage (the second telemetry value) by 256 and multiply by the telemetry word. Then multiply by 100. Then subtract 273.16. For the example packet:

((4.99/256) * 147) * 100 - 273.16 = 13.4

The value is in degrees Celsius (C). To convert to degrees Fahrenheit, multiply by 9/5 and add 32.

13.4 * 9/5 +32 = 56.1 degrees F

Use the same process as for the fourth word. For the example packet, the outside temperature is: 11.4 degrees C or 52.6 degrees F.

A word about the outside temperature measure may be useful. Because of the location of the probe it tends to self-heat some. We�re working on improving its location, but until it�s changed you�ll find that the values at altitude are higher than they should be. This is because the small probe current has more of a heating effect at very low temperatures than it does at higher ones.

The equations presented are still being refined. For example, there is a known error because the equations don�t account for local pressure. If the local barometric pressure is high, the altitude value will be lower than it should and vice versa. We�re working on a fix for this.

In addition, air pressure does not vary linearly with altitude and cannot be modeled accurately with a single equation. This is why the equations to convert the sensor voltage to altitude have power terms in them, and why there are two equations. (We calibrate the sensor data against the downlinked GPS data. The equations are from the data reduced from EOSS-50.)

Also the analog-to-digital converter in the shuttle has eight bits of accuracy. So when the balloon is up high, we�re working with small voltage changes (from 50,000 ft. to 80,000 ft. the sensor output voltage changes by about 0.2 v). So a change of one bit can represent many feet. Specifically, a one-bit change is equal to about 0.0195v (5.00 volts divided by 256 possible word values). So over the 30,000 feet between 50,000 and 80,000 feet, we would see the telemetry word change by a count of 10 (0.2/0.0195) or roughly one bit per 3,000 feet. This makes it tough to achieve an accurate curve fit!

Hopefully this data will increase your enjoyment of the EOSS flights. We are always looking for new ideas or ways to increase the effectiveness of our education mission. If you have an idea (or two or three) or a question, please drop me an e-mail.

Nick Hanks, N�LP

Here is Mike's explanation and data in a text file

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