The aeronautical community must pool its resources and protect its strategic interests if it is to avoid losing the hundreds of millions of dollars invested in satellite navigation. The threat comes from an Inmarsat-sponsored proposal, currently before the International Telecommunications Union-World Radio Council (ITU-WRC), to share frequencies in the global navigation satellite system/radio navigation satellite system (GNSS/RNSS) band of 1,559-1,610MHz.

The GNSS/RNSS band has until now been designated for exclusive use by the aeronautical and maritime communities, but the proposal - originally put forward at last year's WRC 97 meeting and due to be considered more seriously at WRC 2000 - would allow mobile satellite service (MSS) users access to a subset of frequencies in the lower portion of the band, between 1,559 and 1,567MHz.

If the proposal is accepted at WRC 2000, the ITU risks compromising critical aeronautical safety-of-life services that rely on GNSS/RNSS signals because they will have no protection from interference by other band users.

"The spectrum protection issue is as time critical and potentially damaging as the Year 2000 flaw, and suffers from the same lack of foresight, understanding and resources to correct it," says James Miller, a senior analyst with United Airlines. He believes the fact that the spectrum problem is seen as a strategic issue has caused inertia among airline lobby groups such as the Air Transport Association (ATA) and International Air Transport Association (IATA).

"We may not be using [the Spectrum] now, but if you look at the studies done, by 2005 we're going to be looking at air traffic control gridlock," says Miller. "Once that spectrum is gone, it's gone. Airlines have to step up to the plate and put resources into this, because the ATA and IATA are not going to do it unless we say this is important to us."

In economic terms alone, the issue is vitally important. United, for example, has been equipping its fleet with GNSS functionality to comply with Federal Aviation Administration and International Civil Aviation Organisation (ICAO) infrastructure modernisation plans. By the end of 1998, 35 Boeing 747-400s, 34 777s, four 767-300s, 20 Airbus A319s and one McDonnell Douglas DC-10 operated by United will be able to use the GNSS.

If spectrum sharing becomes a reality, that investment is lost, says Miller, and even greater losses will be incurred from the operational impact of not being able to take advantage of satellite navigation technology.

As recently as October, the battle to protect the GNSS/RNSS band from encroachment by MSS users appeared to be swinging in favour of the aeronautical community, with a suggestion emanating from the ITU that the frequency-sharing proposal would be turned down. But there was a sting in the tail: to retain exclusive use of the GNSS/RNSS band, the aeronautical community must present at WRC 2000 a concise and detailed plan for its utilisation .

Currently, the GNSS/RNSS band supports the US global positioning system (GPS) and Russia's Glonass. Future plans foresee a multitude of uses. As well as supporting the US Wide Area Augmentation System, the European Geostationary Navigation Overlay System and Japan's MSAS augmentation system, the band may be needed for "-ADS-B [automatic dependent surveillance-broadcast] and pseudo-satellites for the LAAS [local area augmentation system], so we can do Cat 3 landings," says Miller. "We don't know yet what frequencies those systems are going to use, yet they're forcing us to make decisions on that right away just to fill the band."

There are conflicting views on the efficacy of sharing the band. A study conducted outside the framework of the ITU Radiocommunication Sector, but presented at WRC 97, suggested that appropriate power limits for the MSS would protect GPS and GLONASS as well as future navigation satellite systems, such as the E-NSS-1. "They're trying to minimise the threat to GPS-based applications," says Miller. "Once they [the MSS] get their foot in the door, it's obvious they're not going to get good service unless they crank the power up."

If this happened, the fear is that the selectivity of airborne systems would not be great enough to pick out the weak signals from navigation satellites. Current generation GPS receivers cannot deal with in-band interference. Air Navigation International, a sister publication to Flight International, reported this year that studies conducted by the ICAO found that current minimum specification GPS units were susceptible to interference and could not satisfy the draft accuracy requirements for Category 1 approaches.

The problem is receiver selectivity - a measure of its ability to differentiate between weak signals from geostationary navigation satellites and perhaps stronger signals from MSS users on adjacent frequencies. Fixing the problem would require redesigning the radio-frequency circuitry, possibly bumping up the cost of the receivers in the process.

