Tactical Data Links - Satellite Link 16
Key words and phrases: tactical data links, satellite link 16, RN ship system requirement, Joint Range Extension, S-TADIL J
General
Link 16 is being developed in a number of ways beyond the basic concept of integrating a MIDS/JTIDS terminal on a platform and using Link 16 as the language of that terminal.
Link 16 via Satellite
The Royal Navy investigated the use of the Link 16 message standard on a satellite bearer to overcome the LOS limitations of MIDS/JTIDS. Feasibility studies were undertaken in 1991/2. The Satellite Tactical Data Link (STDL) is now part of the RN Ship System requirement and is to be implemented at the same time as MIDS/JTIDS/Link 16. The intended fitting programme includes the Aircraft Carriers, Type 42 Destroyers, Type 45 AAW Destroyer (T42 replacement and successor to the CNGF), circa 2006, Type 23 Frigate (13 of 16) (2003) and SSNs (2005).
The US Navy are also investing in satellite Link 16 known as Satellite Tactical Data Information Link J (S-TADIL J). However the solution chosen is not the same as that of the RN, thus the two systems are not compatible. The USN also use S-TADIL A, carrying Link 11 data.
A generic format for extended range Link 16 is being developed to use satellites and land line and is known as the Joint Range Extension JRE.
The organisation of access to the satellite bearer for the RN will be TDMA on a single SHF Satcom channel, i.e. the same basis (except for the single channel) as MIDS/JTIDS. In comparison the USN will utilise UHF Satcom via a near-real time Demand Assigned Multiple Access (DAMA) system.
The figure below shows three ways in which satellite communications could be organised. In the broadcast mode, one ship would transmit and all other ships would receive. In the network mode each ship would transmit data in its assigned transmit opportunity and in the inter-group mode one ship in each group would transmit data on behalf of its group.
Possible Satellite Communication Paths
Transferring Link 16 data by satellite presents new challenges. The first major challenge is that, at least initially, the capacity of the satellite channel will be less than the terrestrial communications. The second challenge is that communication via satellite will introduce time delays. There will, of course, be other problems such as increased operator workload but the two major challenges are considered in the next two paragraphs.
Satellite Channel Capacity
The limited capacity available will mean that the data to be exchanged will need to be carefully selected. Surveillance data would seem to be a high priority but voice would be a poor use of scarce capacity. There may be occasions on which a degradation in reporting rate will have to be accepted and updates on tracks will not be transmitted as often as in terrestrial communications. Messages may have to wait in transmission queues slightly longer and the positional data may not be as accurate when transmitted as in the terrestrial case.
Time Delays
The time delays will also have an effect on the accuracy of received data (which may be comparatively old) and receiving systems will have to allow for this. The receiving system may be receiving data on the same track by both satellite and terrestrial communications; some selection logic must be implemented.
Also the Link 16 protocols require a hand-shaking protocol for certain message exchanges. The hand shaking requires that responses are received within certain time limits to be valid. Using satellite communications may delay the responses such that they will arrive outside the normally acceptable time limits and allowance must be made for this.
STDL
The Royal Navy is procuring STDL to help alleviate some BLOS problems. As stated this link utilises a SHF Satcom bearer for Link 16 messages (plus a few special STDL messages, STMs) and uses different timing and gridlocking to Link 16. The Geodetic Gridlocking message J27.0 may be utilised for this purpose. Gridlocking (similar to Link 11) is necessary since no time synchronisation using Round Trip Timing (RTT) or equivalent passive protocols are employed.
STDL remains a real-time data link, however capacity and data rate are reduced, and the maximum number of STDL time slots per frame is 32 supporting a maximum of 16 transmitting STDL units (SatUs). Some RN units (CVSG, T42 and T45) will be capable of forwarding STDL/Link 16, MIDS/JTIDS/Link 16 and Link 11 data. Units designated will act in the forwarding role as FSatUs; e.g. a FSatUJ will be a STDL/Link 16 to MIDS/JTIDS Link 16 forwarder.
