I think that it's gps_protocol.c in new_sat()

In reply to:

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if (old == -1 || gps->sats.time < old_time) {
old = i;
old_time = gps->sats.time;

---

old_time is zero at the first invocation of the function, and all of gps->sat[i].time are also zero. Thus old always = 0, and old_time always = gps->sats[0].time for subsequent invocations of new_sat.