ESA Galileo satellite set for new orbit

Galileo satellites are placed in medium orbits, at 23 222 km altitude along three orbital planes so that a minimum of four satellites will be visible to user receivers at any point on Earth once the constellation is complete. 

The fifth and sixth Galileo satellites, launched together on 22 August 2014, ended up in an elongated orbit travelling out to 25 900 km above Earth and back down to 13 713 km. 

In addition, the orbits are angled relative to the equator less than originally planned. 

Credit: ESA-P. Carril

ESA's fifth Galileo navigation satellite, one of two left in the wrong orbit this summer, will make a series of manoeuvres this month as a prelude to its health being confirmed.

The aim is to raise the lowest point of its orbit, its perigee, to reduce the radiation exposure from the Van Allen radiation belts surrounding Earth, as well as to put it into a more useful orbit for navigation purposes.

Should the two-week operation prove successful then the sixth Galileo satellite will follow the same route.

The Galileo pair, launched together on a Soyuz rocket on 22 August, ended up in an elongated orbit travelling out to 25 900 km above Earth and back down to 13 713 km.

The target orbit was a purely circular one at an altitude of 23 222 km. In addition, the orbits are angled relative to the equator less than originally planned.

The two satellites have only enough fuel to lift their altitude by about 4000 km, insufficient to correct their orbits entirely.

But the move will take the fifth satellite into a more circular orbit than before, with a higher perigee of 17 339 km.

"The new orbit will fly over the same location every 20 days," explains Daniel Navarro-Reyes, ESA Galileo mission analyst.

"The standard Galileo repeat pattern is every 10 days, so achieving this will synchronise the ground track with the rest of the Galileo satellites.

Orbits of the fifth and sixth Galileo satellites launched together by Soyuz on 22 August 2014, in red, compared to their intended position, in dashed green, and the position of the four satellites launched in 2011 and 2012 in solid green. 

This view looks down over Earth’s South Pole, helping to illustrate how the two satellites’ orbital inclination relative to the equator is less than was intended. 

In addition, the satellites are in an elliptical rather than circular orbit, with a maximum altitude of about 25 900 km and a minimum altitude of about 13 700 km. 

This compares to a planned circular orbit of 23 222 km. 

The satellites are in a safe state, correctly pointing towards the Sun, properly powered and fully under control. 

Credit: ESA

"In addition, from a user receiver point of view, the revised orbit will reduce the variation in signal levels, reduce the Doppler shift of the signal, and increase the satellite's visibility.

"For the satellite, reducing its radiation exposure in the Van Allen radiation belts will protect it from further exposure to charged particles.

"The orbit will also allow Galileo's Earth Sensor to hold a stable direction for the satellite's main antenna to point at Earth.

"Right now, when the satellite dips to its lowest point, Earth appears so large that the sensor is unusable. The satellite relies on gyroscopes alone, degrading its attitude precision."

Present orbits of the fifth and sixth Galileo satellites launched together by Soyuz on 22 August 2014 , in red, compared to their intended position, in dashed green, and the position of the four satellites launched in 2011 and 2012, in solid green. 

This view looks side on to the two satellites’ orbital plane, which is off-centre relative to Earth. 

The targeted orbit was circular, inclined at 55º to the equator at an altitude of 23 222 km. 

The satellites are instead in an elliptical orbit, with a maximum altitude of around 25 900 km, a minimum altitude of around 13 700 km and a lower inclination. 

The satellites are in a safe state, correctly pointing towards the Sun, properly powered and fully under control. 

Credit: ESA

The recovery is being overseen from the Galileo Control Centre in Oberpfaffenhofen, Germany, with the assistance of ESA's Space Operations Centre, ESOC, in Darmstadt, Germany.

France's CNES space agency is providing additional ground stations so that contact can be maintained with the satellite as needed.

