pitaya_gps.txt

### Time ###

All of this needs to be done as root.

This assumes the GPS module is connected to the 4th channel of the
FT4232H. If it isn't, you need to edit the DEVICES variable below to
point to the correct one. The connections you need to make are:

    GPS Pin   <-->    FT4232 Pin
    ----------------------------
    3.3V              No connect
      EN              No connect
    VBAT              No connect
     FIX              No connect
      TX              DD1
      RX              DD0
     GND              GND (doesn't matter which one)
     VIN              3V3 (doesn't matter which one)
     PPS              No connect

Wire those up, then connect the FT4232H to the USB port on the Pitaya.

**DO NOT CONNECT POWER IN TO THE 3.3V PIN NOR THE VBAT PIN ON THE GPS
MODULE! Power should be connected to VIN only.** 

Make sure the default timesync daemon is disabled:

    systemctl disable systemd-timesyncd

Run:

    apt-get update
    apt-get install gpsd chrony gpsd-clients

Replace the contents of /etc/default/gpsd with:

    START_DAEMON="true"
    DEVICES="/dev/ttyUSB3"
    GPSD_OPTIONS="-n" 

Replace contents of /etc/chrony/chrony.conf with:

    refclock SHM 0 refid NMEA trust
    
    keyfile /etc/chrony/chrony.keys
    commandkey 1
    driftfile /var/lib/chrony/chrony.drift
    logdir /var/log/chrony

    maxupdateskew 100.0
    makestep 0.1 -1

    hwclockfile /etc/adjtime
    rtcsync

Note: when I say replace, I mean delete the current contents and replace
      them with what I showed. Both of these files should exist already.

Restart the two services:

    systemctl restart gpsd
    systemctl restart chrony
    
Make sure they started without errors:

    systemctl status gpsd
    systemctl status chrony
    
If all is well, make sure they are set to run at boot:

    systemctl enable gpsd
    systemctl enable chrony

Can view GPS status with:

    gpsmon

Can view the synchronization information with

    chronyc sources

and detailed statistics with

    chronyc sourcestats

Can check time with:

    timedatectl

Time zone will be UTC! This is easiest, otherwise you'd have to
change it if you took the detectors somewhere. If you really want to
change the timezone:

    timedatectl list-timezones

to list the available ones, then

    timedatectl set-timezone xxx

where xxx is the timezone you want from the output of the first command.
EST is "America/New_York".

One final note: GPS time does not include leap seconds. Chrony can be set
up to correct for leap seconds, but I have not done this. See the 
`leapsecmode` section of `man chrony.conf`. At present, this means
that GPS time is ahead of UTC by 17 seconds. I did not enable this because
it requires manually updating the leap seconds record whenever a new one
is added. It shouldn't really matter, because all the detectors will still be
synchronized to the same time (within a few ms), which is all that matters.

### Position ###

Again, all as root. The library for GPSD requires Python3. See

    https://github.com/MartijnBraam/gpsd-py3

for info.

Run:

    apt-get update
    apt-get install python3 python3-pip
    pip3 install gpsd-py3

Simple example:

    import gpsd

    gpsd.connect()
    packet = gpsd.get_current()
    print(packet.position)

Remember it's Python3, so this must be run with `python3`, not `python`.
I don't think this script needs to be run as root, but I'm not sure.