# Combining Approximate and Exact Time Synchronization Policies with message_filters

Is there a way to use both Approximate and Exact Time Synchronization policies together in a node?

Specifically, I have 3 topics - let's call them `/a`

, `/b`

and `/c`

, carrying messages of types `A`

, `B`

and `C`

respectively. I want `/a`

, `/b`

to be synchronized using `message_filters::sync_policies::ApproximateTime<A, B>`

policy. And I want `/b`

and `/c`

to be synchronized using `message_filters::TimeSynchronizer<B, C>`

. Basically, I expect `A`

and `B`

type messages to have approximately the same timestamps, but I know that `B`

and `C`

will definitely use the same timestamps, and I want to be certain that I'm accidentally not using `C`

whose timestamp is not matching with `B`

's. Is there a way to set this up with `message_filters`

, or another way to solve this problem? Any help is appreciated! Thank you.