Discussion

Questions about ESA Moon Programme

Question: Berengere Houdou (ESA)

I am the ESA technical officer of the MoonNEXT Phase A study (to start beginning of February) and I am currently working with our study scientist Detlef Koschny and our Science Definition Team to consolidate a kind of strawman payload for both the lander and the rover.

Unfortunately, I could not attend the Radioastronomy workshop that took place at ESTEC beginning of December, but was debriefed by Detlef Koschny and Pierre-Damien Vaujour. Thanks for the summary you distributed: this is a very useful starting point.

We are now in the process of collecting detailed information about each instrument, so that the winning industrial contractors will be able to conduct a thorough design iteration, taking into account a realistic payload and all its associated engineering constraints. All the information will be gathered in the Science and Payload Definition Document (SPDD), to be distributed to the contractors beginning of February.

In this view, Pierre-Damien already extracted from your summary data and text, and implemented them in a template that will consist the chapter of the SPDD dedicated to LRX (see attached document). The 2-antenna configuration (one on the lander, one on the rover) is so far retained.

Could you review and complete the document with maybe consolidated design parameters? We also included comments/questions directly in the text.

The more details available the better in terms of mass breakdown, power (nominal and peak), operational concept, deployment need (and associated hardware if applicable), modes (and their respective power) etc.

There are two specific points I would like to clarify: - the communications architecture between lander/rover/Earth is not clear yet and will be studied in the course of the study:

  • from first estimation, the envisaged communications between the rover and the lander (UHF) will not be continuous, limited in range (5km) and in rate: a few 100s of kbit/s (maybe being very optimistic one could reach 1 Mbit/s at a limited distance)
  • the communications from the lander to Earth will not be continuous (14 days out of 28 days of the Lunar month will be without Earth visibility) and will reach rates in the order of a few Mbit/s
  • the communications from the rover to Earth will probably be of the same order as from the lander to Earth

As a result, the rate of 100 Mbit/s you mentionned in the summary does not seem feasible. What would be the impact of considering a much lower data rate? - I have the same concern for the power: 7.5W for each module is quite high (for the lander, and even more for the rover). What could be done with a power in the order of 3 to 4 W (day operation)?

Answer: Heino Falcke

First of all, it would, of course, be extremely good if indeed a better communication infrastructure could be studied. Maybe we can go beyond UHF communication?

Anyway, a reduced bandwidth means a reduced bandpass (i.e. only narrow-band observing). So, instead of observing over 10 MHz instantaneously you would only observe over 100 kHz or even just 10kHz (but switchable). That would still allow one to do interferometry but substantially reduce image quality. The technology test would still be feasible but some science would inevitably be lost. 100 kHz is more or less approaching a minimum.

The limit of 5 km would simply mean that the resolution would be reduced by a factor of 4 or so - not nice, but given the reduction in bandwidth something one can live with.

For the power: a reduction in bandwidth also means a reduction in power. Hence, the 3-4 W is less of a limitation. The power needs is then driven by the local computing needs and not by the amplifier anymore. I would, however, not recommend to go lower.

In terms of operations: one could indeed drive to a new location, say in steps of 100 m every 24 hrs, observe a couple of minutes at one frequency, step to a new frequency, and transmit the data at the same time to the lander (where the same antenna operates) and correlate there and transmit the integrated signal (factor >10.000 less data) to Earth. Loss of sight to Earth could therefore be handled through some local buffering in the lander – that could be OK. If was just wondering whether there is some experience with solid-state memory (USB sticks/SD cards) in space. That would be ideal (large memory, low power).

The constraint to daylight periods is not a big issue. The longer we can observe the better, of course, but if we can get 1h per location, or say 10% (2.4 hrs) that would already be useful. During that time we would need the full data bandwidth. In the rest of the time we could sit idle, or do local measurements with data buffered and transmitted once in a while (burst mode).

Personally I would much rather reduce the duty cycle of interferometry and then be able to increase the power and data transmission during that time (i.e. have all the power and bandwidth for 10% of the time rather than have 10% of the power for 100% of the time).

In terms of location: we would prefer to be able to go behind a hill/mountain with no direct sight of Earth (but to the lander) to test radio blockage and simulate far-side locations. We are less interested in a direct data communication from the rover to Earth. Maybe some of the burst mode data could be sent directly to Earth, once the rover is out of sight from the lander.

For the rover design it is important to be large enough to carry the antenna without tipping over: 3 x 1.5 m crossed dipoles. The weight is of order 0.5 -1 kg (need the exact number from our colleagues in Poland here!) .

About Frequency Range

Xenophon Moussas:

We can increase the frequency to 200 MHz very easily and it is worth. Then we observe closer to the Sun. On the Moon the noise is very low, hence it is worth having some data.

Heino: Power and the need for down conversion becomes an issue with the larger frequencies. Also the number of LNAs.

Read Out

Xenophon:

We should add cadence in the matrix. I believe that a minimum of 100 spectra per second is a good value. We could have a mode for higher cadence for a short period, for example during activity.

Number of channels

Xenophon :

We sould increase the number of channels minimum to 1000 or 2000. It is very easy to have this. We can transmit averages for most of the time, exept during activity.

Jan Berman:

For averages we could in principle have as many channels as we want, it's just a matter of setting the number of frequency bins (record length) in the FFT.

On the other hand, in my opinion the number of real-time channels should be relatively small, otherwise we will consume too much power. The real time channels should be flexible though, in terms of frequency bandwidth so that we can have both narrowband and wide-band channels. The effective total bandwidth (BW) will be set by the datalink from the receivers to the onboard computer on the lander. For 3 channel 16bits complex (I and Q) data it's easy to calculate. 10 Mbit/s ↔ 100 kHz BW, 100 Mbit/s ↔ 1 MHz BW, and 1 Gbit/s ↔ 10 MHz BW and so on.

See attached datasheet for a 4-channel DDC (Digital Down Converter) from Texas- Instruments that we have been looking at. (There are several others on the market, this is just an example). It consumes 250mW/channel. Perhaps one could do the downconversion more power efficient in an FPGA by tailoring it to our specific needs but I don't know how much one will gain if anything.

projects/lrx/discussion.txt · Last modified: 2017/07/27 15:56 (external edit)
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