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Since all smart phones have antennas, and these antennas are capable of detecting and receiving many types of signals including wifi, 2g, 3g, 4g, and 5g, then "we" should be able to make apps that help us observe the strength and presense of all these signals independent of the SIM card we happen to have and whether or not our phone is in airplane mode, correct? However, the signal-detector type Android apps I see out there only work when the radio transmitter is powered on, and even then it's limited to the particular service addressed to our SiM. Am I missing some vital factoid? I feel I must be. Something is not adding up for me.

I do have a dedicated EMF detector "Electro-smog Meter". It doesn't have transmitters yet it's antennae seem to work just fine.

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  • You can for bluetooth and wifi. For others, the OS has to allow it. I dont know if it does. Jul 16, 2023 at 20:17

2 Answers 2

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Here is my output in /dev:

RMX1911:/ $ ls /dev
ls: /dev/event-log-tags: Permission denied
ls: /dev/.coldboot_done: Permission denied
ls: /dev/cgroup_info: Permission denied
__properties__     gpiochip1            memory_bandwidth   pts                     smd8            v4l-subdev17
adsprpc-smd        gpiochip2            msm-rng            qce                     smdcntl8        v4l-subdev18
adsprpc-smd-secure gpiochip3            msm_aac            qg                      snd             v4l-subdev19
apr_apps2          graphics             msm_aac_in         qg_battery              socket          v4l-subdev2
ashmem             hw_random            msm_alac           qseecom                 stderr          v4l-subdev20
at_mdm0            hwbinder             msm_amrnb          ramdump_a610_zap        stdin           v4l-subdev21
at_usb0            hypnus               msm_amrnb_in       ramdump_adsp            stdout          v4l-subdev22
at_usb1            i2c-0                msm_amrwb          ramdump_cdsp            stune           v4l-subdev23
at_usb2            icesdcc              msm_amrwb_in       ramdump_ipa_fws         subsys_a610_zap v4l-subdev24
avtimer            iceufs               msm_amrwbplus      ramdump_md_a610_zap     subsys_adsp     v4l-subdev25
binder             iio:device0          msm_ape            ramdump_md_adsp         subsys_cdsp     v4l-subdev26
blkio              iio:device1          msm_audio_cal      ramdump_md_cdsp         subsys_ipa_fws  v4l-subdev3
block              input                msm_evrc           ramdump_md_ipa_fws      subsys_modem    v4l-subdev4
btpower            ion                  msm_evrc_in        ramdump_md_modem        subsys_venus    v4l-subdev5
ccid_bulk          ipa                  msm_g711alaw       ramdump_md_venus        tty             v4l-subdev6
ccid_ctrl          ipaIpv6CTTable       msm_g711alaw_in    ramdump_memshare_DIAG   ttyHS0          v4l-subdev7
cg2_bpf            ipaNatTable          msm_g711mlaw       ramdump_memshare_FTM    tun             v4l-subdev8
console            ipa_adpl             msm_g711mlaw_in    ramdump_memshare_GPS    uhid            v4l-subdev9
cpu_dma_latency    ipa_odl_ctl          msm_hdcp           ramdump_microdump_modem uinput          vga_arbiter
cpu_variant:arm    ipa_tethering_bridge msm_hweffects      ramdump_modem           uio0            video0
cpu_variant:arm64  jpeg0                msm_mp3            ramdump_venus           uio1            video1
cpuctl             kgsl-3d0             msm_multi_aac      ramdump_wcss_msa0       urandom         video2
cpuset             kmsg                 msm_qcelp          random                  usb-ffs         video3
dcc_sram           loop-control         msm_qcelp_in       rfkill                  usb_accessory   video32
device-mapper      mdmreason            msm_rtac           rmnet_ctrl              usf1            video33
dpl_ctrl           mdmrst               msm_sps            rpmsg_ctrl0             v4l-subdev0     video4
dri                media0               msm_wma            rpmsg_ctrl1             v4l-subdev1     video5
ecryptfs           media1               msm_wmapro         rpmsg_ctrl2             v4l-subdev10    video6
fd                 media2               mtp_usb            rpmsg_ctrl3             v4l-subdev11    video7
freezer            media3               network_latency    rtc0                    v4l-subdev12    vndbinder
fscklogs           media4               network_throughput sensors                 v4l-subdev13    wlan
full               media5               null               smcinvoke               v4l-subdev14    wwan_ioctl
fuse               memcg                ppp                smd11                   v4l-subdev15    zero
gpiochip0          memdev               ptmx               smd7                    v4l-subdev16

