Monthly Archives: September 2013

Galileo Atomic Clocks

An atomic clock works like a conventional clock but the time-base of the clock, instead of being an oscillating mass as in a pendulum clock, is based on the properties of atoms when transitioning between different energy states.

When an atom is excited by an external energy source, it goes to a higher energy state. Then, from this state, it goes to a lower energy state. In this transition, the atom releases energy at a very precise frequency which is characteristic of the type of atom. Read more…

PHM or Passive Hydrogen Maser (1/2)

The Passive Hydrogen Maser (PHM) is the master clock on the Galileo satellite’s payload. Its extremely good performance makes it the most stable of all clocks currently in space, better than 1 nanosecond per day. Some other features: 18 Kg of mass, 28 liters of volume and 20 years lifetime.

The hydrogen maser uses the properties of the hydrogen atom to serve as a precision frequency reference. But, how does it work? Let’s see what is the process: Read more…

GIOVE-B, Second Galileo Satellite

GIOVE-B (Galileo In-Orbit Validation Element) was launched by a Soyuz rocket from Baiknour on 27 April 2008, carrying the most accurate atomic clock ever flown into space at that moment.

The 500 kg satellite was left into a circular orbit at an altitude of 23,173 km, inclined at 56 degrees to the Equator, 3 hours and 45 minutes after the launch. It makes a complete journey around the Earth in 14 hours and 3 minutes. Read more…

Galileo Signal Polarization

Polarization is a property of electromagnetic waves that can oscillate with more than one orientation. By convention, the orientation of the wave’s electric field at a point in space over one period of the oscillation specifies its orientation.

The magnitude and direction of an electric field is defined by what is called an electric field vector. The Galileo transmitted signals are Right-Hand Circularly Polarized (RHCP), which means that the electric field vectors have a constant magnitude but the direction changes in a rotary manner. If the wave is frozen in time, the electric field vector of the wave describes a helix along the direction of propagation. Read more…

Galileo Frequency bands

The Galileo Navigation Signals are transmitted in four frequency bands (E5a, E5b, E6 and E1) providing a wide bandwith for the transmission of the Galileo signals.

The Galileo frequency bands have been selected in the allocated spectrum for Radio Navigation Satellite Services (RNSS) and in addition to that, E5a, E5b and E1 bands are included in the allocated spectrum for Aeronautical Radio Navigation Services (ARNS), employed by Civil-Aviation users, and allowing dedicated safety-critical applications. Read more…

Galileo Services

Galileo, once fully operational, will offer four high-performance services worldwide:

Open Service (OS) Galileo open and free of charge service set up for positioning and timing services.

Commercial Service (CS) A service complementing the OS by providing an additional navigation signal and added-value services in a different frequency band. The CS signal can be encrypted in order to control the access to the Galileo CS services.

Public Regulated Service (PRS) Service restricted to government-authorised users, for sensitive applications that require a high level of service continuity.

Search and Rescue Service (SAR) Galileo will be an important element of MEOSAR (Medium Earth Orbit Search and Rescue system), and thus a key contributor to Cospas-Sarsat, the international satellite-based search and rescue distress alert detection and information distribution system. Galileo satellites will receive emergency beacons carried on ships, planes or people and send them back to national rescue centres. Also a feedback to a beacon could be sent, being this exclusive with Galileo.

Positioning computation

The key in the positioning computation is the pseudorange or distance to the satellite. The navigation receivers calculate the pseudorange to each satellite under visibility by measuring the elapsed time between the transmission and reception of GNSS signals. To compute the position of a GNSS receiver a minimum of 4 satellites under visibility (4 pseudoranges) are needed, although the accuracy will improve with higher number of satellites. Read more…