Jim Doherty
Senior Analyst, Institute for Defense Analyses; member, GPS Independent Assessment Team
On the question of interoperability between GPS and Galileo, the overwhelming operational issue is that geometry matters. For better geometry, particularly in urban and natural canyons, mountainous terrain, and other challenging situations with limited views of the sky, the more signals available from different satellites, the better.
The second major issue is that signal power and spectrum also matter. In the not-so-distant future GPS and Galileo users will operate in an increasingly challenging noise and interference environment. Adding signal power in space is extremely expensive; however, taking advantage of the spectral diversity of existing and planned signals--for example, civil GPS signals on L1, L2, and L5 as well as planned Galileo signals--would mitigate impacts of much future interference.
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We must keep in mind that GPS is fundamentally a global position and time (positime) distribution network. As long as users receive four GPS signals with good geometry, they can obtain accurate and precise positime tags for events or observations on this common grid anywhere, Galileo, when it becomes operational, will be the same-type network. GPS alone is already extremely good, and user equipment designers currently provide integrated user systems (e.g., automobile navigation systems) that extend the positime grid deep into urban canyons and even parking garages, with inertial sensors, wheel counters, map matching, and other technologies.
Still, there are times when only half the sky is visible or when a new noise or interference source pops up. With more signals in that half of the sky, using both GPS and Galileo together may provide a solution when either alone would not. Similarly, shifting to a slightly different frequency signal might allow operation in the presence of an interference source. Yes, this will make some user equipment more expensive, and lower-end users may not want to pay for more capability. However, for safety-critical applications or for those applications whose failure would have a high economic impact, users would likely be willing to scale up in cost for increased performance using both together.
What are key issues of the interoperability question? Most discussions of GPS and Galileo indicate they will be noninterfering or compatible with each other, and this capability is incredibly important. Nothing indicates the systems will merge to become one. It is, however, technically possible that they remain independent systems with differentiated services for specific user groups but are managed such that others could elect to use them as independent redundant services or as one robust integrated service.
In this regard, interoperability means first setting orbits and slots so that each additional signal source improves geometry for most users. Next, if the system clocks are managed compatibly, then only one "extra" satellite is needed to solve for the second system clock in the receiver. And finally, if the geodetic reference frames are the same, or nearly so, the computational power needed for conversions is minimized.
Borje Forssell
Professor,
Norwegian University of Science and Technology
The question of interoperability and compatibility between Galileo and GPS has been discussed since it became clear that Galileo will be a reality within a few years. The sometimes bewildering discussion has been further confusing because people use various definitions of the words.
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According to my definition, systems are interoperable if they can be used to their full extent and without reduced performance whether they are used separately or together. This includes the provision that neither disturbs the other in a manner noticeable by the user. It also means that two systems used together (integrated) should perform better than or at least as well as the best of the two at that time and place.
The frequencies, signal formats, and power levels of Galileo and GPS (including the modernized version) seem to meet the interoperability requirement according to this definition. A number of studies and simulations have been carried out, and the (uncontested) results show that mutual interference leads to a maximum reduction of signal-to-noise ratio (SNR) of about 0.1 dB, but an average of only a fraction of that value. No user would notice such a small reduction.
Interoperability also includes time and coordinate references to the extent that both systems should use the same references or, if not, transmit the differences through their navigation messages so that the receiver does not need extra satellites to determine these differences. Galileo and GPS are designed to meet these requirements as well.
However, these results have not stopped the discussion. Attention now is perhaps more concentrated on compatibility, but in my opinion compatibility and interoperability are interwoven and should be treated together because they both concern the coexistence of the systems.
According to a definition by the U.S. Department of Defense (DoD), there are two kinds of compatibility: Radio Frequency Compatibility and National Security Compatibility. Two systems are said to be RF-compatible "if it is assured that one system will not cause interference that unacceptably degrades the stand-alone service that the other system provides." By this definition, Galileo and GPS are compatible.
The other DoD definition reads "signals are compatible from a national security perspective if the ability to protect military service while simultaneously preventing hostile use and preserving civil service outside the area of hostilities is preserved through the spectral separation of civil and military signals."
This definition is obviously invented in the context of Galileo PRS and GPS M-code having overlapping spectra in the L1 band. The definition does not tell which national security it concerns, but the United States has made very clear that it is U.S. national security, and that the United States alone can determine, if deemed necessary, when it is applicable.
This leads to a locked-up and very unfortunate situation. The problem should be solved in a spirit of mutual respect, which has been somewhat lacking in talks held. Galileo and GPS as a whole will benefit the navigation community immensely, and short-sighted unilateral military aspects should not be allowed to reduce this benefit.
In the long run, it will be impossible to prevent any user, deemed hostile or not, from using state-of-the-art navigation, in the same way as, for example, state-of-the-art cell phones or cars. The problem must therefore be solved in a way that allows the most-sophisticated users to benefit most, but without preventing the less-sophisticated from benefiting to the best of their ability from the same basics. I am confident that this will be the situation at some point in the future.
Rainer Grohe
Director
Galieo Joint Undertaking
Evolution of user requirements clearly indicates a pressing need to achieve interoperability between GNSS systems for the benefit of both individual users and for all types of applications. In this respect, interoperability means Galileo's capability to be used, either as an alternative or in combination, with GPS and/or other systems.
This objective is written into Galileo's formal "Mission Requirement Document." It stipulates that "the Galileo Global component shall facilitate the combined use of Galileo Open Service and Safety of Life services with the GPS Standard Positioning Service and its evolution, and with the GLONASS SPS Service with its own evolution."
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Mass-market applications
For the personal user and in land mobile applications, the combined advantages of receiving data from different signal sources represents a major opportunity for assistance services and to enhance positioning solutions. This is the case, for instance, when satellite signals are masked in built-up areas. Detailed studies of the simultaneous use of Galileo and GPS have demonstrated all the benefits in terms of accuracy and signal availability in urban environments.
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Safety of Life applications
