ATM Modernization, Commercial

The Issues Orbiting Galileo

By Charlotte Adams | October 1, 2003
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Years after it was first proposed, Europe’s Galileo satellite navigation system finally is moving toward launch. The European Union (EU) and the European Space Agency (ESA) this year jointly funded Galileo’s development and validation phase, providing 1.1 billion euros to support work through 2005. Total cost is pegged at 3.5 billion euros, compared with the more than $10 billion (9.2 billion euros in today’s less valuable dollars) required to develop GPS. Although the Galileo figure may be an underestimate, the European system benefits from the body of knowledge and experience with GPS and will use smaller, less complex spacecraft.

Galileo already has gotten further than U.S. skeptics once thought possible. They have viewed the European system with suspicion, concerned about security and trade issues. If defense and electromagnetic compatibility issues, plus interoperability questions, can be resolved, however, a "super constellation" would provide greater integrity, availability and reliability than either system, alone, could give.

The Galileo system will comprise 30 satellites–27 operational and three spares–flying in three circular medium-Earth orbits at an altitude of 14, 713 miles (23,616 km) and inclined at 56 degrees to the equator. The system is scheduled to be operational in 2008.

Europe plans four navigation services: a free, open service (OS), a safety-of-life (SoL) service, a commercial service (CS) and a governmental public regulated service (PRS). OS, PRS and SoL services are planned at or near the GPS L1 frequency, centered at 1575.42 MHz. The OS and SoL services also would use the planned GPS L5 frequency (1176.45 MHz)–which Galileo calls E5a–and the E5b frequency (1207.14 MHz). The PRS signal is planned for both L1 and E6 (1278.75 MHz).

Galileo has opted not to overlay the GPS L2 frequency (1227.50 MHz), even though L2 is slated to receive a new GPS civil signal. "L1 and L5 offer the maximum freqency gap," comments Rene Oosterlinck, ESA’s project manager for Galileo, referring to the accuracy gain enabled by receiving two widely spaced signals to correct for ionospheric distortion.

As part of a push for international participation, EU negotiators are talking with Chinese officials and hope to wrap up an umbrella agreement describing the scope of areas for future cooperation this year, says Eero Ailio, with the European Commission’s (EC’s) Galileo unit. "The Chinese are very positive about the process–they want to invest in Galileo, to put cash in."

Neither the PRS service nor critical technologies, for which licenses do not exist, are being discussed with China at this time, Ailio says. PRS is vaguely defined, but seems intended for EU national law enforcement and intelligence agencies, and will be usable by European military services.

Gathering Momentum

Much receiver development work is proceeding or in prospect, both in ESA and the EC. The Galileo Joint Undertaking (GJU)–initiated this summer between ESA and the EC–also listed at its Web site a "first call" for bids on preliminary receiver development, with bids due by Oct. 17. The goals of this call include:

  • A development plan for receivers and user terminals, including combinations with other global navigation satellite system (GNSS) and non-GNSS systems;

  • Development of receiver core technologies; and

  • Development of a prototype Galileo receiver, based on the current knowledge of GPS and satellite-based augmentation system (SBAS) receiver technology.

Despite this progress, there are many unknowns. The 2008 operational date seems a little ambitious. And the market for alternative-technology satnav products and services may be less than it seems to be. The example of Airbus, an earlier, now wildly successful, project to create a European airframer–may be a misleading parallel. Satnav "is not as mature a market as aircraft was when Airbus tried to get into it," says Marco Caceres, senior space analyst with the Teal Group. As a more practical matter, "There’s a big difference between launching some system and having an actual system that can be reliably used day in and day out," a U.S. industry observer adds.

U.S. security objections are the biggest hurdle for the embryonic system. Despite Galileo advocates’ frequently asserted desire to build a civil-controlled, European GNSS system, some of them want to overlay the PRS signal on the U.S. military’s new M-code, so that Galileo cannot be jammed without jamming M-code. This would complicate the U.S. policy of "local denial," which would selectively deny enemy forces access to GNSS services in an area of conflict. The first GPS satellite to carry M-Code is expected to be launched in 2006.

The U.S. Defense Department (DoD) is obviously displeased. GPS, possibly in conjunction with inertial systems, is key to NATO’s 2010-to-2015+ navigation system strategy, according to a presentation by a key NATO official. M-code will be crucial for NATO military operations, including navigation and precision strike.

RAIM Through the Roof

It is too early to say exactly what Galileo could add to civil aviation, but if the two systems are compatible and interoperable, a super constellation of some 60 satellites would raise some interesting possibilities. For one thing–quite apart from the projected SoL service–receiver autonomous integrity monitoring (RAIM) solutions, now used by technical standard order (TSO) C129 GPS receivers to verify the integrity of GPS signals, would become much more robust.

Although seven to eight GPS satellites are often available for RAIM calculations, there sometimes are only four or five available at one time. In the latter case, GPS RAIM "totters on the edge of mathematical observability," comments a European industry official. It leaves much to be desired because there are too few satellites to calculate a good integrity solution. But if both satellite systems are used together– and 15 or 16 satellites are pulled into the equation–there would be a higher-confidence mathematical solution.

With two sets of satellites, RAIM solutions could potentially support more demanding protection limits. Using the unencrypted ranging signals of the SoL service, plus the two aviation-protected GPS signals that will be available in the modernized constellation, users could get a very high level of integrity, even if the Galileo SoL integrity message was encrypted. There may be encryption on the SoL service as a means of generating revenue, Oosterlinck notes.

