bnp (04/28/83)
As the Pioneer spacecraft leaves the solar system, it will be capable of transmitting data, but will the receivers tracking it be able to receive the data? I would imagine the signal strengths involved in radio astronomy are less, but are the tracking stations used for Pioneer, equipped to work with the increasingly weak signals from the spacecraft? At what range will contact be lost? Bruce Peters BTL - Naperville, Ill.
alb (04/28/83)
At present, Pioneer emits an 8-watt signal. By the time it reaches the orbiting receiving stations, it has been reduced to about twenty trillionths of a watt. At that point, it is still able to be read out of the background wash. Scientists expect to be able to 'hear' it until it travels about 5 billion miles outside the solar system.
karn (04/28/83)
I just did some calculations on Pioneer's likely maximum range. I had to make some educated guesses on numbers, so my conclusions may be somewhat off; if anyone has the correct figures, let me know and I'll update my calculations. Receiving station assumptions: 200' dish, 50% illumination efficiency (half the intercepted energy is lost) 8 GHz operating frequency; wavelength = 3.75 cm 50 degree Kelvin receiving system temperature T (good, but probably worse than the real figure) 100 Hz receiver bandwidth B (assuming a pretty slow data rate) A 10 db signal-to-noise ratio is required (probably too high) Such a dish would have a gain of +71 dbi (decibels above an isotropic or "point source" antenna). The receiver would have a noise floor of KTB = 1.38E-23 * 50 * 100 = 6.9E-20 watts, or -191.6 dbW. (K = Boltzmann's constant) Pioneer 10 assumptions: 8 watt transmitter output = +9 dbW 5' dish with 50% illumination efficiency == +39 dbi gain EIRP (effective isotropic radiated power) = +9 + 39 = +48 dbW Maximum allowable path loss X would therefore be +48 dbW - X + 71 >= -191.6dbW + 10; X <= 300.6 db The path loss in db is given by 22 + 20*log(distance in wavelengths) Solving for distance, we get 7.9E13 wavelengths, about 3E12 meters or 10,000 light-seconds. Pluto's orbital semi-major axis is about 5.9E12 m, so I'm at least in the right order of magnitude. Keeping everything else constant, the signal-to-noise ratio decreases by 6 db for each doubling of the distance, i.e., going to 6E12 meters would decrease the signal-to-noise ratio to 4 db. In any case, I'm sure that they don't have a lot of signal to spare, and that contact couldn't be kept with such a system very far outside the solar system even if the isotope generator were to last indefinitely. On the other hand, if you were to use the Arecibo dish... Phil