Most GPS manufacturers have a mission planning package included in their software. What is needed is a recent copy of a GPS ephemeris (or almanac); the software can then perform numerical integration to show what satellites are available at the location and time desired.
The example I’m using is the following:
- Quick Plan software from Trimble GPSurvey
- An ephemeris downloaded from Trimble’s website on 10/19/2001
- Surveying in Minneapolis, Minnesota on 12/14/2001
- Elevation mask is 15 degrees above the horizon
Quick Plan makes it easy to select a location. The default position for Minneapolis is N 44 degrees 53', W 93 degrees 13'. The time zone is central standard USA. The ephemeris used showed 28 satellites of the 29 satellites available. Rather than show all tables and graphs available to the user, I’m including only three:
- A table showing constellation changes. This is Table 1.
- A graphic of Table 1. This is Figure 1.
- Three sky plots, each one-hour duration. These are Figures 2, 3 and 4.
Let’s look at Table 1. Shown are satellites visible at specific times. The first line of data starts at midnight, December 13. The time period is 10 minutes. Seven satellites are available (6,10,15,17,23,24,26). At 10 minutes after midnight another satellite becomes visible, #18. These eight satellites are visible until 20 minutes after midnight. This is continued for the 24-hour period of December 14. As long as four satellites are visible at one time, a surveyor can observe static baselines. I believe most manufacturers recommend five or more satellites for RTK. At no time are less than five satellites available.
Figure 1 on page 48 a graphic of Table 1 and needs no further discussion. Since it’s possible to survey anytime on December 14 in Minneapolis, let’s get specific and discuss sky plots. Figures 2, 3 and 4 are plots showing where the satellites are visible to a GPS receiver on the ground. I selected the time from 9:00 a.m. until 12 p.m. in three one-hour segments.
Several years ago, long before the constellation was completed, I worked on a GPS control survey in Rolla, Missouri. We could only observe five hours during each day, so timing was critical. I generated sky plots for each of the five hours.
Click here to download the chart that accompanies this article.