The ICAO is so concerned about the impact of the Inmarsat-sponsored proposal it has stated that, if there is to be spectrum sharing, then GNSS will not be permitted to be used for safety-critical services. Industry consultant Joe Dorfler says, if that policy remains, even the "-augmentation systems such as EGNOS, WAAS and all of the LAAS will not be qualified for safety services".

Dorfler is actively involved in the GPS Risk Assessment Study mandated by the US Congress and being carried out by Johns Hopkins University in Baltimore. He says the ICAO's stand implies "-the door has been closed on the hopes of decommissioning current navigation systems such as VORs, and ILSs during the transition period to GNSS".

The impact of the ICAO's seemingly uncompromising stance should not be underestimated. Without GNSS, there is no communication, navigation,surveillance/air traffic management (CNS/ATM) system, which has been billed as the future for global air navigation. The development of CNS/ATM has already been threatened by the separate, but by no means less important, step taken at WRC 97 to allow sharing in the satellite communication bands between 1,545MHz and 1,555MHz and between 1,646.5MHz and 1,656.5MHz.

Before WRC 97, these bands were allocated exclusively to the aeronautical mobile satellite (route) service (AMS(R)S). A proposal to introduce an allocation to the mobile satellite service by merging aeronautical, maritime and land mobile satellite services into a generic mobile satellite service was presented at WRC 97.

The majority of delegates supported the proposal, with the exception of some of the Asia-Pacific countries, the ICAO and the International Maritime Organisation. The conference adopted the plan and agreed to some footnotes providing extra protection for aeronautical and maritime use.

Proponents of this change argue that safety-of-life communications such as air traffic control messaging would always have priority over other system users, but considerable doubt has been expressed over the ability of technology to ensure instantaneous communication for those users who require it. Space systems would have a pre-emption mechanism that provides priority access to key system users.

"We think it is possible to do this within one system," says Kors Van der Boorgard of IATA. "However, the intention is also to have pre-emption between systems - so-called inter-system co-operation. It means, for instance, that Inmarsat must terminate a phone call because Iridium needs the spectrum for aviation, and requires that the networks of those service providers must be connected and operating in co-operation." Apart from the competition issues that this method of working presents, from a technical perspective it is difficult to achieve because of the differences within each system. There is also a doubt over access time.

Given that co-ordination between systems takes time, a controller will not have the instantaneous ability to talk with a pilot. This has serious consequences for the ability to pass on urgent safety-critical information. Van der Boorgard would like to see a return to pre-WRC 97 status for the satellite communication bands, but believes this is not possible. He says IATA, the ICAO and others are therefore working to obtain cast-iron assurances that "-safety will not be compromised and that aviation has guaranteed access to the spectrum".

A big problem for the aeronautical community is that it has a minority voice within the ITU and is therefore seen as a minority user. Says United's Miller: "My response is that if you consider all of the passengers on all of the world's airlines, then we are the majority users and are thinking of the global safety and future economic development of all the countries."

To help raise this profile, the airline industry has invested more than $100,000 to produce information packs, presentations and a CD-ROM to alert GNSS/RNSS users to the inherent dangers of sharing band allocation with the MSS and calling on them to lobby effectively for exclusivity. Airlines in particular are not content with leaving the spectrum sharing issue to the ICAO.

Miller believes past events have proved that only the airlines themselves will go the extra mile to protect their interests. "Airlines have sufficient political leverage, and the strongest economic incentives, to help preserve the safety and security of the GNSS for aeronautical use."

He says success will depend on ATA and IATA linking up with much stronger organisations such as NATO to build an international alliance interested in protecting and promoting the security and economic benefits of GNSS.

"The optimal scenario is for the ITU to get this item permanently off the WRC 2000 agenda," adds Miller. "As radio spectrum becomes more valuable, however, this becomes more improbable, and the aviation industry will be forced to defend its interests based on economic lobby strength rather than potential safety threats, employing tactics that the MSS lobby has long used."o

United has invested heavily in making much of its fleet, including 34 Boeing 777s, with GNSS functionality. If spectrum sharing becomes a reality, this investment will be lost, it believes

Source: Flight International