| 3 | | == Concepts and Hardware == |
| 4 | | |
| 5 | | The full waveform digitiser (FWD) is supplied as an upgrade to the existing ALS50v2 (and up) sensors, and essentially consists of an upgraded data logger and an intensity digitiser (the FWD). The discrete point recording subsystem is unchanged. In implementation, the FWD is added as a spur after the automatic gain controller (AGC). There is a trigger from the discrete system to the FWD that tells the FWD when to start recording. |
| 6 | | {{{ |
| 7 | | +---> Full waveform digitiser -->--+ |
| 8 | | | ^ | |
| 9 | | Laser receiver -> Automatic Gain Controller -->| | |--> Data logger |
| 10 | | | | | |
| 11 | | +---> Discrete point subsystem -->--+ |
| 12 | | }}} |
| 13 | | |
| 14 | | The FWD is implemented as two sampling boards (A & B) that operate digitise the incoming intensity level and nothing else. Each digitisation board is capable of recording at up to 60kHz and interleave the sampling - hence, the maximum sampling rate is 120kHz. If the laser pulse rate is set higher than 120kHz, the system will FWD one in two pulses (i.e. 120kHz pulse rate = 120,000 FWDs/sec ; 121kHz = 605,000 FWDs/sec). The FWD boards sample at 1 or 2 nanosecond resolution (1ns is approximately 15cm of distance travelled). The laser pulse in the ALS50 system has a 4ns pulse duration (or 9ns if using <100kHz pulse rate mode), so the FWD must have a <= 2ns digitisation to avoid aliasing (Nyquist). |
| 15 | | |
| 16 | | The FWD records either 64, 128 or 256 samples. These samples are recorded from the time of the first return, with a small buffer before that (the "pre-trigger samples"). This buffer allows the capture of the lead-in to the first pulse (otherwise it would start at the first detected peak) and is specified in terms of meters to the operating software (default 5m). This will also help capture (for example) sparse tree tops prior to the first return. |
| 17 | | |
| 18 | | The intensity is digitised to an 8 bit value. This is measured after the AGC amplification has taken place and is nominally the same measurement as in the discrete system (though the discrete value is accumulated over a longer time period so will typically have a higher value). The AGC value for the matching discrete return is the value used for all the related FWD samples - i.e. the AGC value is constant for a laser pulse. |
| 19 | | |
| 20 | | Similarly, the range data (measured to 1.5cm range / 100 picoseconds) is still part of the discrete system and FWD ranges are derived by association with the first return's range. |
| 21 | | |
| 22 | | Processing note: the trigger from the discrete system appears to have a small delay (order of 7ns?), that is different for the two possible laser pulse rates and also may be dependent on temperature (unproven). This delay manifests as an offset between the discrete returns and the matching waveform peaks. At the time of writing, it is corrected by a manual / statistical approach (see below). |
| 23 | | |
| 24 | | |
| 25 | | FWD filestructure: |
| 26 | | * FWD data is recorded to the RawWFD directory, with a folder structure identical to RawLaser (flightline based) |
| 27 | | * files named WFDYYMMDD_HHMMSS_XXXXXXXXXXX{AB}.LWV |
| 28 | | * A or B indicates which board digitised the signal |
| 29 | | * always in A&B pairs (if not, there has been corruption) |
| 30 | | * are always up to 62501KB (64MB?) |
| 31 | | * the .LWV files cannot be easily linked to the .SCN (discrete files) - there is no direct linkage between LWV file 2 and SCN file 2 |
| 32 | | |
| 33 | | flightline log file says if WFD on/off |
| 34 | | |
| 35 | | |
| 36 | | |
| 37 | | == Benefits of full waveform == |
| | 3 | == Concept and benefits of full waveform == |
| | 4 | |
| | 5 | Full waveform digitisation is simply recording the variation in returned pulse intensity over time, instead of computing several discrete returns. |
| | 16 | |
| | 17 | |
| | 18 | == Hardware description == |
| | 19 | |
| | 20 | The full waveform digitiser (FWD) is supplied as an upgrade to the existing ALS50v2 (and up) sensors, and essentially consists of an upgraded data logger and an intensity digitiser (the FWD). The discrete point recording subsystem is unchanged. In implementation, the FWD is added as a spur after the automatic gain controller (AGC). There is a trigger from the discrete system to the FWD that tells the FWD when to start recording. |
| | 21 | {{{ |
| | 22 | +---> Full waveform digitiser -->--+ |
| | 23 | | ^ | |
| | 24 | Laser receiver -> Automatic Gain Controller -->| | |--> Data logger |
| | 25 | | | | |
| | 26 | +---> Discrete point subsystem -->--+ |
| | 27 | }}} |
| | 28 | |
| | 29 | The FWD is implemented as two sampling boards (A & B) that operate digitise the incoming intensity level and nothing else. Each digitisation board is capable of recording at up to 60kHz and interleave the sampling - hence, the maximum sampling rate is 120kHz. If the laser pulse rate is set higher than 120kHz, the system will FWD one in two pulses (i.e. 120kHz pulse rate = 120,000 FWDs/sec ; 121kHz = 605,000 FWDs/sec). The FWD boards sample at 1 or 2 nanosecond resolution (1ns is approximately 15cm of distance travelled). The laser pulse in the ALS50 system has a 4ns pulse duration (or 9ns if using <100kHz pulse rate mode), so the FWD must have a <= 2ns digitisation to avoid aliasing (Nyquist). |
| | 30 | |
| | 31 | The FWD records either 64, 128 or 256 samples. These samples are recorded from the time of the first return, with a small buffer before that (the "pre-trigger samples"). This buffer allows the capture of the lead-in to the first pulse (otherwise it would start at the first detected peak) and is specified in terms of meters to the operating software (default 5m). This will also help capture (for example) sparse tree tops prior to the first return. |
| | 32 | |
| | 33 | The intensity is digitised to an 8 bit value. This is measured after the AGC amplification has taken place and is nominally the same measurement as in the discrete system (though the discrete value is accumulated over a longer time period so will typically have a higher value). The AGC value for the matching discrete return is the value used for all the related FWD samples - i.e. the AGC value is constant for a laser pulse. |
| | 34 | |
| | 35 | Similarly, the range data (measured to 1.5cm range / 100 picoseconds) is still part of the discrete system and FWD ranges are derived by association with the first return's range. |
| | 36 | |
| | 37 | Processing note: the trigger from the discrete system appears to have a small delay (order of 7ns?), that is different for the two possible laser pulse rates and also may be dependent on temperature (unproven). This delay manifests as an offset between the discrete returns and the matching waveform peaks. At the time of writing, it is corrected by a manual / statistical approach (see below). |
| | 38 | |
| | 39 | = Operational details = |
| | 40 | |
| | 41 | This describes a few details from the operations side, though only the parts relevant to processing. |
| | 42 | |
| | 43 | FWD filestructure as stored on the data logger: |
| | 44 | * FWD data is recorded to the RawWFD directory, with a folder structure identical to RawLaser (flightline based) |
| | 45 | * files named WFDYYMMDD_HHMMSS_XXXXXXXXXXX{AB}.LWV |
| | 46 | * A or B indicates which board digitised the signal |
| | 47 | * always in A&B pairs (if not, there has been corruption) |
| | 48 | * are always up to 62501KB (64MB?) |
| | 49 | * the .LWV files cannot be easily linked to the .SCN (discrete files) - there is no direct linkage between LWV file 2 and SCN file 2 |
| | 50 | |