If indicator shows OFF even under the open sky, please check that iPhone's (or cellular iPad) Settings>Privacy>Location services are ON. Then inside location services check that taximeter app has its location switch in "Always" or "When In Use" positions.
In rare cases, to get GPS receiver working properly, it is required to restart the iPhone (or cellular iPad) completely by powering it down (long hold on the power button and confirm the switch off) and then on again (by pressing the power button). Then please try again under the open sky.
Please note that iPods and wifi only iPads don't have GPS chip built in and would require external GPS module for the taximeter (or any GPS app) to function properly. There are many, you can find a comparative review of two available modules here.