Xantos

Hi Guys,
Flying my Bobcat /Jetcentral Cheetah this weekend , I recorded a speed of 533.7 Kmph = 331mph on the Spektrum Telemetry with the GPS sensor.
All the runs were at WOT and in my estimation , downhill speeds were well over 250mph but the max speed recorded seems to be quite a stretch , question is how much error could there be in the max speed reading ?
N

rhklenke

My Feedback: (24)

It depends on how the max speed is calculated - which nobody but the GPU unit's manufacturer knows for sure.

Likely, its the maximum distance between two sequential GPS fixes over the time between fixes. If the GPS unit is a 5Hz unit, than there is 200 msec between each fix. 331 MPH is 485 ft/sec, or 97 ft in 1/5 sec. 250 MPH is 366 ft/sec, or 73 ft in 1/5 sec. The difference is 23 ft. - it is easliy possible that two subsequent GPS fixes could be off by 13 feet each, resulting in a 23 foot total error...

To really calculate a good speed estimate, you need two timed fixes separated by as much distance as you can get. Then of course, you'll need to correct for wind as GPS is speed over the ground...

Xantos

Sounds like a reasonable explanation!
no idea however on the specs of the GPS , It's a little matchbox size black box that plugs into the TM1000 into the Spektrum receiver.

bri6672

My Feedback: (6)

So can the speed be viewable real time? More importantly be voiced with a dx18 gen2??

Jgwright

GPS speed measurement is not accurate as you have found. Much better to fit a simple Pitot tube system. I see that Spektrum sells a Pitot based system and it is half the price of the GPS one!

http://www.spektrumrc.com/Products/D...rodID=SPMA9574

John

Xantos

John,
Ive tried the Pitot but the reading are erratic definitely more so than the GPS

Jgwright

Well I am surprised at that. I would get back to Spektrum.

John

Quandry

As others have mentioned there are difference between pitot and gps monitoring. I have tried both on my DX18 and found the GPS easier to work with, but may not be as accurate. In terms of your own reading, that sounds extremely fast so one of these is the case:

1. your jet is extremely fast
2. the GPS technology isn't well implemented and is lying
3. your actual GPS unit has a bug/issue and it is giving a false reading

My money is on either option 1 or 3. In both cases though we can not tell from the forum which one it is, but you can make a simple test to check by making up an RX/Battery/GPS combo on a board, have a friend drive you down the freeway at 100kph, does your TX output show you 100kph or is it way off? refit to your jet and see if the results correlate.

OK it's not accurate down to the nth degree but it is a simple test to use as a 'sanity check'.

Let us know what you find out!

Xantos

Quandry ,
yes the jet is fast and Im probably flying it at WOT most of the time but the only reason i was asking about the accuracy of the unit is because of the wide variation of max speeds reported over 4 flights although the flying style and environmental factors across all 4 flights was similar.
I saw speeds from 403kmph ,388kmph , 533kmph and 460kmph
so theres too much of a variation.

sidgates

My Feedback: (2)

I have the N2 pitot for the Taranis radio. I was getting slightly erratic results and the designer told me to keep the pitot ports covered for a few seconds when the system is powered up. Now the speed read outs have been very consistent and accurate. This may help on Spektrum too.