Military/Civil Interoperability

Since the U.S. Air Force (USAF) and Navy began equipping aircraft to meet military safety mandates and civil air traffic management (ATM) requirements in the late 1990s, they have focused primarily on cargo, tanker, troop carrier, VIP and general utility aircraft. Less thought was given to fighters, bombers and special mission aircraft. But that’s changing now. Faced with European 8.33-KHz VHF channel spacing and Mode S surveillance mandates, the services are rushing to upgrade their tactical aircraft, too. The cost will be enormous. The Air Force’s interoperability bill for all types of aircraft, many of which are not yet funded, is estimated be about $24.3 billion.

No question, the fighter fleet is playing catch-up. Fighters’ Mil-Std-1553 bus architectures would make civil avionics insertions designed with ARINC 429 interfaces difficult, if there were even room for these boxes. And achieving 8.33-KHz channel spacing–implemented in Europe above FL245 in 1999–isn’t simple, as some tactical aircraft carry only UHF communications. An all-but-certain Eurocontrol extension until 2009 provides some relief on Mode S compliance, but mandates for required navigation performance (RNP) and reduced vertical separation minimum (RVSM) loom ahead. RVSM increases airspace capacity between 29,000 feet and 41,000 feet by reducing aircraft vertical separation from 2,000 feet to 1,000 feet.

Eurocontrol also is considering a mandate for civil aircraft to use controller pilot data link communications (CPDLC) above FL285 by 2009 for new civil aircraft and by 2014 for legacy civil aircraft. State aircraft probably will be exempted, although some military platforms, such as VIP carriers, may be equipped for CPDLC for safety reasons and operational benefits, says Col. Jan Plevka, a Eurocontrol military expert.

The USAF and Navy are well on the way to achieving 8.33-KHz spacing and are making good faith efforts to achieve Mode S. Exceptions will be soon-to-be retired aircraft like the C-141, S-3B, F-14 and older F/A-18A/Bs, as well as trainers, which won’t fly in European airspace. For domestic RVSM the USAF expects to achieve "workarounds" with U.S. authorities to allow trainers to get to and from their practice ranges.

The pioneering KC-135 upgrade, conceived in the mid-1990s, includes capabilities such as RNP to 1.0 nautical mile (nm), automatic dependent surveillance (ADS), and CPDLC. New avionics include a communications management system for data links, flight management system (FMS), Inmarsat satcom radio, multimode receiver with microwave landing system, a second embedded GPS/INS (inertial navigation system), and modifications to existing HF and VHF/UHF radios. Baseline aircraft going into the upgrade already were equipped with RVSM modifications and "Pacer CRAG" updates, including traffic alert collision avoidance system (TCAS), enhanced ground proximity warning system (EGPWS), color weather radar, and a single embedded GPS/INS. Rockwell Collins has configured 51 aircraft and is under contract for 89 more. The company estimates as many as 411 aircraft eventually will be upgraded.

The USAF’s command and control aircraft, the E-3 airborne warning and control system (AWACS), also will get a comprehensive upgrade. Through three different programs the aircraft will receive: secure GPS navigation by 2008; TCAS, Mode S elementary, RVSM and 8.33-KHz spacing by 2010; and Mode S enhanced, CPDLC, ADS-B and RNP/RNAV by 2018.

Mode S

The Air Force regards Mode S as its most urgent communication, navigation and surveillance (CNS)/ATM requirement because the penalty for noncompliance is airspace denial "from the ground all the way up," says Lt. Col. Steve Sorensen, chief of the Air Force’s Global Access and Navigation Safety and CNS/ATM Branch in the Pentagon. A Mode S elementary transponder transmits airframe-specific identification, current altitude and call sign. Mode S enhanced surveillance transmits that information plus heading, air and vertical speed, rate of turn and selected cruise altitude. Mode S-enabled selective interrogation also means higher data integrity, reduced radio frequency interference, and increased air traffic capacity.

Every aircraft will have to equip with Mode S elementary for safety reasons, except for those rarely flying in European airspace. (They will be accommodated on a case-by-case basis.) The Mode S enhanced mandate will apply to modern, fixed-wing aircraft of 12,570 pounds (5,700 kg) or more with digital cockpits and more than 19 seats, including VIP aircraft in the civil route structure. It will not apply to fighters.

