K3TBG - Education in Acoustics

Of the many projects in acoustics education, a few make use of amateur radio. One of these radio-related projects uses RF telemetry of atmospheric properties from balloons. Since these are purely educational exercises, the flights are rare. The telemetry is described below.

Telemetry

  1. Frequency and mode: 433.9 MHz, CW

  2. Power output: 5 - 50 milliwatts into a quarter-wave ground-plane antenna

  3. Platform: meteorological balloons

Telemetry System

An embedded microcontroller (typically a PIC12C509) is used to control a pair of 12-bit A/D converters. The microcontroller also converts the A/D output (in hexadecimal form) to Morse Code and keys a low-power 433.9 MHz transmitter. The transmission is at 12 WPM with 5 pairs of data words transmitted every minute. Morse Code was chosen because of the combination of simplicity of implementation and the potential for reasonable ranges with low-power transmitters.

Coding

Measurements made are typically temperature and pressure. The measured values are converted to three "digit" hexadecimal values and transmitted by Morse Code. The transmission sequence is a repetition of: a separator character, measurement channel 1 (three hex characters), measurement channel 2 (three hex characters). The separator character is a Morse period if the telemetry package is ascending. The separator changes to a string of seven "dits" after the balloon bursts. Every ten minutes, the station call sign is transmitted.

Interpreting the values

Since it is not permissible to transmit information via amateur radio that coded to conceal the content, the following information is provided to interpret the telemetry (and this web site is linked through the QRZ database).
  1. Converting to decimal: Each measurement is coded in three characters, C1, C2, and C3 (in that order). For example, a measurement might be coded as B3E. Each character has a value - the numbers 0 through 9 have their own value; the letters A through F have the values 10 through 15. First, convert the characters into their values. For the example, B3E, the values are 11, 3, 14. The decimal value of the raw measurement is then 256*C1 + 16*C2 + C3. For the example given, the decimal value is 256*11 + 16*3 + 14 or 2878. Some spreadsheets can do this conversion through a "hexadecimal-to-decimal" conversion function.

  2. Converting a temperature value: The decimal value, Dt, from the temperature channel has a gain, Gt, and an offset, Xt. The temperature in degrees Celsius is Gt*(Dt - Xt). The gain and offset are listed below and are specific to an individual flight.

  3. Converting a pressure value: The decimal value, Dp, from the pressure channel has a gain, Gp, and an offset, Xp. The pressure in pascals Gp*(Dp - Xp). The gain and offset are listed below and are specific to an individual flight.

Calibration values

Flight number and date: none current

  1. Temperature gain:
  2. Temperature offset:

  3. Pressure gain:
  4. Pressure offset: