Tactical Data Links, Air Traffic Management, and Software Programmable Radios* B

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TACTICAL DATA LINKS, AIR TRAFFIC MANAGEMENT, AND SOFTWARE PROGRAMMABLE RADIOS* B. E. White, The MITRE Corporation, Bedford, MA

(AC2ISRC) was: Why can’t civil aviation data Abstract link (CADL) requirements be satisfied by Link Background on Link 16, global air 16? Another question is whether a single traffic management (GATM), and the joint CADL can satisfy Air Force GATM needs. tactical radio system (JTRS) is provided. An objective of this paper is to answer Information addressing the ability of: 1) Link these questions. Section 2 briefly introduces 16 to handle evolving civil aviation data link the Link 16 program. CADLs and implications (CADL) waveforms; 2) a single data link to for DOD are discussed in Section 3. Section 4 satisfy Air Force GATM requirements; and 3) highlights difficulties of integrating Link 16 JTRS to incorporate data links of interest to and CADLs waveforms, and relying on a single aviation is offered. Relationships with layered data link to satisfy CNS/ATM data link communication architectures, the Global Grid, requirements. Section 5 discusses SPRs and and software programmable radios (SPRs) are the JTRS. Section 6 has concluding remarks also discussed. and suggested actions.

1. Introduction 2. Link 16 In October 1994 Link 16 was Link 16 is NATO terminology for an designated as the DOD’s primary tactical data anti-jam (AJ), secure, data and voice system, link for all military service and defense agency with standard waveforms and messages to 2 command, control and intelligence (C I) promote interoperability, supported by the Joint systems [1]. The Air Combat Command Tactical Information Distribution System (ACC) has already incorporated Link 16 into its (JTIDS) and Multifunctional Information 2 C and sensor systems and is now planning on Distribution System (MIDS) terminals. installing Link-16 in its fighters and bombers. This commitment entails an expenditure of Figure 1 shows how the number of billions of dollars. platforms to receive Link 16 rapidly rises starting in FY00, and the significant changes in However, there are other data link the increases from FY03 through FY15, based requirements competing for DOD resources, on various planning data [2]. notably those of the International Civil Aviation Organization (ICAO) for communications, navigation and surveillance/ 3. Civil Aviation Data Links air traffic management (CNS/ATM). In 1997 CADLs are primarily associated with the Air Force created a global air traffic ATM systems, and include fixed message sets management (GATM) function in a new built into upgraded but standard system program office (SPO) at Hanscom Air communication procedures to reduce pilot and Force Base (AFB) to include the certification of controller workload and lead to quieter cockpits avionics capabilities from a GATM and airwaves. Also, CADLs on military 2 perspective. A question asked by the aircraft could be used as a resource for C . commander of the Aerospace C2 Intelligence, ICAO’s CNS/ATM concept relies on data links ______Surveillance, and Reconnaissance Center to be the primary means of routine * This work was sponsored by the Electronic Systems Center (AFMC), Hanscom Air Force Base, Massachusetts. 0-7803-5749-3/99/$10.00 © 1999 IEEE 5.C.5-1 communications for air traffic services (ATS) does not yet require dual redundant satellite [3]. Controller-pilot data link communications communication systems for ATC but probably (CPDLC) is the first comprehensive ATS data will. Many potential users, particularly the link application to be implemented. airlines, have advocated HF data link (HFDL) for over the ocean ATC instead of a second Line of Sight (LOS) Data Links satellite system because HFDL is less expensive to install. The author expects a The FAA is advocating a new digital SATCOM/HFDL configuration to become voice and data time division multiple access acceptable for ATC after some further FAA (TDMA) waveform for their next-generation effort to validate HFDL for this application. air-ground communications (NEXCOM) system. This waveform is known as Mode 3 of It is noted that Inmarsat’s geostationary the VHF digital link (VDL-3). In 1995 ICAO satellites do not provide polar coverage, a endorsed VDL-3 as the long-term solution to desirable feature for military applications. worldwide spectrum congestion in the VHF However, the narrowband ICO system will aeronautical band. VDL-3 operates at a 25- provide global coverage. Although there has kHz channel data rate of 31.5 kb/s. The VDL-3 been interest in providing aeronautical service end-to-end delay is no more than about 250 ms, with ICO, this capability is not yet realized. a latency suitable for ATS. HFDL may also support global coverage. Another future CADL, VDL-2, uses the same modulation scheme as VDL-3 but Implications for DOD employs carrier sense multiple access (CSMA) DOD must comply with civil standards instead of TDMA. VDL-2 is attractive to being developed by ICAO. With the assistance airlines for aeronautical operational control of Hanscom’s GATM SPO, the Air Mobility (AOC) where latency requirements for data are Command (AMC) is planning to integrate not so stringent, as compared to ATS. ADS-A, Aero-I, and HFDL, into its operational Any of the time-critical functions of airlift capabilities. CPDLC are more in the bailiwick of ATS, not ACC has been concerned about AOC, and at VHF should be handled by VDL- equipping all their aircraft with avionics 3, not VDL-2. However, VDL-2 could be equipment that would make them compliant around for a long time, and additional ATS with the emerging civil aviation standards. The uses for VDL-2 that do not require near-real- “heavy special use aircraft” that routinely fly as time latency performance could be attractive. general aviation traffic (GAT) aircraft, viz., the Automatic dependent surveillance E-3 (Airborne Warning and Control System broadcast (ADS-B), a CADL system critical to [AWACS]), E-4 (command post aircraft), E-8 “free flight”, is devoted to the LOS broadcast (Joint Surveillance Target Attack Radar System of position, velocity, and intent information. [JSTARS]), and the OC/RC/TC-135 Potential solutions to ADS-B are the Extended (reconnaissance and tanker aircraft) would be Squitter, Self-Organizing TDMA (S-TDMA) fully equipped. However, it was questionable (also known as VDL-4), and the Universal whether it would be necessary to equip fighters, Access Transceiver (UAT). bombers, and EC/HC-130s (transport aircraft). Overall, considerable progress has been Beyond Line of Sight (BLOS) Data Links made in the last two years. The number of GATM communications (comm) functional Inmarsat aeronautical (Aero) satellite units planned for various aircraft between 1999 communications (SATCOM) is certifiable for and 2016 are shown in Figure 2. CMU stands BLOS air traffic control (ATC) use. ICAO for communications management unit.

