Messenger's Communications System - Some Questions Options

[Thu]

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

[Jim from NSF.com]

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.

[hendric]

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.

[tty]

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.

[Thu]

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.

[Thu]

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.

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?