| Subject: | SBAG human exploration road map chapter |
| Author: | Barry (HQ-CG000) Geldzahler <[email protected]> |
| Date: | 23-Aug-2011 11:30:59 |
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Hi Joe:
The short answer to your question is YES. Arrays can and have been used for downlink of s/c data. The earliest example I know of is the use of the Very Large Array: 27- 25m dishes in New Mexico were phased together for the Voyager encounter with Neptune because the 70m wasn�t sensitive enough. Lately, the JPL/DSN folks phase up 2- 34m antennas at a time to synthesize a 48m dish.
There are many advantages for arrays:
� An array is a more reliable resource than a single dish. If the 70m is down for any reason, so too are the tracking and radar facilities. The same is true for the high power klystron tubes. However, with an array, if any given antenna is taken out for maintenance or is in an anomalous condition, little performance is lost. For example, losing a single antenna out of 25 would be a loss of only 2% of the array downlink capability. Hence, availability of the array is more assured and robust to operational �down time� or element failures.
� Spectrum management is not an issue with an array. Since the high power uplink beam forms ~200 km above the earth, the FAA EIRP limit will not be violated obviating the need for a time-consuming coordination among a large number of Agencies.
� The angular resolution of an array is better than that of a single dish because the resolution is determined by the spacing between antennas. With antenna spacings of 60m, an effective diameter of ~300m can be achieved. Increased angular resolution can help characterize NEOs in unprecedented detail.
� Scalability. If still higher resolution or greater sensitivity is desired, additional antenna elements can be added. At roughly $1M per antenna element, increased capability can be added at a low cost.
� Extensibility to Ka band. This would be unique to NASA and provide 16 times the angular resolution of the 70m radar system as well as significantly improved range and range-rate measurement.
� 10-20X increase in data rate capture over today�s Ground Network for Earth Science Decadal Survey missions will be provided at minimal cost.
� High power emergency uplink capability can be provided more quickly (~250 times) since spectrum coordination will not be required. Hence, the uplink will not take ~ 1 month to coordinate with the multiple Agencies as at Goldstone; it can be provided virtually on-demand.
� And of course it costs less to get the same or more capability with an array than a single dish. Above about 14m, the cost of an antenna is dominated by the cost of manufacture and goes as the diameter to the 2.7 power. Below about 10m, the cost is dominated by the cost of the electronics for all the antennas. Example 1: the 100m dish in Green Bank cost $100M when it was built. Most of that cost was in the price of steel. To build the same antenna today would cost 300M [steel has gone up dramatically]. Example 2: SCaN has decided to build 34m antennas at a cost of 50M each [when fully outfitted]. An array of 48 12m antennas [the equivalent of FOUR 34m dishes would cost about 50M. I just got a price for the antennas themselves from the manufacturer: 365K/dish as opposed to the 675K when I bought only 3 for the uplink demo!]. And because you can use sub-arrays, the dishes don�t all have to point in the same direction simultaneously.
These are the major advantages of arrays.
Hope this helps. I'm off to see Yan Fernandez today re the SBAG Roadmap.
Barry
The more crucial the decision, the more crucial the data.
The more crucial the data, the more crucial the measurement.
NASA Chief Scientist, Space Communications and Navigation
202-358-0512 [office]
202-255-6441 [blackberry]
________________________________________
From: Nuth, Joseph A. (GSFC-6900) [[email protected]]
Sent: Tuesday, August 23, 2011 12:50 AM
To: [email protected]
Subject: RE: [SBAG Human Exploration] SBAG human exploration road map chapter
Barry,
How does your array do as a downlink for data from spacecraft?
Joe
________________________________________
From: Geldzahler, Barry (HQ-CG000) [[email protected]]
Sent: Monday, August 22, 2011 5:48 AM
To: '[email protected]'
Subject: Re: [SBAG Human Exploration] SBAG human exploration road map chapter
Paul
As an fyi, I'm meeting with yan fernandez tomorrow in orlando to discuss how our uplink arraying techniques can contribute to neo tracking and characterization.
If this comes up at the sbag mtg, I'd appreciate it if you'd advocate for the uplink arraying in the roadmap. It'd be a big help. Unfortunately, I won't be able to get to the sbag this time
Thanks
Barry
----- Original Message -----
From: Paul A. Abell [mailto:[email protected]]
Sent: Monday, August 22, 2011 05:30 AM
To: [email protected]
Cc: Mark V. Sykes
Subject: [SBAG Human Exploration] SBAG human exploration road map chapter
Mark and All,
Here is the draft of the human exploration chapter. Please feel free to send me any comments, suggestions, or feedback that you may have on this topic.
Thanks!
-- Paul
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