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According to the BLE protocol, when a device is advertising, three channels are used to minimize interference and a channel hopping strategy is put in place.

Probably due to my poor knowledge of radio protocols, I am unable to clearly see if an advertise is transmitted
a) on only one of the three channels at any given time;
b) on at least one of the three channels, meaning the same advertise could be picked up twice by a listener.

From my understanding, the observer device only listens on one of the three frequencies at any given moment.

Could anybody help me how with more insight on this matter?

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The BLE advertisement uses three channels to avoid interference as explained in this answer. There is no 'channel hopping' as such in advertising, it is used when the BLE device is connected.

With regards to the channels usage when advertising, it would depend on the BLE stack used and the configuration. For example, the nRF5x Softdevice stack has option to select which of the three channels would be used for advertisements. The default (or examples) option that I have seen is having all the three channels enabled. Which would mean for every advertisement event, the advertisement packet would be sent on the three channels one after the other. Having all three enabled maximizes the chances that an observer listening on one of these three picks it up.

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  • \$\begingroup\$ Thanks! This explains why I seldom see the same advertise being picked up twice by the same receiver, some times. \$\endgroup\$
    – Daemon Painter
    Apr 20, 2020 at 7:10

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