Mars Reconnaissance Orbiter Reception updated: (15/05/2009 12:55:09 +0100)
As you will have probably read on the Venus Express page, successful reception of that space craft occurred with a home made down converter. Following this achievement, it was decided that reception of the next furthest spacecraft should be attempted.
|The inspiration for building the 8.4GHz down converter
came from a demonstration that was given by
James Miller G3RUH and
de Guchteneire ON6UG at the
UK Microwave Group microwave meeting in November 2005. James and
Freddy have had a fair amount of experience in receiving these
deep-space-probes, and there is a nice page of useful information at the Amsat-DL site
The photograph on the left shows the experts, James and Freddy, using a home made 1 metre mesh dish, an off-the-shelf DB6NT 8.4GHz LNA and down converter. In the photo, Freddy is listening directly to VEX just with open waveguide! The DB6NT DSN converter gives an output IF of 23cms. The signals during the demonstration were consistent and copy able in SSB audio bandwidth, with Venus Express being a lot stronger than the Mars Reconnaissance Orbiter. The feed used on the 1m dish is a dual circular polarisation waveguide with a septum plate in it. The DB6NT LNA shown in the picture has a waveguide input and is coupled off the relevant port in order to achieve reception of RHCP signals from the Orbiter.
The Mars Reconnaissance Orbiter (MRO) was launched on August the 12th, 2005 from the Kennedy Space Centre in Florida. At the time of reception, the one way light time was 4.045806 minutes, making the Orbiter 45,151,194 miles away from Earth. Once the MRO reaches Mars, it will orbit the planet taking photos and performing other science experiments. The MRO transmits on DSN channel 32 which is 8439.444444 MHz - by the time that reaches Earth, due to Doppler the frequency has dropped to around 8439.031 MHz. The MRO has a 3 meter diameter dish antenna driven by a 100 Watt TWTA to transmit signals to Earth - this means that the signal coming in our direction is of the order of 4.2 mega watts of RF.
Using the same receive system as used for Venus Express, the Doppler was first calculated and the receiver was tuned - the dish was adjusted as per the az/el generated by the JPL Horizons software. The picture below shows the MRO receive setup, with Mars and the Moon being visible above the dish.
The signal was clearly visible in the FFT display of the SDR-14 software radio, and just audible in SSB bandwidth of a communications receiver. The signal was consistently about 6 to 8 dB above the noise floor. The FFT picture below was taken in a 2KHz bandwidth centred on 10.7MHz which is the IF of the receiver used. All oscillators are locked to GPS. Technical details of the down converter and LNA are on the VEX page.
There is a rather large WAV file of the signal (1.6Mb) - its best to download the file, then process it in your favourite FFT software in order to see the carrier, as its weak and may not be audible to the untrained ear - the file can be found here. For processing the WAV file, it is worth checking out WaveVue and Spectrum Lab. There is another interesting page written by Rob Hardenberg about receiving the UHF beacon from MRO - its located here.
Just for interest, the picture below is the block diagram of the down-converter used to convert the 8.4GHz signal down to a suitable IF frequency - in this case the IF is around 400MHz. Again, further details of this down converter can be found on the VEX page.
Following some further work on the LNA (a modified Marconi Ku band LNB) by adding a new feed that came from Bertrand Pinel's (F5PL) design which uses two sets of screws for the de-polarisation and replacing the front end GaasFet in the LNA with a MGF4918D, the performance improvement has been astonishing. The spec of the GaasFet shows 0.6dB NF @ 12GHz, with an associated gain of 10dB. The signal is more than 3 or 4 dB higher than the above examples.
Also, from this reception test, there are two WAV file recordings here and here - you can now easily hear the carrier from the MRO. As above, use your favourite FFT program to process the sound files. At the time the wave files were recorded, the MRO spacecraft was more than 53258740 miles away. During the reception, what looked like 2 sidebands, 3.24KHz +- the carrier were detected. James Miller, G3RUH has kindly been calculating CNR's for the recordings - tonight's is 24.7 dB-Hz, which is a staggering 14dB improvement over the 'old' LNA. The author wishes to thank James, G3RUH for his continuing support and help with these receive tests.
Received MRO again this evening, signals were about 4 to 5dB above the noise, and copyable in SSB. The MRO was 69593573 miles away which is 25 million miles further away than when I first copied a signal from it! The photos below show some close-ups of the LHC feed horn with depolarising screws, and the feed horn mounted on the feed arm of the 1m dish.
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