Interference? The fiber-optics option

The use of GPS signals is certainly commonplace in today’s technological age. Various locating systems, tracking systems and precision timing applications all use the common decoded NMEA and 1 PPS signals from a GPS satellite in a multitude of different ways.

When a direct line-of-sight path to GPS satellites is unavailable, the GPS signal must first be received where there is a direct line-sight path, decoded, and then the resulting signals routed to where they are needed. The Luxlink GPSX-1001 has been designed to do exactly that.

LuxLink GPSX-1001 fiber-optic transceiver.

The GPSX-1001 is the result of a specific request by a research group of a midwestern U.S. university for seismic studies in an underground mine. More than 20 units were installed in several branches of the mine and have been in continuous operation successfully for two years.

The GPSX-1001 transceiver is a multifunctional device that can be used as a transmitter or a receiver/repeater. In operation, the NMEA signal and the 1 PPS signal are both multiplexed by the GPSX-1001 (set as a transmitter) and launched into a single optical fiber. The multiplexed signal is then received from the fiber at a second GPSX-1001 set as a receiver/repeater. Here, the NMEA and 1PPS signal are de-multiplexed and available as individual outputs (see Figure 1).

FIGURE 1. GPSX-1001 block diagram.

The original multiplexed signal is also then reapplied to another integral optical transmitter for use at a third receiver/repeater. Additional receiver/repeaters can be connected in the same fashion to allow the signals to be transmitted to numerous locations.

Fiber-optic cable is virtually immune to electrical interference and can be routed wherever convenient without regard to the proximity of electrical noise producers, water or high voltages. Because fiber optic cable is non-conducting, ground loops that can result in loss or corruption of the GPS signals are virtually eliminated. The bandwidth of the fiber and circuitry in the GPSX-1001 is such that the fast rise and fall times of the 1-PPS signal are maintained and the NMEA signal is as noise free as the original input.

Transmission distances using the GPSX-1001 can extend to a mile or more. For longer distances, additional GPSX-1001 units can be added.

The GPSX-1001 is user configured by means of front-panel DIP switches. Integral LED indicators are provided to continuously monitor the NMEA, 1 PPS, power and optical link signals. Power is obtained from simple wall type plug-in adapters or low voltages and need not be regulated because the GPSX-1001 units contain internal regulators.

Figure 2 shows three GPSX-1001 units in a typical GPS signal distribution system. The NMEA interface can be RS-422 or RS-232, depending on the requirements of the signal source. The 1 PPS signal is 50-ohm TTL compatible. Each transceiver pair will produce signals over distances in excess of several miles and will operate from –35° to +75° C (–31° to 167° F), allowing them to be used both indoors and outdoors. Units are available for use with multimode or single-mode fiber and with standard fiber-optic connectors.

FIGURE 2. GPS NMEA/1 PPS transmission system.

Irwin Math is president of Liteway Inc. and has more than 30 years of experience in the design and development of fiber-optic transmission systems. He was also the founder of Math Associates Inc., one of the pioneering firms in fiber-optic transmission system technology in the early 1980s.