Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

What stops Mars Express getting lost in space?

30.01.2004


Determining the three-dimensional position of Mars Express in space with as much precision as possible, at a distance of 155 million kilometres away from Earth, is no simple business.



If the Solar System were shrunk down so that Earth was only the size of a soccer ball, then Mars Express would be a speck of dust floating well over two kilometres away!

ESA ground stations have kept track of the spacecraft’s range and velocity ever since launch.


By measuring the ‘Doppler shift’ undergone by the signal coming from the distant spacecraft (like the changing sound of a passing police car siren), they can calculate the spacecraft velocity component along the line of sight to Earth down to a few millimetres per second.

‘Ranging’ data can be acquired simply by sending a signal to Mars Express then measuring how long it takes to come back.

These data are combined together by the mission’s Flight Operations and Navigation team with information on the various gravitational influences of the Sun and all the planets.

Even the slightest acceleration due to solar radiation pressure is used to model the spacecraft trajectory.

But ‘ranging’ only shows where the spacecraft is in a single dimension. To independently cross-check the three-dimensional position of Mars Express at any one time requires another extremely accurate navigational procedure. This is called ’Delta differential one-way ranging’, or ’Delta DOR’ for short, and is carried out on Mars Express twice a day.


Delta DOR involves performing a triangulation measurement on Mars Express by acquiring its signal with two separate ground stations located on either side of the Earth.

This is done using tracking stations at Goldstone in California, Canberra in Australia and Madrid in Spain, located about 120 degrees apart. They can measure extremely precisely the difference in acquisition time between ground stations, accurate to one thousand millionth of a second.

Radio waves travel at the speed of light, so the difference in acquisition time between the two stations can be used to get separate measurements of distance and a shared point of signal origin.

There are some errors caused by differences in the two stations’ clocks as well as atmospheric effects, but these are cancelled out with calibration measurements of quasars whose positions are extremely well known.

This technique can be used to decrease the degree of uncertainty about a spacecraft’s position down from many tens of kilometres down to a few kilometres or less.

Seeing stars keeps Mars Express in line

Besides position and velocity, the other crucial parameter is the spacecraft’s attitude, or which way it is pointing. Mars Express stays linked to ESOC with a high-gain antenna that must be kept tightly pointed at Earth to an accuracy of at least one degree.

The only time the spacecraft temporarily lost this connection was when a manoeuvre – such as the Mars Orbit Insertion on Christmas Day – required Mars Express to be pointed away from Earth. Maintaining a high level of attitude control is crucial to the mission.


The spacecraft uses a ‘star tracker’, which is basically a camera that takes pictures of the star field it is pointing at twice a second . An automatic on-board system identifies stars in each image, working from a stored library of some 3,227 stars.

The library used by Mars Express’s star tracker comes from data acquired by the ESA star-mapping mission Hipparcos in the early 1990s, which measured the position and distance of more than a million stars in space.

The star tracker can identify a maximum nine stars at any one time, identifying them in sets of three. Any pointing error or drift that is identified can be corrected using reaction wheels on the spacecraft.

All the ESOC mission team have to do is give attitude guidance – telling the system where the Sun is, where the Earth is and what to look at for any one time. This system gives a pointing accuracy of at least one hundredth of a degree.

The same star tracker hardware will help guide ESA’s Rosetta after its launch in February 2004; however the software has been updated and includes pattern recognition systems to distinguish stars from the cometary dust the spacecraft is expected to encounter as it nears its target, Comet 67P/Churyumov-Gerasimenko.

Guido De Marchi | ESA
Further information:
http://www.esa.int/export/SPECIALS/Mars_Express/SEMS5N474OD_0.html

More articles from Information Technology:

nachricht World's thinnest hologram paves path to new 3-D world
18.05.2017 | RMIT University

nachricht Internet of things made simple: One sensor package does work of many
11.05.2017 | Carnegie Mellon University

All articles from Information Technology >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Wafer-thin Magnetic Materials Developed for Future Quantum Technologies

Two-dimensional magnetic structures are regarded as a promising material for new types of data storage, since the magnetic properties of individual molecular building blocks can be investigated and modified. For the first time, researchers have now produced a wafer-thin ferrimagnet, in which molecules with different magnetic centers arrange themselves on a gold surface to form a checkerboard pattern. Scientists at the Swiss Nanoscience Institute at the University of Basel and the Paul Scherrer Institute published their findings in the journal Nature Communications.

Ferrimagnets are composed of two centers which are magnetized at different strengths and point in opposing directions. Two-dimensional, quasi-flat ferrimagnets...

Im Focus: World's thinnest hologram paves path to new 3-D world

Nano-hologram paves way for integration of 3-D holography into everyday electronics

An Australian-Chinese research team has created the world's thinnest hologram, paving the way towards the integration of 3D holography into everyday...

Im Focus: Using graphene to create quantum bits

In the race to produce a quantum computer, a number of projects are seeking a way to create quantum bits -- or qubits -- that are stable, meaning they are not much affected by changes in their environment. This normally needs highly nonlinear non-dissipative elements capable of functioning at very low temperatures.

In pursuit of this goal, researchers at EPFL's Laboratory of Photonics and Quantum Measurements LPQM (STI/SB), have investigated a nonlinear graphene-based...

Im Focus: Bacteria harness the lotus effect to protect themselves

Biofilms: Researchers find the causes of water-repelling properties

Dental plaque and the viscous brown slime in drainpipes are two familiar examples of bacterial biofilms. Removing such bacterial depositions from surfaces is...

Im Focus: Hydrogen Bonds Directly Detected for the First Time

For the first time, scientists have succeeded in studying the strength of hydrogen bonds in a single molecule using an atomic force microscope. Researchers from the University of Basel’s Swiss Nanoscience Institute network have reported the results in the journal Science Advances.

Hydrogen is the most common element in the universe and is an integral part of almost all organic compounds. Molecules and sections of macromolecules are...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Dortmund MST Conference presents Individualized Healthcare Solutions with micro and nanotechnology

22.05.2017 | Event News

Innovation 4.0: Shaping a humane fourth industrial revolution

17.05.2017 | Event News

Media accreditation opens for historic year at European Health Forum Gastein

16.05.2017 | Event News

 
Latest News

New approach to revolutionize the production of molecular hydrogen

22.05.2017 | Materials Sciences

Scientists enlist engineered protein to battle the MERS virus

22.05.2017 | Life Sciences

Experts explain origins of topographic relief on Earth, Mars and Titan

22.05.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>