The GPS Observer
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.Looking at Figure 2, 11 different satellites are visible at some time during this hour. The length of line beside each satellite number shows how far each satellite has moved, and in what direction during the one-hour period. Figure 3 shows the configuration from 10:00 a.m. to 11:00 a.m. Figure 4 is from 11:00 until 12:00 p.m. As stated earlier, all times are central standard USA.I’m sure you can see the usefulness of sky plots for static surveying. In areas where trees, buildings, etc. cause obstructions, the surveyor can be selective on when to observe at a particular station. Let me tell you of one particular survey where sky plots proved valuable.
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.The primary horizontal control point for this survey was located on the campus of the University of Missouri at Rolla, and two huge oak trees a few feet to the west obstructed everything to the west; we could only observe satellite east of the monument. We looked at the sky plots, and only one showed enough satellites east (first and second quadrants) to observe from that control point. We arranged our observing sessions to have a GPS receiver on that control point during that time period. Without the sky plots we could have lost one day’s observations.
Click here to download the chart that accompanies this article.