How to receive from ³Cat-2

The ³Cat-2, as a satellite operating in an amateur bands, transmits periodically an open beacon signal (VHF band) that carries telemetry data of the satellite and which can be easily decoded with minimum amateur RF equipment. There already exists a big community of radio amateurs which continuously listen and decode signals from ³Cat-2 and help us track the satellite, and we would really appreciate it if you became one. The beacon signal contains basic information about the satellite status such as the mode in which the satellite is operating, the average temperature, battery voltage, current consumption and the pointing of the satellite (attitude).

The ³Cat-2 VHF beacon is transmitted every 20 or 40 seconds (depending on the battery state of charge) at the frequency of 145.970 MHz in compliance with the AX.25 protocol and using a BPSK modulation at 9k6 bps and with no Forward Error Correction mechanisms. The satellite transmits in linear polarization from two redundant transmitters with different call-signs.

To receive and properly decode beacon messages from ³Cat-2 a VHF antenna tuned at the 144-146 MHz band is needed. Despite an omnidirectional antenna is, in some cases, enough to receive the signal from the satellite, a Yagi (or similar) antenna in circular polarization is recommended to obtain the best SNR. After the antenna it is recommended to use a selective RF chain to properly filter and amplify the signal. The RF chain should be designed and tuned depending on the antenna temperature and interference in your area. However, in general, we recommend to have a selective (narrow-band) LNA followed by a filter. In case of using a wide-band LNA or having interference in your area, we recommend to add a very selective filter before the LNA. This worsens the noise figure of the receiver but avoids inter-modulation products created by the interferences inside the LNA. After the RF stage a receiver is needed to demodulate and decode the signal. We recommend to implement a BPSK demodulator with a Software Defined Radio (SDR)-based receiver. There exists a large variety of SDRs in a wide price range and many of them might be good enough to receive from ³Cat-2. We normally use the USRP B200 or B210 from Ettus Research in our ground stations but something in a lower price range might be appropriate as well to properly receive and decode the beacon signal.

Right now, we don't have confirmation that an RTL-SDR is enough to receive from ³Cat-2 but it may be possible if using a selective enough RF chain.

In our main ground station in Barcelona we use a high gain VHF Yagi antena working in circular polarization and with 14dBi of gain followed by a band pass filter centered at 144-146 Mhz with about 20Mhz of bandwidth. After the filter there is a selective LNA with approximately 20dB of gain and 0.9dB NF. In Barcelona we have a strong interference and a high noise floor and that is why we prefer to keep the filter before the LNA. After the LNA we have a USRP B200 receiver working along with a GNU-Radio based software to decode the received packets.

The ³Cat-2 beacon is a very short and simple telemetry downlink which includes critical information from the on-board sensors. An example of a beacon signal can be:

In which values are separated by spaces and tabs (\t). The meaning of each value is the following:

1. Mode. Possible values: (1) survival mode, (2) sun-safe mode, (3) nominal mode, (4) TX communication incoming (data downlink), (5) RX communications (command uplink), (6) and (7) payload mode.
2. Battery voltage in mV. In the example 7.781 V.
3. Current consumption in mA. In the example 245 mA.
4. EPS temperature (in ºC). In the example 7 ºC.
5. Antenna temperature (in ºC). In the example 6 ºC.
6. Status of the ADCS system. (0) means Detumbling enabled. (1) means Nominal ADCS.
7. Control flag of the ADCS routine. Possible values: (0) automatic ADCS transitions, (1) manual ADCS transitions.

The next 3 fields are floating point numbers. If detumbling is enabled, they correspond to magnetomer values in nT for the axes X, Y and Z respectively. If detumbling is not enabled, they correspond to the sun vector, axes X, Y and Z.

The last 3 fields correspond to the control voltages for axes X, Y and Z, regardless of whether the magnetorquers are enabled or not.