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Note:

This document was originally called “Bird’s eye view of Motus data” and was was been adapted from a version written by John Brzustowsky in 2017. It provides an overview of the fundamentals of how Motus data processing works. The document has been adapted to incorporate more recent developments, particularly the addition of new digital tags and base stations manufactured by Cellular Tracking Technologies (CTT), who took over the development from Cornell University.

Basic terms

Hardware

  • Tags - Attached to an animal, emits radio signals
    • There are two types of tags, Lotek and CTT, which operate and are processed differently.
  • Receivers - On the landscape, receive signals collected by antennas
  • Antennas - Attached to a receiver, detect different signals emitted by tags
  • Nodes - Special antennas for CTT receivers

Data

  • Hits - An individual detection of a tag signal by an antenna
  • Runs - A series of hits for specific tag
  • Batches - A collection of hits/runs which were processed together

To better understand these terms, let’s look at a visual of the data.

Understanding the data

Hits and Runs

Here’s a segment of data from a receiver (with a two antennas):

Receiver R
                                     Time ->
       \=============================================================\
Tag A: / 1-----1--1----1-----1-----1         4---4----4--4------4--4-/ <- antenna #1
       \                                                             \
Tag B: /               3-----3--3--3--3--3-----3----3--3             / <- antenna #1
       \                                                             \
Tag C: /  2--2---2--2--2--2--2--2--2--2--2--2--2--2--2--2--2--2--2   / <- antenna #1
       \                                                             \
Tag C: /            5--------5--5--5--5--5--5-----5-------5-------5  / <- antenna #2
       \=============================================================\
  • Each row represents an individual tag (left) on a specific antenna (right)
  • Each individual number (digit) represents a tag hit (also called detection)
  • Each series of numbers represents a run
    • A run is a series of hits for an individual tag
    • For example, Tag A shows two runs (1 and 4), each with 6 hits
  • A tag may be detected by more than one antenna at a time, and therefore have multiple runs that overlap in time
    • For example, Tag C is detected on both antenna 1 and 2

Overlapping Runs

Technically, a specific tag should never have overlapping runs on a single antenna, but this can happen in noisy environments with Lotek tags (see Chapter 5 - Data Cleaning: Preliminary Filtering for how to address these false positives).

Overlapping runs would not happen with CTT tags because of the way runs are built, but the rate of hits would still be expected to be around 1 per second or lower.

Determining tag identity

For Lotek tags, multiple tags transmit the same ID, therefore a distinctive period (time between hits) is used to differentiate among tags with the same ID. Since Lotek tags cannot be identified by individual hits, this means that runs are the fundamental unit of detection.

The spacing between two hits is considered compatible with a particular tag if it represents a multiple of the period length (this allows for occasional missed hits). However, a run can have gaps (missing hits) only up to a certain size. Beyond that size, measurement error and clock drift are large enough that we can’t be sure that the next detection is compatible or not.

For example, in the runs for Tag A (above) we’re not sure the gap between the last detection in run 1 and the first detection in run 4 is really compatible with tag A, so run 1 is ended and a new run is begun. We could link runs 1 and 4 post-hoc, once we saw that run 4 was being built with gaps compatible with tag A, but at present, the run-building algorithm doesn’t backtrack.

In contrast, for CTT tags, there are 232 unique codes, which ensures that each is unique, and the period is not distinct as it may vary with power available from the photovoltaic cell.

Runs are assembled based on consecutive hits on a single antenna (or node, if applicable), as long as they are not spaced by more than an arbitrary period (600 seconds).

Batches

A batch is the result of processing one collection of raw data files from a receiver. Batches are not inherent in the data, but instead reflect how data were processed. Batches arise in these ways:

  • a user visits an isolated receiver, downloads raw data files from it, then uploads the files to motus.org once they are back from the field

  • a receiver connected via WiFi, Ethernet, or cell modem is polled for new data files; this typically happens roughly every hour, with random jitter to smooth the processing load on the motus server

  • an archive of data files from a receiver is re-processed on the motus server, because important metadata have changed (e.g. new or changed tag deployment records), or because a significant change has been made to processing algorithms.

