Space blog

SSTL’s second Galileo contract, GIOVE-A2, has progressed through a successful Critical Design Review (CDR). The small satellite is a follow-on mission to the successful GIOVE-A mission launched two years ago.

The two-day CDR meeting, attended by ESA representatives at SSTL’s headquarters in Guildford, reviewed the design changes between GIOVE-A and GIOVE-A2. The next stage in the project will progress towards manufacture.

ESA awarded the follow-on contract to SSTL in March 2007. The project covers initial activities leading to the construction of a further spacecraft for the Galileo satellite navigation system, based on SSTL’s proven GIOVE-A technology.

A pioneering experiment was launched onboard UK-DMC in 2003, that uses signals reflected from the GPS signals already in orbit to monitor the weather at sea. The joint team experiment team, comprising SSTL and the University of Surrey members has now succeeded for the first time in capturing a Galileo signal from SSTL's GIOVE-A satellite reflected off the ocean surface. Although the signal was weak, because the equipment is optimised for receiving GPS signals, this is an important achievement demonstrating the potential for determining the weather at sea with remote sensing satellites.

What's more, it seems this new development is well timed - the Partnership for Observation of the Global Oceans (POGO), a distinguished partnership of international scientists is calling for a stable network of satellites for surveying vast extents of the surface of the ocean to enable societal benefits' (see POGO release, BBC).

Coincidentally, HRH Duke Of Kent was visiting SSTL at the time and was given a whistlestop tour which touched on "bistatic radar" and "forward scatterometry" (keep reading...).

The pioneering GPS Reflectometry Experiment was launched onboard SSTL’s UK-DMC satellite in 2003 to demonstrate the use of GPS reflections to determine the roughness of the ocean, using a method called “bistatic radar” or “forward scatterometry”. This experiment has now successfully detected a Galileo satellite navigation signal reflected by the ocean’s surface. GIOVE-A, the first Galileo demonstration satellite, also built by SSTL, was commissioned by the European Space Agency and has been transmitting prototype Galileo signals since its launch in December 2005.

In early November, 20 seconds of data were captured in orbit above the Arafura Sea, north of Australia, and downloaded to Surrey for processing. Whilst the orbiting experiment on UK-DMC is not optimised for Galileo signals, enough of the reflected signal energy was received to allow the detection and plotting of the weak signal after processing by University of Surrey PhD student, Philip Jales. The shape of the reflection gives an indication of the sea roughness and hence the weather at that place and time, where the wind speed was around 14 mph (22 km/h).

SSTL's Global Navigation Satellite Systems (GNSS) / GPS head, Dr Martin Unwin (centre left) explained

“This is an important achievement in remote sensing and demonstrates the potential offered by Galileo for scientific purposes. A constellation of small satellites could be deployed at low cost to take measurements over the oceans where there are large gaps in forecast knowledge at present. An improved measurement system in space could be used to warn mariners of storms and to provide data for global climate change models - potentially even to detect Tsunamis.”

Dr Unwin is also enthusiastic about benefits of inter-system cooperation:

“Signals from Galileo, in conjunction with GPS and the Russian and Chinese systems, Glonass and Compass, can all be used as part of a new tool for ocean sensing. The future high bandwidth signals transmitted by Galileo, in particular, will enable higher resolution measurements of special interest to scientists, for example, in resolving wave heights”

GPS Reflectometry is of great interest to engineers and scientists as a cost effective means of remote sensing. Firstly, a special transmitter is not required because GPS signals are already broadcast to the Earth 24 hours a day. Also, a satellite dedicated to GPS reflectometry would only need to carry a modified miniaturised GPS/Galileo receiver and an antenna, which could potentially be accommodated on a tiny 10 kg satellite platform at low cost, enabling multiple satellites on a single launch.



The concept is shown in the animation shown to the left (this is freely available and can be embedded into blogs from YouTube).

The UK-DMC Reflectometry Experiment has also previously been used to detect GPS signals reflected off ice and, surprisingly, dry land. The value of these measurements has yet to be fully explored but they may be used as inputs for climate modelling.

A future revision of the experiment, the “GNSS Reflectometry Instrument” is now being designed at Surrey with a view to flight on a future satellite mission. It is being designed specifically to receive Galileo signals as well as those from GPS, with the intention of real time processing. Dr Unwin's final words on the matter, “The sooner Galileo is up and transmitting the better”

SSTL’s Group Executive Chairman, Sir Martin Sweeting stated in a press release today that he believes the increased competition resulting from the June proposal by the European Commission to fund the Galileo satellite navigation system through the public sector will reduce the cost and risk involved.

”The increased competition will have significant benefits,” stated SSTL’s Group Executive Chairman, Sir Martin Sweeting. “The public sector will soon be in a position to place contracts that give the European taxpayer better value for money and step up the pace of delivering the system”.

