Recalcitrant GPS

[rogellis]

I have two PAWs running next to each other.  One will boot up in 2 mins, the other in 30 mins. 
The reason is the latter will not find enough GPS satellites. 

They are the same PAW units, in the same boxes, with the same wiring, but the GPS on No2 always sees 4 fewer satellites than No1.  Is that a known fault with GPS dongles?  I cannot see any other reason for the difference.  I tried shielding the dc converter and the speaker amplifier, but that made no difference. 

These are the log messages.

Yes, I know, I am sad (it is midday on the 25th......)

R

[Ian Melville]

Can you swap the GPS dongles between the units and try again?

[exfirepro]

Rog,

Just reading back through your posts and noticed that the GPS module in the photo of one of your units over in the GPIO thread is ‘wrong side up’, which might be significant. The GPS antenna in the UBlox dongle is on the side of the board where the case has the black print. Fitting it as shown in your pic means that the antenna is screened between its own pcb and the main PilotAware boards which will certainly reduce its ability to pick up satellites.

Impressed by your ‘tenacity’ and determination to test PilotAware’s operational limits BTW. Interesting to read how far you needed to go to to get PAW to ‘fail’. Just glad I don’t keep a cooking pot in my flexwing LOL   

Best Regards

Peter

[rogellis]

Peter.

Not sure if that was the problem or not, as gps reception was still not too good when I flipped it over.  I then swapped the dongles between the boxes, and they both came up with good results.  I then swapped them back, and again they are both working normally. 

I was looking on the Amazon comments site, which said that when first switched on the gps may take 15 minutes to acquire a position.  But subsequent initialisations will be much shorter.    Is that true? 

Anyway, both appear to be working normally.  But I have ordered a cheapo spare from Ebay just in case, at some ridiculous price of £6.

R

P.S.   How can you fly without a pressure-cooker...  ;-)

.

[Ian Melville]

R

When a GPS is first powered up it does not know the time or where it is, from that it will not know how any satellites there are out there their frequencies or locations. It has to find one or more satellites by trial and error, then download an alimanc (all the sat data it needs). It will then can start working out where it is. This data is kept alive on the GPS by a small battery, so next time you power up it has a head start. If you leave it off for too long the data in the alimanc will expire, or be lost of the battery runs down, or if the unit is move al long distance while off, it will need to start the process again. So yes it is true.

[exfirepro]

Hi Rog,

The only thing I would add to Ian’s well detailed explanation is that a poor ‘sky view’ such as indoors or inside a metal hangar can considerably increase the time taken for a GPS unit to establish (or re-establish) its ‘ephemeris data’, which is why we always suggest initial powerups be done in the open - especially if difficulty is being experienced. Metallic coatings/additives in some types of (conservatory) glazing can also block or significantly reduce satellite acquisition, considerably increasing the ‘time to first fix’, or even preventing the fix being acquired at all. The other tip is to try not to move the unit until it has got its ‘fix’ as a moving unit can significantly extend the ‘TTFF’. Best if you can to sit for a few minutes until a good GPS fix has been established before moving off.

Best Regards

Peter
p.s. I love pressure cookers.....      but for weight reasons my in flight catering is usually restricted to the odd Mars bar* and bottle of water (*other chocolate snacks are available), though I did fly back to Montpezat from Peyresourde Altiport (Pyrenees) one time with 8 large pizzas and 2 bottles of wine in the back seat. (a long and slightly scary story with a happy ending) 

[rogellis]

If you leave it off for too long the data in the alimanc will expire, or be lost of the battery runs down, or if the unit is move al long distance while off, it will need to start the process again. So yes it is true.

Thanks for the info Ian.  Yes, I suppose my car GPS does something similar.

But I still have one lazy GPS.  I started them up simultaneously this morning - one gained a fix in 2 minutes, and the lazy one in 12 minutes.  Perhaps the lazy one's internal battery is not working, so it has to start again from scratch?  I have ordered a spare, just in case it continues to play up.

I do like the way they chirp to each other:  "traffic ten meters, same level...."   
Not the sort of thing you want to hear.....!

R

(The term 'chirp' is probably appropriate.  Many aeons ago I was told the first IFF transponders had the acronym POLLY, and so they 'squawked'.  Since PAW is smaller, it will 'chirp'....)

.

[PaulSS]

Sorry Roger but, just to extend the bit of thread drift, someone has already beaten you to your use of 'chirp'  . We used to use it (in the Fleet Air Arm) for our I Band transponders. The fighter controllers would quite often instruct us to strangle our gadget, strangle the parrot and chirp. I think they had something against Norwegian Blues 

[rogellis]

Sorry Roger but, just to extend the bit of thread drift, someone has already beaten you to your use of 'chirp'  . We used to use it (in the Fleet Air Arm) for our I Band transponders. The fighter controllers would quite often instruct us to strangle our gadget, strangle the parrot and chirp. I think they had something against Norwegian Blues 

Perhaps the PAW should be called 'Sparrow' or 'Finch'.....   


R