AbstractThe need for satellite navigation came out of military requirements in the early 1960's.
The first system of this kind was TRANSIT.
It was more a positioning rather than true navigation system.
The satellites were well designed and remained operational long over their target life time.
The two signals were in the VHF frequency range.
At a certain moment the usage of TRANSIT was released for the civilian maritime community.
The system has provided a limited but useful service for almost 30 years.The program was decommissioned by the end 0f 1996.
Meanwhile the development of technology, in particular space qualified atomic clocks opened the possibility to design and fly systems with largely improved performance and true precision navigation capabillity, fullfiling a broad range of new military requirements (missile guidance,in flight refuelling,guided bombs etc).
The first system of this kind was NAVSTAR/GPS.
The Block 1 demonstration configuration was launched between early 1978 and late 1985 (11 satellites).
It was followed immediately by the operational Block 2, completing the initial Block 1 with 27 satellites launched between early 1989 and end of 1996.
The full 24 satellite constellation was completed in early 1994.
GPS transmits the navigation service on two L-band frequencies.
The coarse acquisition code C/A is intended for civilian use as well,but without any guarantee of continuous availability or precision.Moreover the C/A code was long time deliberately degraded(selective availability).
This degradation gave the impetus in Europe to start in the early 1980's the design and development of the system known as EGNOS.
The European Geostationnary Navigation Overlay System.Similar systems were developed subsequently in other parts of the world(WAAS/MSAS etc.)
It might be this very fact who helped the responsible US authorities to give up the selective availability of the C/A code in May 2000.
More or less at the same time begun in the USSR the design and development of the GLONAS system.
It uses the same frequency L-band as GPS,but the signals are not compatibles.
As GPS, GLONAS transmits two services,the high precision (HP) available to the military,and the Standard Precision Service (SPS) released in 1988 to civilian users.
It is to be noted that both GPS and GLONAS although available for civilian usage do not provide any guarantee of continuos availability and precision.
Hence none of them satisfy the requirements dictated by the safety of civilian navigation,be it in the air or to sea.
The first global system thought from the beginning to satisfy the very strict requirements of civilian navigation and positioning is the European system under development.
System studies and hardware development started already in the 1980's under the name NAVSAT and evolved into what is at present known as GALILEO.
This paper intends to explore the system evolution from TRANSIT to GALILEO as a function of availability of technology,of system architecture and user requirements.