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datatrak:intro [2020/05/31 11:47] – [Lane identification in Datatrak] philpem | datatrak:intro [2020/05/31 12:36] – philpem | ||
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Generally this is done by using a lower-frequency " | Generally this is done by using a lower-frequency " | ||
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===== Datatrak basics ===== | ===== Datatrak basics ===== | ||
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+ | ===== Datatrak LF signal format ===== | ||
==== Basic signal format ==== | ==== Basic signal format ==== | ||
+ | |||
+ | This is the simplest possible Datatrak signal. | ||
The signal for the F1 chain (for basic Datatrak) looks like this: | The signal for the F1 chain (for basic Datatrak) looks like this: | ||
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^ $F_1$ | Sync and timing | ^ $F_1$ | Sync and timing | ||
^ $F_2$ | ... | | ^ $F_2$ | ... | | ||
+ | |||
+ | This only allows for a single chain, and is only used for illustration. | ||
The receiver (also known as a // | The receiver (also known as a // | ||
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Each navigation slot is 80ms long -- with 40ms transmitted at a higher frequency ($F_1+$ and $F_2+$) and 40ms at a lower frequency ($F_1-$ and $F_2-$). | Each navigation slot is 80ms long -- with 40ms transmitted at a higher frequency ($F_1+$ and $F_2+$) and 40ms at a lower frequency ($F_1-$ and $F_2-$). | ||
- | === Adding more slots: Interlacing === | + | |
+ | ==== Adding more slots: Interlacing | ||
The basic signal encoding is fine, but the limit of eight slots restricts the number of transmitters which can exist in the network. It's obvious from the timing diagram above that only half of the transmitter capacity is being used. This is easily fixed: | The basic signal encoding is fine, but the limit of eight slots restricts the number of transmitters which can exist in the network. It's obvious from the timing diagram above that only half of the transmitter capacity is being used. This is easily fixed: | ||
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^ $F_2$ | Sync and timing | ^ $F_2$ | Sync and timing | ||
- | The disadvantage | + | This allows us to have 16 transmitters, |
+ | When planning the network, care must be taken to keep slot collisions to a minimum as Locators move around the signal area. | ||
- | === Adding even more slots: Dual Cycle Interlaced mode === | ||
- | Interlacing can be extended further by linking two 1.68-second //cycles// into a larger cycle: | + | ==== Adding even more slots: Dual Cycle Interlaced mode ==== |
+ | |||
+ | Interlacing can be extended further by linking two 1.68-second //cycles// into a larger | ||
^ $F_1$ | Sync and timing | ^ $F_1$ | Sync and timing | ||
^ $F_2$ | Sync and timing | ^ $F_2$ | Sync and timing | ||
- | This expands the system to a maximum of 24 navigation slots -- the measurements for slots 1 to 8 are updated every 1.68-second cycle, | + | This expands the system to a maximum of 24 navigation slots, with a further caveat: |
- | Slot collisions are still an issue, but the rules are relaxed somewhat: | + | Slot collisions are still an issue, but the rules change slightly: |
* While receiving slot $1 \leq N \leq 8$: | * While receiving slot $1 \leq N \leq 8$: | ||
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* It is not possible to receive slot $N-16$ (1..8) | * It is not possible to receive slot $N-16$ (1..8) | ||
- | ==== Three-step navigation process ==== | + | |
+ | ===== Three-step navigation process | ||
(See also [[references# | (See also [[references# | ||
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* Initial " | * Initial " | ||
- | * Secondary " | + | * Secondary " |
- | * Final " | + | * Final " |
This system has one massive advantage: the entire navigation solution can be obtained in three stages from the same input data. | This system has one massive advantage: the entire navigation solution can be obtained in three stages from the same input data. | ||
- | Once the position of the receiver is known, successive positioning calculations may only need the " | + | Once the position of the receiver is known, successive positioning calculations may only need the " |
This works because a phase measurement at one frequency subtracted from a phase measurement at another frequency will result in a phase measurement taken at the difference in frequency between the two signals -- 80Hz in the case of the super-coarse fix. | This works because a phase measurement at one frequency subtracted from a phase measurement at another frequency will result in a phase measurement taken at the difference in frequency between the two signals -- 80Hz in the case of the super-coarse fix. | ||
- | < | + | To put this in perspective, the relative accuracies of the different phases are: |
- | </ | + | |
+ | ^ Phase ^ Frequency | ||
+ | | " | ||
+ | | " | ||
+ | | " | ||
+ |