I have some questions about Messenger's communication subsystem, could anybody help?
1. It is said that the advantage of phased array antenna is that they can be electronically steered - which mean Messenger can keep communicate with the Earth even while moving/rotating without using its thrusters to stabilize?
2. What is the gain for this antenna compared to a parabolic one? It is said that phased array antennas have lower gain than parabolic ones at the same power, is it correct?
3. As I saw in the s/c design diagram at http://messenger.jhuapl.edu/the_mission/sp...aft_design.html, Messenger has 2 phased-array antennas mounted on opposite sides of the s/c. Are they simply providing redundancy or are both required for the s/c to have full sky coverage?
4. Regarding the above question, what is the sky area (in degree) that one antenna can cover?
5. Do you know of any planned deep-space mission that's going to use the same antenna as Messenger's?
6. Having to know about the phased array antennas, I see they're really magnificent things. But do they have any major drawbacks for use in a deep-space mission? Do they require any special requirement on the ground station?
Thanks,
Thu
Full Version: Messenger's Communications System - Some Questions
1. yes
3. It is so Messenger can keep the sun shade always pointed at the sun. A standard dish (like Deep Impact's) would not work because it would need to be shaded and the shade would block transmissions at times.
3. It is so Messenger can keep the sun shade always pointed at the sun. A standard dish (like Deep Impact's) would not work because it would need to be shaded and the shade would block transmissions at times.
2. Yes, the gain is lower for a phased-array antenna vs a parabolic. For a parabolic, you get your energy directed in the direction you are going. For a phased-array, you are essentially building a photon wavefront electronically. Because of this your losses are higher. But you gain some advantages by not having a big dish, such as weight, and directionality without mechanicals.
5. Don't know about deep-space, but the Iridium constellation uses phased array antennas. This is the source of the (in)famous Iridium flare. The arrays were covered in a flat reflective material that essentially turned them into mirrors. Because of their precise positioning and direction requirements, it's possible to predict very accurately when and where flares will happen.
6. I think their drawback for deep-space missions is their lower gain than a parabolic dish. I don't think they require any special ground-station requirements.
5. Don't know about deep-space, but the Iridium constellation uses phased array antennas. This is the source of the (in)famous Iridium flare. The arrays were covered in a flat reflective material that essentially turned them into mirrors. Because of their precise positioning and direction requirements, it's possible to predict very accurately when and where flares will happen.
6. I think their drawback for deep-space missions is their lower gain than a parabolic dish. I don't think they require any special ground-station requirements.
3/4
No, two arrays is not enough for whole-sky coverage. In theory a phased array can cover a hemisphere (180 degrees), but it doesn't work in practice.
The smallest number of array that could cover the whole sky would probably be four arranged as a tetraeder, 120 degrees coverage being quite feasible.
6.
They also have lower gain as receivers than a parabolic dish of the same diameter, so if there is no separate receiver antenna more power would be required at the ground station.
No, two arrays is not enough for whole-sky coverage. In theory a phased array can cover a hemisphere (180 degrees), but it doesn't work in practice.
The smallest number of array that could cover the whole sky would probably be four arranged as a tetraeder, 120 degrees coverage being quite feasible.
6.
They also have lower gain as receivers than a parabolic dish of the same diameter, so if there is no separate receiver antenna more power would be required at the ground station.
Thank you all for the answers, it's much clearer to me now. Recently I saw that most mobile phone towers are equipped with phased array antennas - actually they've been there long before but I haven't noticed until now 
Hope that they can have a place in space soon.
Hope that they can have a place in space soon.
QUOTE (tty @ Aug 15 2007, 03:32 AM) 
3/4
No, two arrays is not enough for whole-sky coverage. In theory a phased array can cover a hemisphere (180 degrees), but it doesn't work in practice.
The smallest number of array that could cover the whole sky would probably be four arranged as a tetraeder, 120 degrees coverage being quite feasible.
No, two arrays is not enough for whole-sky coverage. In theory a phased array can cover a hemisphere (180 degrees), but it doesn't work in practice.
The smallest number of array that could cover the whole sky would probably be four arranged as a tetraeder, 120 degrees coverage being quite feasible.
Regarding my 4th question, I found an article on Messenger's antenna system http://www.mwjournal.com/Journal/article.asp?HH_ID=AR_110
"...The low gain Antennas each have patterns that cover a hemisphere with maximum gains of 6 dBic. The medium-gain Antennas each provide a 90° by 7.5° fan-beam with a peak gain of 15 dBic, while each high gain antenna electrically scans a 12° by 2.5° beam within a quadrant with a peak gain 28.5 dBic3 (the beamwidth specifications are for the 3 dB points). The medium- and high gain antenna patterns on the front and back of the probe sweep through a hemisphere of space as the probe is rolled about the sun-probe line. Thus the medium- and high gain Antennas can provide a communication link for any location of the Earth at data rates that are medium and high, respectively, compared with the data rate of the low gain Antennas...."
A quadrant - does this mean the Earth must be within the 90° field of view from Messenger's HGA in order for the antenna to scan? Can anybody explain this?
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