ESA's Rosetta Mission                                                                                                                      updated: (13/11/2009 09:15:10 -0000)

Update 13/11/2009 09:00 GMT  - Rosetta has just completed its final Earth swing by, and in the course of doing so, provided some excellent signals on S-Band, which were tracked by amateur-DSN enthusiasts;

As can be seen in the above image, the Doppler plot clearly shows a curve, caused by acceleration of the space craft. The image below shows the S-Band signal complete with the sidebands containing the TT&C data.

Update 4/5/2006 12:36 BST   - Rosetta test results with a Peltier cooled LNA. The noise floor drops by 0.5dB and hence increases the SNR of the received signal.

Update: 1/05/2006 16:29 BST

After having my DSN LNA measured at the RAL microwave meeting and being a little unhappy with the results, I put together a proper noise figure measurement setup and started again with my LNA's tuning. The result is that I can now HEAR Rosetta in the head phones and use the signal to peak the dish, it seems to work very well indeed. The FFT Spectran picture below shows the Rosetta carrier clearly visible out of the noise.

Update: 31/03/2006 11:35 BST   

Rosetta reception has been achieved using the small 1.8m dish. The signal was easily visible in the FFT, but not audible. Its current frequency is 8421.873 MHz and the space craft is 242,198,000 miles away from earth, not bad DX at all. The picture below shows the Spectran FFT plot, with the 'vertical' white line being the signal from the space craft.

 

Today (26/02/2006) we decided to test DSN reception with a 3.7m dish that was available. Using the recently rebuilt DSN / X-band down converter, signals were copied from the MRO space craft at amazing strength, but also the Rosetta spacecraft. Rosetta is on its way to visit a comet called "Comet 67P/Churyumov-Gerasimenko" - it will arrive there in 2014, so we can have this nice signal source for a few more years yet. At the time of receiving the signal, the space craft was 243,174,748.9 miles away - a considerable distance. The antenna on the Rosetta space craft is 2.2m diameter - its not known what the transmitter power is.

The picture on the left shows the recently rebuilt DSN down converter. There are now two phase locked oscillators to give non-inverted IF's when tuning the x-band and the DSN band. Front end band pass filters are switched in to eliminate image noise, and the output from these filters is buffered by a single stage low noise gaasfet preamplifier before feeding the mixer - this LNA was added to help over come the losses in the filter, which total about 2dB including switching.

The IF amplifier has also been rebuild using a MAR-6 super and this gives a reasonably flat response from 100MHz to 1GHz, so ideal in this IF amplification application - it was noted that with the previous Avantek gain block, the shot noise was crippling the AGC of the receiver, this resulted in what sounded like very high oscillator noise. With the MAR-6 super, this problem has completely gone away.

The 'new' 8.4GHz band-pass filter was obtained in the UK from abex.co.uk

The block diagram on the right shows the 'final version' of the dual band down-converter. As can be seen the overall system diagram is shown, with the out-door unit being shown in the red box. The ODU is fed with +24v supply, a 100MHz reference signal, and it returns a 10MHz to 1GHz IF.

The switching is arranged so the LO is always on the low side of the received signal, resulting in the correct sidebands being received. The appropriate band pass filter is also switched in. After the initial build, it was decided that a post filter preamplifier should be incorporated to overcome the losses that are caused by the filters. At worst, the filters give 2dB loss, but the preamp has 10dB gain at 8GHz so is more than enough to overcome any losses (also losses in the two SMA relays).

The 7GHz phase locked oscillator requires -24v to run - this is obtained by a DC-DC converter that can generate about 600mA.

The DSN channel info for Rosetta is as follows:

DSN tracking spreadsheet DSN  Spacecraft TX  Velocity Doppler corrected Doppler shift
Mission Name Channel frequency (MHz)  (KM/s) (MHz) (MHz)
Rosetta 19 8421.790123 2.56475 8421.718074 -0.072049

Today's frequency was 8421.718074 as per the calculation.