I was really annoyed to get to the end of this article and not find any information about frequencies, bandwidth, power, or anything else that would enable a practical consideration of the problem. Not only am I annoyed with the journalist (who I presume has a science degree), I'm even more annoyed with NASA for omitting this basic factual information from their statement in favor of pablum that appears to be tailored to small children.
Thanks for finding that. Apparently the receive antenna has a gain of 0dbi. The use of PCM/FSK for the modulation schemes means you need to achieve a high signal-to-noise ratio on the receiver for the message to be coherent to the spacecraft. I don't know the spectral power density of a PCM/FSK, but this most likely describes everything you'd want to know about the scheme: http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=109174... . The abstract specifically mentions NASA.
If the issue is that all of the existing gear is wideband and you need to send something narrowband - build a narrowband exciter, use an upconverter to drive the wideband amp.
PCM/FSK-AM/PM doesnt seem to be really a complicated thing. Its a pretty easy way to send data, and should be build-able.
They say they need to use the DSN to do the comms. This is a set of three stations, in the US, in Spain, and in Australia, that has a 70 meter dish each, plus some 34 meter dishes.
It's not regular ham equipment in many ways, obviously. Besides power, aperture, and accurate pointing, which have been mentioned here, there is also the need to hand off transmitting/receiving as the Earth rotates. You also need a receiver and transmitter velocity model to adjust for Doppler.
People are not giving the difficulty of this problem adequate respect.
How hard would it be for access to be granted to the DSN? The challenge to build the transmission equipment does not seem insurmountable nor even particularly hard.
It's the logistics and red tape to make it happen.
I'd also like to point out, that this satilite will be doing a near earth flyby - you could probably get away with something smaller, maybe even a 3.5m dish versus a you know, 70m dish.
The engineers at GSFC say you will need the DSN. The smallest DSN antenna is 26m and that is used only for LEO (a couple thousand km).
You talk about "red tape" like moving a huge radio telescope is some kind of formality, and you quoted a figure of 3.5m with no apparent engineering basis. I find this ridiculous.
There are lots of other issues involved, but the DSN picked up the carrier signal in 2008, and it's much closer now. The article also mentioned that people can listen to the carrier as it goes by. I don't think dish size is one of the problems.
well, other than 0db gain antenna on the sat. But, ideally, if we can hear it, all we need to do its match the equivalent ERP on this end that the sat is sending (gain of the TX antenna on the sat, plus the minimum gain of the RX antenna required to successfully receive) it should work.
The previously linked pdf puts the up/downlinks around 2.0/2.2GHz; the same band used for the Apollo missions, which figures given the era (http://en.wikipedia.org/wiki/Unified_S-band).
Currently, the entire Deep-Space Network is on ~8.4GHz.
Re-equipping a multinational network for S-band could quite possibly a significant budgetary issue given how little love have been NASA getting in recent years.
Yeah, it could be implemented inexpensively in SDR using GNU Radio and a USRP. Of course you'd still need the front end hardware; high gain antenna, LNA, and PA. Not something out of the ordinary for some hams though. It's probably NASA not having the manpower to devote to it.
Apparently the highest gain antenna NASA has is 61.7 dbi at the relevant frequency. If they used 5 watt transmitters, that would be like running 4.5 megawatts into a conventional dipole antenna. The reality is NASA probably used many tens or even hundreds of watts to contact the spacecraft. As an amateur radio operator I assure that is out of reach for most of us.
> Apparently the highest gain antenna NASA has is 61.7 dbi at the relevant frequency.
Don't forget angular resolution. Because of the antenna's geometry, it's misleading to describe its gain without also mentioning that the gain applies to a very small angle -- which, depending on the circumstances, may be a great advantage if it needs to reject interfering sources.
61.7dbi, while it represents a peak rather than a more descriptive statistic, does go a ways toward describing 'angular resolution', or rather, how tight you can make the beam of the transmitter.
It's easiest to think about it thusly: You steal part of the power that would normally go out equally to a sphere ("isotropic radiator") and you redirect that power to a smaller part of the sphere. If you can successfully redirect the entire power into only half of the sphere (1 hemisphere), you get +3db. Keep slicing that in half and you add +3db each time.
61.7dbi gain necessarily means that the power felt at the center of the target is 1545883 times as powerful as it would be if you were using an isotropic radiator, given the same number of input watts. If you have successfully concentrated the power that much, something at the target's range isn't going to detect sidelobes much at all.
This isn't a technical summary - radiation patterns are never absolute step functions and there is indeed a distinction between peak power and usable area... it's just not as relevant as the huge number represented by 61.7dbi, for any remotely gaussian distribution of signal.
What are you on about? All the relevant information is there. What need is there for details? That would be interesting, sure, but it is not in any way necessary when simply reporting on the state of this mission. Something the linked article does.
Not everything needs to always contain all information.
Agreed I've been sitting here trying to figure out what relevancy frequencies, bandwidth and power have to the primary issue which is cost. The article described the problem and why there isn't a solution quite clearly to me.
The cost is a function of the technical criteria, and we have no idea what the cost is either. It might be quite small, just not within the unallocated budget at NASA. Without specifics, there's no way to gauge the economic question.
It's not uncommon for NASA to include a list of TV satellites and frequencies that stream their video releases in a press release, so why not for research satellites as well?
It's also missing basic information about orbital approach.
Just how far away is it going to be on this 'return'? What would be the dV to an Earth capture?
edit: It will apparently "enter the earth-moon system" in August of 2014.
edit2: After its second or third serendipitous mission, it was deliberately put into an orbit that would put it on a Lunar flyby-capable trajectory with closest approach August 10, 2014.
Which seems to be paper-copy-only conference proceedings, despite the claim that it's located on NTRS (whose search function seems to be embargoing my IP, but whom friends note says "No Digital Version Available").
Maybe it was one of the things removed when NTRS was mysteriously taken offline in March due to some sort of security concerns?
