[X] My Assistant

[Paolo]

I thought it was time to start a new topic on NASA's next Moon mission
incidentally, there is a new mission update out today

[CryptoEngineer]

I look forward to LADEE's launch tonight.

One of the more intriguing bits of new kit is the LLCD - a laser based communication system. It has a number of advantages over radio; smaller size & power, greatly improved data rates, and it doesn't require exclusive use of a chunk of radio spectrum.

But there are still things I wonder about.

1. How do the two ends find each other? At lunar distance the 'spot' illuminated by the laser is only a few miles across.
2. There are three ground sites - one in California, one in New Mexico, and one in Tenerife; how does it know where to look?
3. The laser operates in the near infra-red. To what extent can it deal with cloud?

I presume there's some beacon mechanism to indicate 'look more closely here', but its not described.

To what extent can it deal with slew? Would this mechanism be useful for Earth-orbit-to-ground communications?

CE

[djellison]

1. How do the two ends find each other?

No different to radio - you need to know where you are, and where the station is. MRO has to know where the Earth is. Goldstone has to know where MRO is, for example.

2. There are three ground sites - one in California, one in New Mexico, and one in Tenerife; how does it know where to look?

Same as radio - by programming in the appropriate information. It's a simple geometry problem.

3. The laser operates in the near infra-red. To what extent can it deal with cloud?

I don't believe it can. Higher freq radio struggles with rain. The increase in bandwidth more than makes up for the times when you can't communicate (i.e. 10x faster, but maybe you drop 1 day in 10 is still a 9 fold increase)

To what extent can it deal with slew? Would this mechanism be useful for Earth-orbit-to-ground communications?

No different, again, to radio - you just need slightly tighter pointing control. Spacecraft-to-Spacecraft laser has already been tested, as has orbiter to ground with Alphasat and ESA intends to use it in their version of TDRS. LRO has received data via laser into LOLA.

Doug

[mcaplinger]

No different to radio...

Actually, I think the spot size is small enough that a separate wider-field (or scanned, maybe, not clear) acquisition beam has to be sent by the ground station so that the flight system can adjust its pointing dynamically -- see http://dspace.mit.edu/openaccess-dissemina....1/61673‎