ATM Modernization, Business & GA, Commercial, Military

Avionics System Design: Project 25 Radios: A Welcome Standard

By Walter Shawlee 2 | June 1, 2001
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If you have anything to do with public safety, law enforcement, government agencies or contract/leased aircraft, Project 25 radios will be a reality in your life this year. And these new radios will grow steadily in importance as a key avionics item in your aircraft cockpit. They soon will replace older-generation FM radios in many applications both on the ground and in the air.

Aviation has been a pivotal tool for a myriad of government agencies, from the California Highway Patrol to the U.S. Forest Service. Unfortunately, due to the proliferation of incompatible radio systems, establishing useful communication between air and ground or maritime units, as well as among agencies, has been a problem. It has, that is, until the arrival of Project 25.

Project 25 is a watershed achievement, allowing participating members to share intellectual property, user requirements and network technology to create a truly interoperable international communications system. Needless to say, customers, not vendors, created this environment. Project 25- (or P25) compliant radios are not the domain of a single vendor, but conform to a shared standard administered by the Telecommunications Industry Association (TIA). Full details of the project standards can be found online at (select group TR-8).

TIA engineering committee members who manage Project 25 include key government agencies and public safety groups, plus industry representatives from most radio suppliers, including Nortel, Nokia, ComNet/Ericsson, Motorola, Thales, Technisonic, Daniels, Transcrypt, DVSI, E.F. Johnson and others. Pooling both technology and customer requirements has allowed a far-reaching communication plan to be translated into workable technology. It returns intellectual property (IP) revenues to all contributors in the form of license fees, so all contributing players benefit, even if they do not sell the radio. P25 technology addresses the entire range of land-based systems, handhelds, repeaters and airborne units, and provides the compatible long-term operational framework for all of them.

There has been a not-so-subtle attempt by radio manufacturers to hijack standards and deftly direct them into proprietary areas by "improving" or "extending" them at the expense of real standardization. This left radio operators in a communication limbo, unable to speak to key groups. The problem started years ago with simple tone signaling and squelch technology. But eventually it expanded into encryption and a maze of incompatible formats, trunking schemes and frequencies. Today the result is a vast range of technology that essentially acts as an impediment, rather than an aid, to inter-agency communication.

What caused this problem? The origin is the fact that the various agencies’ communication systems all began with land mobile equipment, including repeaters, frequencies, etc. This procurement took place over many years and in widely separated areas. Local area issues and whoever got the sale dictated the procurement decision.

The land mobile industry is especially aggressive in terms of marketing. Its vendors pursue large contracts with a whole-hearted vigor, and they seek to make customers "captive" in terms of specific proprietary technology and features. Thus, swapping suppliers can be difficult.

Nine years ago, the problem came to a head, and so, drowning in a sea of incompatible technology, government agencies launched Project 25. Its intent was to create a single, umbrella standard with backwards compatibility, to define all radios purchased in the future.

The U.S. government was the only customer with enough clout to mandate this standard in the face of towering supplier reluctance. Its crystal clear message to all suppliers was "comply with our interoperable standards or lose your ability to supply systems in the future." Many local agencies now benefit from the federal government’s influence and participation in the Project 25 standards committee.

Project 25 has four key system objectives. Derived from the February 1999 Statement of Requirements, they are:

  • Obtain maximum radio spectrum efficiency;

  • Ensure competition in system life-cycle procurements;

  • Allow effective, efficient and reliable intra-agency and inter-agency communications; and

  • Provide user-friendly equipment, requiring the least amount of mental and physical interaction by the operator.

The key issue is to provide all agency users with the best possible use of available spectrum at the best price possible. The standardized system should be easy for users to understand, allowing all relevant groups to communicate effectively. This goal may seem obvious, but our systems are mandated by existing standards, and the land mobile world has had a much looser and less defined format. Plus, land mobile communication, especially for government and law enforcement users, has had a long-standing requirement for privacy and security not common in the aviation world, and that has encouraged divergent technology.

It is important to recognize that Project 25 does not define a specific transmission method (although AM is not used) or frequency. Instead, it defines a complete radio architecture that can be deployed on any approved land mobile radio service (LMRS) bandsplit, with operation that provides both legacy support and improved new operation.

A key element is that Project 25 requires tight control of spectrum space and the ability to operate in narrow-band FM environments as a default. It begins with 12.5-KHz channel spacing in Phase I (including backwards compatibility with 25-KHz spacing). And it expands to tighter 6.25-KHz channels in the proposed Phase II.

This dictates very tight frequency control and stability (especially critical at UHF), superior design of the radio front end, and low spurious emissions from the transmitter as de facto basic requirements. These design considerations are not trivial. When full band programmable operation is added, some serious technical challenges emerge. Indeed, no conventional aircraft transceiver could meet these performance criteria, including 8.33-KHz channel units.

The P25 project’s clear and immediate goal was to expand channel availability. A finer division of frequency allocations was the first step in the process. Also, more channel access was created by re-use of dormant UHF-TV spectrum space, and the "800-MHz" band now extends down to 764 MHz as part of the P25 operational plan.

P25 technology is primarily a digital medium, although it provides analog transmission as legacy support. It can seamlessly respond in the correct mode in fully compliant radios, a major improvement in terms of user interaction and wasted control motions. This seamless transition between analog and digital modes, narrow and wideband operation, and clear and encrypted transmission represents a pivotal difference between old radios and P25 hardware.

Digital operation also provides users with immediate security from casual interception. It makes detection and eavesdropping significantly more difficult, even without encryption. It also provides the key to features like sophisticated group calling, paging and direct-unit access, as well as a reliable data pathway for new services and functions.

Actual modulation and encryption capability can take many forms within the overall P25 plan, from clear analog to encrypted QPSK digital data. And the radio itself can be defined as being one of four basic types with regard to its encryption security:

  • Type 1 is for classified national government communications;

  • Type 2 is for unclassified national security-related communications;

  • Type 3 is for unclassified sensitive government communications (e.g. public safety); and

  • Type 4 is for other purposes (including export).

Unlike simpler radios of the past, Project 25 systems support many security and control features, from Electronic Serial Number (ESN) transmission and validation to several levels of encryption. These include FIPS 46-2 DES (Data Encryption Standard) and FIPS 140-1 for wide compatibility, and NSA specification V23-94-1 for Type 1 secure applications. In addition, Project 25 provides legacy support for Federal Standard 1023 analog encryption systems.

Unlike previous systems, degraded range or voice quality is not permitted in encrypted modes, and testing must include both male and female voices. These represent major advances for radio operation with regard to encryption, which has had a questionable performance record.

New Project 25 radios support the group and network functions that are important for both incident coordination and security. They also support many operational modes that are quite unfamiliar to aviation personnel.

Next month, we will look deeper into the Project 25 architecture and what the real flyable hardware looks like. Avionics manufacturers have taken very divergent paths to deliver P25 capability in the airframe, and in the next issue you will see exactly how this affects equipment operation and capability.

Walter Shawlee 2 may be reached by e-mail at [email protected].

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