This 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.
(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:
- Leica comments inline in italics
For each return the system digitises and records:
- 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.
- The AGC-scaled output voltage to the discrete system (0-255, stored in discrete intensity fields)
- The AGC-scaled output voltage to the FWD system (0-255, stored in full waveform amplitude packets)
- Leica: all correct
As we understand it, the two voltages digitised in 2 & 3 should be identical, i.e. we should theoretically see a full waveform amplitude that matches the corresponding discrete return intensity. Differences in sampling time presumably mean this doesn't align exactly, but the two voltages are otherwise the same?
- 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.
There 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 value also used in the discrete system?
- 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.
The intensity values stored (discrete and Raw Waveform Amplitude values) need to be corrected to provide relative values. This is done with the GBIC (Gain Based Intensity Correction) approach, i.e. the Excel file you gave us. While GBIC was designed for discrete data, it should work for waveform data too?
- Leica: correct
The GBIC rectifies the signal so that different shots have comparable intensity values. This procedure uses the digitised AGC gain voltage as an entry to a look up table to look for the amplification factor (AF).
- Leica: correct
The result of the GBIC are relative intensity values that can be compared with one another. If we want absolute values to relate directly to 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.
Is the GBIC approach your final solution for the intensity or are you able to produce an equation to calculate the amplification factor (AF) from the voltage applied to the AGC?
- Leica: we do not plan to change the lookup table other than possibly the format of the table.
Many thanks and best regards,
Ana (PML)