0-7803-5749-3/99/$10.00 © 1999 IEEE 5.C.5-2 4. Integration of Link 16 and CADLs resides in the JTIDS/MIDS terminal. A given Section 1 questions are now addressed. Link 16 platform's host computer contains the required databases, various controls, message processing, interface, input/output, and other Layered Communication Architecture functions. Thus, if a hardware or software A "layered" communication system change outside the terminal becomes necessary, architecture is advocated as a tool for analyzing all the host platforms must be modified. the integration problem. Ideas of a MITRE Although layering implies modularity, colleague, Mike Butler, are freely used. He has the reverse is not necessarily true. However, elaborated on several key attributes of layered two or more adjacent layers can be combined in architectures: 1) technology neutrality; 2) some situations to save space. One just has to functional encapsulation; 3) interface be careful that when layers are combined, standardization; 4) independent resources; and future flexibility is not precluded. 5) extensible systems. Layering facilitates performance vs. flexibility tradeoffs as Contrasts Among Link 16 and CADL technology and system needs change. This Waveforms helps ensure the architecture accommodates system evolution over a long period. As an exercise in preparing this paper, the author allocated some basic functions of An example of a general, layered Link 16 and VDL-3 to the ISO layers. In doing communication architecture is the 7-layer this, it became apparent that Link 16 and model of the International Standards CADLs of interest have many fundamental Organization (ISO). This model is well known differences. Link 16 employs an L-Band, 3 and has been applied successfully. MHz-bandwidth, fast-frequency-hopping, AJ- A tenet of layering is to partition the coded, multi-pulse waveform with encrypted implementation of functions so that each self- messages. VDL-3 is a VHF, 25 kHz-channel, contained physical entity within a system non-hopping, non-AJ, multi-mode, waveform realizes only functions within the same layer of with un-encrypted messages. The time slot and the architecture. An example is when a radio coding structures of the two schemes are very contains only the hardware and software different. Furthermore, the ground necessary for performing modulation/ infrastructure for interconnecting the two demodulation and coding/decoding associated systems does not exist. This does not bode with the physical layer and data link layer, well if Link 16 is to accomplish all the respectively, of the OSI model. The functionality of the CADLs becoming required JTIDS/MIDS radios (which are contained in by the FAA and civil aviation authorities the terminals) already follow that precept in (CAAs). There are too many practical hurdles part. to overcome that hinder the cost-effective integration of these waveforms. This answers However, in general, the Link 16 the first question of Section 1. system does not have a full layered architecture because the tenet of layering stated above is not always followed. First, portions of the network A Potentially Attractive Approach to and cryptographic management functions Accommodating Link 16 and CADLs reside within the message signal processor of Instead of attempting to have Link 16 the “radio”. The rest of these two functions accommodate CADL waveforms, a better reside in the network interface computer of the approach may arise from asking: What can be terminal outside the radio. Furthermore, not done to protect the investment in Link 16 everything required by the Link 16 system

