ccurately determining one's position has been a recurrent problem in history [1]. It even precedes the first deep-sea navigation attempts of ancient civilizations and reaches the present time with the issue of legal mandates for the location identification of emergency calls in cellular networks and the emergence of location-based services. The science and technology for positioning and navigation has experienced a dramatic evolution [2]. The observation of celestial bodies for navigation purposes has been replaced today by the use of electromagnetic waveforms emitted from reference sources [3].There is a large variety of radio-navigation systems, ranging from legacy ones dating from the middle of the last century, such as Decca or Loran, to the ones relying on the transmissions from wireless local area network (WLAN) base stations or from the devices found in wireless sensor networks. However, the systems based on satellite transmissions are the ones that play a prominent role today. They are gathered under global navigation satellite systems (GNSS). This term refers to all systems (some of them operational, and others under development) that provide users with positioning information

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BIOGRAPHY Enrique Domínguez received a M.Sc. degree in Telecommunications Engineering in 2000 and a Master in Space Technologies in 2009, both from the Polytechnic University of Madrid. He joined GMV in 2000 working first in the development of EGNOS and Galileo and since 2009 in GNSS software receivers, multi-sensor fusion algorithms and integrity algorithms.

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In a scenario in which several GNSS systems, GPS, Galileo, GLONASS, COMPASS, QZSS, are planning to coexist in the same frequency band, spectra/signal structure management and relative power levels are the main instruments to counteract the possible impact of intersystem interference, and the ability to increase the transmit power on individual signal components from GNSSs system providers has become an important feature of the modernized satellite payloads. Among the modernizations that GPS Block IIR-M andIIF Space Vehicles payload underwent, one of them is the capability of programmable power output also known as ¨flex power¨ capability. The flex power capability enables operators of the system to increase the signal strength of the military P(Y) and M-code signals, if needed by Department of Defense (DoD) and allied GPS users. The 2nd Space Operations Squadron (2SOPS) experimented, over the course of five days during the month of September, the use of this programmable power output capability in several SVs. This paper provides an analysis of the C/No measured from sensor stations where the transmit power level changes that GPS satellites underwent during the days from the 7th of September 2010 to the 12th can be assessed. Moreover, a theoretical analysis based on real measurements collected with the STFC (UK) 25m Radio Observatory [1] is presented, providing a possible explanation on how the flex power capability may have been implemented on board.