GPS isn’t perfect and many factors can cause jumps, especially indoors. The GPS modules need ephemeris data which is data transmitted via satellites containing information about their current and future position in the sky. 

It uses that along with timing information to calculate its position. In fantastic signal (outdoors with a good view of the sky) it takes at least 30 seconds to download this information from each satellite so it can then calculate a position. It needs to download this information from at least 4 strong signal satellites so it can triangulate its position and calculate a 3D fix.

That is very power-hungry and takes even longer when in poor signal. We use GPS-aiding data provided by the GPS manufacturers, which is ephemeris data predicted over the next 7 days (i.e the predicted locations of the satellites over the next 7 days). 

The GPS can then skip the ephemeris downloads and can begin trying to calculate its position almost immediately. This allows for much faster and power efficient fixes.

However, satellite conditions are constantly changing, and the predicted ephemeris can be incorrect. If the ephemeris data is wrong, the GPS module will be assuming the GPS satellite is in a different location than its true location and will triangulate an incorrect fix. 

To mitigate that risk, we download fresh aiding data every 5 days, but the risk is still there that a satellite will deviate from its predicted path.

That brings us to the minimum initial satellite parameter. The idea for the feature is to ensure we are using more satellites than the bare minimum to build redundancy into the system, so that if one satellite happens to be off the predicted course it won’t throw the triangulation out.

Initial Satellite Strength Configuration

These additional parameters are available on the GPS Settings Parameter tab. A use case that would use these parameters is to avoid GPS jumps outside Geofences/ignore fixes while in a safe inside a store but also accurately trip track while in transit.

The idea of these settings is that we can configure the device to be more certain that the fix it accepts is valid, by requiring minimum numbers of satellites and that the signal strength is above a threshold.

Require 3D Fix

This requires that a position fix use 4 satellites, and calculate a height as well as latitude and longitude, before being considered valid. It is recommended, as using only 3 satellites gives less redundancy in the rare event of a satellite failure, increasing the risk of a position fix giving an erroneous result. However, disabling this option can allow fixes to proceed faster, and in adverse conditions such as urban canyons. The default is Yes.

Discard First Fixes (0-32)

This option discards the first few valid position fixes from the GPS module. This gives the module some time to refine and cross-check its estimate, reducing the risk of an erroneous fix in the rare event of a satellite failure. We recommend the default value of 3.

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 difficult 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 need the Minimum Initial Satellites features, the best place to start is with a low value.

Testing

The more satellites above 4 are required for the first fix, the more redundancy and the less likely an off-predicted course GPS satellite will cause a jump in position. It is recommended to have at least 5 (ideally 8) set for the min initial satellites @ -10 signal margin.

Initial PDOP, Position Accuracy and Speed Accuracy

The following devices support  “initial speed accuracy”, “initial position accuracy”, and “initial PDOP”. The idea is that requiring the initial fix to be very accurate should filter the bad indoor positions out, but after the initial fix the GPS settings will be loose to help with live tracking when it is stolen. 
Supported Devices:

• Oyster3
• Oyster3-BLE
• Remora3
• Yabby3

If the parameter “Use Initial Fix Settings” is set to yes, then no fix is considered valid until the initial GPS fix parameters are met. Once the first valid fix is achieved, it then uses the regular parameters. By using this they can require the first fix when the GPS is turned on to be very accurate to avoid GPS jumps when the device is indoors, but subsequent fixes can be less accurate so the success rate for fixes in recovery mode trips won’t be adversely affected.

The parameters below produced the results seen in DMA office window (around 1200 positions in that tight group).