Venus Express / Mars Reconnaissance Orbiter Monitoring                                                                 updated: (15/05/2009 13:04:55 +0100)

The goal of receiving an inter planetary space probe has finally been realised ;-) - the primary objective of building a working 8.4GHz DSN receiver culminated with the reception of the tracking signal from Venus Express at 8.4GHz today the 5th of December 2005 at 15:50UTC. The secondary objective is to improve the receiver and listen for and hear the MRO signal which is also at 8.4GHz. The inspiration for this project came from the amazing demonstration of MRO and VEX reception by James Miller G3RUH and Freddy de Guchteneire ON6UG at the UK Microwave Group meeting held in November. To see the 'current' status of amateur reception of VEX, click here.

Introduction.

Venus Express was launched by the European Space Agency on the 9th of November 2005 from the Baikonur Cosmodrome in Kazahkstan. Today (5th December 2005) its fairly distant, with the one way light time being around 0.361356 minutes, this means that the approximate distance is just a touch over 4 million miles - that is good DX by any standard. Venus Express transmits a tracking / telemetry beacon on DSN channel 17 which is 8419.074074 MHz when it leaves the space craft. Since the space craft is flying away from Earth at a fair speed, there is a doppler shift which has to be taken into account when tuning for the signal. The antenna on the space craft is 1.3m diameter, with an approximate gain of 41.23dBi, making the EIRP about 516Kw.

The receive system basically consists of the same LNA and down converter that is used for 7GHz to 8GHz reception but with the addition of a 8420MHz band pass filter.

The receive antenna used for this test was a 1m fibreglass offset dish, with circular polarity feed. The feed with 'integrated' LNA can be seen below. The LNA has been removed from a Marconi Ku band LNA used for satellite TV reception. The LNA stages have been retuned for 8GHz by tabbing them with copper foil, and an 8GHz RF output has been added after the third stage. (details at the bottom of this page)

For this test, the down converter was loosely tied to the feed arm of the dish. The oscillator in the down converter runs at 8GHz and is fed with an 80MHz reference from a GPS locked source / frequency synthesiser. The frequency at the time (15:30 UTC 5/12/2005) of reception was 8419.0010 MHz. The JPL horizons website was used to calculate the azimuth / elevation of the space craft, then using a reference from an X-band communications satellite, the appropriate azimuth / elevation was calculated.

The two pictures below show the band pass filter that was designed with the G3JVL filter software. It is a 2 cavity filter built in WG16. Once the filter had been tuned up, it had 0.5dB through loss at 8420MHz, with a bandwidth of about 50MHz. The plot on the right shows a sweep of the filter - the peaks can clearly be seen. Checks on the image frequency of 7580MHz showed that the filter has over 50dB of rejection - this should solve the image noise problem, and make the receive signal a lot better - in fact hopefully it'll allow the reception of the MRO probe too!

The spectrum plot below clearly shows the carrier drifting in frequency as the relative position / motion of the Earth and the Venus Express probe changes. The image below was captured using an RFSpace SDR14 Software radio. The first picture was taken in a 3KHz span, whilst the next picture was taken in a 5KHz span. The excellent thing about using the SDR radio is that your actual receiver doesn't have to be spot on frequency, yet you can still see the signal in the SDR and peak up the dish etc, then when satisfied that everything is aligned, tune the receiver to hear the carrier.

LNA modification details.

The LNA is made by modifying a Marconi Ku band LNB. Basically the modifications involve chopping out the DRO, IF amplifier, 11GHz filter and the mixer assembly, removing the tuning tabs around each Gaasfet, and replacing the waveguide / antenna assembly with one more suitable for 8GHz. For the new waveguide, I used 28mm copper water pipe couplers soldered together.  The LNA is tuned up on a weak signal by experimenting with the positioning of the tabbing around the active devices, whilst monitoring the output on a receiver or spectrum analyser. The red circles in the photos below indicate where a modification needs to take place, by adding tuning tabs or removing tracks. The output of the 3 stage LNA is coupled to coax using a suitable DC blocking SMD capacitor - in this case it was from the same board, located next to the mixer. The 1/4 wave probe also has to be lengthened due to the slightly lower frequency. The photos below show the modifications performed.

Results after adding the 8.4GHz band pass filter.

The waveguide filter described above has made about a 4 to 5 dB improvement in the received signal, presumably due to the removal of the image noise. The two images below show the down converter with added filter, and the improvement in received signal. The probe is further away today, at 4.17 million miles. If you want to listen to what the signal sounds like, click here

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