I have two examples of Intentservice. One is the Download example in the commonsware book. the other is at http://www.vogella.com/articles/AndroidServices/article.html#ser
You can achieve this in either of two ways,
AlarmManager is android's in-buite class that allows you to execute certain action on particular time peroid.
TimerTask does same thing as AlarmManager, you can repeat certain action of your code again and again.
However AlarmManager is ligher in the execution so i suggest you to go with AlarmManager class.
Create an AlarmManager that fetches the GPS Co-ordinates and post them to server on regular interval basis.
Have a look at to this AlarmManager Example.