During flight, my newly purchased Phantom-4 starts to come down or goes upward without any explicit command from my side. Any idea?

Try calibrating the sensors using 'DJI Assistant 2' downloadable from Here

I had the same problem. After I calibrated my VPS sensor it was back to normal. This is the cal you need to do in front of a computer screen lining up the blue/red boxes. It kept failing at 66% during the cal. I finally tried another monitor and it worked.

My symptom was when I let off the ascend stick it would continue to climb, sometimes 100 feet more, then slowly descend back to where I originally centered the joy stick. When I landed my distance from the ground was way off.

Please try to calibrate VPS sensor as Aardvark mentioned above, and check whether you have the latest firmware or not, if no, please upgrade it firstly.
Then please calibrate IMU before flight.
If still the same case, we'll guide you to export the flight data for analysis.

Roadstar1700 Posted at 2016-12-9 05:56
I had the same problem. After I calibrated my VPS sensor it was back to normal. This is the cal you need to do in front of a computer screen lining up the blue/red boxes. It kept failing at 66% during the cal. I finally tried another monitor and it worked.

My symptom was when I let off the ascend stick it would continue to climb, sometimes 100 feet more, then slowly descend back to where I originally centered the joy stick. When I landed my distance from the ground was way off.

Thanks dear.

I tried it few days back but could not complete that as at some point during the vps calibration, the tracking of the box on the screen was not getting made. It was like a lock at some position and beyond that it was not following the box. I was using my laptop screen so following yours advise I will try the same on some desktop screen.

Keeping a fixed height is critical in my case as I am using my Phantom-4 to scan different crop fields. I have indigenously mounted the following items on my Phantom-4: a multispectral sensor, gps receiver, mounting mechanism, etc. All this payload amounts to almost 300-400 grams.  

fansc53c1d1e Posted at 2016-12-9 07:36
Thanks dear.

I tried it few days back but could not complete that as at some point during the vps calibration, the tracking of the box on the screen was not getting made. It was like a lock at some position and beyond that it was not following the box. I was using my laptop screen so following yours advise I will try the same on some desktop screen.

Have you anything mounted below the aircraft, underneath the VPS sensors ?

If so remove and try again anything on the lower part of the landing gear 'might' interfere with the sensors.

It might also struggle with that payload. Thats an extra 30% weight to carry.

Aardvark Posted at 2016-12-9 14:42
Have you anything mounted below the aircraft, underneath the VPS sensors ?

If so remove and try again anything on the lower part of the landing gear 'might' interfere with the sensors.

https://forum44.djicdn.com/data/attachment/forum/file:///D:/UnmannedAerialSystem-Activities/Phantom-4%20pics/1.jpg[img]https://forum33.djicdn.com/data/attachment/forum/file:///D:/UnmannedAerialSystem-Activities/Phantom-4%20pics/3.jpgfile:///D:/UnmannedAerialSystem-Activities/Phantom-4%20pics/4.jpg[/img]

fansc53c1d1e Posted at 2016-12-9 23:14
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Please see the two pics attached. These show the mounted ADC-Micro sensor on my Phantom-4. Is there any problem for the VPS sensors with this mounting?

fansc53c1d1e Posted at 2016-12-9 18:21
Please see the two pics attached. These show the mounted ADC-Micro sensor on my Phantom-4. Is there any problem for the VPS sensors with this mounting?

That could well be causing interference with the ultrasonic sensors. The only way to know for sure would be to remove and see how stable the aircraft was.

If you are flying like that then don't use RTH or pull the throttle full down before landing, and tell it to hover on lost signal not RTH!   

It may well think it is on the ground at all times due to the ultrasonics bouncing off your equipment.

Aardvark Posted at 2016-12-10 01:03
That could well be causing interference with the ultrasonic sensors. The only way to know for sure would be to remove and see how stable the aircraft was.

But the VPS is generally believed to work at when no GPS signal is available, e.g., indoor flying. In our case, we fly in an open experimental area where GPS signal has no problem. Is there a way to disable VPS to avoid the problem?

fansc53c1d1e Posted at 2016-12-10 10:42
But the VPS is generally believed to work at when no GPS signal is available, e.g., indoor flying. In our case, we fly in an open experimental area where GPS signal has no problem. Is there a way to disable VPS to avoid the problem?

The visual positioning can be switched off in DJI Go, but I think the ultrasonic sensors still operate and measure the height above ground up to 10m. You could try it and see what happens. But I would take great care when landing (or flying), if it gets a false signal due to equipment fitted then a full down left stick might cause the motors to stop. Which is not what would normally happen.

VPS works with GPS when it is near the ground since it gives more accurate positioning and doesn't drift around by a few meters like GPS does.

How does it know it is near the ground and thus should use VPS?  - because it is reading a short distance on the ultrasonic sensors.   If the ultrasonic sensors are seeing your equipment then you will have problems.

Aardvark Posted at 2016-12-10 17:05
The visual positioning can be switched off in DJI Go, but I think the ultrasonic sensors still operate and measure the height above ground up to 10m. You could try it and see what happens. But I would take great care when landing (or flying), if it gets a false signal due to equipment fitted then a full down left stick might cause the motors to stop. Which is not what would normally happen.

Gentleman, Thank you very much.
We will re-design our mounting.

Nigel_ Posted at 2016-12-10 19:17
VPS works with GPS when it is near the ground since it gives more accurate positioning and doesn't drift around by a few meters like GPS does.

How does it know it is near the ground and thus should use VPS?  - because it is reading a short distance on the ultrasonic sensors.   If the ultrasonic sensors are seeing your equipment then you will have problems.

Gentleman, Thank you very much.
We will re-design our mounting.