identify process sending a signal to a shell script

I've written a shell script that uses the trap feature to output its progress when it receives a SIGUSR1 signal, but I don't see how to have the script be able to output the progress to the STDOUT of the process which sent the signal. So, if the script is running in terminal emulator /dev/pts/10 and I send the process a signal from a terminal emulator on /dev/pts/11, the output is going to /dev/pts/10, but I want to send to /dev/pts/11. I do NOT want to have to resort sending the progress message via wall or hard-coded shell redirection to a specific STDOUT.