Most Recent Advisory
15 Sep 2023

How Dangerous Is It To Use A Single Point Prediction For An Entire Region?

A common question among mission planners is ‘how applicable to the surrounding region is a GPS prediction for a specific point?’ Here at the Operations Center, we like to compare GPS performance to the weather. If you are standing in your front yard and it’s raining, you can be reasonably assured that it’s also raining in your back yard. The problem is in determining what’s happening on the other side of town, the other side of the county, or the other side of the state. As would seem obvious, the further you get from your front yard, the less likely you are to be correct in your weather prediction. GPS is much the same. To complicate matters, GPS performance boundaries often act like weather fronts. To illustrate, imagine yourself standing in the middle of an open field. You are on the very leading edge of a large rainstorm. To your left is rain; to your right is blue sky. Front and back, the cloud line extends to the horizon. If you were to walk a few hundred feet to your right, the weather would improve substantially. Walk the same distance to your left and you’d get drenched. Straight-ahead or straight back and you’d just get a light drizzle. Likewise, the geometry of the GPS constellation can often result in these ‘fronts’. Figures 1 through 4 illustrate this effect.
Figure 1 (click to enlarge) Figure 2 (click to enlarge)
Figure 3 (click to enlarge) Figure 4 (click to enlarge)
If you were to run a prediction at the specific Lat/Lon for Point 1, you’d assume that the region being planned for has an unacceptable spike at 1000Z. Point 2 would lead you to conclude that there is a spike, but that it is possibly within tolerances, while Point 3 shows no spike at all. To give an idea of scale, the distance between Points 1 and 2 is 90 miles. The distance between Points 1 and 3 is 28 miles. Much as meteorologists must have an understanding of localized weather patterns before they can make reasonable weather forecasts, so too must mission planners familiarize themselves with the standard GPS performance in their region. The GPS Operations Center provides both global and regional maps so mission planners can see where these fronts occur. The maps show the maximum DOP attained throughout the region. If a uniform color is shown across the region, mission planners can be reasonably assured that a prediction at a specific Lat/Lon will be applicable to the entire region. If multiple colors are apparent, mission planners will need to perform multiple predictions to determine the exact time and magnitude of the spike in each colored area.