0-7803-5749-3/99/$10.00 © 1999 IEEE 5.C.5-3 radios while providing a more affordable way radios. An aircraft platform might be of handling CADLs? configured more easily to suit particular missions. First the military’s “Global Grid” is mentioned as being relevant to the architectural However, the author adds some words discussion. The Global Grid vision – any user of caution: Before abandoning the present communicating with any other user – is derived system configurations, the possibilities from DOD’s Joint Vision 2010. Because of the promised by this approach should be explored increasing demand for more bandwidth to in detail to be assured that all essential system accommodate higher data rates, the means for functions are preserved and acceptable attaining this goal is based primarily on performance requirements are met. wideband capabilities promised by SATCOM, One way of thinking about this problem microwave, and fiber optic communications is depicted in Figure 3. Each “slice” represents media. a distinct viewpoint within the ISO-model It is important to distinguish what is – layer, data link system, or performance/ from what is not – part of the Global Grid. The flexibility parameter plane. Just a few systems Global Grid encompasses only the bottom four and parameters are illustrated; there may be layers, the physical, link, network, and many more of importance. Within each transport layers of the OSI model. The session, parameter there can be various aspects presentation, and application layers are on the corresponding to different layers of the model. users’ side of the architecture. The Global Grid Several of these are shown for vulnerability “merely” transports already-composed and latency. messages as reliably and speedily as possible. One idea concerning Figure 3 is as If done thoroughly the functions of Link 16 follows. The goal of integrating different data will probably map to all levels of a layered links from the point of view of the layered architecture, and therefore from the outset, model may be facilitated by “scrubbing” Link 16 is not compatible with the Global Grid performance-flexibility tradeoffs among bottom-four-layers concept. systems. The benefit of greater flexibility in What about isolating the radio functions applying commercial standards and technology from the application functions in both the Link- may be worth giving up some performance. 16 system and the CADLs? In this approach the data link “essence” of Link 16 and the Comparisons/Contrasts Among Civil Aviation CADLs would not have anything to do with the Data Links networking (or above) layer(s). One would Now the second question, whether the have “plug-in” “Link-16 ” and “CADL” PC- Air Force should employ only one CADL is like cards that could provide the physical and considered. Various CADLs have distinctive data link layer functions. All upper-layer characteristics. Important features and functions would be provided by a standard limitations of Aero C, H, I, and L data links, protocol stack. HFDL, VDL- 2, 3, and 4, UAT, Extended If only the true physical and data link Squitter, and Mode S data link are summarized layer functions resided in the JTIDS/MIDS in Table 1. radios, it may be possible to build them smaller Essential characteristics of these and cheaper; and similarly for CADL radios. CADLs are arranged in Table 1 with respect to Also, one might be able to choose the messages attributes of considerable interest for civil and the media independently making possible aviation communications. Some attributes the passing of Link 16 messages over CADL relate to the application being addressed; others radios, and aeronautical messages over Link 16

