Modern GPS chipsets do a fantastic job of reducing time to first fix (TTFF), a parameter that drives many applications. They have so many receiver channels that they can search for satellites very quickly, and the newer satellites are more powerful leading to faster capture times. The almanac and ephemeris are sent in parallel on the satellites with data striping so you can download the entire data quickly even though it takes 12.5 minutes for one satellite to do it by itself. It should never take 2 hours.
Here’s a test:
https://www.skydelsolutions.com/en/resources/app-notes/app-note-gnss-basic-tests-ttff/
The punch line is this chart for a cold start, one with no history or data on board to help it start, and without a data link to get data from some other path:
By 35 seconds or so, most units have locked on. If the unit has almanac (valid for 180 days), and ephemeris (valid for 4 hours), it can lock on in a few seconds. In your use case, you can probably afford the power to boot the GPS every 4 hours, get a lock and new ephemeris, and go to sleep. Then your relock time is very quick when the drone contacts the tag/sensor.
I’ve not used it, but this looks interesting:
https://www.u-blox.com/sites/default/files/ZOE-M8B_DataSheet_(UBX-17035164).pdf
Cold start time is 26 seconds, hot start time is 1 second (have time, almanac, ephemeris, and haven’t moved more than 100 km). It can do this with a 72 channel receiver which means parallel download of data among all visible satellites.
It has options for aiding (download almanac and ephemeris via radio link) which reduces TTFF.
If you can tolerate a cold start, just turn it off until you need it. Then the power is nothing when not in use. How much power you use is determined by how often you want a location.
If you want a more responsive system, you can keep the GPS “hot” with almanac, ephemeris, clock, and last known location. This allows a “hot” start and captures in a second or so. To do that, you power it up for 1 minute every two hours, which should be much more than you need since that would do a cold start. The data sheet says that will be 81 mW during acquisition. The datasheet says you need 36 uW otherwise to keep RTC and chip state. Net result is an average power usage of 711 uW of energy. So can your application handle 0.7 mW average and 81 mW peak power usage?
The exact design will depend on the use case you need, how many locates you need to do, and what response time you need for the location.
Mike Ciholas, President, Ciholas, Inc
3700 Bell Road, Newburgh, IN 47630 USA
mikec@ciholas.com
+1 812 962 9408