From 7c4b59c5ed34ace9e3a4afd6b52c48a396d3c6f7 Mon Sep 17 00:00:00 2001 From: wl-estensd <142801766+wl-estensd@users.noreply.github.com> Date: Wed, 26 Aug 2026 08:58:08 +0200 Subject: [PATCH 1/2] Update modem-m16-faq.md Updated ranging FAQ --- docs/modem-m16/modem-m16-faq.md | 25 ++++++++++++++++++++++--- 1 file changed, 22 insertions(+), 3 deletions(-) diff --git a/docs/modem-m16/modem-m16-faq.md b/docs/modem-m16/modem-m16-faq.md index cd9e9a34..427e3f3c 100644 --- a/docs/modem-m16/modem-m16-faq.md +++ b/docs/modem-m16/modem-m16-faq.md @@ -6,9 +6,28 @@ Below are some frequently asked questions about the Modem M16. Click on a questi ## 1. Can the M16 modem measure the range/distance between two modems? -No, the modem's uses an unsynchronized method of sending data meaning that an time of flight estimate would give large errors in the range estimations. The acoustic signal is not synced but known to be within some time frame of 25ms. - -However if looking at the [Signal to noise ratio (SNR)](../modem-m16/modem-m16-diagnostic-report.md) you can use it to indicate that the distance between the modems is fare or close. For example would a grater SNR (better signal) indicate that it is most likely close as it has so good signal. +No. The modems are unsynchronized and the M16 is not designed for time-of-flight +ranging. Range estimates based on its timing will have large errors, for two +reasons: + +- **Transmit side:** The exact time of the acoustic transmission is not + available on the interface. The TX_COMPLETE flag is raised within + approximately 25 ms after the acoustic transmission, but the exact delay + varies. +- **Receive side:** The modem does not use a sync word. The receiver + continuously decodes the incoming signal, and the decoding is robust enough + to recover a message from as little as half of the acoustic waveform. The + point where a packet is registered as received is therefore not fixed + relative to the acoustic arrival, and can vary by up to roughly half the + duration of the acoustic transmission. + +In water, these timing uncertainties correspond to path errors from tens of +metres up to several hundred metres, which makes time-of-flight ranging +impractical. + +However, the [Signal to noise ratio (SNR)](../modem-m16/modem-m16-diagnostic-report.md) +can give a rough indication of whether the other modem is near or far: a higher +SNR generally means the modems are closer to each other. --- From 796dc1c698028a0ae52e8d8d9f1346902dd34201 Mon Sep 17 00:00:00 2001 From: wl-estensd <142801766+wl-estensd@users.noreply.github.com> Date: Wed, 26 Aug 2026 09:19:43 +0200 Subject: [PATCH 2/2] Update modem-m16-faq.md update --- docs/modem-m16/modem-m16-faq.md | 10 ++++------ 1 file changed, 4 insertions(+), 6 deletions(-) diff --git a/docs/modem-m16/modem-m16-faq.md b/docs/modem-m16/modem-m16-faq.md index 427e3f3c..be4701f1 100644 --- a/docs/modem-m16/modem-m16-faq.md +++ b/docs/modem-m16/modem-m16-faq.md @@ -14,12 +14,10 @@ reasons: available on the interface. The TX_COMPLETE flag is raised within approximately 25 ms after the acoustic transmission, but the exact delay varies. -- **Receive side:** The modem does not use a sync word. The receiver - continuously decodes the incoming signal, and the decoding is robust enough - to recover a message from as little as half of the acoustic waveform. The - point where a packet is registered as received is therefore not fixed - relative to the acoustic arrival, and can vary by up to roughly half the - duration of the acoustic transmission. +- **Receive side:** The modem decodes packets continuously and does not use a sync word. The +  point where a packet is registered as received is therefore not fixed +  relative to the acoustic arrival, and can vary by up to roughly half the +  duration of the acoustic transmission. In water, these timing uncertainties correspond to path errors from tens of metres up to several hundred metres, which makes time-of-flight ranging