| 65 | | The LIDAR works by firing a (4ns or 9ns) laser pulse downwards and measuring the roundtrip time for the light pulse to return, then converting this to a distance. The pulse isn't modulated by a carrier - it's just an on/off pulse. There are four timing cards ("range cards"), so up to 4 return pulses can be detected, with the intensity of the return measured only on the first 3 returns. A minimum time separation between two returns means the minimum distance between two returns must be at least 2.7m for them to be counted as independent. The expectation for the number of returns is 1 return ~100%, 2 returns ~10%, 3 returns ~1%, 4 returns ~0.1% of points - obviously this varies with the terrain. |
| 66 | | |
| 67 | | The intensity of a return is measured as an 8 bit value (0=dark (water), 255=bright) and relates to the reflectivity of the illuminated surface. The value is amplified by an automatic gain controller, and is not related to a physical measure (can it be?). The intensity can be used in various processing algorithms to help distinguish transitions between surfaces. The AGC tries to keep the intensity in the range 100-150 or so. |
| 68 | | |
| 69 | | There is also a "MPiA" (Multiple Pulses in the Air) mode, which fires two pulses evenly separated, rather than waiting for the first to come back before firing another [SPiA mode, times out in case the pulse is eaten]. If a seagull gets in the way of the second pulse before the first pulse has returned, things will mess up ;) (on an edge of a very unluckily placed cloud, this would look a bit like it merging into the ground). |
| | 68 | The LIDAR works by firing a (4ns or 9ns) laser pulse downwards and measuring the roundtrip time for the light pulse to return, then converting this to a distance. The pulse isn't modulated by a carrier - it's just an on/off pulse. There are four timing cards ("range cards R1-R4") running for a pulse, so up to 4 returns can be detected. The system has a "MPiA" (Multiple Pulses in the Air) mode, which fires two pulses evenly separated, rather than waiting for the first to come back before firing another [SPiA mode, times out in case the pulse is eaten]. To measure this, there are actually two banks of timing cards (bank A and bank B, both with R1-R4 cards), so there are 8 timing cards in total. |
| | 69 | |
| | 70 | A minimum time separation between two returns means the minimum distance between two returns must be at least 2.7m for them to be counted as independent. The expectation for the number of returns is 1 return ~100%, 2 returns ~10%, 3 returns ~1%, 4 returns ~0.1% of points - obviously this varies with the terrain. |
| | 71 | |
| | 72 | The intensity of a return is measured only for the first 3 returns (R1-R3), and is an 8 bit value (0=dark (water), 255=bright) relating to the reflectivity of the illuminated surface. The value is amplified by an automatic gain controller, and is not related to a physical measure (can it be?). The intensity can be used in various processing algorithms to help distinguish transitions between surfaces. The AGC tries to keep the intensity in the range 100-150 or so. |
| 209 | | * Two datasets required: |
| 210 | | * BIT (Built-In Test) mode data, where the range cards are all fed with identical fake data representing the same distance. All cards should give the same result, so differences are used to calibrate each card against the others. |
| 211 | | * A real dataset with |
| | 212 | |
| | 213 | Range offset correction (+range card calibration). |
| | 214 | * Correction for the slightly different timing of the 4 range cards in the system. |
| | 215 | * At a set distance, the range cards should all return the same result. |
| | 216 | * Measured by checking the first return pulse against a known distance and by computing the timing errors for each range card so as to calibrate them against the first pulse. |
| | 217 | * '''Measured by Leica but also measured and verified in calibration procedure.''' (see below) |
| | 218 | |
| | 219 | Two datasets are required: |
| | 220 | 1. BIT (Built-In Test) mode data, where the range cards are all electronically fed with identical fake data representing the same distance. All cards should give the same result, so differences are used to calibrate each card against the others. |
| | 221 | 1. A real dataset with a well known distance (in the factory this will be a target, in the world it'll be a site with accurate GCPs) |
| | 222 | |
| | 223 | First, we need to determine the timing differences between the 4 range cards (R1-R4). To do this, we use BIT mode data, where all range cards receive the same pulse at the same instant. Averaging these numbers gives the timing offsets between the cards |