| 14 | | The FWD is implemented as two sampling boards |
| 15 | | |
| 16 | | The FWD digitises the incoming intensity level and nothing else. It records either 64, 128 or 256 samples at 1 or 2 nanosecond resolution (1ns is approximately 15cm). The 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). |
| | 14 | The FWD is implemented as two sampling boards (A & B) that operate digitise the incoming intensity level and nothing else. It records 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 nominally the same value as shown in the discrete system and is measured after the AGC amplification has taken place. 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. |
| 30 | | - no range gate yet but may be added (phil) |
| 31 | | - "pre-trigger" samples = buffer before first return peak to ensure you get lead in to pulse, defaults to 5m |
| 32 | | - especially important if there's low signal before first real peak, e.g. sparse tree top |
| 33 | | range data is measured independently to achieve 1.5cm range (100 picosec). |
| 34 | | - fwd data goes to RawWFD, structure identical to RawLaser, files named WFDYYMMDD_HHMMSS_XXXXXXXXXXX{AB}.LWV |
| 35 | | - A or B indicates which board digitised the signal |
| 36 | | .LWV files |
| 37 | | - are always up to 62501KB |
| 38 | | - always in A&B pairs |
| 39 | | - can't link single scn files to lwv files within a flightline |
| | 29 | |
| | 30 | FWD filestructure: |
| | 31 | * FWD data is recorded to the RawWFD directory, with a folder structure identical to RawLaser (flightline based) |
| | 32 | * files named WFDYYMMDD_HHMMSS_XXXXXXXXXXX{AB}.LWV |
| | 33 | * A or B indicates which board digitised the signal |
| | 34 | * always in A&B pairs (if not, there has been corruption) |
| | 35 | * are always up to 62501KB (64MB?) |
| | 36 | * 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 |
| | 37 | |