Communicating with Rosetta space probe -

Discussion in 'Off-Topic' started by GopherT, Nov 28, 2014.

  1. GopherT

    Thread Starter AAC Fanatic!

    Nov 23, 2012
    6,073
    3,856
    - Conceived in the 1990s
    - Final design and construction in the early 2000s
    - Launched in 2004
    - Designed for digital communication range of 1 billion km (600 million miles).
    - Commuicate via S-band (2 GHz) and X-band (8 GHz)
    - Antenna is 35 meter diameter (near Perth, Australia)
    - Data transmission rate is 10 to 22k bits per second

    http://www.esa.int/Our_Activities/Space_Science/Rosetta/Long-distance_communication

    Now, my expectation is higher than 22k Hz and i do not expect to have a 35 meter antenna but it is 10 to 15 years later and my new, top rated ASUS router also uses 2.4 GHz bands but cannot send a WiFi signal from my basement to the second floor!
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,399
    497
    I do have a question. How long does the signal takes to travel from the comet to Earth? How long does it take to cover the 1 billion km?
     
  3. GopherT

    Thread Starter AAC Fanatic!

    Nov 23, 2012
    6,073
    3,856
    About 55 minutes at the full billion kilometer range. It is not currently that far away. About half hour right now.
     
    Last edited: Nov 28, 2014
  4. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,399
    497
    Thank you.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    It's all about antennas, power levels, and propagation along the path.
    In amatuer radio antennas are everything.
     
  6. GopherT

    Thread Starter AAC Fanatic!

    Nov 23, 2012
    6,073
    3,856
    I also think it is the two steel beams used as a design feature in my house but that was intended to be a secondary comment. I am just amazed at the billion km range. I've seen the animations of the Rosetta probe, the antenna is not huge. 1/d^2 is big effect after a billion km. The on board power is not unlimited either. I just want to see what type of signal/noise they get and the filters and gating system they use to extract the weak signal and separate it from the interference.
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    When you know what you're looking for it makes thing easier. It is common for GPS signals from 22,500 miles to be pulled up out of the noise floor.

    Routers have omni-directional antennas. Dishes are highly directional. The difference is HUGE - LOL
     
  8. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,162
    The Earth stations sending data into space have much more power than any OTS router.

    GPS satellites orbit the earth at approximately 16,500 miles and have an 11 hour and 58 minute orbit. Back in 1979, it took 12 hours to get a sub-meter fix as the number of space vehicles were few.

    Geosynchronous satellites are positioned with 1 degree spacing above the equator at approximately 22,200 miles to achieve an approximate 23 hour, 56 minutes, and 4 second orbit, matching the earth's rotation.

    Voyager's comms are slow by todays standards at forty bits per second.

    Attached is some info on voyager's communications. The principles are the same for Rosetta.
     
    Last edited: Nov 29, 2014
    BR-549 and ISB123 like this.
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,913
    2,186
    http://blogs.esa.int/rosetta/2014/08/05/tracking-the-spacecraft-following-a-comet/
    I think Rosetta uses coherent data downlinks but what seems like pure RF noise is often not.
    This is about hiding signals in noise but the same principles apply when detecting a known very weak signal below the noise threshold.
    http://www.radioeng.cz/fulltexts/1995/95_02_06.pdf
    http://www.ausairpower.net/OSR-0597.html
     
  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,399
    497
    There was documentary on PBS about Rosetta. One of the sections of the documentary was the stage where Rosetta came out of hibernation and was supposed to send a signal to Earth verifying that it woke up. They showed the screen with the noise floor and them waiting for the signal spike to show up. They had 2 dishes looking for the signal, one in Australia, one in US. If I remember right, Rosetta was 6-10 hours overdue (you can imagine the commo team pulling their hair out), they showed that US dish got the spike, but Australian dish did not, so they were not sure if they actually got the right signal. Then both dishes showed spikes and this confirmed that the Rosetta woke up and ready to approach the comet.

    Papa is right. They were looking at specific frequency. The screens that they showed in the documentary had a marker at that specific frequency. They knew what to look for, where to look for, when to look for. All they had to do is sit and wait for it to show up. Which it did about 6-10 hours late.
     
  11. GopherT

    Thread Starter AAC Fanatic!

    Nov 23, 2012
    6,073
    3,856
    I cannot imagine that a signal is "late". If anything is easy, time is easy in electronics. What I think took time was the tuning and filtering and gating and signal conditioning and antenna alignment or what ever was happening behind the scenes at the antenna dishes that took six hours before they could identify the signal.
     
  12. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,399
    497
    Well, the documentary was a bit dramatized... American style... they talked about hibernation and how it was later add on, and how they were not sure if the Rosetta would wake up at all... all very dramatic...
     
Loading...