I would separate the project in several phases with a step by step approach:
Phase 1) Feasibility study and prototyping:
Core service must be based on a SW layer which is able retrieve and store data related to users. I would prefer modular design based on Servlet containers (i.e.: Tomcat) with http rest API layer + modules for data access.
Database technology to be defined for data storage on a separate tier
Client application able to communicate via http rest with the core service
Once a PoC will be up and running the next point will be to define the ID domain and how all users should register and authenticate to the system. This topic must be very well defined as it has big impacts on UX.
The outline of first phase described above should rely on backends, with possibly no UI available.
Phase 2) UI and UX
Web and mobile UI to be defined for the available components and features
I would include here the development of all needed features to make the whole presentation layer completely flexible to fast branding process (i.e.: give us your logo and you have your app branded by the push of a button)
After phase 2 a first version of the product should be deployed in the white label version (i.e.: named iTrack and with standard colors) thus, if the project gains the expected audience, then all features needed for first generally available (GA) version should be defined, planned and released based on dev cycles to be defined.
Additional notes and details:
By my experience the best would be to start building a prototype based on simple android application (all smartphones have a GPS and a way to retrieve data from it is a consistent way) and a first API core base to send and receive the position sent by the app. I would use Json in the communication, but I am open to suggestions.
Further phases: due to the different applicability fields I believe once the service will be up and running for smartphones, other devices can be included for a wider audience; try to think about the GPS modules available for Arduino and Raspberry: if the software is built in the right way, access to the service can be easily extended to those devices. John Rodrigues