This applies to most Digital Matter cellular devices.
It is often useful to understand how well the GPS module is performing. Enabling GPS debugging can provide:
- Time To First Fix (TTFF) information
- Satellite Signal Strength (SNR) information
- Orbital download information.
Steps to enable GPS debugging:
- Login to www.oemserver.com
- Select the devices - explained here
- Set the debug level - explained here
- To enable debugging, be sure to set the GPS module to Info
- Wait for the device to connect and receive the logging change instruction.
Once enabled, the device will collect data and upload it on the next connection.
Useful notes for interpreting the data:
- Debug[GPS][Warn]: GPS manager - Fix timeout: this indicates the GPS module has failed to get a fix in the allocated time. The device may sleep or try again.
- Debug[GPS][Info]: TTFF=133s PDOP(x10)=33 3Dfix=1: this states that the device obtained a fix in 133 seconds.
- Debug[GPS][Info]: Valid=135s PDOP(x10)=19 3Dfix=1: the device obtained a valid fix in 135 seconds. This is after the fix has met the minimum requirements for a good fix. These are usually set-able in System Parameters.
- Debug[GPS][Info]: SAT4 42322,38000,35322,29322: signal strengths reported by the module.
The number immediately after the "SAT" is the number of satellites the receiver is tracking.
The first 2 digits of the numbers represent the "Carrier To Noise" ratio (C/No) in dBHz and is an indicator of the received GNSS signal strength. The higher the C/No value is, the better is the strength, and therefore the quality of the received signal. In our testing we consider values above 40 as excellent, but chances are that you will only get above 40 when conditions are near perfect - so don't stress over these numbers. The main reason we have surfaced these values via the debug messages is that values at extremes indicate something is wrong with the GNSS / antenna subsystem, and any values between those extremes are OK.
These messages are logged every 60 seconds when in debug mode. This example message suggests it is tracking 4 satellites. In this example the highest signal strength is 42, which is considered strong.
The other numbers are displayed for internal debugging purposes.
Notes on Dilution of Precision (DOP) And Specifically PDOP
DOP is a description of the purely geometrical contribution to the uncertainty in a position fix and has a number of specific applications. Some examples are PDOP, GDOP, HDOP, VDOP, TDOP and RDOP.
DOP is a function expressing the mathematical quality of solutions based on the geometry of the satellites.
Position Dilution of Precision (PDOP) is the most common and we use it as a useful indication of the uncertainty of the 3 dimensional position (3 coordinates) - ie how accurate do we expect the position to be?
PDOP has a best case value of 1, with higher numbers being worse.
A low number of PDOP (2) is good, a high number (typically > 10) is considered to be bad. The best PDOP would occur with one satellite directly overhead and three others evenly spaced about the horizon.
PDOP could theoretically be infinite, if all the satellites were in the same plane.
Most Digital Matter devices have a parameter for the maximum PDOP value to consider for a valid fix. This is generally a high value as default and you can 'tighten' up this parameter if needed. Note that this might mean that position fixes take longer resulting in higher consumption of energy which might impact on battery-powered devices, so it is a trade-off between accuracy and battery usage.
For more info on the details of GPS performance see this article by UBLOX: https://www.u-blox.com/sites/default/files/the_gps_dictionary.pdf