ExoMars/TGO ESA Hangout – replay and follow-up questions

We had a blast(!) yesterday in the ESA Hangout here at ESOC, talking with Flight Director Michel Denis, Deputy Spacecraft Operations Manager Silvia Sangiorgi and Mission Analyst Michael Khan, some of the team here working on ExoMars/TGO; we also had Ian Thomas, project manager for the NOMAD instrument, join us remotely from Brussels.

The live video chat ran from 16:00-17:05 CEST and the discussion provides a lot of information on mission progress to date, as well as details on what’s happening specifically for tomorrow’s deep space manoeuvre. We are also able to answer multiple questions submitted by the public via Twitter (#askESA), Google+ and YouTube.

You can watch the replay below, which is live in ESA’s YT channel.

In addition to the questions answered in  the Hangout, we spotted a couple of nice ones submitted just as time ran out, so we asked our experts to reply to these this morning; the written answers are below the video player in this post.

Thank you to everyone who watch, and especially to those who took time to submit a question.

Background info on #ExoMars:
http://www.esa.int/Our_Activities/Space_Science/ExoMars
http://www.esa.int/Our_Activities/Operations/ExoMars_TGO_operations


(Question submitted via YouTube) Which type of risks might be faced during orbiting and landing? #askESA

There are a wide range of risks and potential failures for both the EDL (entry, descent and landing) of the Schiaparelli lander demonstrator module and the orbit entry of TGO. Some of these would have only a minor effect on the respective missions or even none at all, if recovered in time; others could cause complete loss of the mission. Some of these would be quite serious if they occurred at one moment, but largely harmless if they happened at a different time.

In very general terms, these risks include:

Schiaparelli

  • Failure to separate from TGO , or late separation
  • Entry into the atmosphere at an incorrect angle (entry accuracy must be within 0.4 degrees of the 12.4-degree target value)
  • Failure of the landing systems (parachute, braking thrusters)
  • Loss of communication after landing
  • Loss of one or more instruments after landing

TGO

  • Failure to separate from Schiaparelli, or late separation
  • Failure of the main engine prior to or during the orbit-raising manoeuvre on 17 Oct (without this manoeuvre, TGO will impact the surface)
  • Failure of the main engine prior to or during the orbit-entry manoeuvre on 19 Oct (without this manoeuvre, TGO will perform a flyby and depart Mars on a heliocentric trajectory)

And in general:

  • Failure of a ground system (ground stations, mission control system, etc.) at a critical moment

To a very great extent, all of the systems on board both craft and on ground (which are critical for uploading commands and receiving real-time status info) are fully redundant, and in some cases triply so. Also redundancy is built into the operational scenarios wherever possible, for instance the second chance to separate Schiaparelli and land successfully in case of any anomaly during the first separation attempt. However, even this level of redundancy may not help if, say, the TGO engine fails on 19 October, when every minute counts.

Nonetheless, both craft are very well designed, all systems are functioning perfectly nominally to date, the teams on ground are very well trained and led by experienced operations managers and, so, with a little luck, we expect everything to go well in October.

More details on the complete landing and orbit entry sequence, mentioning some possible problems, are described in the Rocket Science blog post here.


(Question via YouTube): I don’t know if I’m too late for this but does ESA uses data sent from the spacecraft to calculate orbital mechanics or does the deep space network provide the tracking data or both? Orbital mechanics fascinates me but I don’t know just what is used to get information on trajectories (KSP is great for learning basics but it sadly lacks realism).

Reply courtesy Michael Khan

Conventional orbit determination for interplanetary spacecraft uses two types of radiometric measurements to determine the trajectory. ‘Radiometric’ means that the data is extracted from the radio link between a ground station and the spacecraft. These two types are ‘ranging’ and ‘Doppler’.

Ranging means that the time needed by a signal to travel from the ground station to the spacecraft and back is measured. As the speed of light and the time needed to return the signal via the spacecraft electronics is known, the travel time gives the distance.

Doppler means that a change in the frequency received by the ground station with respect to the known frequency at which the signal was transmitted by the spacecraft is measured. This change happens if the spacecraft is moving towards or away from the ground station. The Doppler effect is a well-known and much-used effect in physics. For instance, when the police uses radar to prove that you were speeding, then you get fined courtesy of the Doppler effect.

In short, ranging measures the relative distance between the ground station and the spacecraft, also known as the ‘slant range’, or ‘range’, for short. Doppler measures the relative speed between the ground station and the spacecraft. This is the same as the rate of change of the range, or ‘range rate’ in techspeak.

Measuring range and range-rate a few times will not tell you the spacecraft’s trajectory. You need to have a large set of ranging or Doppler measurements, or both, and a first notion of what the spacecraft trajectory could be. This first notion, an initial guess, is then refined (and refined, and refined…) until the resulting trajectory matches the set of range and range-rate measurements. This process is called orbit determination. If this sounds like a lot of tedious math, then that is because it is a lot of tedious math.

But computers do most of the work.

So far, so easy. In fact, no. There is a problem: Ranging and Doppler both measure only in one direction: the line of sight between the ground station and the spacecraft.

However, a spacecraft traveling through the infinite void of interplanetary space could be hundreds of kilometres off to either side of the line of sight, and that would not show up in the ranging, nor the Doppler data. The relative distance and velocity would still be the same. The error in the position would show up eventually, but only after some time. So you would need a lot of added ranging and Doppler measurements, and even then, the orbit determination would still not be very accurate. You simply would not be able to compute where your spacecraft is at any given time as accurately as you would like to, but there would not be much you could do about it.

Not when using only ranging and Doppler, that is.

What is needed is a way to measure the position perpendicular to the line of sight, or, again in techspeak, in the ‘plane of sky’. Neither ranging nor Doppler can do that, so something different is needed.

The solution comes with a name that is as complex as the method it describes: Delta-Differential One-Way Ranging, or Delta-DOR, for short.

Delta-DOR implies that the spacecraft emits a series of defined signals or tones. While it does that, it is observed by not one, but two ground stations on Earth. These two ground stations must be very far apart. The farther, the better, such as, one ground station in Europe, the other in South America or Australia. That far apart.

When these two ground stations capture the tones, the radio waves that carry the signal are captured in very fine detail. Radio waves are like a sine curve, an undulating oscillation. This is true of all waves. By comparing the signals received at both of the ground stations, one can see that at any given time, the signal received at the first ground station will be on a different position of the sine curve than the signal received at the second ground station. From this difference in position on the sine curves the direction in the plane of sky from where the signal came can be computed. It’s tricky, and it requires that the time is measured extremely precisely and that an enormous amount of data is processed, but it can be done.

The reasons why it can be done are because there is a means of calibrating the measurements. Calibration implies first measuring something for which the result is known. For Delta-DOR, before measuring the direction in the plane of sky towards the spacecraft, first the direction of a quasar near the line of sight toward the spacecraft is measured. Quasars are extremely distant, but very powerful sources of radio waves in space, and their directions have been measured very accurately.

To summarize: With the calibrated Delta-DOR result, plus ranging and Doppler (in practice, Doppler is more important than ranging because it the the more precise of the two), it is possible to determine the spacecraft position and velocity not only in the line of sight, but also in the plane of sky. There is a lot more effort involved, but most of that is done by computers, and the benefit is that in the end, the trajectory of a spacecraft cruising through the solar system many hundreds of millions of kilometres away will be known to within an uncertainty that is no larger than a few city blocks.

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