Eurocontrol is drafting a civil/military CNS/ATM interoperability roadmap, which it hopes to issue in June. The document will address certification procedures for military aircraft and transition arrangements for European mandates.

An Air Force identification friend or foe (IFF) replacement program is buying APX-119 transponders, as well as APX-114 interrogators, for F-15s. Fielding will begin in January 2006, and 457 F-15C/D/Es are to be equipped by 2009. For F-16 Block 50s the service plans to buy BAE Systems’ APX-113 combined interrogator/transponders that are Mode S-capable. For F-16 Block 40s the service is studying a transponder-only solution. A corresponding software upgrade will be fielded by 2007. Newer Block 50 F-16s have been funded, Block 30 aircraft have been partly funded, and Block 40s have not yet been funded. "What allows us to be compatible with CNS/ATM is the transponder piece," says Col. Michael Williams, commander of the Fighter/Attack Systems Wing at Wright-Patterson AFB, Ohio.

While the USAF focuses on the minimum fighter requirement of Mode S elementary, the Navy plans to push further, to something more than elementary but less than enhanced. Since some of the information needed for enhanced surveillance is already on the aircraft’s Mil-Std-1553 data bus, the Navy aims for "elementary Mode S surveillance plus," says Chris Hoover, CNS/ATM integrated program leader for the Naval Air Systems Command (NavAir).

The Navy’s lead tactical CNS/ATM aircraft, the P-3C Orion submarine hunter, will receive 8.33-KHz VHF radios, enhanced Mode S surveillance systems, and RNP/RNAV for departure through en-route and non-precision approach operations–from RNP 4 down to RNP 0.3. The Navy also plans to equip its C-2A delivery and E-2C command and control aircraft with Mode S, 8.33-KHz spacing and similar RNP capability, starting in early FY07 and mid-FY08, respectively.

8.33-KHz Spacing

VHF 8.33-KHz channel spacing is required in Europe at FL245 and above and is being planned for above FL195 by 2007. As of mid-January 2005, Eurocontrol was considering a mandate for state aircraft to comply with the requirement above FL195 by March 2009. The proposed mandate would cover VIP, tanker, cargo, maritime patrol and airborne early warning platforms.

The USAF, meanwhile, is buying 583 8.33 KHz-capable ARC-210 radios for models A through E of its F-15s, a step that will provide basic 25-KHz spacing by the end of this year. This procurement is driven by the fighters’ need to communicate with FAA ground facilities and civilian airliners for homeland defense. The operational flight software to move to 8.33-KHz spacing will be fielded between November 2008 and January 2009. F-16s already have ARC-186 VHF radios with 25-KHz spacing and can waive the 8.33-KHz requirement. But F-117s are a concern, as they have no VHF capability, according to Sorensen.

Navy F/A-18s, except for the older A and B models, and C-2As have been equipped for 8.33 KHz. P-3Cs and E-2Cs will be made compliant over a period from FY06 through FY11.

FM immunity has been a big issue because high-power FM stations in Europe have encroached on civil ILS frequencies, raising safety concerns for aircraft landing at civilian airports. The fix, which has been largely completed in the fighter world, calls for ILS systems that are filtered to prevent FM radio interference. Air Force F-15s and F-16s have been outfitted. In most cases the Navy has procured ILS filters for existing systems. Where ILS units were not upgradeable, as in older P-3s, multimode receivers with built-in protection will be used. F/A-18s, which have military ILS systems, don’t have this problem.

RNP/RNAV

Basic RNAV (B-RNAV), including RNP 5.0, has been required in Europe since 1998. Compliant aircraft have to have navigation equipment that can achieve an accuracy of 5 nm with a 95 percent confidence level. However, Eurocontrol plans to implement precision RNAV (P-RNAV) of +/- 1 nm in terminal airspace by 2004/2005 and in en-route airspace by 2010. "Four-dimensional" RNAV (4D-RNAV)–with an additional timing function–may be required gate-to-gate by 2015.