STDL requires that Time Slots are distributed over a frame. Frame duration will vary with the aggregate data rate as follows:
| Frame Duration | Aggregate Data Rate |
|---|---|
| 640 ms | 19.2 Kbps |
| 1.28 s | 9.6 Kbps |
| 2.56 s | 4.8 Kbps |
| 5.12 s | Lower Date Rates |
| All aggregate data rates are: - 19.2 Kbps, 9.6 Kbps, 4.8 Kbps, 2.4 Kbps, 1.2 Kbps, 600 bps, 300 bps, 150 bps and 75 bps. However date rates <300 bps are not used for the TDMA mode of operation. | |
The number of transmit Time Slots per frame is dependent upon the aggregate data rate. E.g. at 19.2 Kbps the maximum 32 Time Slots can be designated per frame, supporting a maximum of 16 transmitting SatUs. Multiple Time Slots for the same user must be contiguous in order to increase the transmission capacity of that unit. Time Slot structure within the TDMA frame and Time Slot allocation to SatUs is determined by the Universal Modem (UM) system Network Implementation Plan (NIP). Meanwhile satellite access planing is under the auspices of the Satellite Planning Authority (SPA). Encryption is provided by the BID 1650 and error detection encoding is achieved via Cyclic Redundancy Check (CRC).
The Satellite Control Shore Function (CSF) address, 100008 (not a track number) is exchanged on the UM Virtual High Speed Control Circuit (VHSCC) along with STDL Technical Messages (STMs). The STMs employed are as follows:
| STM 01 | Network Configuration |
| STM 02 | Request Bearer Status |
| STM 03 | Capacity Overload |
| STM 04 | Bearer Status (Response to STM 02) |
| STM 05 | Request Network Configuration Design (Add/Delete SatUs) |
| STM 06 | Interleaving and Range delay |
| STM 08 | Baseband Control and Traffic Exchange (BCTE) Control |
In addition the Link 16 J0.1 SatU Test Message reply to the J0.6, and the J0.6 Communications Control Message are transmitted via the Baseband Control and Traffic Exchange (BCTE) user port. The remainder of the Link 16 messages are transmitted via the STDL circuit in accordance with laid down protocols.
The Link 16 messages used on STDL include all of the Link 16 catalogue except the following: J0.* messages with the exception of the J0.1 and J0.6 via BCTE as stated above, the J1.* series, Synchronisation messages (Round Trip Timing (RTT)) and the non-C2 messages J12.*. Link 16 Precise Participant Location and Identification (PPLI)< J2.* messages, are known as PLI – Participant Location and Identification on STDL in common with Link 22.
Equipment
Shore equipment consists of the UK Military Satellite Communications System (UKMSCS), STDL Network Design Function (SNDF), Satellite Planning Authority (SPA), and includes Controller Shore Function (CSF), Interim System Planning Computer (ISPC) plus Network Implementation Plan (NIP) and Network Controller Terminal (NCT).
Ship equipment consists of the following:
- Network Terminals (NT) which will be the Universal Modem (UM)
- Network Terminal Computers (NTC) which is the Data Link Processor System (DLPS) which is a front end processor handling all RN data links and data forwarding in a similar manner to the USN C2P model 5.
- NT (UM) – NTC (DLPS) link is an ISO 8802/3 Ethernet LAN
- The Network Control Initialisation Data Preparation Facility (NCIDPF), known as the "Nicky Duff" handle both MIDS/JTIDS Link 16 and STDL/Link 16 Initialisation Data (ID).
The latter is in the form of Network Configuration Identifiers (NCIs) numbered 1 to 64 which specify the Time Slot structure. Within these structures Network Reference Indicators (NRIs) (1-16 for Surface) are unique to each design. There are a minimum two slots per user with user position defined as 1 to 32 within a TDMA frame. Some slots may be designated Priority interrupt slots, in a similar manner to Link 22.
Network Design and Distribution
The Type 23 Frigate will have some restriction on its operation of STDL in comparison to that of the other units. For instance, the Surface Ship Command System (SSCS), with which they are fitted was developed with the 19 bit, 5 character alphanumeric track numbers in mind. However it is intended that these units will continue to operate the 12 bit, four digit octal numeric numbering system used by Link 11 and can thus only operate STDL within that limit despite the Link 16 message standard. The Type 23 is not fitted with MIDS/JTIDS Link 16 and thus does not have the NCIDPF with which to initialise STDL. Therefore a specialised device, the Peripheral for Initialisation and Control of STDL (PICS), which is linked to the Data Link Processor System (DLPS), has been procured from Data Sciences for this purpose.
Ship Equipment Configuration
Modes of Operation
Satellite Tactical Data Link (STDL) will be configurable for the three modes of operation mentioned earlier as defined below. These are:
- Network mode
- Group Link Mode
- Broadcast mode
Network Mode
A group of STDL equipped units utilise the Time Division Multiple Access (TDMA) mode of operation of the Universal Modem (UM); each participant transmitting its own information, using a Link 16 message format, in a composite transmission according to allocated timeslots and receiving similar transmissions from other STDL Units (SatUs). As stated elsewhere the number of active units is limited to 16 (or possibly fewer dependent upon the achievable aggregate data rate). The initial deployment of STDL in the Royal Navy will not allow the operation of the TDMA mode, which will not be available until the Universal Modem (UM) becomes available, perhaps as late as 2004.
