Haar measure on $p$-adic unit circle

Question

I am trying to wrap my head around the haar measure on the unit circle in $\mathbb{Q}_p$ for $p< \infty $, that is the haar measure in $T_p = \{x \in \mathbb{Q}_p: |x|_p =1 \}$. Can anyone refer me to a resource about this?

Answer 1

$$\Bbb{Z}_p^\times = \bigcup_{a=1,p\ \nmid\ a}^{p^n} a+p^n\Bbb{Z}_p,\qquad \mu( a+ p^n \Bbb{Z}_p)=\frac{1}{p^{n-1}(p-1)}$$