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


1 TACTICAL DATA LINKS, AIR TRAFFIC MANAGEMENT, AND SOFTWARE PROGRAMMABLE RADIOS* B. E. White, The MITRE Corporation, Bedford, MA 2 ISRC) was: Why can’t civil aviation data (AC link (CADL) requirements be satisfied by Link Abstract 16? Another question is whether a single Background on Link 16, global air CADL can satisfy Air Force GATM needs. traffic management (GATM), and the joint 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 I) command, control and intelligence (C 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 and sensor systems and is now planning on C Distribution System (MIDS) terminals. installing Link-16 in its fighters and bombers. Figure 1 shows how the number of This commitment entails an expenditure of platforms to receive Link 16 rapidly rises billions of dollars. 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 3. Civil Aviation Data Links communications, navigation and surveillance/ CADLs are primarily associated with air traffic management (CNS/ATM). In 1997 ATM systems, and include fixed message sets the Air Force created a global air traffic built into upgraded but standard management (GATM) function in a new communication procedures to reduce pilot and system program office (SPO) at Hanscom Air controller workload and lead to quieter cockpits Force Base (AFB) to include the certification of and airwaves. Also, CADLs on military avionics capabilities from a GATM 2 aircraft could be used as a resource for C . perspective. A question asked by the 2 ICAO’s CNS/ATM concept relies on data links Intelligence, commander of the Aerospace C to be the primary means of routine Surveillance, and Reconnaissance Center _______________ * This work was sponsored by the Electronic Systems Center (AFMC), Hanscom Air Force Base, Massachusetts. 5.C.5-1 0-7803-5749-3/99/$10.00 © 1999 IEEE

2 does not yet require dual redundant satellite communications for air traffic services (ATS) communication systems for ATC but probably [3]. Controller-pilot data link communications will. Many potential users, particularly the (CPDLC) is the first comprehensive ATS data airlines, have advocated HF data link (HFDL) link application to be implemented. for over the ocean ATC instead of a second satellite system because HFDL is less Line of Sight (LOS) Data Links 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) It is noted that Inmarsat’s geostationary system. This waveform is known as Mode 3 of satellites do not provide polar coverage, a the VHF digital link (VDL-3). In 1995 ICAO desirable feature for military applications. endorsed VDL-3 as the long-term solution to However, the narrowband ICO system will worldwide spectrum congestion in the VHF provide global coverage. Although there has aeronautical band. VDL-3 operates at a 25- been interest in providing aeronautical service kHz channel data rate of 31.5 kb/s. The VDL-3 with ICO, this capability is not yet realized. end-to-end delay is no more than about 250 ms, HFDL may also support global coverage. a latency suitable for ATS. Another future CADL, VDL-2, uses the Implications for DOD same modulation scheme as VDL-3 but DOD must comply with civil standards employs carrier sense multiple access (CSMA) being developed by ICAO. With the assistance instead of TDMA. VDL-2 is attractive to of Hanscom’s GATM SPO, the Air Mobility airlines for aeronautical operational control Command (AMC) is planning to integrate (AOC) where latency requirements for data are ADS-A, Aero-I, and HFDL, into its operational not so stringent, as compared to ATS. airlift capabilities. Any of the time-critical functions of ACC has been concerned about CPDLC are more in the bailiwick of ATS, not equipping all their aircraft with avionics AOC, and at VHF should be handled by VDL- equipment that would make them compliant 3, not VDL-2. However, VDL-2 could be with the emerging civil aviation standards. The around for a long time, and additional ATS “heavy special use aircraft” that routinely fly as uses for VDL-2 that do not require near-real- general aviation traffic (GAT) aircraft, viz., the time latency performance could be attractive. E-3 (Airborne Warning and Control System Automatic dependent surveillance [AWACS]), E-4 (command post aircraft), E-8 broadcast (ADS-B), a CADL system critical to (Joint Surveillance Target Attack Radar System “free flight”, is devoted to the LOS broadcast [JSTARS]), and the OC/RC/TC-135 of position, velocity, and intent information. (reconnaissance and tanker aircraft) would be Potential solutions to ADS-B are the Extended fully equipped. However, it was questionable Squitter, Self-Organizing TDMA (S-TDMA) whether it would be necessary to equip fighters, (also known as VDL-4), and the Universal bombers, and EC/HC-130s (transport aircraft). Access Transceiver (UAT). Overall, considerable progress has been made in the last two years. The number of Beyond Line of Sight (BLOS) Data Links 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. 5.C.5-2 0-7803-5749-3/99/$10.00 © 1999 IEEE

