Getting the data back – Store and Forward

This video shows the view of Mars Express from the Earth before, during and after the Curiosity landing. It demonstrates perfectly why we need to use a method called 'store and forward' to get the recording of the descent back to Earth.

At the start and end of the video, you can see Mars Express' big 1.6-m High Gain Antenna (the grey circle on the front of the spacecraft) pointed right at us. We need that to be pointed at us to be able to talk to Mars Express from Earth.

Unfortunately, to support the landing of Curiosity, we need to point our Melacom antennas at the incoming lander, and they're fixed perpendicular to the High Gain antenna. That's why during the middle of the video you see the spacecraft turn the High Gain Antenna away from us - it's so it can get the best possible view of the incoming lander.

In order to relay the recording of the descent, we store the data in our on-board memory - a bit like saving a picture to the memory card on your digital camera.

We have 12 Gigabits of on-board memory, which might sound small compared to your home computer, but it's plenty of space for what we need. Once we turn back to Earth, we can tell the spacecraft to forward the recorded data back to Earth, just like plugging in your camera and downloading the results from the memory card. In fact, due to the criticality of the Curiosity recording, we'll transmit it to Earth three times to make sure it reaches us safely.

So when you're watching the landing tomorrow, note that's why it'll take us a bit of time to swing the spacecraft around and dump the recorded data to ground. The JPL orbiter Mars Reconnaissance Orbiter will do the same thing and so will experience a similar delay.

In contrast, the live relay from Mars to Earth will be provided by JPL's venerable Mars Odyssey orbiter, the oldest spacecraft currently operating around Mars. It uses a different mode, called 'bent pipe', where it takes the incoming data and 'bends' it around and blasts it back towards Earth more or less simultaneously.

If all goes according to plan, this direct relay will be NASA's first confirmation of a successful landing, and the detailed recordings made in 'store and forward' by the other two orbiters will follow shortly after to provide us a full picture of this historic landing.

What is Open Loop Recording?

How Mars Express will listen to Curiosity

3-D waterfall diagram showing the open loop recording made by Mars Express of MER-B (Opportunity) during the rehearsal overflight for Curiosity EDL.

You'll see a lot on our coverage of the Curiosity landing about Open Loop Recording,' something which was hinted at in a previous post about the difference between 'signal' and 'data'.

OLR refers to the type of recording that will be made by Mars Express as Curiosity descends towards Mars, and in parallel by ESA's New Norcia station here on Earth.

In open loop recording, we don't try to decode the bits and bytes being sent by the descending lander but instead try and listen to as much of the radio spectrum as we can, hopefully detecting the tone of the lander's transmissions within this spectrum. Think of it like listening to a crowd of people – you can either focus on the words one person is saying, or listen to the whole crowd to get a full picture of what's going on; that's what we'll do with open loop recording.

On Mars Express we'll use our UHF Melacom radio to listen in on the UHF part of the spectrum – usually used on Earth for radio and television transmissions; it's also used at Mars as the frequency that different orbiters and landers use to talk to each other.

From New Norcia we'll be listening to the X-Band part of the spectrum – used on Earth mainly for radar systems but also as a way of communicating with spacecraft across the solar system (Mars Express uses X-Band for its main link back to Earth).

Each of these parts of the spectrum is actually a wide range of frequencies and in open loop we listen to as many as possible, creating a diagram like the one in the picture above.

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What time is it?

How we solve the problem of multiple time zones

If you saw our descent timeline article, you'll have noticed that we speak about different time zones (of course with acronyms!). If you've also been following the NASA coverage for MSL arrival at Mars, then you'll see it gets even more confusing. In case you're wondering what they all are, then we're here to try and explain!

World time zones

World time zones

First of all, we have to deal with different time zones here on Earth - something you've no doubt experienced if you've taken a long distance flight.

Here at ESA's operations centre, ESOC, in Germany, we use CEST – Central European Summer Time – the time zone most of Europe is on during the summer. Over at JPL in California, they are 9 hours behind, on PDT – Pacific Daylight Time – summer time for the west coast of the United States.