0-7803-5749-3/99/$10.00 © 1999 IEEE 5.C.5-4 refer to desired performance within a given instead of hardware; greater flexibility and cost application; and others refer to schedule and/or effectiveness might be achieved. costs associated with implementation. In the This has led to the concept of SPRs or interest of brevity this table is primarily software defined radios (SDRs) [5]. In turn this qualitative. The value judgments indicated by has spawned a joint service program for the “colors” are the sole responsibility of the acquiring future radios with a new author. evolutionary, open-system, JTRS architecture. Based on material developed for One of the first examples of a military Section 3 and the entries of Table 1, the author SPR was SPEAKeasy, a prototype concludes that the Air Force cannot expect to development conducted jointly by the Army satisfy all its CADL needs with a single CADL. and Air Force, and later under the auspices of Every CADL has at least one “red” (R) entry in DARPA. Another such effort is the Air Force’s a critical row. For example, in just considering Airborne Information Terminal (AIT). The ATS and ADS-B, applications necessary for Army and Navy also have other candidate radio flying in terminal airspace and free flight, programs heading in this direction. respectively, no CADL can readily accomplish both functions. The JTRS modes and capabilities are based on the JTRS operational requirements Given this conclusion, what should the document (ORD) [6] for handheld, dismounted, Air Force do? The author recommends that the vehicular, maritime/fixed, and airborne Air Force plan to acquire VDL-2 radios that operational domains. have the assured capability of being upgraded to VDL-3. Several vendors have developed The author notes that radio capabilities suitable VDL-2 radios, and prototype VDL-3 for ATC, i.e., HFDL, 8.33 kHz, and the VDL radios have been successfully demonstrated. If are included. HFDL and VDL are not required VDL-3 does not materialize by 2007, as for the JTRS until FY03. Also, there is some expected, at least the Air Force would have a concern in noting that SATURN, the NATO good CADL capability for C2 operations, version of the fast-frequency-hopping upgrade albeit, VDL-2 is non-real-time and not to Have Quick II, is scheduled for airborne appropriate for time-critical ATS messages. operations as late as FY04. One would hope Regarding ADS-B, the author recommends the that SATURN would be implemented earlier to Air Force analyze the results of last summer’s better test the capabilities of the JTRS Safe Flight 21 tests and plan for the best single architecture. It is also noted that UHF DAMA, ADS-B data link implementation. and other AIT capabilities are included in the ORD. Several L-Band waveforms are Also, the Air Force might benefit by included, notably, Link 16 and Mode S Level 4, continuing to plan for HFDL and Aero-I, and but there is no mention of the Extended waiting a little longer before reassessing and Squitter, the UAT, or S-TDMA. deciding whether any of the emerging commercial satellite systems would provide The author wonders whether too many adequate alternative data link capability. JTRS waveforms are being contemplated. A late 1998 Defense Science Board (DSB) 5. Software Programmable Radios recommendation suggested that the JTRS program concentrate on new data link and Commercial radio technology has networking capabilities, and implement only a progressed to where more waveform processing small subset of the waveforms in the ORD. functions can be accomplished with software

