GPS mission planning software
A GPS mission consists of three phases. planning, execution, and post-processing. Mission planning software packages run on a desktop system at your office and assist with planning and post-processing phases.
Why you need to plan
For any given location there will be times when the satellite coverage is not good. If you are doing fieldwork and you have spent a lot of time getting to the field location only to find that your GPS data will be useless, you will regret not spending 15 minutes in planning.
Run the mission planning software, set up the vicinity you will be working in the position file, and the date you want to work, and it will generate plots showing the PDOP levels you can expect. Then just avoid working when the PDOP is unacceptably high.
The two commercial programs that I know of are tightly integrated with the field software so they are not all that useful unless you buy in to the whole enchilada -- data collector, proprietary data collector software, and "office" software.
Planning for a GPS mission should include checking for suitable conditions. This includes knowing that the satellites will be well positioned when you will be in the field.
I use Trimble's free planning program for this. You can download it from Trimble. The only bad part is that it is available only for Windows systems but it's handy enough that I use it anyway.