This can get really confusing when agencies like ESA and NASA work together on time-critical activities like MSL landing. At NASA, Curiosity will land on 5 August – but here in Europe it'll land on the 6th! So not only is the time of landing different, but it happens on a different day depending on where you are!

To solve these problems, the space industry (and many other organisations facing similar issues) use a standard time zone called UTC – Coordinated Universal Time.

This time zone was standardised in 1961 to allow our increasingly networked world to work better together. It represents GMT (Greenwich Mean Time), the zero reference for all time zones, but with no daylight savings time shift – so it never changes throughout the year.

At ESOC our short-hand for this time-zone is to put a letter 'Z' after the time, which is where UTC gets its nickname of "Zulu Time" (Z = Zulu in the phonetic alphabet).

So when Curiosity lands, Europe (CEST) will be 2 hours ahead of UTC and JPL (PDT) will be 7 hours behind. Thanks to UTC, though, we can coordinate and communicate pretty well together, allowing multiple agencies and nations around the world to work together on this important event.


Mars Express – timeline for MSL support

Early on 6 August, Mars Express will receive crucial signals from NASA’s Mars Science Laboratory mission as it delivers the car-sized Curiosity rover onto the Red Planet. The ESA spacecraft will begin tracking the NASA mission 45 minutes before it enters the martian atmosphere; an ESA ground station will also record vital signals.

The highlight of ESA’s support for NASA’s Curiosity landing happens at 06:29 on Monday, 6 August, when the Mars Express Lander Communication (MELACOM) system is switched on. Recording of the radio signals transmitted by the Mars Science Laboratory (MSL) is planned to begin at 07:09 and end at 07:37 (all times shown as ground event time in CEST).

ESA’s ground tracking station in New Norcia, Australia, will also listen and record signals from the NASA mission at the same time.

CEST = UTC + 2 hours
Earth time = Mars time + 13min:48sec
MEX: Mars Express
MSL: Mars Science Laboratory
NNO: ESA New Norcia station
AOS: Acquisition of signal
S/C: Spacecraft
All times subject to change

Event Earth CEST Earth UTC S/C UTC Notes
DSS-15 (G/S) AOS 4:03:00 2:03:00
NNO AOS MEX 4:03:00 2:03:00
NNO AOS MSL 4:05:00 2:05:00
DSS-15 LOS MEX 6:05:00 4:05:00
NNO LOS MEX 6:05:00 4:05:00
NNO start recording MSL signals 6:25:00 4:25:00
MEX starts slew to point at MSL 6:06:30 4:06:30 3:52:42
MEX MELACOM reciever ON 6:28:48 4:28:48 4:15:00
MEX ends slew to point at MSL 6:36:38 4:36:38 4:22:50 Now pointing at MSL
MEX starts recording MSL signals 7:08:48 5:08:48 4:55:00
MSL Cruise Stage separation 7:14:34 5:14:34 5:00:46 MSL starts transmitting
MRO UHF TM capture starts 7:21:34 5:21:34 5:07:46
MSL Atmosphere entry 7:24:34 5:24:34 5:10:46
MSL Start plasma attenuation 7:26:13 5:26:13 5:12:25
ODY UHF bent pipe relay start 7:26:34 5:26:34 5:12:46
MSL End plasma attenuation 7:27:13 5:27:13 5:13:25
MSL Parachute deploys 7:28:46 5:28:46 5:14:58
MSL Heat shield separation 7:29:07 5:29:07 5:15:19
MSL Backshell separation 7:30:40 5:30:40 5:16:52
MSL Curiosity separation 7:31:17 5:31:17 5:17:29
Curiosity touchdown 7:31:37 5:31:37 5:17:49 Planned
MEX MELACOM reciever OFF 7:36:48 5:36:48 5:23:00
ODY UHF bent pipe relay end 7:37:37 5:37:37 5:23:49
MEX starts slew to point at Earth 7:38:58 5:38:58 5:25:10
NNO stop recording MSL signals 7:40:00 5:40:00
MEX ends slew to point at Earth 8:09:46 6:09:46 5:55:58 Now pointing at Earth
MEX transmitter ON 8:09:48 6:09:48 5:56:00
MEX start sending TM 8:15:00 6:15:00 6:01:12
MEX start recording download 8:15:31 6:15:31 6:01:43
MEX stop recording download 8:40:31 6:40:31 6:26:43
ESOC team passes data to NASA 8:42:00 6:42:00

ESA, NASA, Parkes: Big ears on Earth will listen to MSL descend

Editor's note: We're delighted to post this update on the international, behind-the-scenes cooperation to implement the MSL 'ground listening' campaign. It highlights the hard work, months of preparation and terrific cooperation between ESA, NASA and international partners on a technical, operational and interpersonal level. Thanks to NASA's Susan Kurtik and ESA's Wolfgang Hell for kindly providing information for this report.

