ion satellites GPS signals GPS signals GPS signals GPS receiver with augmentation capability Correction Receiver uses correction and receiver uplink integrity signals Monitoring station(s) Figure 3-15 Basic Mechanisms for Space-Based Augmentation Systems Almost all these systems currently augment the GPS system and SO consist of numerous ground sta- tions widely positioned around the coverage area, which continuously monitor GPS signals received from space. These locations are precisely surveyed, SO the monitoring receivers know exactly the solution that they should be calculating from GPS. Any anomalies from the ideal solution can be determined, correction data calculated and transmitted to the augmentation satellites. Those GEO satellites then broadcast this "correction" signal to the users on the ground (or aircraft trying to get onto the ground) as a "GPS-like" signal. GPS receivers equipped with the additional augmentation capability can use this data to fine-tune the solution they have calculated. This allows the receivers to be much more confident about the signals they are receiving and improves accuracy because ground stations are giving corrections to any position errors caused by, for example, chan- ging atmospheric conditions. This especially helps aircraft, because most GNSS systems are not as accurate for positioning in the vertical plane (determining the height) as they are in the horizontal plane (latitude and longitude). But to use GPS for aviation, having an accurate value for height is critical, even more SO when us- ing it to approach an airport runway. Finally, these systems give very fast feedback (in the region of a few seconds) to the user if the GPS was somehow broadcasting information that could lead to hazardous positioning errors. This in- spires confidence in using GNSS systems for "safety of life" applications. Because of these needs for the aviation industry, most of these SBAS systems have regional or national civil aviation organ- izations as partners. The two major space-based au