Cassini Huygens mission to Saturn and Titan
Here is errata on the way Cassini operates the radio transmission system both to
send and receive commands. Note that the RF (radio frequency) modulation is in
GHz (Giga Hertz) not MHz (Mega Hertz).
According to Daniel Vågberg of Umeå University, The powerful high gain transmitters on the Cassini probe operate
at very high frequencies... in the Giga Hertz range.
Download the PDF file for more information:
http://www.tp.umu.se/space/Proj_05/Daniel.W.pdf
"The Radio Frequency Subsystem (RFS) provided the telecommunications facilities for the spacecraft and was used as part of the radio science instrument. For telecommunications, it produced an X-band carrier at 8.4 GHz, modulated it with data received from the CDS, amplified the X-band carrier power to produce 20 W from the Traveling Wave Tube Amplifiers (TWTA), and delivered it to the Antenna Subsystem (ANT). From ANT, RFS accepted X-band ground command/data signals at 7.2 GHz, demodulated them, and delivered the commands/data to CDS for storage and/or execution."
"The Ultra Stable Oscillator (USO), the Deep Space Transponder (DST), the X- band Traveling Wave Tube Amplifier (TWTA), and the X-band Diplexer were elements of the RFS which were used as part of the radio science instrument. The DST could phase-lock to an X-band uplink and generate a coherent downlink carrier with a frequency translation adequate for transmission at X-, S-, or Ka-band. The DST had the capability of detecting ranging modulation and of modulating the X-band downlink carrier with the detected ranging modulation. Differenced one-way ranging (DOR) tones could also be modulated onto the downlink. The DST could also accept the reference signal from the USO and generate a non-coherent downlink carrier."
SOURCE: University of Michigan
The conversion factor is 1000
megahertz * 1000 = gigahertz
gigahertz / 1000 = megahertz
1 Giga Hertz is equivalent to 1000 Mega Hertz (GIGA bytes is 1000 Megabytes and so on and so forth)...
Or in other words, 864.5 MHz would be around .8645 GHz. Well below the range of Cassini.
Probably because a wireless transmitter would have a longer wavelength and would lack the
capability to cross the vastness of space within the timeframe necessary
for transmission of commands and the reception of probe data.
Various transmitters are stationed around the earth to enable communication with the satellite as long as it is
within the line of site and not on the opposite side of the planet. A low-gain antenna with a wider beam is used to orientate the probe.
This enables the high-gain antenna with the narrower and more powerful beam to send and receive
the vast amounts of data required to operate the computer systems on the Cassini
probe.
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