On 6 August, ground stations and a radio telescope operated by multiple nations will be listening to signals from NASA's MSL mission as it descends through the Martian atmosphere to deliver the Curiosity rover safely onto the Red Planet's surface.

ESA's first 35-metre deep-space ground station is situated at New Norcia, 140 kilometres north of Perth in Australia. The 630 tonne antenna will be used to track Rosetta and Mars Express, the latter to be launched in 2003, as well as other missions in deep space. The ground station was officially opened on 5 March 2003 by the Premier of Western Australia, Hon Dr Geoff Gallop.  Credits: ESA

ESA's first 35-metre deep-space ground station is situated at New Norcia, 140 kilometres north of Perth in Australia.
Credits: ESA

The stations involved are all in the Land Down Under, since that's the bit of Earth that will be facing Mars at around 01:00 CEST next Monday morning, as MSL approaches its nail-biting plunge into the Mars atmosphere to touch down in Gale Crater.

During this crucial phase of the mission, MSL will transmit two radio links – one direct to Earth in X-band, which is also being used  for routine telecommanding during the cruise to Mars, and a ‘proximity link’ in the UHF band for direct communication with spacecraft orbiting Mars.

To get a good idea of the importance of the ground campaign, let's first look at the in-flight tracking efforts for the proximity link.

International fleet tracks MSL from Mars orbit

"The primary monitoring of MSL's Entry, Descent and Landing – EDL – phase will be provided by two NASA spacecraft in orbit around Mars: Mars Reconnaissance Orbiter [MRO] and Mars Odyssey. These two 'platforms' will be backed up by ESA's Mars Express [MEX]," says Michel Denis, MEX Spacecraft Operations Manager and responsible for MEX tracking support to MSL at ESOC.

(See our earlier post, 'Mars Express to track 7 minutes of terror' for details – Ed.)

Odyssey is the only one of the three that can provide the so-called 'bent-pipe' (or real-time) relaying of signals and is expected to give the first indication to NASA that Curiosity has arrived; confirmation of safe landing is expected by NASA at around 07:31 CEST.

Odyssey over Mars' South Pole: NASA's Mars Odyssey spacecraft passes above Mars' south pole in this artist's concept illustration. The spacecraft has been orbiting Mars since October 24, 2001. Credit: NASA

Odyssey over Mars' South Pole: NASA's Mars Odyssey spacecraft passes above Mars' south pole in this artist's concept illustration. The spacecraft has been orbiting Mars since October 24, 2001. Credit: NASA

In contrast, MRO and MEX can only track, store and then forward recorded signals later.

Artist's concept of the Mars Reconnaissance Orbiter. Image credit: NASA/JPL

Artist's concept of the Mars Reconnaissance Orbiter. Image credit: NASA/JPL

"Only Odyssey can receive, decode and then relay to Earth the actual telemetry data coded into the those signals. Conversely, MRO and MEX will save on board 'open-loop' recordings," says Denis.

This means they will record only the spectra of the radio signals and the related Doppler variations in signal intensity, and not the encoded telemetry.

(NASA's Susan Kurtik, MSL Mission Interface Manager at JPL, adds: In fact, we will be able to extract telemetry from the MRO open-loop recording, although it will take ~8 hours to process - Ed.)

(The variations in signal strength due to the Doppler effect are explained by the rather famous ambulance siren analogy.)

Ground tracking campaign provides crucial support

Now, let's look at the ground tracking campaign, the crucial, 'behind-the-scenes' activity in support of Curiosity's arrival at Mars provided by stations on Earth.

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