Venus Express Options

[tedstryk]

If all goes well, Venus Express will be a major topic for discussion in this forum a year from now. Does anyone know how good the surface coverage will be from VIRTIS and VMC? My understanding is that VIRTIS will obtain low resolution multispectral maps, and that VMC will, in addition to cloud monitoring, have one channel that can see the surface, but I don't know at what resolution or at what quality. It will be nice to have some non-radar images of Venus' surface besides the Venera snapshots and the shadowy images from Earth and Galileo's NIMS.

Ted

[brellis]

hi cndwrld

thanks for providing a window into the dataflow workings

"There was some discussion about getting data down from spacecraft, and frustration with the amount of time it takes. I think I finally figured out how to post images, so I'll put up a couple examples from VEX to show how this stuff gets downloaded.

When we do our planning, our ESA proprietary simulation tool will create a prediction of how much data the instruments accumulate each orbit, as well as how much we are able to downlink during each comm pass with the Cebreros Station in Spain."

Is the storage capacity on V.E. heavily-burdened?

As to transmission back to earth, I wonder if there could be a software on the spacecraft similar to the program that updates a web page, looking for numbers that change. For example, once a normal temperature range in a given area over a given time period has been established, the craft prioritizes data that appears outside the norm for sending back to earth.

Similarly, I've wondered how "smart" the Mars orbiters can get as they "pushbroom" their way around the planet. Perhaps they can automatically notice small changes like new gullies.

[cndwrld]

>>Is the storage capacity on V.E. heavily-burdened?
Well, yes and no. The instruments (particularly the imaging spectrometer VIRTIS and the VMC camera) could take much more data than they do. We are usually limited by our data rate, and our ground contact periods, not the SSMM volume.

>>I wonder if there could be a software on the spacecraft similar to the program that updates a web page, looking for numbers that change.

I'd see two problems with this. First, it only would work with housekeeping telemetry, not with science data. And the housekeeping is much less in volume than the science data. We dump a fixed 216 Mbits per orbit for housekeeping data; we dump about 4200 Mbits per orbit of science data. It might be a useful idea in some circumstances, though. Say, on spacecraft on long duration missions in cruise phase, where all you have housekeeping telemetry and no science, or commercial spacecraft.

Two, you'd only get data when it changed. But in that case, if you are looking at telemetry to find a problem, you would have to trust the numbers you don't have. You'd have to believe that if no data came down, that everything was working and the number you got last week is still the number you have now. I would have real trouble doing that. In my experience, data that doesn't exist has an unknown value, and the item generating the data doesn't work unless it proves it works by putting out data. Things 'fail silent' all the time.
Again, though, in certain circumstances, it might be useful.

[dvandorn]

>>Is the storage capacity on V.E. heavily-burdened?
Well, yes and no. The instruments (particularly the imaging spectrometer VIRTIS and the VMC camera) could take much more data than they do. We are usually limited by our data rate, and our ground contact periods, not the SSMM volume.

So, here's a question -- is the download rate limited more by intrinsic limits on data transfer rates (i.e., the spacecraft's transmitter power and the size of its dish), or by the relatively short periods of time you have the Cerberus facility available to receive downlink? (I know you have some attitude issues which result from VEX being designed to orbit Mars and not Venus, but I'm looking for average data transfer rates, here, not those that are constrained by other parameters.)

What I'm getting at, here, is whether or not a vastly expanded Deep Space Network would aid appreciably in getting more data off of the vehicle, thereby letting you take as much data as the instruments can actually collect.

I hate to see a valuable resource being under-utilized because of bottlenecks in our ability to get data back from it. And I'm wondering if it just isn't time for an expanded DSN to be started. If it could be funded by an international consortium that gets its seed money from something other than charging rapacious hourly rates, that would be even better... but I keep hearing more and more issues with the ability to get data back from our spacecraft that are out there exploring the unknown, and it just seems bass-ackwards to spend billions of dollars on a well-designed probe and then hamstring its science return because we have an antiquated DSN that can't keep up with demand and yet still charges thousands of dollars (or Euros, or what-have-you) an hour for the privilege of using it.

Maybe we ought to have everyone convert their unused Dish Network dishes into a vast distributed amateur Deep Space Network and offer its services to all comers, first-come-first-serve...?

-the other Doug

[cndwrld]

So, here's a question -- is the download rate limited more by intrinsic limits on data transfer rates (i.e., the spacecraft's transmitter power and the size of its dish), or by the relatively short periods of time you have the Cebreros facility available to receive downlink? (I know you have some attitude issues which result from VEX being designed to orbit Mars and not Venus, but I'm looking for average data transfer rates, here, not those that are constrained by other parameters.)

