How to Extend an Antenna Cable with GPS over Fiber Technology
Sometimes the distance from outdoor antenna to indoor GPS repeater/receiver is too long to consider using coaxial cable. Losses through attenuation, even with the use of in-line amplifiers, can adversely affect the signal, potentially making it unusable.
The cable length can be increased by inserting an amplifier in-line with the cable. However, physical distance isn’t always the main issue; if a cable must run through an area where there are high levels of electromagnetic interference (EMI), say in the vicinity of power cables, the signal can be badly affected.
Low-loss coaxial cable can be expensive and difficult to install due to its thickness and weight. For example, LMR400, a commonly used cable, is approximately 10.5 mm in diameter and weighs 10 kg per 100 meters.
How optical fiber can help
In the context of providing indoor GPS, the length of a coaxial cable generally is limited to around 100 meters, due to the effects of attenuation and increased noise. Even a well-screened, low loss coaxial cable can be affected by EMI, causing delays and data packet corruption. By comparison, the distance from antenna to receiver can be up to 10 km over single mode fiber.
Optical fiber is an inert material, based on glass (silica) or plastic materials, making it immune to the external effects of RFI/EMI. Therefore, it can be installed near high voltage power cables without any unwanted interference affecting the signal.
Optical fiber cable is thin and lightweight and can be installed more easily than a thicker, low loss coax.
Components of a GPS over fiber system
The outdoor antenna in a GPS over fiber system is exactly the same as used in any “standard” GPS over coax system. Tallysman TW3xxx antennas have a built-in low noise amplifier (LNA), with gain ranging from 28 to 40 dB. However, any antenna of the correct specification will work in this situation.
The antenna connects to an RF to optical converter (usually referred to as a transmitter or Tx) via a low-loss coaxial cable. The Tx unit converts the incoming RF signal into a light signal before transmitting it into the single-mode optical fiber. The fiber can be a single strand cable, specifically installed for the antenna link, or a spare strand in the existing in-building infrastructure.
At the far end, the reverse happens: an optical to RF converter (referred to as a receiver or Rx) converts the light signals back to RF.
DC power considerations
The fiber optic cable is non-conductive, which means that the GPS repeater/receiver will not be able to provide 5 V DC to the antenna LNA as it would normally do with a coaxial cable connection. The Tx unit performs this function, to ensure that the LNA in the antenna receives power.
Similarly, the Rx unit terminates the DC voltage from the attached receiver, by providing a resistive load that simulates an antenna present. If the antenna fails, or is disconnected, the Tx unit will sense that there is no antenna bias current drawn and assume that there is no antenna present. The Tx will then switch off its laser to prevent light being unnecessarily transmitted to the fiber.
If the Rx unit detects that there is no (or low) light input from the fiber, the DC load is removed from the circuit and the status LED shows red. The open circuit connector indicates to the receiver that the antenna is not present and will cause an alarm indication in most GPS receivers and time servers.
GPS over fiber case study
A simple, but effective, GPS over fiber link was installed in a seismology monitoring station, deep under the Austrian Alps.
The two main problems that were overcome were (1) physical distance and (2) the presence of high voltages surrounding the hydro-electric generator.
Written by Phil Whitting, FalTech GPS