OK a couple of geostationary tests resolved the issue, in the 2hrs with display on, it didn't miss a beat, continuous 1sec recordings. But 1/4 hr with display off it had typical big gaps up to almost 2 minutes long.
But like the xperia, and unlike the Pixie, it doesn't make big speed spikes while the display is off.
I'm still not sure what caused the drop outs, when I'm fairly sure the display was on, it was on top of the car in hot sun, so there may be a heat issue. I don't want to test for this, in case I damage the phone, we'll let Fin do that!
So as long as there's no heat problem, I think this is a better phone than the Pixie and the Xperia.
It has the high res of the Pixie but the absence of big spikes like the xperia
Here's a car ride with the GT31 and ZTE next to each other on the roof racks.
Red is GT31, yellow ZTE
The green centre line is where the ZTE has a drop out, that's why the yellow line takes a short cut across the bottom of the red peak. Apart from that the two lines are very close.
And this next one is a bike ride with the GT31 in the rear carrier with the ZTE on one arm and the Xperia on the other.
Blue ZTE, red GT31 yellow Xperia
Again the centre line is where the ZTE had a drop out, but you can see how much better it is than the xperia, when it's not dropping out.
Both the Pixie and Xperia, don't give good results on my arm when I'm bike riding, because my arms are more vertical than when sailing, and my body shields the satellites more, doesn't seem to worry the ZTE as much, but it's still not as close to the GT31 as it is when on top of the car with a good sky view.