The UK’s Vega Group–which leads the international consortium providing the Galileo program’s system simulation facility–has conducted studies on RAIM. These analyses assume the existence of Galileo and a modernized GPS system, with the availability of a second, aviation-protected civil signal on the GPS L5 frequency. Vega’s work suggests that "civil aviation users will be able to use RAIM and similar techniques to get a level of integrity broadly equivalent to that currently expected for GPS-plus-SBAS, or indeed Galileo’s own safety-of-life service," says John Loizou, manager of the company’s Systems Engineering Space Business Unit.

The number of satellites in view–it could be 15 or 16, with the addition of Galileo, rather than seven or eight with GPS–would provide more redundant information from which integrity can be derived. And the independence of the two systems "would provide a degree of defense against a number of failure modes and feared events," Loizou says. For example, the independence of the two systems’ time references adds some protection against system-time errors, he says. Potentially, Galileo and GPS together could be used for Category I precision approaches without the geometric constraints imposed by a geostationary SBAS system, which limits usability at very high latitudes.

There is debate about whether RAIM, alone, could ever be used for precision approaches with high demands on aircraft required navigation performance (RNP). In other words, should "the system"–GPS plus its wide area augmentation system (WAAS), and Galileo plus its integrity monitoring apparatus–provide external integrity monitoring or should the onboard receiver calculate and evaluate integrity, using RAIM and RAIM-like processes internally?

Loizou stresses that SBAS systems will be necessary for the indefinite future, given that GPS will require externally monitored integrity at least until the second, aviation-protected civil frequency is fully fielded. (The first satellite carrying the L5 civil signal is expected to be launched in 2006.) Even with two aviation-protected GPS signals, the GPS ionospheric corrections provided by WAAS would also be an important backup if there is interference on either frequency. Europe’s EGNOS SBAS system, expected to be certified by 2006, would continue to be used for many years after Galileo comes on stream for the same reason. And aviation users who will be equipping with common WAAS/EGNOS avionics, would not want to replace this gear any time soon. (EGNOS stands for European Geostationary Navigation Overlay Service.)

Galileo is designed to provide a global network of integrity monitors, feeding information to a central integrity computation facility at the Galileo system’s ground control station, with integrity messages uplinked and broadcast from the Galileo satellites. Galileo officials also envision the opportunity for geographic regions to perform their own integrity calculations and broadcast their own integrity messages through the European constellation.

If Galileo comes to be fielded, the question of how many separate means of providing integrity–or whether GNSS receivers alone can guarantee integrity–is up to the regulatory authorities, Loizou stresses. RAIM-type mathematics executed in an appropriate receiver–using a combined Galileo and GPS constellation–would provide integrity to the level required for precision landings, Loizou says. "But philosophically, whether people would accept that integrity could be decided entirely by a ‘receiver autonomous method,’ without ground monitoring of the signals to broadcast integrity ‘flags,’ is a separate issue," and one, Loizou agrees, that seems unlikely at the moment.

The issues of designing a hypothetical common GPS/Galileo receiver are not trivial. It would have to do more signal processing to deal with a multitude of new Galileo signals, even though Galileo would operate in the same frequency range–at L band–making antenna designs easier than would be the case otherwise. Roughly compatible signal structures would make the receiver’s task easier. And the more closely the two systems are coordinated, the easier the receiver burden would be.

Signal Strife

The following options are among those that have been under consideration for the PRS service, according to a presentation last year by Robert Bell, then NATO’s assistant secretary general for defense support:

  • Direct overlay of M-code using the same modulation scheme,

  • A slightly different signal that centers on the M-code frequency but extends beyond the M-code "sidelobes" in each direction,

  • A flexible modulation scheme similar to the first option but reprogramable to a different modulation, and

  • A totally different modulation scheme.

The first two concepts would not allow selective jamming, and the third would require extensive, time-consuming (and not necessarily successful) coordination with European authorities. The fourth approach might allow selective jamming. M-code, designed for GPS L1 and L2, was invented to spread the military signal outside and away from the civil GPS signal, allowing the civil GPS signal to be jammed without harming the military signal. M-code’s spectrum peaks on either side of the present GPS L1 open access signal.

The two sides are discussing "several types of [PRS] modulation, where we use the [L1] frequency band in a different way," Oosterlinck says. But the U.S. has had trouble with a lot of proposed PRS signals, he adds. "We are trying to calculate the interference level, the power density, and all of those things." Finding a solution to the problem "depends on where the United States will put the limit [for acceptable degradation to M-code]." If PRS should overlay M-code and if the United States should want to jam PRS with an M-code degradation of "0.0000," it could not be resolved, technically, Oosterlinck adds.

The U.S. has suggested that the Europeans consider moving the PRS service to the Glonass G1 frequency–also in the L band–or somewhere between L1 and G1, according to a DoD official. (Glonass is the Russian GNSS system.) "These alternatives are just as robust," the official asserts, although they could be jammed without jamming M-code. When asked about the requirements for PRS service at L1, EC officials have said they are classified, the official says. There is concern that European arms manufacturers might want to export PRS-guided munitions.

The question need not be resolved before Galileo test satellites are launched because the program is developing an onboard "flexible signal generator," allowing signals to be reprogrammed from the ground. It would be good to maintain this flexibility on production satellites, Oosterlinck says, even though it adds to the system’s complexity.

Oosterlinck stresses the legality of an M-code overlay. Legally, "Europe could launch Galileo tomorrow with an overlay of the M-code." In the end, however, it’s a political matter, he concedes, which Europe wants to resolve with the U.S.

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