For RNP/RNAV the Navy will certify the total navigation system error to allow access to civil airspace by using the military GPS precise positioning system (PPS). Navy aircraft will use dual-frequency military GPS to achieve GPS ionospheric correction and inertial navigation systems for navigation continuity if the GPS signal is lost. The Navy will be able to accomplish this goal because it will replace existing 5-channel GPS receivers with all-in-view, 12-channel receivers that can perform receiver autonomous integrity monitoring (RAIM) of GPS data. NavAir’s CNS/ATM program has funded GPS integrity receivers for the P-3, E-2C and C-2 aircraft, as retrofits, and for the F/A-18E/F, as forward fits. Other programs are putting in GPS integrity receivers that the CNS/ATM program will leverage for RNP/RNAV, Hoover says.

The Navy’s P-3C is intended to meet existing and future RNP/RNAV requirements, from RNP 4 down to 0.3 in the terminal and approach areas, Hoover says. This will require new avionics, such as Rockwell Collins’ CDU-7000 mission computers, including FMS functions. The existing Smiths data loader will be upgraded, and the inertial system will incorporate a military GPS card to form a tightly coupled GPS/INS system. The first aircraft is slated for installation in the fourth quarter of FY06. The mission computer, navigation system, displays and aeronautical database, coupled with containment and alerting algorithms, will help the P-3Cs meet RNP/RNAV requirements.

On the Air Force side, both F-15s and F-16s have INS systems that will maintain RNP 5 for two hours, which is longer than most fighter missions. When they cross the ocean, the fighters generally accompany tankers, Williams explains. The tankers, which are equipped for RNP over long distances and for RVSM, can "tow us across the pond," keeping the fighters in the correct air lane, he says.

F-16s, however, cannot currently use GPS to refresh INS systems because the GPS sensors in their embedded GPS/INS–or EGI–systems and GPS receivers do not comply with B-RNAV integrity requirements. F-15Es already have INS and EGIs, and because the EGI does not automatically update the INS, these aircraft will be capable of B-RNAV operations. One hundred and seventy-nine F-15C/Ds are being upgraded with EGIs that will include GPS integrity sufficient for B-RNAV.

Although the Air Force initially doubted the B-52 could reach RNP 5.0, engineering studies and the fact that the aircraft carries two navigators, convinced the service to certify the bomber at that level. The KC-10 meets RNP 0.3 and the E-4 airborne command post, RNP 1.0, Sorensen says.

RVSM and Beyond

RVSM compliance is less pressing for the Air Force and Navy, as aircraft that don’t meet the mandate will simply be routed to lower levels. Navy fighters do routinely fly above FL290, but retrofit solutions are not deemed cost-effective, and Eurocontrol has said that state aircraft will be accommodated. RVSM will become more of a concern, however, if the corresponding avionics modifications come to be required below FL290. The Air Force has no RVSM plan for its fighter aircraft, nor has the Navy funded tactical RVSM modifications.

Many of the large USAF Air Mobility Command aircraft can or will meet RVSM requirements, Sorensen says. Most pressing among Air Combat Command’s fleet are the unfunded B-2 and B-52 bombers. They won’t drop bombs in RVSM airspace but need to train at those altitudes.

Beyond these immediate mandates, the Air Force is taking a hard look at technologies like ADS-B, Sorensen says. It’s not a requirement now, but many countries probably will adopt it in the next decade. From a homeland security perspective, alone, it would make sense for the services to adopt the technology as it catches on in the civilian world in order to improve their situational awareness respecting air threats. The airborne command post, Air Force One, other VIP planes, and the C-17s already have ADS-B. CPDLC is another system to watch. The airborne command post has it, and the KC-135 is getting it.

Navy Guidance

After rushing to prepare for Eurocontrol’s requirement for 8.33-KHz VHF radio channel spacing, the U.S. Navy began to track CNS/ATM interoperability requirements for its tactical aircraft much more closely. NavAir has developed an analytical framework for tactical aircraft and has given its PMA 209 combat electronics office planning and budgetary authority–quite different from the federated USAF approach.

PMA 209’s analytical framework for tactical aircraft stresses maximum use of existing avionics or iterative upgrades. This approach is necessitated by the power, space and weight limits of tactical aircraft. NavAir has created a set of performance-based functional requirements documents and related test point matrices (TPMs) for 8.33-KHZ channel spacing, Mode S surveillance, RNP/RNAV and RVSM.

"Eurocontrol is doing as much as it can to accommodate state aircraft, but when you’re flying in the same airspace with civil aircraft, there’s only so much they can do," says Hoover. Missing requirements may result in rerouting, delays or even denial of airspace. Now in draft form, the TPMs provide checklists for compliance testing.