Batches are artificial divisions in the data stream, so runs of hits will often cross batch boundaries. Adding this complication to the picture above gives this:

Receiver R
                                     Time ->
       \=============|==============================|==================\
Tag A: / 1-----1--1--|--1-----1-----1         4---4-|---4--4------4--4-/
       \             |                              |                  \
Tag B: /             |  3-----3--3--3--3--3-----3---|-3--3             /
       \             |                              |                  \
Tag C: /  2--2---2--2|--2--2--2--2--2--2--2--2--2--2|--2--2--2--2--2   /
       \============================================|==================\
       <-- Batch N ->|<-------- Batch N+1 --------->|<--- Batch N+2 --->

False positives

False positives (apparent detections of tags actually caused by other sources) need to be taken into account. These can happen for quite a number of reasons, and will sometimes affect Lotek and CTT tags in different measure. False positives are difficult to identify, but there are approaches to identify conditions in which the are more likely to occur, and potential ways to mitigate them.

If you have any suggestions about techniques you are using to separate true detections from false positives, we encourage you to share them with us!

Noisy environments

Radio noise from interference can create bursts that look like tags. As the amount of radio pulses from other sources increases in the environment, the expected number of false positives will also increase.

See:

Bit errors

Actual tag signals may be incorrectly transmitted. Those should rarely produce valid ID’s, but they will be more likely with CTT tags, mostly because where is a very high number of possible combinations (4 billions). They should still rarely produce an ID of a tag actually manufactured. In CTT tags, bit errors have been found to lead more often to specific patterns (e.g. the last trailing digits being all 7 or F’s: xxxFFFF due to only a partial signal being received). These patterns are excluded from the list of valid tags.

Aliasing

Aliasing occurs when the combination of multiple tags create the appearance of a tag that is not present but matches another known tag. This can happen in at least 2 ways:

  1. 2 tags with distinct ID’s, but with the same period (both Lotek and CTT).
  2. 2 tags with the same ID, but with distinct periods (only for Lotek).

In the first case, if the burst of both tags overlap, they may interfere with each other and create the appearance of a new tag for a time. Assuming that the 2 tags do not have exactly the same period, their bursts should eventually drift apart and the alias is not expected to persist over a very long run.

In the second case, if you have multiple tags with the same ID but distinct periods, this could also result in new periods, but those would exceed 2 consecutive hits (run length = 2) only rarely.

Both types of aliasing is mostly problematic in environments where you have many tags present simultaneously which increases the incidence of overlapping tags (e.g. colonies).

Manufacturers for both Lotek and CTT tags have integrated various methods to reduce the incidence of false detections into their proprietary technologies. Data collected by Sensor Gnomes are processed by the “Tag finder”, which looks at properties of the signal to exclude likely false positives (e.g. higher deviation in the frequency of pulses within a burst). The parameters can potentially be adjusted, but an aggressive approach aimed at reducing false positives will also result in an increase in false negatives.

See:

Run length

With both types of tag technologies, the likelihood of obtaining false positives should decrease as the run length increases.

For Lotek tags, we generally recommend ignoring runs comprised only of 2 or 3 hits. Some probably relate to real tag detections, but the vast majority probably do not.

For CTT tags, we do not yet have a suggested minimum threshold. In most cases, given that tag period is short, one would expect that true single-hit runs would be quite rare, so those should be excluded. With the exception of some specific tag IDs more prone to error, the likelihood of obtaining the same false ID in consecutive detections is probably very low. Runs of 2 detections or more are probably safe in most instances.

Missed detections

False positives should more often lead to gaps in detections during a run. A run that contains several gaps in detection would therefore be deemed less reliable. A simple metric for this would be to divide the number of hits in a run (the run length) by the duration of the run (tsEnd - tsStart). Longer runs with few or no gaps in detection would be optimal.

Overlapping runs

Runs of the same tag on the same antenna should also be an indication of false positives, but this should be highly correlated with the number and/or ratio of short runs described above.

Spatio-temporal models

State-space models and other approach can be used to assess whether movement make sense from a biological point of view, and can help assign probabilities that individual detections are valid.

False negatives

False negatives are usually even more difficult to detect.

  • Faulty equipement or installation is always possible of course. Please refer to the installation guidelines to make sure you follow all the recommendations.

  • DO NOT FORGET to register your tags! (details here)

  • DO NOT FORGET to activate your tags before deploying them!

  • Make sure that you report your tag and receiver deployment details BEFORE uploading your data. Motus tag finder only seek tags that are known to be deployed.