0-7803-5749-3/99/$10.00 © 1999 IEEE 5.C.5-5 Because Link 16 has been selected as the Contributors and decision-makers tactical C2 data link of choice for all of DOD, should work on the concept of relaxing and is being implemented on thousands of Air requirements and increasing architectural Force platforms, this system will be around for a flexibility where possible. long time. Therefore, a Link 16 capable waveform should be included in the JTRS. According to the JTRS implementation plan, References Link 16 is to be realized within a JTRS SPR by [1] June 1996, Joint Tactical Data Link FY03. The author thinks this is good but would Management Plan, Command, Control, like to see Link 16 accommodated sooner. Communications, Computers, and Intelligence (C4I), Department of Defense, USA 6. Conclusions [2] 29 October 1998, Air Force Link 16 Master Plan, AC2A/CC, AF/XOR, AF/AQI, prepared The Link 16 system cannot satisfy the by Electronic Systems Center Link 16 System emerging CADL requirements in a cost- Integration Office (ESC/DIAJ) in cooperation effective manner. with AC2A/C2GN No single data link will satisfy the Air [3] October 1998, “Controller Pilot Data Link Force’s CADL needs. Communication (CPDLC) Technology White Although none of the CADLs are Paper,” ESC/GAT Engineering, item on required before 2003, VDL-2 will likely be Electronic Systems Center (ESC/GAT) website implemented before then. VDL-3 is to be [4] December 1996, Air Mobility Command implemented starting in 2007. A CADL for CNS/ATM Study: CNS/ATM Interoperability ADS-B may be implemented sooner than Requirements Report, Air Traffic Control and expected. The Air Force should acquire VDL- Landing Systems Program Office, Electronic 2 radios that have the assured capability of Systems Center, Hanscom AFB, MA (Rev. 1) being upgraded to VDL-3, plan for ADS-B, and continue to follow but await further [5] March 1999, Architecture and Elements of commercial SATCOM developments. Software Defined Radio Systems as Related to Standards, SDRF Technical Report 2.0, There is high-level commitment to Link Software Defined Radio Forum (SDRF) 16 as the tactical data link of choice for DOD C2I systems. Large expenditures of U.S. [6] 23 March 1998, Operational Requirements taxpayer dollars are planned for the phased Document (ORD) for Joint Tactical Radio implementation of this system on-board (JTR) thousands of aircraft critical to the national defense and warfighting capability of the U.S. Acknowledgment Hence, Link 16 should be accommodated by The author is grateful to several MITRE the Global Grid and JTRS programs. colleagues for reviewing work leading to this The JTRS program should emphasize paper, and providing many beneficial Link 16, arguably the most challenging and comments. Special thanks go to Mike Butler, most important wideband waveform of the Cliff Danielson, Mary Girard, Chuck Marley, foreseeable future. One suggestion might be to Darrell Trasko, and Warren Wilson. include a task for detailing the relationship between Link 16 and the OSI model as part of both the Link 16 and JTRS technological roadmap efforts.

3000

2500

2000 20-Nov-97 1500 29-Oct-98 14-May-99 1000

500

0 Fiscal Year 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 Figure 1. Number of Air Force Data Link Platforms Programmed for Link 16

Number of Units

2000

1800

(Total No. Aircraft) 1600

1400 Airlifters (1044)

1200

Tankers (649)

1000

OSA* (285)

800

SMA** (50)

600

* Operational support aircraft

400

** Surveillance monitoring aircraft

200

0 Comm Function CMU HFDL ADS-A Voice 8.33 kHz ON/OFF Data Link Data SATCOM SATCOM Figure 2. Planned Communications Functions for Various Air Force Platforms (Through 2016)

Latency Electromagnetic Compatibility Availability Performance/FlexibilityVulnerability Parameter Application Set-up Delay Cryptographic Security Presentation

Session Queuing Delay Message Error Rate Layer of ISO Model Transport End-to-End Delay

Network

Data Link A D S - B D S - A L i n k 1 6 n k i L Anti-Jam Protection Physical V D L Mode 3 V D L Mode

