Raw data access and appropriate API

[EmmanR]

Good afternoon, 

I am looking for a watch model that would be used in a group of children to monitor their activity during rehabilitation in a research project. We have several requirements : 

- I will need to develop an appropriate app to collect the data without need of a personal phone.

- We need to collect raw cardiac frequency (PP interval to analyze the data for our own HRV indices), raw accelerometer data, and distance and gait velocity. 

I have then 3 questions : 

- which API (device or Web) would you recommend, and which Fitbit model enables the access to the raw data ?

- what do you suggest us to properly gather our data ? 

- how are distances computed in Fitbit apps (GPS or accelerometers) depending on the chosen activity mode ?

I already searched in the API and this forum but I could not find my answers. 

Thank you very much for your advice !

[Gondwana]

There are no Fitbit models that meet your requirements. All Fitbit watches need to the paired with a separate phone or tablet. PP interval isn't provided; only sampled heart rate readings every few seconds. The Device API can provide raw accelerometer data, whereas the Web API does not. However, getting sensor data from the Device to an external app is very difficult (and requires phones).