Complications

The picture above is complicated by several facts:

  • receivers are often deployed to isolated areas so that we can only obtain raw data from them occasionally

  • receivers are not aware of the full set of currently-active tags

  • sensorgnome receivers do not “know” the full Lotek code set; they record pulses thought to be from Lotek coded ID tags, but are only able to assemble them into tag hits for a small set of tags for which they have an on-board database, built from the user’s own recordings of their tags. This limitation is due to restrictions in the agreement between Lotek and Acadia University for our use of their codeset.

  • Lotek receivers report each detected tagID independently, and do not assemble them into runs. This means a raw Lotek .DTA file does not distinguish between tag 123:4.5 and tag 123:9.1 (i.e. between tags with ID 123 and burst intervals 4.5 seconds and 9.1 seconds).

This means that:

  • raw receiver data must be processed on a server with full knowledge of:

    • which tags are deployed and likely active

    • the Lotek codeset(s)

  • raw data must be processed in incremental batches

  • processed data should be distributed to users in incremental batches, especially if they wish to obtain results “as they arrive”, rather than in one lump after all their tags have expired.

Receiver Boot Sessions

A receiver reboots when it is powered down and then (possibly much later) powered back up. A boot session is defined as the period between reboots. Reboots often correspond to a receiver:

  • being redeployed
  • having its software updated
  • or having a change made to its attached radios,

Motus treats receiver reboots in a special way:

  • a reboot always begins a new batch; i.e. batches never extend across reboots. This simplifies determination of data ownership. For example, all data in a boot session (time period between consecutive reboots) are deemed to belong to the same Motus project. This reflects the fact that a receiver is (almost?) always turned off between the time it is deployed by one project, and the time it is redeployed by another project.

  • any active tag runs are ended when a receiver reboots. Even if the same tag is present and broadcasting, and even if the reboot takes only a few minutes, hits of a tag before and after the reboot will belong to separate runs. This is partly for convenience in determining data ownership, as mentioned above. It is also necessary because sometimes receiver clocks are not properly set by the GPS after a reboot, and so the timestamps for that boot session will revert to a machine default, e.g. 1 Jan 2000. Although runs from these boot sessions could in principle be re-assembled post hoc if the system clock can be pinned from information in field notes, this is not done automatically at present.

  • parameters to the tag-finding algorithm are set on a per-batch basis. At some field sites, we want to allow more lenient filtering because there is very little radio noise. At other sites, filtering should be more strict, because there is considerable noise and high false-positive rates for tags. Motus allows projects to set parameter overrides for individual receivers, and these overrides are applied by boot session, because redeployments (always?) cause a reboot.

  • when reprocessing data (see below) from an archive of data files, each boot session is processed as a batch.

Incremental Distribution of Data

The motus R package allows users to build a local copy of the database of all their tags’ (or receivers’) hits incrementally. A user can regularly call the tagme() function to obtain any new hits of their tags. Because data are processed in batches, tagme() either does nothing, or downloads one or more new batches of data into the user’s local DB.

Each new batch corresponds to a set of files processed from a single receiver. A batch record includes these items:
- receiver device ID - how many of hits of their tags occurred in the batch - first and last timestamp of the raw data processed in this batch

Each new batch downloaded will include hits of one or more of the users’s tags (or someone’s tags, if the batch is for a “receiver” database).

A new batch might also include some GPS fixes, so that the user knows where the receiver was when the tags were detected.

A new batch will include information about runs. This information comes in three versions:

  • information about a new run; i.e. one that begins in this batch
  • information about a continuing run; i.e. a run that began in a previous batch, has some hits in this batch, and is not known to have ended
  • information about an ending run; i.e. a run that began in a previous batch, might have some hits in this batch, but which is also known to end in this batch (because a sufficiently long time has elapsed since the last detection of its tag)

Although the unique runID identifier for a run doesn’t change when the user calls tagme(), the number of hits in that run and its status (done or not), might change.

Reprocessing Data

Motus will occasionally need to reprocess raw files from receivers.
There are several reasons:

  • New or modified tag deployment records

    The tag detection code relies on knowing the active life of each tag it looks for, to control rates of false positive and false negative hits. If the deployment record for a tag only reaches the server after it has already processed raw files overlapping the tag’s deployment, then those files will need to be reprocessed in order to (potentially) find the tag therein. Similarly, if a tag was mistakenly sought during a period when it was not deployed, it will have “used up” signals that could instead have come from other tags, thereby causing both its own false positives, and false negatives on other tags. (This is only true for Lotek ID tags; CTT should be unaffected, provided deployed tags are well dispersed in the ID codespace.)

