Mission planning is carried out prior to field work in order to identify the optimal times for collecting GPS data. The quality of GPS data is dependent on the geometry between the receiver and satellites (this includes the number of satellites that can be "seen" by the receiver and the angle between the receiver and satellites - a satellite near the horizon usually provides a lower quality signal because of greater atmospheric interference and the increased likelihood of the signal reflecting from surface features such as lakes - this is known as "multipath" error). The effect of geometry on GPS quality is measured by PDOP (position dilution of precision) - this is the overall measure of the precision obtainable with a given satellite geometry. A PDOP of 4 or less yields excellent precision. A PDOP between 5 and 7 is acceptable; a PDOP of 7 or more is poor.

The Quick Plan utility (part of Pathfinder Office) can predict PDOP for a selected day and identify the optimal times for collecting data. Quick Plan requires a current Almanac file to make these predictions. The Almanac file contains information on satellite position and health; these files are continuously transmitted by the GPS satellites and automatically recorded by GPS receivers in use in the field.

You can tell if your GeoExplorer has received a complete almanac file by switching it off. As the screen is counting down the time to shut down, the letter a is displayed at the bottom of the screen. An upper case A indicates that a complete almanac file was recorded; a lower case a indicates that a complete almanac file was not recorded. For mission planning you should use an almanac file less than 30 days old. The simplest way to get a current almanac file is to take your GeoExplorer outside and allow it to calculate positions. It will automatically acquire a new almanac file in about 45 minutes. You do not need to instruct the GeoExplorer to record an almanac file. The almanac file can be transferred to the PC by selecting Utilities|Data Transfer, selecting "Almanac" as the file type and clicking on "Transfer".

To start Quick Plan select Utilities|Quick Plan. This opens the date selection window where you select the day that you plan to do field work; next the point window opens where you supply the location of the field work (this can be specified as a point on a world map, selected from a list of U.S. cities, or entered as latitude and longitude coordinates). Entering this information brings up the status window:

Figure 1. The Quick Plan Status window for
Denton on June 25th 1999, with a threshold
elevation of 15 degrees.

The "threshold elevation" of 15 degrees means that if the angle between the receiver and a satellite is less than 15 degrees (above the horizon) the satellite will not be used (this value can be changed under Options|Elevation mask). Selecting Graphs|Number of SV's and PDOP will construct a graph showing number of satellites "visible" and the PDOP value for the selected day:

Figure 2. Number of satellites and the PDOP value for Denton on June 25th 1999.

You can now have Quick Plan identify optimal mapping times based on your specified conditions by selecting Options|Auto Time:

Figure 3. The Auto View Time Selection window.

In this example, the minimum number of satellites is set to 5, the working hours are 800 to 1900 hours, the maximum PDOP is 4, and the minimum observation time is 60 minutes. Quick Plan calculates the optimal mapping times based on these conditions:

Figure 4. Optimal Mapping Times Calculated By Quick Plan.

In this example, you could schedule lunch for 1130 - 1200 and an afternoon coffee break for 1510 - 1550 (both times with unacceptable PDOP values).