My Employer Requires Me To Install A Tracking App On My Personal Phone

Location: California. I work as a field technician. Our company has always used a paper sign-in sheet at the depot and a separate work-order system for service visits. This week management told us to install a new app on our personal phones. It asks for location access set to "always" and says it can collect data in the background. The presentation said the purpose is to confirm arrival at customer sites, which makes sense during a shift. I asked whether I could turn off location access when I clock out, and the trainer said the app has to remain active so it can sync overnight. She did not explain why a work visit needs my location when I am at home. We were told that anyone who refuses to install it will be taken off the schedule next week.

I have not installed it yet. I asked for the privacy policy and a written explanation of what data is retained and who can see it. HR sent me a link to the app developer's general policy, which covers several products and does not clearly describe our company's settings. We have company tablets in two vehicles, so there may be another way to check in, but management has not offered one. I don't want to lose shifts because I asked a question, and I also dont want to hand over round-the-clock location access without understanding what is collected or how long it stays there. Are there California rules that address an employer's use of tracking software on a worker's own phone? If the company can require the app during work hours, can it also require "always" access after work? What should I ask HR to put in writing before deciding how to respond?