Data Link System Figure 3. One View of Integrated Data Link Solution Space

0-7803-5749-3/99/$10.00 © 1999 IEEE 5.C.5-7 Table 1. Civil Aviation Data Link Characteristics Civil Aviation Data Link Application, Aero C Aero H Aero I Aero L HFDL VDL – 2 VDL – 3 VDL – 4 UAT Extended Mode S Performance, or Squitter Schedule/Cost Attribute Primary Oceanic Oceanic Oceanic Oceanic Oceanic Over Land: Over Land: Over Land Over Land Over Land Over Land: Operational Areas Over Land Over Land Over Land Over Land Over Land Ground sites Ground sites Oceanic Oceanic Oceanic Ground – B – B – B – B – B – Y – Y – B – B – B sites – Y Coverage BLOS: BLOS: BLOS: BLOS: BLOS: LOS LOS LOS LOS LOS LOS: ≤70 o N/S ≤ 70 o N/S ≤ 70 o N/S ≤ 70 o N/S Propagation – G – G – G – G – G ≥ 15 kft latitude latitude latitude latitude anomalies – Y – G – Y – Y – Y – Y Terrestrial Design Long Long Long Long Long 200 nmi 200 nmi No info 100 nmi 100 nmi 100 nmi Range (based on – B – B – B – B – B – G – G – G – Y – Y – Y link budgets) ATS No Yes Yes Yes Possibly Only for Yes No No No No – R – G – G – G – Y Non-Time – G – R – R – R – R Critical –Y AOC No Possibly Possibly Possibly Possibly Yes Yes No No No No – R – Y – Y – Y – Y – G – G – R – R – R – R ADS-A No Yes Yes Yes Yes Yes Yes No No No Possibly (two way) – R – G – G – G – G – G – G – R – R – R – Y ADS-B No No No No No No No Yes Yes Yes No (one way) – R – R – R – R – R – R – R – G – G – G – R Frequency Band L-Band L-Band L-Band L-Band HF: VHF: VHF: VHF: L-Band: 966 L-Band: L-Band: (color indicates – G – G – G – G 3–30 MHz 118–137 118–137 120–150 MHz (could 1030 MHz 1030 MHz propagation – R MHz MHz MHz move) up; 1090 up; 1090 effects, e.g., – Y – Y [4, p. 52] – G MHz down MHz down external noise, – Y – G – G multipath, etc.) Channel 5–10 kHz 5–10 kHz 5–10 kHz 5–10 kHz 3 kHz 25 kHz 25 kHz 25 kHz ≈2 MHz ≈8 MHz ≈2 MHz Bandwidth – R – G – Y – R – R – Y – Y – Y – G – G – G Information Facsimile Data Voice Data Voice Data Data Data Data Data Data Data Data Service(s) E-mail – B – B – G – G – G Voice – G – G – G – G –Y – B User Data Rate 600 b/s 9.6 or 64 2.4 or 4.8 600 b/s 2.4 kb/s ≤ 31.5 kb/s ≤ 19.2 kb/s: 19.2 kb/s 1 Mb/s 1 Mb/s 1 Mb/s – R kb/s kb/s – R (typical) No system Up to 4 – G – B – B – B – G – Y – Y management TDMA data – G time slots @ ≤ 4.8 kb/s each – G Latency Includes Includes Includes Includes Includes Non-Real Near-Real ≥ 250 ms ≥ 250 ms ≥ 250 ms ≥ 250 ms media Time: Time: ≤ 1s – G – G – G – G round trip round trip round trip round trip delays Packet end-to-end – Y overlaps delay delay delay delay for 90+ % – R – R – R – R – Y – G System Available Available Available Available 2000 + 2000 + 2007 + 2003 + 2003 + 2003 + Available Availability – B – B – B – B – G – G – R – Y – Y – Y – B Schedule Airborne Terminal Low High Medium Medium Low Medium Medium Medium Low High (if no High (if no Costs (B-kit) – G – R – Y – Y – G – Y – Y – Y – G Mode S Mode S already) already) – R – R Airborne Terminal Low High Medium Medium Low Medium Medium Medium Medium Medium Medium Costs (A-kit) – G – R – Y – Y – G – Y – Y – Y – Y – Y – Y Service Provider Medium Medium Medium Medium Medium Medium Low Low Low Low Low and/or Other Costs – Y – Y – Y – Y – Y – Y – G – G – G – G – G

Relative Qualitative Definitions: Color Description for Intended Application Blue (– B) Exemplary; Needed; Inexpensive; etc. Green (– G) Good; Desirable; Affordable; etc. Yellow (– Y) Satisfactory; Possibly; “Pricey”; etc. Red (– R) Deficient; Undesired; Expensive; etc.