In areas of sky scrapers there is a phenomenon called "reflection", where the signal from satellites reflects through the buildings and may cause the location to be off.
We cannot eliminate the GPS errors, but we can indicate to you that accuracy is limited. Our analytics will also filter out many of the false exits from indoors locations due to GPS reception issues.
We have evaluated many devices, picked the best one and on top of it built sophisticated analytics that build the daily schedule and create reliable alerts.
Still, in downtown areas the solution will have accuracy limitations. What this means is that if your child is lost downtown, the GPS may have more significant errors. It is the limitations of the core GPS technology, regardless of AngelSense.