Telemetry on the W5VSI GPS/APRS package
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,
NØLP
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:
First word: battery voltage
Divide the telemetry word by 0.1 and the answer is in volts. For the example
packet, the battery voltage is 8.7 volts.
Second word: reference voltage
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.
Third word: barometric pressure
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 * BP2
– 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.
note: in the above formulae, the numbers
following BP are exponents
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
Fifth word: outside temperature
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.
More about Barometric Pressure
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, NØLP
Here is Mike's explanation and data in a text
file
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