Is it possible to create a minature and very inexpensive GPS online and offline tracker which is OwnTracks-compatible? Yes, it is possible.
During a bout of craze^W inspiration, I obtained a u-blox Neo 6M GPS module (EUR 9.30) and connected it to an Wemos-D1 mini ESP8266 module (EUR 3.35). Within the hour, I was happily publishing OwnTracks JSON location messages to a local broker over WiFi. The device is so small I named it Pico. (The rubber bands you see in the photo below are part of the cough design cough to make it vehicle-ready, and the LED is simply for additional blinkenlights of course. The black box is a USB battery pack.)
The idea would have been to use mobile phone WiFi tethering or a MiFi to take the result along in a vehicle, but that would have been a bit boring. I learned during my experience with Homie that the ESP8266 has an SPIFFS file system, and you can probably guess what's going to happen.
When the Pico loses connection to its MQTT broker or disconnects from WiFi, it continues in offline mode, recording to the file system. Once a minute it attempts to reconnect, and if it succeeds, it publishes what it's collected so far to the configured MQTT broker. This appears to work quite reliably.
The only slight handicap in Pico is, that the PubSubClient MQTT library has a default maximum MQTT message size of 128 octets which is too little for the information we want in OwnTracks JSON. (We don't just publish lat and lon, but also velocity, altitude, distance travelled, and a few other bits.) We could easily change definition in PubSubClient, but I opted in the interest of portability and in the interest of maximizing the number of offline location publishes to use OwnTracks CSV which we developed for our Greenwich devices, and which is already fully supported by the Recorder. This means, with a 3MB file system on the ESP8266 we can store in excess of 60,000 points.
The CSV content may look a bit strange because we hexalize (is that a word?) the time stamp, unfloat (is that also a word?) lat and lon (we save the space of two decimal points) and do a few other optimizations (in the interest of conserving valuable GSM data).
The Pico publishes (or serializes to the file system if it's offline) a location when it detects it's moved mindist meters (100 by default) and 5 seconds interval have elapsed. When it's stationary, it'll publish a ping location (that's what we call it) once an hour, and on booting it publishes a first location to tell us it's alive.
A challenge would now be to take a EUR 4.40 GSM/GPRS module and allow the Pico to be a fully capable online tracker which publishes MQTT over the Internet via GPRS. Any takers?