The two satellites were previously Sun-pointing. "On 3 November that changed for the fifth satellite, as it transitioned to normal Earth-pointing mode," adds Daniel.

During November, some 15 manoeuvres will take the satellite into its new orbit. Once there, it can formally begin in-orbit testing. The host satellite's health is checked first, followed by more detailed navigation payload testing.

NASA Van Allen Probes observations helping to improve space weather models

NASA's Van Allen Probes orbit through two giant radiation belts that surround Earth. 

Their observations help improve computer simulations of events in the belts that can affect technology in space. 

Credit: John Hopkins University Applied Physics Laboratory /NASA

Using data from NASA's Van Allen Probes, researchers have tested and improved a model to help forecast what's happening in the radiation environment of near-Earth space, a place seething with fast-moving particles and a space weather system that varies in response to incoming energy and particles from the sun.

When events in the two giant doughnuts of radiation around Earth, called the Van Allen radiation belts, cause the belts to swell and electrons to accelerate to 99 percent the speed of light, nearby satellites can feel the effects.

Scientists ultimately want to be able to predict these changes, which requires understanding of what causes them.

Now, two sets of related research published in the Geophysical Research Letters improve on these goals.

By combining new data from the Van Allen Probes with a high-powered computer model, the new research provides a robust way to simulate events in the Van Allen radiation belts.

Geoff Reeves

"The Van Allen Probes are gathering great measurements, but they can't tell you what is happening everywhere at the same time," said Geoff Reeves, a space scientist at Los Alamos National Laboratory (LANL), in Los Alamos, N.M., a co-author on both of the recent papers.

"We need models to provide a context, to describe the whole system, based on the Van Allen Probe observations."

Prior to the launch of the Van Allen Probes in August 2012, there were no operating spacecraft designed to collect real-time information in the radiation belts.

Understanding of what might be happening in any locale was forced to rely mainly on interpreting historical data, particularly those from the early 1990s gathered by the Combined Release and Radiation Effects Satellite (CRRES).

Imagine if meteorologists wanted to predict the temperature on March 5, 2014, in Washington, D.C. but the only information available was from a handful of measurements made in March over the last seven years up and down the East Coast.

That's not exactly enough information to decide whether or not you need to wear your hat and gloves on any given day in the nation's capital.

Artist's rendition of the Van Allen Probes in orbit. Credit: NASA

Thankfully, we have much more historical information, models that help us predict the weather and, of course, innumerable thermometers in any given city to measure temperature in real time.

The Van Allen Probes are one step toward gathering more information about space weather in the radiation belts, but they do not have the ability to observe events everywhere at once.

So scientists use the data they now have available to build computer simulations that fill in the gaps.

The recent work centers around using Van Allen Probes data to improve a three-dimensional model created by scientists at LANL.

The project was called DREAM3D, the Dynamic Radiation Environment Assimilation Model in 3 Dimensions. Until now the model relied heavily on the averaged data from the CRRES mission.

The Dynamic Radiation Environment Assimilation Model (DREAM) was developed at LANL to understand and to predict hazards from the natural space environment and artificial radiation belts produced by high altitude nuclear explosions.

DREAM was initially developed as a basic research activity to understand and predict the dynamics of the Earth's radiation belts. 

It uses Kalman filter mathematical techniques to assimilate data from space environment instruments with a physics-based model of the radiation belts.

DREAM can assimilate data from a variety of types of instruments and data with various levels of resolution and fidelity by assigning appropriate uncertainties to the observations.

Data from any spacecraft orbit can be assimilated but DREAM was originally designed to work with input from the LANL space environment instruments on geosynchronous and GPS platforms.

With those inputs, DREAM can be used to specify the energetic electron environment at any satellite in the outer electron belt whether space environment data are available in those orbits or not.

Even with very limited data input and relatively simple physics models, DREAM specifies the space environment in the radiation belts to a high level of accuracy.

DREAM is currently being tested and evaluated as we transition from research to operations.