  • Bug fixes or improvements in the tag finding algorithm

  • Corrections of mis-filed data from receivers

    Sometimes, duplication among receiver serial numbers (a rare event) is only noticed after data from them has already been processed. Those data will likely have to be reprocessed so that hits are assigned to the correct station. Interleaved data from two receivers having the same serial number will typically prevent hits from at least one of them, as the tag finder ignores data where the clock seems to have jumped backwards significantly.

The (eventual) Reprocessing Contract

Reprocessing can be very disruptive from the user’s point of view (“What happened to my hits?”), so Motus reprocessing will be:

  1. Optional: users should be able to obtain new data without having to accept reprocessed versions of data they already have.

  2. Reversible: users should be able to “go back” to a previous version of any reprocessed data they have accepted.

  3. Transparent: users will receive a record of what was reprocessed, why, when, what was done differently, and what changed

  4. All-or-nothing: for each receiver boot session for which users have data, these data must come entirely from either the original processing, or a subsequent single reprocessing event. The user must not end up with an undefined mix of data from original and reprocessed sources.

  5. In-band: the user’s copy of data will be updated to incorporate reprocessed data as part of the normal process of updating to obtain new data, unless they choose otherwise. We expect that most users will want to accept reprocessed data most of the time.

Initially, Motus data processing might not adhere to this contract, but it is an eventual goal.

Reprocessing simplified

Raw data records from an arbitrary stretch of time are complicated to reprocess, because runs which cross the reprocessing boundaries might lose or gain hits within the reprocessing period, but not outside of it. This might even break an existing run into distinct new runs.

This would be challenging to formalize and represent in the database if we want to maintain a full history of processing. For example, if reprocessing deletes some hits from run 2, how do we represent both the old and the new versions of that run?

Therefore, for simplicity we choose reprocessing periods that cross no runs. Currently, that means a boot session, as discussed above.

Distributing reprocessed data (eventual)

The previous section shows why we only reprocess data by boot session. Given that, how do we get reprocessed data to users while fulfilling the reprocessing contract?

Note that a reprocessed boot session will fully replace one or more existing batches and one or more runs, because batches and runs both nest within boot sessions.

Replacement of data by reprocessed versions should happen in-band (point #5 above), so one potential approach is this:

  • the batches_for_XXX API entries should mark new batches which result from reprocessing, so that the client can deal with them appropriately. This can be done by adding a field called reprocessID with these semantics:
    • reprocessID == 0: data in this batch are from new raw files; the normal situation
    • reprocessID == X > 0: data in this batch are from reprocessing existing raw files.
    • X is the ID of the reprocessing event, and a new API entry reprocessing_info (X) can be called to obtain details about it.
    • if the user chooses to accept the reprocessed version, then existing batches, runs, hits and GPS fixes from the same receiver and boot session are retired before adding the new batches.
    • if the user chooses to reject the reprocessed version, then X is added to a client-side blacklist, and the user will not receive any data from batches whose reprocessID is on the blacklist.
    • later, if a user decides to accept a reprocessing event they had earlier declined, then the IDs of new batches for that event can be fetched from another new API reprocessing_batches (X), and the original batches will be deleted
  • to let users more efficiently fetch the “best” version of their dataset (i.e. accepting all reprocessing events), we should also mark batches which are subsequently replaced by a reprocessing event. For this, we add the field replacedIn with these semantics:
    • replacedIn == 0: data in this batch have not been replaced by any reprocessing event
    • replacedIn == X > 0: data in this batch have been replaced by reprocessing event X. Then the client can ignore any batches for which replacedIn > 0. We could also add a new boolean parameter unreplacedOnly to the batches_for_XXX API entries. It defaults to false, but if true, then only batches which have not been replaced by subsequent reprocessing events are returned.
  • users can choose a policy for how reprocessed data are handled by setting the value of Motus$acceptReprocessedData in their workspace before calling tagme():
    • Motus$acceptReprocessedData <- TRUE; always accept batches of data from reprocessing events
    • Motus$acceptReprocessedData <- FALSE; never accept batches of data from reprocessing events
    • Motus$acceptReprocessedData <- NA (default); ask about each reprocessing event

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