Dear Christian Mätzler,

First, thank you very much for your in-depth comments. We have adressed or are adressing them. I would like to try the interactive possibilities of this open review to adress one question/clarification. It concerns these comments :

5) No information is given on the scattering and absorption cross sections of the tags used, nor of the supporting structures.

6) No information is given on the method used to discriminate the responses and the backscattered signals from different tags, and how this discrimination may be linked with the phase determination.

Mainly, I do not see the motivation behind your suggestion to characterize the Radar Cross Section of the tags and supporting structure (comment 5). I think comes from a misunderstanding of the difference between a radar target and an RFID tag (comment 6), that we have clarified in the text. The answer to comment 6 is straightforward: one tag at a time is requested by the interrogator (using standard the communication & anti-collision protocol EPC Gen2) to modulate the signal it backscatters, while all the other tags and the environment backscatter a non-modulated wave (as it would do with a standard radar). The phase difference of arrival is measured between the two modulation states of the received signal, therefore linked only to the singulated tag.

Indeed in RFID, an important parameter is the DeltaRCS of the tags, which is the difference of the tag's radar cross-section between these two modulation state. We have characterized the DeltaRCS for the model of tag used (depending on frequency and input power). I can add this characterization in the 'instrumentation' section if you find it useful.

In the end,knowing how the response of each tag is discriminated, we do not see the reason for characterizing the cross-section of the tags and supporting structure (which would make sense however with a radar). Can you confirm me, or otherwise explain the motivation behind the comment 5 ?



Sincerely yours,

Mathieu Le Breton, on behalf of all the coauthors.

Dear referee, first, thank you very much for your review, we appreciate your work.

Please find below some answers, and a  question (your original text is in bold)



In short, your comments showed that we need to clarify our text, to avoid any misunderstanding (in particular, the fact that we used only commercial off-the-shelf readers and tags, without customisation), which we will do.

1) the authors claim to use standard RFID, that this is not really true a standard RFID do not be equipped with sensors, you must design a customised tag. The same for the battery you can't connect a battery to a standard tag. You should provide more information related to the Considered RFID schema.

We will clarify this point. In particular, what is 'standard' is the communication protocol (EPC-Gen2) and the RF channel (ETSI-302-201).  Then we used 'commercial off-the-shelf' devices, both tags and reader. We have modified the tag (UHF tag, model Survivor B, from Confidex, that embeds an EM4325 chip from EM electronic, an a small battery) ony superficially: first, we configured its memory bank wirelessely, in order to make temperature measurements, and to increase its power sensitivity. Second, we painted the tag in white to reduce the radiative heat transfer.

So we bought an industrial tag, but we have not designed a customised tag.

2) you perform a measure of phase difference but it is not clear how. If you used inductive coupling rfid it is quite difficult. I suppose that you used a RF tag, and you claim that the reader is able to detect the phase difference. Can you please better explain how? You have to analyse the signals at the demodulator  in order to detect the phase difference of did you use another technique? Please explain.

We used an UHF RFID (working around 868 MHz). The reader (Impinj R700) has the capacity to read phase difference of arrival when reading a tag, out-of-the-box. Several other readers have this capacity, such as the ImpinjSR420 or the ThingMagic M6. Measurements of phase difference of arrival is largely used for RFID localization, see for example the introduction of this review :

Le Breton M, Liébault F, Baillet L, Charléty A, Larose É, Tedjini S. Dense and long-term monitoring of earth surface processes with passive RFID — a review. Earth-Science Reviews. 2022 Nov 1;234:104225.   



We do not know, nor need to know, how the Impinj R700's electronic is designed in order to read the phase. For clarification, we will add a reference to research papers describing techniques to read the phase from a reader.

3) did you take into account the attenuation introduced in the RF signal  by the snow? 

Could you please clarify ? For which aim should we take the attenuation into account ?

There are several sources of signal loss, such as, indeed, attenuation in the snow, but also detuning of the tag, reflexion at the surface, or destructive multipathing interferences. We have measured the signal strength data for each tag, but the data is messy: using signal strength is often a difficult method to exploit for RFID sensing outdoors because of its many influence factors (see again the review Le Breton et al. 2022). Indirectly, the signal strenght received by the tag can slightly influence the phase (and therefore our SWE measurement), when the tag enters non-linear behavior (for example when it receives very powerful signal). The non-linear behavior depends strongly on model of chip. With our tag, non-linearity occurs only at high power that are not reached in the study. And in any case, that would be a small source of noise.

In short: The present study exploits only the phase difference of arrival (related to wave slowness), not the signal strength (related to attenuation).

4) I suppose that you modified the reader, don't you? If yes please report the introduced customisations.

No, we have used a commercial off-the-shelf reader, not customised. The method works with any reader that is able to read the phase difference of arrival.