Group Link Mode
Here two or more dispersed groups, i.e. Task Forces/Groups with overlapping areas of operation or in a support configuration, may be interconnected using STDL. Each group would normally be exchanging data within their group via other data links such as MIDS/JTIDS Link 16, Link 11 or Link 22. Two way inter-group data exchange may be achieved in either of two ways. Firstly by splitting the slots between the two transmitting units in the TDMA mode (again not available without the UM) and secondly by the use of two separate Satcom channels, the transmitter of one group on the receive channel of the other group, thus providing full duplex operation.
This latter method will be available before the UM is available, dependant on Sattelite channel capacity allocated. Regardless of method nominated units will function in the data forwarding role and will be known as Forwarding SatUs (FSatUs). Units forwarding data to/from specific data links may have additional letters appended to this term dependent upon the connected data links. For example a FSatUJ would forward data to/from Link 16. The terminal letters so used are:
- J Link 16
- A Link 11
- B Link 11B
- N Link 22
The forwarding capability in the RN may not function until the advent of the UM
Broadcast Mode
In this mode a single nominated STDL equipped unit continuously broadcasts data which all other, silent, SatUs receive. The UM of the transmitting unit is selected to the Continuous Access mode of operation facilitating the broadcast of data over a wide area. The broadcaster may of course be a data forwarder, providing information from other sources such as AEW&C aircraft. The use of TDMA with most slots allocated to the desired broadcaster, but with priority interrupt slots nominated may provide greater flexibility. That, however, would be a variation on the TDMA mode – not the broadcast.
The operational date for STDL in the Royal Navy without the UM is likely to be 2001/2. Note that this system is not compatible with either the USN S-TADIL J (described next) or the USAF Joint Range Extension (JRE).
S-TADIL J
Satellite Tactical Data Information Link (TADIL) J (or S-TADIL J) operates at UHF and was developed by the United States Navy (USN) to provide the following facilities:
Fall back when Link 16 relay connectivity is lost.
Continuous connectivity without JTIDS or Link 11 range limitations.
Exploits Command & Control Processor (C2P) Data Forwarding and Concurrent
Operations modes to maintain a common surveillance picture with seamless
connectivity.
Announced reporting (modified token passing) protocol for a network of up to 16
units.
S-TADIL J (US Satellite Link 16)
The US system uses a near-real time transmission method, whereby data is collected into packets, known as Demand Assigned Multiple Access and it operates via UHF Satcom. This compares with the UK Satellite Tactical Data Link (STDL) which uses real rime Time Division Multiple Access (TDMA) as for Link 16 and transmits at Super High Frequency (SHF) Satcom.
History
The programme began in 1994 under SPAWAR with an investigation of the feasibility of developing a satellite interface for the Command & Control Processor (C2P) to operate Joint Range Extension (JRE). This was to meet a requirement to support up to 16 units in a network and meet a 20 second surveillance track update rate. It is also required that there be a seamless and uninterrupted exchange of data between JTIDS/Link 16 and S-TADIL J.
In August 1995 the first in-house demonstration of S-TADIL J took place. This involved the exchange of J2.0 (Indirect PPLI), J3.2 (Air Track), J3.3 (Surface Track) and J7.0 (Drop Track Messages) using the following transmission speeds and formats:
- 2400 & 4800 bps DAMA
- 2400, 4800 and 9600 non-DAMA
The initial C2P modules developed could not operate S-TADIL J simultaneously with JTIDS, but a demonstration was conducted in USS Nimitz and USS Callaghan involving Link 11 and S-TADIL J. Later in 1995 the name changes from the original SATLINK-16 to S-TADIL J. The earlier demo was followed in 1996 by another involving the USS Vinson Battle Group. This consisted of four ships with concurrent Link 11, JTIDS and S-TADIL J capability exchanging J2.0, J2.2, J2.3, J2.4 and J7.0 messages on S-TADIL J. Later the same year the Vinson Battle Group conducted another trial involving a full set of S-TADIL J surveillance messages. In July 1997 the Operator Text Message (OTM) was added to support co-ordination beyond HF voice range and in August of the same year Theatre Ballistic Missile Defence (TBMD) messages were added.
In September 1997 USS Nimitz completed a receive only demonstration which was followed by verification and acceptance testing of the Model 4 C2P with S-TADIL J.