Based on the provided list, the following entries are related to the modem cellular:

  • at_mdm0: This entry typically represents the modem device in Android systems. It is commonly used for communication with the modem for tasks such as AT commands.
  • ipa: This entry refers to the Internet Protocol Accelerator (IPA), which is responsible for handling various network-related tasks, including cellular connectivity.
  • mdmreason: This entry is associated with the modem and is used for managing modem reset and initialization operations.
  • mdmrst: This entry is also related to the modem and is used for modem reset functionality.

These entries suggest the presence and functionality of the modem cellular in the device.

Rooting a phone can grant greater access to system files and settings, potentially allowing you to access modem devices and perform actions like reading signal strength without a SIM card. However, the ability to interact with modem devices after rooting depends on the specific device, operating system version, and modifications made during the rooting process. Tools like Android Debug Bridge (ADB) commands or modem-specific utilities can be used to retrieve signal strength information, but caution is necessary as accessing and modifying modem devices can lead to instability, loss of functionality, or even damage to the device. Additionally, the availability of signal strength information without a SIM card may vary depending on the device's hardware and software capabilities. Thorough research, consulting reliable sources, and proceeding with caution are recommended before attempting any modifications or accessing modem devices.

Based on most voted answer in this (https://stackoverflow.com/questions/12855482/sending-at-commands-via-adb-android), you can perform AT command from given modem device in /dev directory, in my phone case is at_mdm0

echo -e "AT\r\n" > /dev/at_mdm0

Now, try AT+CSQ to check siqnal quality with modem without SIM card

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  • This does not answer the question. Please edit it provide an answer. Jul 17, 2023 at 7:42
  • I can see I am getting in over my head quickly, but I hear you saying it is possible to check signals quality without a SIM card as long as you have enough permissions. It sounds like older scrap phones could be retro-fitted with a custom Android OS that was focused on utilizing the hardware merely for signal analysis / metering. Does that sound true?
    – hoatzin
    Jul 17, 2023 at 10:47
  • My original question was basically, "Can we make apps" that would do this stuff? And what I am hearing is, "Yes, but those apps would have to be running on a very specialized OS that has very different permissions than a normal Android OS." Am I getting it?
    – hoatzin
    Jul 17, 2023 at 10:50
  • Your output showed "Permission Denied" because you were not rooted?
    – hoatzin
    Jul 17, 2023 at 10:54
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Smartphones do have the hardware capability to detect various signals, the ability to access and observe this information is controlled by the phone’s operating system and hardware. This is deliberate design for security reasons.

Most signal detector apps available on the Google Play Store work by accessing the phone’s built-in signal strength measurement tools. These tools are typically only active when the phone’s radio transmitter is powered on, which is why these apps may not work when the phone is in airplane mode or when the SIM card is removed.

I included some helpful links above as well as here and here May you can try to tweak a custom recovery like TWRP and include a command line functionality in it for this purpose.

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  • Three of your links point to articles about low-tech cell phone detectors. I may have been unclear in my question. My curiosity is about using smart phones especially in non-emitting (airplane) mode as signal detectors. I'm not so curious about building devices to detect cell phones.
    – hoatzin
    Jul 17, 2023 at 3:36
  • "Smartphones do have the hardware capability to detect various signals, the ability to access and observe this information is controlled by the phone’s operating system and hardware." This is what I'd like to know more about. What if an Android operating system really tapped into that information and displayed it? That would be super useful (to a certain select group of EMF connoisseurs).
    – hoatzin
    Jul 17, 2023 at 3:41
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    That is a security constraint within the Android / i)S or any other cellphone OS.- in a custom recovery you are at root and source is open. If you know how to write API calls to the hardware you can write a command line Util that displays this but once the full OS boots up - forget about getting this info. Jul 17, 2023 at 6:36

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