GPS Satellite “Watch-Outs”

1. Signal Blockage – Landscape and Buildings. “Accurate GPS and DGPS measurement require a clear line of site to the source, so having a clear view of the sky at all times grealy increases the accuracy of the signal,” explains Ryan Pieper, product manager, GPS and sensors with Trimble. Trees, buildings, and hilly terrain can cut off the signal, forcing the receiver to reestablish and reconnect with the satellites. Depending on the quality of the receiver and the extent of the blockage, this can take anywhere from a few minutes to a half hour.

One way to reduce the incidence of signal loss is to mount the antenna as high as possible on the cab. Marlin Melander, marketing manager for Raven Industries’ Flow Control Division, provided this field example.

“I had one customer that complained that his light bar would work while traveling in one direction but would display an accuracy warning signal when he turned around. It turned out that he mounted his GPS receiver on the hood of his tractor insead of on the roof. The receiver would lock onto the Wide Area Augmentation signal (WAAS) while traveling south but when he turned north, the cab would block the signal. Once he moved it to the roof, the problem disappeared.

2. Signal Blockage – Multi-Path. “Multi-path is the reflection of GPS signals often caused by metal surfaces or wet foliage,” explains John Bohlke, product marketing manager for Hemisphere GPS. “To minimize the effects of multi-path, the user should mount the GPS antenna at the highest point possible, above obstructions and away from reflective surfaces.”

3. Satellite Location and Position. High quality receivers today are much better at compensating for inferior satellite position in the sky, but the position of the constellation can still cause problems.

Notes Graeme Kennelly, marketing manager, KEE Technologies, these satellites are on the move, circling the Earth about every 12 hours in different planes.Their locations at the time of your use of the signal can impact the signal quality. For example, a phenomenon called the dilution of position (DOP) can occur if satellites are grouped together rather than spaced apart. For example, equally space GPS satellites in the sky give a much better position than if they are all grouped together near the horizon.

Finally, dramatic changes in the ionosphere, such as solar storms, and daily sunrise and sunset can also degrade accuracy, says Bohlke. “Users can try to avoid those times but that may not be practical.”

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