I'm having trouble getting GPS data from my modem consistently. Normally I can get the GPS in Python with the following code:
import time
from serial import Serial
# If a "port" is given, then the port will be opened immediately.
ser = Serial(port="/dev/ttyUSB3", timeout=2, write_timeout=2)
# The following prints as "True"
print(ser.is_open)
def read_until_timeout(ser, timeout=5):
"""Turn GPS on and read until either we find the line we want,
or the timeout occurs
"""
# Turn GPS on
ser.write(b"AT+CGPS=1,1\r\n")
# Request GPS
ser.write(b"AT+CGPSINFO=1\r\n")
# Start the loop
time_start = time.time()
while ((time.time() - time_start) < timeout):
print("Trying ser.readline().decode('ascii', errors='replace')...")
try:
# Read from the serial port
line = ser.readline().decode('ascii', errors='replace')
except serial.serialutil.SerialException as err:
print(f"Problem reading data: {err}")
if "+CGPSINFO:" in line:
print(f"Found '+CGPSINFO:' in line: {line}")
break
On a working gateway, that last print statement would return a line like the following, which contains the GPS latitude and longitude:
print(read_until_timeout(ser, timeout=5))
>>> '+CGPSINFO: 5100.506298,N,11404.432885,W,031120,234519.0,1080.1,0.0,204.6\r\n'
Sometimes the output from running this function is as follows, which I think just means the GPS can't get a "fix":
print(read_until_timeout(ser, timeout=5))
>>> Found '+CGPSINFO:' in line: +CGPSINFO: ,,,,,,,,
But sometimes it says it's "busy" as follows:
print(read_until_timeout(ser, timeout=5))
>>> Problem reading data: read failed: device reports readiness to read but returned no data (device disconnected or multiple access on port?)
Is the above because more than two applications can't access the serial port at the same time?
If I try sending AT messages to the modem from Linux Bash, it says the modem is "busy"...
$ echo 'AT+CGPSINFO=1' | socat - /dev/ttyUSB3,crnl
>>> 2020/11/20 19:18:24 socat[3251] E open("/dev/ttyUSB3", 02002, 0666): Device or resource busy
So I check which devices are using the modem:
$ sudo lsof /dev/ttyUSB3
>>> COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
ModemMana 800 root 12u CHR 188,3 0t0 449 /dev/ttyUSB3
python 1706 root 3u CHR 188,3 0t0 449 /dev/ttyUSB3
But if I shut down ModemManager with sudo kill 800 the cellular internet connection dies, and I can't have that because these are remote IoT devices far in the wilderness (I have about 50 of them in operation around North America).
Is there a way to share the modem with ModemManager so I can access the GPS results from Python or from the shell, while on a cellular internet connection?
This question is related so I put a bounty on it too: https://superuser.com/questions/1455327/trying-to-read-a-serial-port-device-or-resource-busy
Update Nov 22, 2020:
@GitFront helpfully suggested using ModemManager directly. Great suggestion. Neat trick with the gi library too.
From Ubuntu Bash, here's what I get when I try to get the GPS from ModemManager directly:
user@user-Default-string:~$ mmcli -m 0 --location-status
/org/freedesktop/ModemManager1/Modem/0
----------------------------
Location | capabilities: '3gpp-lac-ci'
| enabled: '3gpp-lac-ci'
| signals: 'no'
Notice under location capabilities, no gps-raw, gps-nmea, agps shows up for some reason, only 3gpp-lac-ci, but I know this gateway/SIMCom 7600 modem has GPS/GNSS capabilities, and it's got a passive antenna hooked up properly.
Does anyone know why gps-raw, gps-nmea, agps are not showing up as capabilities?
The exact same gateway/modem/antenna combination in my office does have gps-raw, gps-nmea, agps capabilities, and everything works perfectly for it... The only difference is the SIM card, which should not matter. The gateway in my office reports the following capabilities, by contrast:
user@ax1000008:~$ mmcli -m 0 --location-status
------------------------
Location | capabilities: 3gpp-lac-ci, gps-raw, gps-nmea, agps
| enabled: 3gpp-lac-ci
| signals: no
------------------------
GPS | refresh rate: 30 seconds
I request to enable gps-raw, gps-nmea with:
user@ax1000008:~$ sudo mmcli -m 0 --location-enable-gps-raw --location-enable-gps-nmea
successfully setup location gathering
Then I request the GPS successfully (on the gateway/modem in my office...) with:
user@ax1000008:~$ sudo mmcli -m 0 --location-get
--------------------------
3GPP | operator code: 302
| operator name: 220
| location area code: 2BC4
| tracking area code: 0000
| cell id: 04B68931
--------------------------
GPS | nmea:
| $GPGGA,183553.00,5100.511216,N,11404.429721,W,1,06,1.7,1078.0,M,-16.0,M,,*6C
| $GPRMC,183552.00,A,5100.511138,N,11404.429729,W,0.0,341.3,221120,15.4,E,A*10
| $GPGSA,A,2,01,03,21,22,31,32,,,,,,,2.0,1.7,0.9*3C
| $GPGSV,4,1,15,01,45,295,34,03,11,291,29,10,31,109,20,11,,,36*4F
| $GPGSV,4,2,15,12,08,056,19,21,53,258,35,22,33,292,32,23,02,111,15*79
| $GPGSV,4,3,15,25,06,092,22,31,33,163,23,32,70,066,33,08,02,236,*7C
| $GPGSV,4,4,15,17,04,344,,20,,,,24,,,*48
| $GPVTG,341.3,T,325.9,M,0.0,N,0.0,K,A*2B
| $PQXFI,183553.0,5100.511216,N,11404.429721,W,1078.0,9.61,5.64,0.13*4E
| utc: 183553.00
| longitude: -114.073829
| latitude: 51.008520
| altitude: 1078.000000
On the non-working gateway in the field, here's what I get when I try to enable gps-raw and gps-nmea:
user@user-Default-string:~$ sudo mmcli -m 0 --location-enable-gps-raw --location-enable-gps-nmea
error: couldn't setup location gathering: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.Unsupported: Cannot enable unsupported location sources: 'gps-raw, gps-nmea''
from Can two IoT applications access a cellular modem at the same time so I don't have to kill ModemManager and lose my internet connection?
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