Many digital modes (such as WSPR and FT8) require reasonably price clocks which may be assisted by GPS technology.
Hans G0UPL observes that WSPR does not require GPS. However:
1) WSPR requires precise frequency, the transmission must be within the 200Hz WSPR sub-band.
2) WSPR requires precise timing: transmissions start on the 2nd second of even minutes, +/- a few seconds. An ordinary crystal or worse still, the ceramic resonator on some Arduinos, will not provide sufficient timing accuracy to run WSPR for long.
A GPS is a very convenient and easy way to achieve both of these objectives. On the Ultimate3S kit http://qrp-labs.com/ultimate/u3s the latitude and longitude are also converted to Maidenhead Locator which is encoded live, into each WSPR transmission. Something that is necessary if you want to use WSPR for tracking something (ship, balloon) for example the Canadian C3 expedition http://qrp-labs.com/c3
Speaking of which: Jim N2NXZ’s U3S-22 balloon (running modified Ultimate3S firmware) reached Kazakhstan today. For the second time! It’s already been flying for 23 days, at a little over 9,000m altitude, and completed one world circumnavigation! See http://qrp-labs.com/flights/u3s22 for details and live tracking (using WSPR) during daytime. Big congratulations to Jim!