Changes between Initial Version and Version 1 of Sensors/LeicaLIDAR/FWMikesNotes/FWGainDiscussion

Apr 10, 2013 7:09:26 PM (7 years ago)



  • Sensors/LeicaLIDAR/FWMikesNotes/FWGainDiscussion

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     1This page documents some discussions with Leica summarising how the AGC alters the values stored and how this can be reversed (to some extent).  Source is an email from 11 Aug 2010.
     4(PML email) Thanks for your emails, I think between your replies I have now a clear picture of the situation. Let me know if this is correct:
     5 * ''Leica comments inline in italics''
     7For each return the system digitises and records:
     8 1. The voltage applied to the AGC gain circuit (0-255, stored in Flight Line id field).  The gain circuit then produces an AGC-scaled voltage that is fed directly to the discrete and full waveform subsystems.
     9 2. The AGC-scaled output voltage to the discrete system (0-255, stored in discrete intensity fields)
     10 3. the AGC-scaled output voltage to the FWD system (0-255, stored in full waveform amplitude packets)
     11    * ''Leica: all correct''
     13As we understand it, the two voltages digitised in 2 & 3 should be identical, i.e. we should theoretically see a full waveform amplitude
     14that matches the corresponding discrete return intensity.  Differences in sampling time presumably mean this doesn't align exactly, but the two
     15voltages are otherwise the same?
     16 * ''Leica: not exactly.  The discrete intensity digitizer has an offset of about 110mv meaning that a digitized value of 0 has a voltage of 110mv.  You would need to do a calibration comparing the peak of the full waveform to the discrete digitized intensities to determining the exact function to do this conversion.''
     18There is also a "digitizer gain" stored in the "Waveform Packet Descriptor" that the spec says can be used to go back from waveform amplitude to a voltage value by applying: VOLTS= OFFSET + GAIN * Raw_Waveform_Amplitude.  However, there is only ever one value for this, so the voltage retrieved is just the AGC-scaled output?  Is this digitiser gain a fixed value or measured in any way? And finally,is this
     19value also used in the discrete system?
     20 * ''Leica:  The digitizer is similar to an oscilloscope.  You can choose the x and y scales and offsets.  The scale and offset are chosen to maximize the usable scale. The typical range of voltage being digitized by the FWD are 0 - 2.5 volts. The most appropriate scale that the digitizer supports of 0-4.3 volts = 0-255 counts.  Therefore the scale for the waveform is 0.0179 volts per bit.  This is a fixed value.  This is not taking into account any scaling done by the AGC.  Applying this conversion will get you the post gain signal voltage.  To remove the scaling done by the AGC you still need to apply the GBIC correction.''
     22The intensity values stored (discrete and Raw Waveform Amplitude values) need to be corrected to provide relative values. This is done with the
     23GBIC (Gain Based Intensity Correction) approach, i.e. the Excel file you gave us.  While GBIC was designed for discrete data, it should work for
     24waveform data too?
     25 * ''Leica:  correct''
     27The GBIC rectifies the signal so that different shots have comparable intensity values. This procedure uses the digitised AGC gain voltage as
     28an entry to a look up table to look for the amplification factor (AF).
     29 * ''Leica:  correct''
     31The result of the GBIC are relative intensity values that can be compared with one another. If we want absolute values to relate directly
     32to ground measurements we would need to do a calibration, and this is the current research of many organisations. PG:  correct, there are several papers on the internet on this topic.  The GBIC correction only corrects for one of many factors that affect the return intensity.
     34Is the GBIC approach your final solution for the intensity or are you able to produce an equation to calculate the amplification factor (AF)
     35from the voltage applied to the AGC?
     36 * ''Leica:  we do not plan to change the lookup table other than possibly the format of the table.''
     38Many thanks and best regards,
     40Ana (PML)