Post by ka9q on Jul 20, 2011 4:42:44 GMT -4
:-) Actually, I once shared that misconception too.
Water does have some microwave resonances higher up, e.g., around 24 GHz. If it resonated as low as 2 GHz, then S-band wouldn't be quite so ideal for space communications. US satellite radio broadcasting, i.e., Sirius/XM, operates near 2350 MHz, between the 2200 MHz Apollo band and just below the 2400 MHz WiFi/microwave oven/Bluetooth band.
The trend in space communications for decades has been to higher microwave frequencies. S-band (2200 MHz) was pretty novel for Apollo because VHF simply wouldn't work acceptably well at lunar distances. Now it's the old, slow, fallback technology. I think Voyager was the first to seriously use X-band (8.4 GHz); now it's the workhorse for interplanetary exploration. Now there's a push to Ka-band (20/30 GHz).
Not only does the available bandwidth increase with frequency, but the net effect on path loss and parabolic antenna gain improves the link margin 6 dB per octave. That is, for a pair of dishes of a given size and a given transmit power and receiver noise level, going from 2 GHz to 8 GHz increases link speed 16 times! The receiving dish still covers the same fraction of the sphere surrounding the transmitter, but the transmitting dish can focus a tighter beam on the receiver. (Throw in modern digital technology and HDTV from a LM on the moon would be a cinch. And it wouldn't require a huge radio telescope like Parkes.)
The big problem is rain fading, which gets worse as you go up. Fortunately, NASA had the foresight to site its Deep Space Network stations in arid locations like Madrid, Spain; Goldstone, CA; and Canberra, Australia. Same with sites doing radio astronomy in the high microwave region.
Eventually you do hit resonances in other atmospheric gases. The most important are the strong diatomic oxygen absorption lines at 60 GHz and 120 GHz that make the atmosphere virtually opaque. If/when 60 GHz RF hardware gets cheap I expect it will become very popular for unlicensed short-range WiFi-like networks with much more bandwidth than the existing 2.4 and 5 GHz allocations. These frequencies are also ideal for intersatellite linking, as they don't pass through the atmosphere. Interestingly enough there's a 'window' between 60 and 120 GHz where absorption is relatively low (except when it rains) and the FCC has opened up much of it to very high speed (multi gigabit) terrestrial point-to-point links.
Interplanetary links will ultimately have to go optical. To avoid clouds they'll have to be sited in space, and that could get expensive. A really good interplanetary optical transceiver will look a lot like the HST, and that could obviously get pretty expensive.
Water does have some microwave resonances higher up, e.g., around 24 GHz. If it resonated as low as 2 GHz, then S-band wouldn't be quite so ideal for space communications. US satellite radio broadcasting, i.e., Sirius/XM, operates near 2350 MHz, between the 2200 MHz Apollo band and just below the 2400 MHz WiFi/microwave oven/Bluetooth band.
The trend in space communications for decades has been to higher microwave frequencies. S-band (2200 MHz) was pretty novel for Apollo because VHF simply wouldn't work acceptably well at lunar distances. Now it's the old, slow, fallback technology. I think Voyager was the first to seriously use X-band (8.4 GHz); now it's the workhorse for interplanetary exploration. Now there's a push to Ka-band (20/30 GHz).
Not only does the available bandwidth increase with frequency, but the net effect on path loss and parabolic antenna gain improves the link margin 6 dB per octave. That is, for a pair of dishes of a given size and a given transmit power and receiver noise level, going from 2 GHz to 8 GHz increases link speed 16 times! The receiving dish still covers the same fraction of the sphere surrounding the transmitter, but the transmitting dish can focus a tighter beam on the receiver. (Throw in modern digital technology and HDTV from a LM on the moon would be a cinch. And it wouldn't require a huge radio telescope like Parkes.)
The big problem is rain fading, which gets worse as you go up. Fortunately, NASA had the foresight to site its Deep Space Network stations in arid locations like Madrid, Spain; Goldstone, CA; and Canberra, Australia. Same with sites doing radio astronomy in the high microwave region.
Eventually you do hit resonances in other atmospheric gases. The most important are the strong diatomic oxygen absorption lines at 60 GHz and 120 GHz that make the atmosphere virtually opaque. If/when 60 GHz RF hardware gets cheap I expect it will become very popular for unlicensed short-range WiFi-like networks with much more bandwidth than the existing 2.4 and 5 GHz allocations. These frequencies are also ideal for intersatellite linking, as they don't pass through the atmosphere. Interestingly enough there's a 'window' between 60 and 120 GHz where absorption is relatively low (except when it rains) and the FCC has opened up much of it to very high speed (multi gigabit) terrestrial point-to-point links.
Interplanetary links will ultimately have to go optical. To avoid clouds they'll have to be sited in space, and that could get expensive. A really good interplanetary optical transceiver will look a lot like the HST, and that could obviously get pretty expensive.