What unit of measurement is rxwaveform from the GEDI L1B dataset in?
Geographic Information Systems Asked by Bwoods on March 25, 2021
Rxwaveform is listed in GEDI’s data level product L1B’s data dictionary as “The corrected receive (RX) waveforms. Use rx_sample_count and rx_sample_start_index to identify the location of each waveform.
” This can also be seen in the attached image. I understand that waveforms are typically measured by amplitude as a function of time(or height), but are the values listed in rxwaveforms amplitudes? It doesn’t explicitly say amplitude in the data dictionary, and what unit of measurement is amplitude in (if it is indeed amplitude). The second image is the data values of rxwaveform in matlab; and what is the significance of rx_sample_count in relation to rxwaveform?
One Answer
AFAIK rxwaveform is a count from the ADC (analog to digital converter). This is, the GEDI must have an analog sensor which measures the returning photons as some signal, but it seems that it is not calibrated to any form of known measurable energy, instead it is just transforming the analog signal to some digital count.
As rxwaveform is variable (the higher above ground artifacts there are more information in the rxwaveform for the footprint) you need to use rx_sample_start_index and rx_sample_count to know where the data is.
For example, say we have this data:
| shot_number | rx_sample_start_index | rx_sample_count |
|---|---|---|
| 19640119100108615 | 1 | 760 |
| 19640119300108616 | 761 | 755 |
| 19640119500108617 | 1516 | 760 |
| 19640119700108618 | 2276 | 765 |
| 19640119900108619 | 3041 | 759 |
| 19640120100108620 | 3800 | 759 |
| 19640120300108621 | 4559 | 791 |
| 19640120500108622 | 5350 | 759 |
| 19640120700108623 | 6109 | 763 |
| 19640120900108624 | 6872 | 776 |
| 19640121100108625 | 7648 | 823 |
| 19640121300108626 | 8471 | 767 |
| 19640121500108627 | 9238 | 778 |
| 19640121700108628 | 10016 | 762 |
| 19640121900108629 | 10778 | 776 |
| 19640122100108630 | 11554 | 777 |
Then if I want to retrieve the waveform for the shot_number 19640120100108620 (6th row), I would need to slice the rx_waveform starting from rx_sample_start_index with rx_sample_count elements, so from 3800 to (3800 + 759).
Answered by caiohamamura on March 25, 2021
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