Airbus Defence and Space

Atomic Clock Monitoring Unit

The CMCU is a high-performance 10.23 MHz master clock generation unit for satellite navigation systems.

Two frequency synthesizers can select independently between two input 9.99 to 10.01 MHz Atomic Clock frequency
references as a basis for the master clock generation. One of the two synthesizer outputs is selected to provide the master clock for the payload and is distributed to up to four identical outputs. The output signal of the second synthesizer provides a hot redundant alternative for the master clock with seamless switching (no distortion of the output signal). A phase meter monitors the output phase of the active synthesizer against the hot redundant one. Maximum reliability is achieved through a design without microcontroller. The TM/TC interface is implemented as hard wired logic and can be addressed by serial and high level commands. A phase synchronous output switching between active and redundant synthesizer is available. Optional features like parameter-based clock drift compensation or synthesizer adaptation to source characteristics can be implemented. The CMCU can be operated with rubidium, passive hydrogen maser and caesium clocks to form a complete timing subsystem. The CMCU in the Galileo configuration operates with passive hydrogen maser and rubidium clocks.


Key Features

  • Master timing reference generation (10.23 MHz) for NAV payloads
  • Direct digital frequency synthesis with hyper-fine tuning steps to correct frequency drifts of atomic clocks while not degrading their performance
  • Frequency comparison system determines drifts between two atomic clocks
  • Seamless switching capability
  • Frequency distribution networks
  • High phase stability over temperature drifts
  • 4 x 2 switch matrix with high isolation to prevent degradation of clock stability
  • Capability to monitor discrete clock Interfaces

Main Application Fields

  • Navigation payloads (global or regional)
  • High-performance communication systems
  • Scientific applications
  • GNSS overlay systems

Customers / Applications

  • Galileo FOC
  • DLR
  • ISRO (Indian regional navigation satellite system)


Power bus

26–48.5 V (adaptable)

Clock inputs

9.99...10.1 MHz (adaptable) 5..13 dBm


Analogue, digital and thermistor inputs for clock telemetry

Output signal

10.23 MHz, -1...7 dBm (adaptable)

Control & monitoring

MIL-STD-1553B or RS422 high level command.


Discrete analogue & digital telemetry.


  • EGSE including high-precision timegeneration and verification equipment
  • Expertise in high-precision timing subsystems Budgets
  • Mass 5.2 kg
  • Volume 270 x 216 x 137 mm3

Environments / Reliability




-15 °C to +45 °C


-40 °C to +60 °C

Radiation tolerance:

up to 100 kRad


0.9927 over 12 years

Power (average) 21 W