Hawk - GPS Settings
Written by Matthew Clark-Massera
Updated at February 3rd, 2023
Table of Contents
The Hawk has on-board GPS for scheduled location updates. This can help with locating the install location - it reduces the overhead of needing to log where each and every Hawk is installed for a deployment - instead we can simply run the GPS to obtain this information quickly.
The Hawk parameters available allow for fine-grained control of the GPS behaviour to meet battery life and data usage requirements.
Performance Limits
The cellular version of the Hawk uses the Nordic nRF9160 LTE + GPS modem. The on-board GPS does not perform as well as the high quality external GNSS we use on other products.
The Hawk can run a GPS fix as part of a task, but it is not a 'tracking' device - able to detect trip start and endpoints.
The GPS cannot be used if we set the device to Stay Connected to the cellular network permanently
Key Parameters
GPS Basic Settings
Enable/Disable the GPS and Aiding Data, and set the GPS fix timeout.
There is the option to hardcode the location if it is known. The 'Default' position will be sent in device data records if configured.
GPS Multiplier on Task
The Hawk operates by configuring Task Schedules. Up to 4 schedules can be configured. A set of measurements is taken with the frequency defined in the schedule. i.e sample the SDI 12 and I2C sensors every 30 min.
By default, a GPS fix is obtained on each task. We can adjust the frequency of obtaining fixes (including if at all).
Examples
Default - Log fix each task
GPS Task Default Params
GPS is enabled and will attempt to get a fix for 90 sec each task.
Under the Task parameters, setting the GPS multiplier to 1 means a fix is attempted every 1 log
Disable GPS Aiding Data to save data
GPS Aiding speeds up getting a fix from a cold start. So downloading it uses data, but will increase battery life somewhat. If we are hard-wired and battery life is not an issue, we can disable it and save on some data. (80kb every 5 days)
Disable GPS
If we set the GPS multiplier to 0, GPS fixes won't be attempted when running a task.
Disable GPS, but hard-code a default Lat/Long
This Lat/Long will be used with each record
GPS Settings (Advanced)
We additionally have parameters to control the accuracy requirements of a fix (what is accepted or discarded)
NB: We can set the device to require more accuracy to accept a fix - it doesn't mean we'll get improved accuracy - we might just get a lot of discards.
Most users do not need to adjust these parameters. Leave on default unless you know what you are doing
PDOP, Position Accuracy and Speed Accuracy are the minimum accuracy requirements for a fix to be considered 'valid'. When the device attempts to get a GPS fix, turns on it's GPS module and attempts to get a fix each second. It may in fact get a rough location fix, but it will not be accepted or logged, and the device will keep trying until it has one that meets this requirement, or the fix fails. We log the first fix that meets the requirements and turn off the GPS after this.
As such lowering these values doesn't automatically result in more accurate fixes, it will often result in more failures. The defaults are a good starting point - fixes are generally accurate to within 10m when in decent signal.
If we are having trouble acquiring a fix we can loosen these fix requirements e.g.
Static Hold
This has the device 'hold' it's current position (lat/long) if the uncertainity in position is below this value. The idea being if we get subsequent fixes only a few metres apart, this is more likely due to the uncertainty vs actual motion - so no need to update position and have the unit appear to 'wander'.
Require 3D Fix
Setting to NO will mean we can get a fix with 3 satellites instead of 4 - but altitude isn't returned. Setting to No may help in acquiring a fix in low signal conditions.
Discard First N Fixes
Discards a configurable number of valid fixes from the GPS before accepting a final fix. This gives the GPS some time to improve its accuracy, and lowers the chance of outlying GPS fixes slipping through the filters. Increasing this number means the device is more 'sure' of it's position - reducing will allow fixes to be acquired more easily (but potentially less accurate)
Minimum Initial Satellites
This option allows you to discard all GPS positions until a certain number of satellites have been acquired at a specified signal strength. Like the Discard First Fixes option, this gives the module more time to refine and cross-check its estimate, reducing the risk of erroneous fixes. It is intended for advanced use cases that require very high position certainty (ie. mission critical geofencing). Since it can affect fix times badly, the default is zero (disabled)
Minimum Initial Signal Margin
This is the minimum signal strength for the Minimum Initial Satellites option. Setting it to a low value (-20) makes the Minimum Initial Satellites function simply count the number of satellites in the fix, rather than requiring that they be high power. A value of zero requires a decent signal strength, and 5 requires a strong signal. It is generally difficulty to get more than a few satellites with strong signals, and most benefit comes from acquiring many satellites rather than a few strong ones. So if you really need the Minimum Initial Satellites features, the best place to start is with a low value.