SSTL aims to support the European Commission and European Space Agency (ESA) by building on its experience gained through the successful GIOVE-A mission, to provide best value in the operational phase of Galileo. GIOVE-A was developed under a €28M contract signed with ESA in the second half of 2003. The mission’s primary aim was to broadcast Galileo signals from space so that Europe could claim the frequencies filed for Galileo with the International Telecommunications Union (ITU). The satellite was designed, built, tested and launched before the end of 2005 – on-time and on-budget. GIOVE-A has transmitted Galileo signals for over 18 months and remains the only Galileo spacecraft in operation. Following the success of GIOVE-A, ESA placed a further contract with SSTL in March 2007 for a second satellite named GIOVE-A2.

Sir Martin added:

“It’s clear that a second source of operational satellites is needed, both to provide healthy competition and to reduce risk to the schedule of the Galileo system. SSTL has begun discussions with other key European suppliers, with the aim of forming a team capable of supplying a significant portion of Galileo whilst maintaining the winning formula employed on GIOVE-A. We expect these discussions to be finalised in October, in time to provide the public sector with a much needed competitive option”.

Galileo is a joint initiative between ESA and the European Commission. When fully deployed in the early years of the next decade, it will be the first non-military positioning system to offer global coverage.

Earlier this week, the SSTL built GIOVE-A satellite successfully transmitted its first navigation message, containing the information needed by user receivers to calculate their position. Prior to reaching this milestone, the satellite had been broadcasting only the data needed for measuring the receiver-to-satellite distance.

The first Galileo navigation message was created by the navigation signal generator unit on board GIOVE-A, using content prepared by the GIOVE Mission Segment. This week-one navigation message was uplinked to GIOVE-A on 2 May from the Guildford ground station operated by Surrey Satellite Technology Ltd (United Kingdom) and then transmitted from the spacecraft to the users. The objective of the test was to demonstrate an end-to-end link between the Mission Segment and the user receivers. The navigation message is being generated for demonstration purposes only – no service guarantee is provided.

The complete radio transmission from GIOVE-A carries a navigation signal and a navigation message. The navigation signal contains the information needed to accurately measure the distance from the satellite to the user receiver. The navigation message provides the timing and spacecraft orbit data needed to calculate the time and exact position of the satellite. One of the main tasks of the GIOVE Mission Segment is the generation of this navigation message.

GIOVE Mission Segment

The GIOVE Mission Segment comprises two main elements, a world-wide network of 13 Galileo experimental sensor stations and the GIOVE Processing Centre (GPC) located at ESA’s European Space Research and Technology Centre (ESTEC) in Noordwijk, The Netherlands.

Note: The GPC is operated by European Satellite Navigation Industries (ESNIS) – the new name for Galileo Industries.

In order to generate the navigation message, the sensor stations track the signal-in-space from GIOVE-A and send the tracking information to the processing centre. The GPC computes, among other things, the precise satellite flight path and on-board clock model. It then generates a prediction of the orbital path and clock performance for the near-term future, which is the basis of the navigation message. Next, this message is transmitted to GIOVE-A which retransmits it to the user.

Aside from navigation message generation, the primary tasks of the GIOVE Mission Segment are performance characterisation of the on-board clocks, orbit modelling, and operations planning & telemetry analysis for the navigation payload.

Another validation step completed

On 12 January 2006, GIOVE A transmitted the first Galileo signals, thereby securing the frequencies allocated to Galileo by the International Telecommunication Union. Now, the transmissions are carrying a navigation message – this means that the satellite is providing information that is needed by a receiver to calculate its position.

Although the navigation message structure of GIOVE-A is slightly different, the GIOVE-A navigation transmissions are fully representative of those that will be sent by the operational Galileo system. The operating principles are identical and the achievement of the first navigation message is another important step in the validation of the Galileo system design.

GPS interoperability

Following this successful test, in the near future GIOVE-A will begin to continuously broadcast the navigation message, with the message content being updated whenever the satellite is visible from the Guildford uplink station. Additionally, the message content will be extended to include the time offset between GPS and the experimental Galileo system. Knowing this offset, the Experimental Galileo-GPS Time Offset (E-GGTO), will allow the user to build a position fix using GPS satellites and GIOVE-A.

Galileo is a joint initiative between ESA and the European Commission. When fully deployed in the early years of the next decade, it will be the first civilian positioning system to offer global coverage.

ESA press release: GIOVE-A transmits first navigation message

What next?

SSTL were awarded a second contract on March 5th 2007 for initial activities leading to the construction of a second spacecraft for the Galileo satellite navigation system. Named GIOVE-A2, the new satellite (estimated total value 25 to 30 million Euros) will be based on the company’s proven GIOVE-A technology. From now on, there will always be a European navigation satellite in space.

ESA press release: GIOVE-A2 to secure the Galileo programme