I got a SWARM M138 Modem kit from Sparkfun and let to share my experience before you decide to buy yours too.
First thing you need to know: it is not an always ON communication channel. Actually the interval between the satellites passing over your head is around 3 hours. You can use the SWARM checker to see the right moment that the satellites will pass: https://kube.tools.swarm.space/pass-checker/
So it is not a device for realtime tracking like SPOT Gen from Globalstar, but it is more affordable and could be used as a backup for emergency applications.
Fortunately Sparkfun created a nice Swarm M138 GUI to let you to test the modem just clicking in some buttons.
After connecting the modem you could see some booting messages:
[0m Swarm MPCIE (MPCIE) Bootloader Copyright (c) 2019-22 Swarm Technologies, Inc Version: 2022-05-16T21:46:13,v2.0.0 ...... Checking image MD5 in STM32 flash... valid Identifying SPI flash... FOUND (Mfr 0x9d ISSI Type 0x6017) Checking image MD5 in SPI flash... INVALID Transferring control to application... [0m$M138 BOOT,POWERON,Swarm M138 (M138)*44 $M138 BOOT,POWERON,Copyright (c) 2019-22 Swarm Technologies, Inc*2d $M138 BOOT,POWERON,LPWR=n,WWDG=n,IWDG=Y,SFT=n,BOR=n,PIN=Y,OBL=n*43 $M138 BOOT,VERSION,2022-05-16T21:45:08,v2.0.2*2f $M138 BOOT,DEVICEID,DI=0x003xxx*2c $M138 BOOT,RUNNING*2a [0m Swarm MPCIE (MPCIE) Bootloader Copyright (c) 2019-22 Swarm Technologies, Inc Version: 2022-05-16T21:46:13,v2.0.0 ...... Checking image MD5 in STM32 flash... valid Identifying SPI flash... FOUND (Mfr 0x9d ISSI Type 0x6017) Checking image MD5 in SPI flash... INVALID Transferring control to application...
Let’s to send a message:
$TD "Hello World!"*31 $TD OK,4611393290264*28 $TD SENT,RSSI=-105,SNR=-5,FDEV=2255,4611393290264*75