S-TADIL J Messages
- PPLI: J2.0, J2.2, J2.3, J2.5, J2.6
- Surveillance: J3.0, J3.1, J3.2, J3.3, J3.4, J3.5, J3.6, J3.7
- ASE/EW/Intel: J5.4, J6.0, J14.0, J14.2
- Info’ Management: J7.0, J7.1, J7.2, J7.3, J7.4, J7.5, J7.7, J8.0, J8.1
- Mission Management: J9.0, J10.2, J10.3, J10.5, J10.6
- Status: J13.2, J13.3, J13.4, J13.5
Multi-Link Operation
The C2P enables Automatic Data Forwarding (ADF) and concurrent operation of Link 4A, Link 11 (including dual Link 11), Link 16 and S-TADIL J using a real time data base.
C2P Concurrent Operation of S-TADIL J and Other Links
The Model 4 C2P exchanges data with the Tactical Data System (TDS) at the level of information and precision supported by Link 11 and Link 4A. This is improved to the Link 16 standard in The C2P Model 5.
Ship installation S-TADIL J operates in a synchronous, half duplex mode at 2400/4800 bps DAMA or 9600 bps non-DAMA.
| STGU | Satellite TADIL Gateway Unit. S-TADIL J capable unit. |
| STGC | Satellite TADIL Gateway Controller. Dynamically assigns STGU transmitting order and time allocations (NTR). |
| SNCT | Satellite Network Cycle Time. Time required for the STGUs to complete one full cycle (modified Token Passing protocol). |
| Real-Time S-TADIL J data | SNCT <20 sec. Prevents R2 conflicts on commonly held radar tracks. |
| Throughput | Track loading which can be accommodated by S-TADIL J. |
Network Protocol
All STGUs receive a Time Slot List (TSL) from the STGC. The first STGU transmits immediately after reading the number of 32 bit words listed in the STGC’s Network Header with no delay for DAMA processing. Each STGU transmits in the order of the TSL until the list is completed, and the order of the TSL is determined by the quality of each unit's response. Where the strongest (lowest # of errors) is first and the weakest last.
S-TADIL J Network
STGU Message Structure
The Network Header provides transmitting STGU specific data for protocol control. It includes total length of transmission, counted in 32 bit words, and in the case of the STGC the TSL is included. The Transport Header identifies JU/PU reporting responsibility for the J Series data to follow. (This is a terminal function in MIDS/JTIDS). Multiple 70 bit J series words are mapped into 9 bytes. Up to 25 J-words are packed behind each header.
STGU Message Transmission
S-TADIL J Network Performance
Given the real time limit of a 20-second network cycle, there are constraints on the number of active units, particularly using DAMA. Since non-DAMA operations are not recommended and are not validated, the maximum number of units is restricted to eight if the 20 second real time criterion is to be achieved. The number of tracks and STGUs that can be accommodated is shown in the figure, S-TADIL J 20 second throughput figures, below.
S-TADIL J 20 Second Throughput Figures
The throughput capacity above assumes an average of 2.4 J series words per track. It should also be noted that large track loads result in more efficient packing and higher track capacity at high transmission rates.
Joint Range Extension
Joint Range Extension (JRE) is intended to provide a means of exchanging J-Series Link 16 messages over media other than MIDS/JTIDS, primarily to achieve Beyond Line-Of-Sight (BLOS) communication.
This becomes important in order to:
- Operate widely dispersed forces.
- Provide the picture to a JICO/Command Base which may be in the rear echelon.
- Avoid the saturation of MIDS/JTIDS capacity by the utilisation of multiple relay hops.
- Overcome the requirement to necessarily provide dedicated airborne MIDS/JTIDS relays when the tactical situation would not naturally demand their deployment.
- Provide Link 16 data communication between surface / ground units when no airborne relays are available.
The US is developing a JRE Applications Protocol (JREAP) Mil-Std, Annexes of which will reflect protocols in respect of UHF SATCOM (Token Passing), Point-to-Point SHF SATCOM or Landline (Full Duplex) and Internet Protocol (IP) based e.g. SIPRNET or EHF-MDR SATCOM TDMA Multicast.
JRE is intended to interface up to four JRE media with one MIDS/JTIDS terminal / network simultaneously and encompass data forwarding filters based on geography, track identity and or J-Series message numbers. Receipt / Compliance protocols will need to reflect the chosen media time latencies and the monitoring and net management facilities will also be specific to the chosen medium.
The USN S-TADIL J will also be accommodated by the JREAP and thus become another
JRE format.
Data Link descriptions: Tactical Data Links | Link 16 | Link 11 | Link 22 | IJMS | Link 1 | Link 14 | Link 4 | Other protocols | CDL, TCDL & HIDL | Satellite Link 16 | Variable Message Format - VMF
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