Telemetry on the KC0YA-11 GPS/APRS Package

by Rick von Glahn, N0KKZ

EOSS currently has two payloads that have onboard KPC3+ TNCs, this one and the AE0SS-11 system. Because the KPC3 can transmit telemetry, we take advantage of that and equip the payload systems with three sensors. Inside temperature, outside temperature and barometric pressure. There are other channels available on the KPC3 and we sometimes take advantage of them to monitor other values, most commonly, the bus voltage, but sometimes we add another channel to monitor if a signal has been received or a relay fired.

The KC0YA-11's current configuration sends down the bus voltage, internal and external temperatures, barometric pressure and a 5th channel that is currently monitoring whether or not a piece of equipment has functioned.


Figure 1.  Temperature Data from EOSS-103

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:


The components of the packet are:

  • Call sign, in this case KC0YA-11
  • The unproto destination field
  • Telemetry packet number, in this case number 122 (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 first telemetry word immediately follows the telemetry packet number.

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

Telemetry Frame Number

It's 122

First word: bus voltage

Multiply the telemetry word by 0.0236 and the answer is in volts. For the example packet, the battery voltage is 4.9796 volts.

Second word: Internal Temperature

Multiply the telemetry word by 1.952941 then subtract 273.15 to produce degrees Celcius.

For the example packet:

(138 * 1.952941)- 273.15 = -3.64142 deg C.

Third word: External Temperature

Same as internal so for the above example packet

(119 * 1.952941)- 273.15 = -40.75 deg C.

Fourth word: Baro Pressure

This value is directly converted to altitude in feet above sea level. But we refer to it as baro pressure because it is not a direct altitude measurement system (GPS, Radar). Because this is a new unit the equation is very rough. It will be refined over future flights.

Altitude = 89563 * 2.7182818 ^ (-0.0097665 * telemetry word)

So, for the example telemetry string:

89563 * 2.7182818 ^ (-0.0097665 * 48) = 56044.74 ft. ASL

Fifth word: outside temperature

Currently (EOSS-105) monitoring a new B-LAN (balloon local area network) system. Reports back if commands have been successfully passed along the b-lan.