Introduction

Global Positioning System (GPS) signals are now extensively used in many Positioning, Navigation and Timing (PNT) applications. Critical infrastructure, in particular, often utilises GPS signals and whether this is for testing, timing or repeating the GPS signal inside a building, this necessitates fixed antenna installations outside buildings or cabinets.

Highly Specialised Skill

Chronos has been installing GPS antennae for many years with thousands of installations worldwide. One outcome from the SENTINEL research project, was a document drawing on this expertise entitled ‘Criteria for the Installation of GPS Antenna on Static Infrastructure’.  GPS installations are a highly specialised skill and this Application Note highlights the key features of best practice, illustrating this with some extreme examples of bad practice.

Antenna Location

Since the 32 GPS satellites are approx. 12,000 miles away in space it seems obvious that GPS antennae should be mounted with a clear 360°sky view and well away form each other. However this simple requirement is often ignored resulting in degraded performance.

The case for 360°view is obvious; the case for  mounting too close together less so. Externally mounted antennae are subject to extreme weather conditions ranging from intense heat to rain and snow. 

Degradation

Over time, the antenna housing or cables and connectors may degrade which can affect the way the antenna behaves.    The antenna receives the native GPS signal from the sky, it is then amplified and passed to the receiver somewhere in the building.  Sometimes, weather extremes will degrade the electronics resulting in the amplified signal being reflected back through the antenna and impacting other GPS antennae in close proximity.  Nearby timing receivers may now start to exhibit relatively large time changes.  These time variations could be many hundreds of nanoseconds. This can easily be avoided by ensuring that the antennae are placed well apart from each other and not as shown here.

Surprisingly, 360°sky view is frequently ignored. North facing walls are often used giving even less satellite view.