Most accurate altimeter + OSD combination?

[Airbender]

Friends,

I need some advice regarding the altimeter and OSD combinations

Recently completed building my "cameraship" - Trex 600 with and Askman .50 APLG mount.

I will be getting a wireless camera downlink equipment soon....now I am focusing on the OSD equipment.

More specifically, I want to be able to very accurately record my height above the ground while having the camera looking out at a straight and level view from the bottom of the helicopter.

In short, I am going to record "balcony" shots and I need to be able to accurately measure the height of the subject(s) I am viewing.

I have seen several OSD systems on the market. Most use a GPS antenna to record the height...but just how accurate are they?

There is another called the VIZION and t uses a VLOG MOD3 altimeter which claims to have a resolution of +/- 1 foot. Again, are these really that accurate?

It is imperative that I have the most accurate altimeter reading possible and be able to overlay the readings on the video thru an OSD system.

Any suggestions from the experts?

TIA to all..

[Mr.RC-Cam]

Both measurement methods have merit, but I would favor the Zlog altimeter for your application.

GPS provides very stable values, especially over the long term. That is, they are not affected by changes in weather. But unlike GPS coordinate data (which is very accurate), its altitude precision is not always ideal. For applications like yours, you would probably expect ±10 meters of error worse case.

The Zlog uses barometric pressure to determine altitude. It indeed can resolve 1 foot resolution. I can walk up stairs and see the climb register on it. For short periods of time, such as over the course of a typical flight, it does very well; I think you will see less than ±3 meters of error overall.

But one issue that you need to plan for is that the rotor wash can act as air pressure and affect the readings. So placement requires experimentation in some installations. Most heli photographers zero the Zlog at ground level, but then make a mental note of the alititude at a low hover and use that value as the real "zero." That way any error from rotor wash can be accounted for.

The Zlog MOD3 is manufactured by Hexpert Systems and they are ending production. It will soon be a very difficult item to find. So if you are interested in using one, now is the time to buy it. For the sake of full disclosure, I'm involved with the Vizion OSD design and dpcav.

[Wavess]

But one issue that you need to plan for is that the rotor wash can act as air pressure and affect the readings. So placement requires experimentation in some installations. Most heli photographers zero the Zlog at ground level, but then make a mental note of the alititude at a low hover and use that value as the real "zero." That way any error from rotor wash can be accounted for.

That´s the same "issue" that Mikrokopter has with it´s pressure sensor, but there are a few solutions to make it work properly, such as.

Place a tape on the hole of the pressure´s sensor (where the mebrane it´s located) and make a little hole with a pin in it.

Adapt pressure´s sensor to place a "what is called on the Air Conditioning control systems" tubing. This is used on this field to have a homogeneus reading on diferential presure and it has very stable readings. The way to do it it´s only to adapt a silicone fuel tube on the sensor´s hole (where the membrane it´s located) and place it where there isn´t any air turbulence.

[Blue Sky]

There is something to be said for Eagle Tree Systems OSD.

http://www.eagletreesystems.com/OSD/OSD.htm

It would give you both barometric and GPS readings as well as

data logging. There are alot of accessories for it.

They provide good service and they aren't likely to go out

of buisness any time time soon.

They have a new OSD in the works that looks great!

http://www.rcgroups.com/forums/showthread....018&page=23

Hopefully it will be out soon.

Their barometric altimeter is only rated for 1 meter resolution

but that may be about all you can expect to get in your application.