Cumulative error between SYSTEM_TIME and companion computer time in Dronekit

Question

similar to this guy, I have been trying to get GPS time from a Pixhawk into a Raspberry Pi companion computer.

Before trying out the @vehicle.on_message('SYSTEM_TIME') approach, I used a wildcard to catch all messages and dumped the results into a file. Examining the file, I saw that the only type of message I am getting from the GPS is GPS_RAW_INT. Most importantly, I am not getting the SYSTEM_TIME_UTC message which, according to the documentation, fetches UTC time from the GPS.

Now, using the @vehicle.on_message('SYSTEM_TIME') works fine, I am getting something. However, what is that something? The documentation claims it is:

The system time is the time of the master clock, typically the computer clock of the main onboard computer.

OK, I presume the "main onboard computer" is the Pixhawk. But how does it get the time if there is no GPS message containing it. The GPS_RAW_INT does not carry any time info.

Investigating further, I wrote the following handler:

@vehicle.on_message('SYSTEM_TIME')
def listener(self, name, message):
    rpi_time = time.time()
    mavlink_time = float(message.time_unix_usec)/1000000
    print( str(rpi_time) + " - " + str(mavlink_time) + " = " + str(rpi_time-mavlink_time) )

rpi_time comes from the Raspberry Pi and is correct since it is set by NTP at boot time. I wanted to verify that mavlink_time (which I presumed has to be GPS based time) would be pretty close. I was wrong.

Running this multiple times, I notice that, at first, GPS time is always about two seconds ahead. As time goes by, it drops behind. Sample output:

1483754011.89 - 1483754014.72 = -2.83385300636
1483754012.16 - 1483754014.96 = -2.79546308517
1483754012.44 - 1483754015.2 = -2.75652289391
...
1483754154.69 - 1483754044.5 = 110.186414957
1483754173.72 - 1483754044.76 = 128.95413518
1483754184.04 - 1483754045.02 = 139.016195059

In this case, where rpi_time (which is correct), says 172 seconds have elapsed, mavlink_time claims only 30. Other trials showed wild variations (the discrepancy is by no means fixed).

So, my questions are:

1. What is the SYSTEM_TIME message based on? Realistically, it can only be GPS (it's the only time data available to the Pixhawk) but, if that is the case...
2. why is it so inaccurate and why do I not also get SYSTEM_TIME_UTC messages?

Answer 1

1. Yes, system time is based on GPS.

2. Are you doing your testing inside? The GPS module computes position, velocity and time together. If it does not have a good lock, it might not produce a good time.

I think the SYSTEM_TIME_UTC message was deprecated.