What I'm getting at, here, is whether or not a vastly expanded Deep Space Network would aid appreciably in getting more data off of the vehicle, thereby letting you take as much data as the instruments can actually collect.

Well, let me mumble a bit while I try to think of something to say. Hmmph, hmm.

The limits on the downlink data volume are basically data rate and time of contact.

How to increase time of contact for an orbiting planetary spacecraft? We could have a steerable antenna, so that the only time we didn't have the ability to point at Earth was when we were occulted. We could transmit data and point the instrument platform to take new data at the same time.

We'd have more contact time, and more flexibility, if we could get as much time on the Earth networks as we wanted. VEX is limited to using the Cebreros station. But if Europe had its full network up (i.e. the third antenna, in addition to New Norcia, Australia and Cebreros, Spain), and each station had multiple antennas large enough for planetary probe reception (> 30 meter, say), and we could jump from station to station as the stations rotated out of view, then we could continue our downlinks as long as we wanted. It is worth keeping in mind that most stations are quite heavily booked these days. So it isn't just about upgrading networks, but expanding support would mean building additional dishes.

To increase the data rate, the spacecraft (steerable) antenna should be larger (while also pointing very accurately). And the ground station antennas should be larger, while also pointing very accurately. We use Cebreros, which is 35 m. DSN can give us 70 meters, which we used once and it let us pump a lot more data through. Transmitter power isn't really a problem for VEX, because power is not a problem. On some probes, it might be an issue. But its a design issue; it is easily scaled up for most missions, probably with minimal weight increases. The data rate also changes with distance from Earth, but there's not much we can do about that. I'm gonna assume for the foreseeable future that antennas will be fixed in size.

The problem with all of this, of course, is cost. No surprise there. A 35 meter antenna, with sufficient accuracy and senstivity, is incredibly expensive. A 70 meter is, well, more. Mulitple sets of antennas in the same location multiplies the price. Bigger antennas on spacecraft are expensive, but more (I am guessing) for launch cost than construction cost. You've got to get it in the launch fairing. The extra weight is expensive. And if you are too heavy and get pushed from one class of vehicle to another (say, Delta to Titan, or whatever), the difference in cost can be enormous. The ability to steer larger antennas accurately gets more expensive: motors, sensors, etc. Also more expensive in operations, although it can be largely autonomous now, I think (i.e., if you're on star trackers, you should be able to just say, 'point to Earth'.)

Can we simulate a bigger antenna on a spacecraft if we use two or three small antennas, separated from each other? Someone will have to tell me the highs and lows of that idea. But one promising thing is somehow getting a bigger antenna for smaller launch cost. Some commercial telecom spacecraft (like the Thuraya cell phone system) and military death stars use collapsible (or expandable) antennas, but I don't know much about them and I'm not a comm guy. Since they don't seem to have taken off, I'm assuming there are reasons for that. Cost again, probably.

I would spend all the money that people are willing to pony up, of course. I'm an engineer, and more expensive toys are always more fun. I happen to feel that we are already spending an awful lot of money, and we are getting the science down. So if we hit some constraints along the way, it doesn't bother me too much. When I worked on Magellan mapping Venus (or in my case, looking at radar telemetry while smart people mapped Venus), it was great fun. I hear that Magellan mapped about 73% of the surface. If we had spent an extra $20 million and gotten 80%, would it have changed very much? I guess the answer to that is up to the individual, but I'm pretty happy with 73%. With VEX, we should outlive our design life, and meet all our science goals, for an amount of money that Europe was willing to pay. And they are paying for Rosetta, and Bepi-Columbo, and ExoMars, one of which I hope to have an opportunity to work on. I guess I'd rather have mulitple missions with reasonable contraints than one or two more perfect missions. I'm biased, of course; I need a job.

I hope that answered some of your questions, Doug.

[JRehling]

When I worked on Magellan mapping Venus (or in my case, looking at radar telemetry while smart people mapped Venus), it was great fun. I hear that Magellan mapped about 73% of the surface.

From the Magellan page on Wikipedia: <<By the end of its first such eight-month orbital cycle between September 1990 and May 1991, Magellan had sent to Earth detailed images of 84 percent of Venus' surface. The spacecraft then conducted radar mapping on two more eight-month cycles from May 1991 to September 1992. This allowed it to capture detailed maps of 98 percent of the planet's surface.>>

I think your recollection of the low-ish figure around 73% may be based on one of the three distinct imaging campaigns covering (only) that much of the planet. Magellan had unexpected operational issues that cost some of the expected coverage. Eventually those issues were worked out and the orbiter could have been kept running to finish the job had funding been available. As it happens, there was decent "fill" data for the 2% Magellan didn't cover, so no one's lost much sleep over it. The next radar mission to Venus will be about higher resolution coverage of what Magellan did map.