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GPS over fibre system for antenna cable extension

Extend the reach of an antenna feeder cable up to 10Km, and overcome the limitations of a coaxial cable system.

Background

To provide an input signal to a GPS repeater, or a device such as a Network Time Sychronization unit, an outdoor antenna would generally be installed at the highest point on the outside of the building.

The length of the coaxial cable from antenna to repeater/receiver may have an adverse effect on signal strength and quality.

Coaxial cable limitations

  • In-line amplifiers and up-rated cable can be used to overcome signal losses caused by attenuation but may introduce unwanted noise and distortion.
  • Low-loss cable is often several millimetres in diameter (RF400 is ~11mm outer diameter) and is heavy and difficult to install over long distances.
  • Good quality low-loss coaxial cable can be expensive.
  • Even when using well-screened cable, signal can be affected by electromagnetic interference (EMI), especially if it is positioned near power lines.
  • Coaxial cables can be adversely affected by temperature changes, severe weather (especially lightning) and moisture.

Any, or all, of these factors may mean that it becomes impractical continually to increase the cable length.

 

GPS over fibre – a reliable and practical alternative

GPS over fibre schematic diagram

A point-to-point optical link consists of three main components:

  • An optical transmit (TX) module
  • A length of single-mode optical fiber cable
  • An optical receive (RX) module

The transmit module (TX) converts RF signals from the outdoor antenna into light pulses and transmits them down the optical fiber link.

The receiver module (RX) at the far end converts the light pulses back into RF signals and passes them to the GPS equipment (repeater | receiver | timing module).

Key features:

  • All GNSS GPS frequencies passed (1000 to 1800 MHz)
  • The transmit module provides a DC voltage to the low noise amplifier (LNA) in the GPS antenna
  • The receive module terminates the DC voltage output from the GPS repeater/receiver and presents an impedance that simulates the presence of a GPS antenna
  • Standard optical connector type is SC/APC (FC/APC on request)
  • Power for each unit is provided by an external +12V power adapter
  • Both TX and Rx units provide connections for serial data transmission (RS232, RS485 & TTL) for monitoring and control

GPS over fibre system customised to your requirements

A typical system, as supplied to many FalTech GPS customers, consists of the following:

  • GNSS active antenna (Tallysman TW3742; 40dB gain)
  • 10m RF240 low-loss coax to connect antenna to TX unit
  • ViaLiteHD Blue OEM TX module with PSU
  • 200m optical fibre, pre-terminated
  • ViaLiteHD Blue OEM RX module with PSU
  • 10m RF240 low-loss coax to RX unit to GPS repeater/receiver

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