Using GPS for spotting will give you more information than just sticking your head out the door. It will tell you your groundspeed so you can figure out proper separation between groups. Using the groundspeed and your known True Airspeed you can get an idea of the wind speed at that altitude. Plus you will know if you are having to crab to stay on jump run track that wind might be shifting. This would mean a correction in jump run might be needed. If you are expecting a crab angle during a crosswind jump run and it is less than you were expecting or less than the last load you flew then you know there has been a change.
But the most beneficial thing you can get from a GPS is the exact location of where your jumpers went out. Compare their comments about the jumprun with what you know you actually flew and you can very precisely correct for the next run.
Many jumpers will hate what I’m about to put here. I DON’T WANT ANY JUMPERS SPOTTING THE LOAD WHEN I’M FLYING. I know what run I want to fly just from my experience. If they want something specific then they can tell me before we open the door what that is. But if they look at the ground and see the plane pointed away from where they think they want to go they will start giving me bad corrections. It just may be that I’m flying a crosswind jumprun.
The Garmin III GPS handheld is what I use. There are many out there that are just as good. The things that I think are important are: 1) A moving map display that zooms in automatically as you approach your target, 2) Simultaneous display of Groundspeed Time to Destination, Distance in Feet off course left or right, and Distance to Destination. The moving map is really handy when doing crosswind and hook pattern jump runs. Before flying, I will look at the airport layout and see how detailed the programing has been by the manufacturer. If there is not a lot of detail, then I will add some. Walk around the airport laying waypoints that you might think are useful. The very ends of the runway; the center of the landing area; corners of the airport property; hazards; prominent landmarks; and any other suggested marks by the jumpers. With practice and use you will find out what you really need on the screen and what is just clutter.
For formation flying, a good HSI page on the GPS can be invaluable. This can show you your actual track over the ground and whether it is changing or not. You can make small corrections before anyone notices that you are getting off course. Remember, you are only spotting for one group so it shouldn’t be too hard to put it spot-on. As long as you anticipate the climbout time correctly it should be cake.
GPS is the ultimate “bombsight.” It gives you a highly accurate (plus or minus 30 feet) position report over the ground. As long as you know where you put them out, you can correct if necessary or at make a correction before you get too long thus preventing an unnecessary go-around.