What is Common Data Link?

Common Data Link (CDL) is a secure US military communications protocol to enable a full duplex, jam-resistant digital data link between various intelligence, surveillance, and reconnaissance (ISR) platforms and ground terminals in a network. This protocol was established by the US department of defense (DoD) in 1991 as the military’s primary protocol for communicating imagery and signal intelligence between ISR platforms and ground stations. The US DoD launched the CDL program that established data link standards and specifications to identify compatibility and interoperability requirements between various ISR platforms and ground terminals.

CDL communication usually involve wideband real-time downlink transmission of sensor data from ISR platforms to ground terminals and real-time uplink transmission to sensor payloads in space (e.g., satellite) and other airborne systems. Downlink sensors transfer infrared, electro-optical, and synthetic aperture radar (SAR) imagery and signals that support US defense services, NATO partners, and other allies. The purpose of these organizations is to securely connect various airborne and space platforms with surface or ground terminals such as ships, low-flying aircrafts, and other military command and control (C2) platforms. The uplink is narrowband compared to downlink capacity and is used to control ISR platforms and payloads integrated into them from the ground. The CDL protocol uses the Ku-band from 14.5 to 15.35 GHz for both uplink and downlink. The Ku-band is typically used for fixed satellite communication services, direct broadcast satellite services, and VSAT systems on ships. CDL delivers a data rate from 200 kbps to 45 Mbps in the uplink and a data rate of 10.71-45 Mbps, 137 Mbps, and 234 Mbps in the downlink.

The implementation of CDL in various mission-critical applications offers a secure way to provide decision-making in military operations on the ground using accurate situational awareness. An authorization act was passed that requires the use of CDL protocol for all types of ISR missions. However, waivers are granted under special circumstances such as the inability to carry a very heavy radio system on a small aircraft. Research is being carried out to continuously design and develop lighter radios (around 2 pounds or lower) to incorporate CDL and establish a secure military network.

Types of Links Supported by CDL

The CDL family currently supports five different types of links based on the type of ground terminal and range of the link.

1. Class 1 – Ground-based applications (land surface terminal, sea-based data link) with airborne platforms with a speed of Mach 2.3 and operating at an altitude of up to 80000 ft.

2. Class II – Ground terminals with a speed of up to Mach 5 and operating at an altitude of up to 150000 ft.

3. Class III – Ground terminals with a speed of up to Mach 5 and operating at an altitude of up to 500000 ft.

4. Class IV – Satellite terminals orbiting at 750 nm

5. Class V – Terminals in relay satellites operating at higher altitudes.

Operation and Architecture of a Common Data Link (CDL)

The primary elements of a CDL link consist of the ground terminal (CDL surface communication element) and ISR platform (CDL platform communication element) as shown in the above figure.

Both the ground terminal and the ISR platform consist of transmitter and receiver chains to transmit and receive uplink and downlink data. The primary components include multiplexer (MUX)/demultiplexer (DEMUX), communication security (COMSEC)-related devices, encoder/decoder, interleaver/deinterleaver, spread spectrum modulator/demodulator, up/down converter, traveling wave tube (TWT)/LNA amplifier, diplexer, and an antenna terminal.

In transmit mode (ISR platform or ground terminal), a stream of data/control signals is multiplexed at the MUX block into a single message block and sent to the COMSEC unit. The COMSEC is responsible for adding security features such as encryption/decryption and other authentication mechanisms to secure the data. The encrypted data is sent to the encoder block for encoding the message with additional bits to assist during error correction at the receiver. The interleaver block inter-mixes the bits in the codeword (or the message) to assist the receiver in easily recovering the lost bits due to communication channel impairments such as noise. The spread modulator modulates the interleaved digital signal, converting it to an intermediate frequency (IF) signal in the 1700 MHz band. This block uses conventional digital modulation techniques such as OQPSK, BPSK, and others. This IF signal is then relayed to the upconverter to upconvert this signal to an RF signal in the Ku-band after which the signal is amplified. The RF signal is sent to the diplexer. The diplexer contains filters that effectively isolate the transmit frequency from the receive frequency, thereby allowing it to share a common antenna terminal. The RF signal is filtered across the Tx passband and sent to the antenna for transmission.

The receiver (ISR platform or ground terminal) receives this RF signal via its antenna and performs reverse operations of the transmitter.

With the continuing trend in the reduction of size and weight of sensors/devices, CDL developers have also been able to significantly improve the performance and adaptability to evolving platforms, making the ISR sensor platforms more modular and flexible. The CDL systems may also be required to seamlessly integrate with legacy and next-generation systems as and when they are introduced, thereby preventing delay in installation and operations. The next-generation capabilities may vary from one application to another, but will most likely include, AES-256-embedded security (TRANSEC), high-speed multiplexing, encoding, and encryption, new integrated circuits (ICs) based on advanced materials, and improved satellite connectivity. With support for advanced signal processing capabilities, interoperability, and a secure network interface, the CDL offers a very secure digital data link that can cater to a wide range of current and future mission-critical applications on land, air, and sea, providing a tactical edge in the battlespace.