The SAA-2N is a knockoff/clone of the NanoVNA V2 Plus and you should NOT buy one.
Order an official one from https://nanorfe.com/nanovna-v2.html
However, if one for some reason falls into your hands, it might be unobvious how to find a firmware that compiles for it. I'm not sure if there are mutliple sub-versions of the SAA-2N but this one has a 4" screen.
sudo apt install gcc-arm-none-eabi python3-serial
git clone -recursive https://github.com/nanovna-v2/NanoVNA2-firmware
cd NanoVNA2-firmware/
git checkout d9c768b298677d2fecc98fb37ec9cd4395dc781e # optional. latest version as of this writing
Now uncomment the following line in Makefile
:
CPPFLAGS += -DDISPLAY_ST7796
and then:
make -j4 BOARDNAME=board_v2_plus EXTRA_CFLAGS="-DSWEEP_POINTS_MAX=201 -DSAVEAREA_MAX=7" LDSCRIPT=./gd32f303cc_with_bootloader.ld
git checkout -- gd32f303cc_with_bootloader.ld
make -j4 BOARDNAME=board_v2_plus EXTRA_CFLAGS="-DSWEEP_POINTS_MAX=201 -DSAVEAREA_MAX=7" LDSCRIPT=./gd32f303cc_with_bootloader.ld
# yes you have to repeat that make command
Now hold the left-most button while turning on the VNA and then
python bootload_firmware.py -f ./binary.bin