3 resides in the JTIDS/MIDS terminal. A given 4. Integration of Link 16 and CADLs Link 16 platform's host computer contains the Section 1 questions are now addressed. 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 As an exercise in preparing this paper, system evolution over a long period. 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- coded, multi-pulse waveform with encrypted A tenet of layering is to partition the messages. VDL-3 is a VHF, 25 kHz-channel, implementation of functions so that each self- non-hopping, non-AJ, multi-mode, waveform contained physical entity within a system with un-encrypted messages. The time slot and realizes only functions within the same layer of coding structures of the two schemes are very the architecture. An example is when a radio different. Furthermore, the ground contains only the hardware and software infrastructure for interconnecting the two necessary for performing modulation/ systems does not exist. This does not bode demodulation and coding/decoding associated well if Link 16 is to accomplish all the with the physical layer and data link layer, functionality of the CADLs becoming required respectively, of the OSI model. The by the FAA and civil aviation authorities JTIDS/MIDS radios (which are contained in (CAAs). There are too many practical hurdles the terminals) already follow that precept in to overcome that hinder the cost-effective part. 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 A Potentially Attractive Approach to always followed. First, portions of the network Accommodating Link 16 and CADLs and cryptographic management functions 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 5.C.5-3 0-7803-5749-3/99/$10.00 © 1999 IEEE

4 radios. An aircraft platform might be radios while providing a more affordable way configured more easily to suit particular of handling CADLs? missions. First the military’s “Global Grid” is However, the author adds some words mentioned as being relevant to the architectural of caution: Before abandoning the present discussion. The Global Grid vision any user – system configurations, the possibilities communicating with any other user – is derived promised by this approach should be explored from DOD’s Joint Vision 2010. Because of the in detail to be assured that all essential system increasing demand for more bandwidth to functions are preserved and acceptable accommodate higher data rates, the means for 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 layer, data link system, or performance/ It is important to distinguish what is – flexibility parameter plane. Just a few systems – from what is not part of the Global Grid. The and parameters are illustrated; there may be Global Grid encompasses only the bottom four many more of importance. Within each layers, the physical, link, network, and parameter there can be various aspects transport layers of the OSI model. The session, corresponding to different layers of the model. presentation, and application layers are on the Several of these are shown for vulnerability users’ side of the architecture. The Global Grid “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 Comparisons/Contrasts Among Civil Aviation the data link “essence” of Link 16 and the Data Links CADLs would not have anything to do with the 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. UAT, Extended HFDL, VDL- 2, 3, and 4, Squitter, and Mode S data link are summarized If only the true physical and data link in Table 1. layer functions resided in the JTIDS/MIDS 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 5.C.5-4 0-7803-5749-3/99/$10.00 © 1999 IEEE

5 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 2 good CADL capability for C 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, and other AIT capabilities are included in the ADS-B data link implementation. 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) recommendation suggested that the JTRS 5. Software Programmable Radios 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 5.C.5-5 0-7803-5749-3/99/$10.00 © 1999 IEEE

6 Because Link 16 has been selected as the Contributors and decision-makers 2 tactical C 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. References According to the JTRS implementation plan, [1] June 1996, Joint Tactical Data Link Link 16 is to be realized within a JTRS SPR by Management Plan, Command, Control, FY03. The author thinks this is good but would Communications, Computers, and Intelligence like to see Link 16 accommodated sooner. (C4I), Department of Defense, USA [2] 29 October 1998, Air Force Link 16 Master 6. Conclusions 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 Paper,” ESC/GAT Engineering, item on Although none of the CADLs are Electronic Systems Center (ESC/GAT) website required before 2003, VDL-2 will likely be 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 [5] March 1999, Architecture and Elements of continue to follow but await further Software Defined Radio Systems as Related to commercial SATCOM developments. 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 2 [6] 23 March 1998, Operational Requirements I systems. Large expenditures of U.S. C Document (ORD) for Joint Tactical Radio taxpayer dollars are planned for the phased (JTR) implementation of this system on-board 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 paper, and providing many beneficial The JTRS program should emphasize comments. Special thanks go to Mike Butler, Link 16, arguably the most challenging and Cliff Danielson, Mary Girard, Chuck Marley, most important wideband waveform of the Darrell Trasko, and Warren Wilson. foreseeable future. One suggestion might be to 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. 5.C.5-6 0-7803-5749-3/99/$10.00 © 1999 IEEE

7 Number 3500 3000 2500 2000 20-Nov-97 29-Oct-98 1500 14-May-99 1000 500 0 Fiscal Year 1996 2008 2010 2012 2014 2006 2004 2002 2000 1998 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 SATCOM SATCOM Figure 2. Planned Communications Functions for Various Air Force Platforms (Through 2016) Latency E lectro magn etic Co mpatib ility Availab ility Vu lnerability Performance/Flexibility Parameter App lication Set-up Delay Cryptographic Se curity P resentation Session Queuing Delay Message Error Rate Transport Layer of ISO Model End-to-End Delay Network Data Link A D S - B L i n k 1 6 A nti-Jam Protection Physical V D L Mode 3 Data Link System Figure 3. One View of Integrated Data Link Solution Space 5.C.5-7 0-7803-5749-3/99/$10.00 © 1999 IEEE

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

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