The link you provided answers that question:
"The built in high sensitivity GPS receiver is able to lock multiple satellites from within multiple buildings or from a window location, eliminating the requirement that an outdoor antenna be installed".  If you're still worried about your specific use-case, I recommend contacting the manufacturer. 


On Wed, May 1, 2019 at 2:25 PM Mehmet Akcin <mehmet@akcin.net> wrote:
hey there Nanog,

I am trying to buy a GPS based NTP server like this one 

https://timemachinescorp.com/product/gps-time-server-tm1000a/

but I will be placing this inside a data center, do these need an actual view of a sky to be able to get signal or will they work fine inside a data center building? if you have any other hardware requirements to be able to provide stable time service for hundreds of customers, please let me know.

mehmet