[babz]

I put a revo in a little foamy,when the plane is connected with usb,the GPS lock sats but when i start without usb,the revo doesn't go in STAB and ATTI mode because the gps is with a red cross.
what have i done wrong ?

[TheOtherCliff]

That is exactly backwards. 

It only has power when the flight battery is connected.  Do you have some strange jumpers on your FC maybe?

[Brian]

Assuming the GPS is actually powered in both configurations, it could be a ground plane issue.

I've seen situations with OPLink where the RSSI dropped significantly when the OPLink was powered from battery vs being powered from USB connected to a computer, or even just having a USB cable plugged in and not connected to anything.  The reason is that small antennas don't have a very big ground plane, which is not optimal.  Plugging in a cable, and especially a that cable is connected to a larger grounded surface (your computer) drastically increases the size of the ground plane, which improves the reception of the antenna.

[TheOtherCliff]

Constant "red cross" means bad cable, no power, fried GPS, etc.  It means you have a hardware problem, not just a reception problem. 

Red block means working but no usable fix.

[babz]

But it becomes green with usb

[TheOtherCliff]

Do you have some strange wiring on your FC that allows USB to power the FC mainport flexiport?  Ahh... that and maybe you have opto ESCs that do not send power on the red ESC "servo" wire.

If you have opto ESCs you will need a way giving 5V power to the FC so it can give 5V to the GPS.  How is your RC receiver powered?  Are you using a 3.3V satellite receiver?

[babz]

i've a standard ESC with bec,it's a plane,if i connect outside the plane with connected usb,it locks and initialise the card then i can remove the plug and it keeps this statement.If i only plug the battery as it should be,the GPS(wich has it's own led status) has led status with locked sats but the card doesn't initialise,if i plug USB ...it goes

[TheOtherCliff]

So you get constant red cross on GPS if you boot just with flight battery.

If you boot with USB power and no battery the GPS works correctly, and if you then plug in the battery and unplug USB, it continues to work fine.

Is the GPS plugged straight into the FC (mainport or flexiport or flexiIO) with nothing else (OSD?) connected to it there, no power wires going elsewhere?

Do you have something strange configured for telemetry?  Do you use OSD?

Maybe just post your uav file (File -> Export UAV Settings) and describe all your wiring, and I can see if there is an issue.

Wiring should be just:
battery to ESC
ESC servo wire to FC
ESC motor wires to motor.
FC to RC receiver (what protocol, SBus, PPM, PWM, etc.) where is it connected?
FC to some servos.
FC to GPS (which port on FC?) (what kind of GPS)
FC to mag on GPS (do you have an I2C aux mag connected?)

[babz]

here are the settings
servos and esc are connected to the right place,gps(m8n) on main,no mag on gps,receiver with PPM on rc inputs.
no OSD

[TheOtherCliff]

It works exactly as it should work.

I put those settings on a Revo, enabled oplink telemetry (since I don't have it in a vehicle to test), plugged in a ubx gps and i2c mag and everything worked as it should.  The GPS started out red block (as it should be with no sats yet found), not red X.  After a few minutes it went orange, then green.  Mag works fine too.  USB not connected.

I powered it off/on (with battery again) and it was still fine.

I powered it off and connected via USB only (no battery power) and it did as expected.  Red X on GPS and also on I2C (and other health colors because of that).

How do you know it is red X with battery and no USB since you don't have telemetry set up?  Maybe you are assuming you have a GPS problem because it won't arm?  It needs more that just a GPS fix with the light blinking.  It needs at least 7 satellites.  It needs PDOP to be no more than 3.5.  It also needs green mag.  Then it will arm.

[TheOtherCliff]

Small GPS sometimes don't have power off storage and so each power on is a cold start.  Cold start can take as long as 15 minutes to download the almanac (list of satellites to make sure it knows which sats are visible).  After that long, INS13 has gone stale and maybe the mag is not green because it is sitting on the ground.  Even with normal GPS it is good practice to wait 15 minutes before the first flight of each day.  I admit I do not do that (if it arms it has checked sats>=7 and PDOP<3.5) but I am not currently having problems.

Look at the FC LEDs.  At first you should see blue (1 second), orange (1 second), repeat.  That means not ready to arm.  When you see blue (one second), but very short orange blip, then it is ready to arm.  Or are you using "always armed"?

So after model sitting for 15 minutes, pick it up and do a pirouette with your body to move the aircraft around in a circle.  This will fix stale INS13, and also may find a spot where mag is green.  As soon as mag is green (with GPS already green and >=7 sats and PDOP<3.5) it will become ready to arm (you can tell because the orange LED is now a short blip) and stay that way even if gps or mag is no longer green.

[babz]

it's allways armed, i will try to move as you say

[f5soh]

Can you post a picture of your setup ?