Strange jump in IGC file recorded on iPhone

Last Sunday, I gave 3 rides in the club's L-23 and recorded them using the SeeYou app on my iPhone 4S. Flights have been uploaded to SkyLines with the 3rd one (https://www.skylines-project.org/flights/26873/) showing the anomaly.

Near the time of my release, the altitude jumps almost 1,000' and position a half mile between 15:37:05 and 15:37:06 PDT.

It seems the takeoff altitude is correct, but then the tow seems to stop climbing for a while as recoded, then continues about 1,000' below actual altitude until the jump, where it catches up.

The position jump is also interesting as it appears the position drifted for a while, then suddenly caught up to reality.

Is this a result of the iPhone dealing with a poor GPS signal? I was in the rear seat with the phone in my breast pocket, so a good view of the sky forward and up, but the wings and fuselage could be blocking some satellites to the sides and rear.

-Tom

Hi Tom,

I saw a NOTAM regarding GPS testing around Alamogordo a few days ago. It's
supposed to run through March. My CAI302 had trouble connecting on Friday.
You're probably out of range of such activity, but I thought I'd mention it.

wrote in message
...
Last Sunday, I gave 3 rides in the club's L-23 and recorded them using the
SeeYou app on my iPhone 4S. Flights have been uploaded to SkyLines with
the 3rd one (https://www.skylines-project.org/flights/26873/) showing the
anomaly.

Near the time of my release, the altitude jumps almost 1,000' and position
a half mile between 15:37:05 and 15:37:06 PDT.

It seems the takeoff altitude is correct, but then the tow seems to stop
climbing for a while as recoded, then continues about 1,000' below actual
altitude until the jump, where it catches up.

The position jump is also interesting as it appears the position drifted
for a while, then suddenly caught up to reality.

Is this a result of the iPhone dealing with a poor GPS signal? I was in
the rear seat with the phone in my breast pocket, so a good view of the
sky forward and up, but the wings and fuselage could be blocking some
satellites to the sides and rear.

-Tom