CNS/ATM Mandate Schedule

* Proposed for civil aircraft **Proposed for state aircraft

(Sources: U.S. Navy and Eurocontrol)

Making C-130 Ready for Europe

The C-130 avionics modernization program (AMP)–covering all types except the C-130J–was kick-started by the need for civil interoperability, says Mike Knisely, the U.S. Air Force’s chief engineer for the project. It is worth as much as $4.3 billion if all 490 aircraft are modernized. So far, engineers have developed a basic design and are writing code. The AMP includes mission as well as interoperability upgrades. Civil interoperability modifications are intended for the combat delivery, tanker, Combat Talon, and H- and U- model gunship versions of the C-130.

The aircraft will have the capability of the service’s pathfinder platform, the KC-135, Knisely says. But having absorbed lessons from previous communication, navigation and surveillance/air traffic management (CNS/ATM) efforts, the C-130 team plans a more integrated acquisition approach. When the development program is complete in 2007, AMP’d C-130s will achieve required navigation performance (RNP) from 10 all the way down to RNP 0.3. The Air Force hopes to begin installing the upgrades in 2008 after completing operational test and evaluation.

New avionics systems include ARC-210 UHF/VHF radios with 8.33-KHz channel spacing, communications management systems, VHF digital link 2 (VDL-2), Inmarsat Aero I satcom as the automatic dependent surveillance link, high-frequency data link, printer, traffic alert collision avoidance system (TCAS), terrain awareness warning system (TAWS), flight management system, autopilot, ILS protection, and enhanced Mode S, as well as new "glass" displays to modernize the cockpit.

The aircraft also will meet reduced vertical separation minimum (RVSM) requirements. C-130s don’t have air data computers, so the Air Force will have to install them, Knisely says. The new altimeter displays will have the required accuracy, and the tolerances of the components will combine to meet the overall system accuracy requirement.

The Mandates

Four mandates drive most of today’s air traffic management upgrades of U.S. military aircraft, to enable continued access to civil-controlled airspace. The mandates are intended to safely accommodate increasing traffic volumes.

8.33-KHz: Modification of VHF air traffic control radios to divide each 25-KHz channel into three 8.33-KHz channels, relieving spectrum congestion.

Mode S Surveillance: Modification of transponders to add data linking with permanent, unique aircraft addresses, reducing radio frequency interference and increasing air traffic capacity.

RNP/RNAV: Upgrade of navigation systems to guarantee an aircraft remains on its prescribed lateral path.

RVSM: Addition of air data computers and other modifications to allow vertical separation to be reduced from 2,000 to 1,000 feet at prescribed altitudes.

(Source: Naval Air Systems Command)

Navy’s Civil Derivative Aircraft

The U.S. Navy started equipping civil-derivative aircraft (CDA) like the C-9B (DC-9) with traffic alert collision avoidance systems (TCAS), ground proximity warning systems (GPWS), 8.33-KHz channel spacing and protected ILS back in the late 1990s. Since then, these aircraft also have added TCAS II, enhanced GPWS and reduced vertical separation minimum (RVSM).

Only $120 million over the next eight years is needed to upgrade the CDA fleet, estimates Peter Weigand, CNS engineer with PMA 207, the Navy’s lead office for CDA interoperability. But these ARINC 429-based aircraft can plug in civilian avionics systems and be quickly certified through the FAA process. Other CDA platforms include C-12Bs (Super King Air 200s), C-20D/Gs (Gulfstream 3s, 4s) C-37A/Bs (Gulfstream 5s and 550s), C-26Ds (Fairchild Metros), C-35C/Ds (Cessna Ultras and Encores) and C-40As (Boeing Business Jet 700s [BBJs]).

The C-9Bs, Gulfstreams, Cessnas and C-40As are implementing Mode S elementary and enhanced surveillance, automatic dependent surveillance-broadcast (ADS-B), controller pilot data link communications (CPDLC) and RNP/RNAV. The C-12 and C-26 turboprops will add this equipage, as well as TCAS II and terrain awareness warning systems (TAWS). The C-12s are all but complete. They already had Mode S. The C-26s are expected to be updated by the end of 2005, but will have waivers in place to cover contingencies. The C-9Bs, Gulfstreams, Cessnas and C-40As currently can fly to RNP levels 2 and above.