Why does GPS hate me? (And other GPS questions) - RC Groups
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Jun 18, 2011, 12:54 AM
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BushmanLA's Avatar

Why does GPS hate me? (And other GPS questions)

I always seem to have a problem with GPS reception on my aircraft. I've tried the GPS with the Remzebi OSD, RVOSD, and the one that came with the DOSD bought here.

I think it's the LS20031.

Anyway, getting a decent lock is darn near impossible inside my old house or my new workshop. Outside is better and possible but sometimes it decides to never happen sometimes it takes 10 mins, sometimes just a min or two.

What really gets me is that I can whip out my android phone with a GPS checker app and get a 8 or so sats in all of thirty seconds. All this in the same place as the aircraft still scratching to get three sats.

Shouldn't my OSD lock even easier with that nice big antenna it has?

I'm mostly trying to get better performance from the LS20031 since it is the only one I use.
The problem is there with or without my RC Tx on and nearby.
The problem is there with or without my 900Mhz Video Tx on, filtered or unfiltered.
Any suggestions?

Whats the go to GPS unit to have these days?
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Jun 18, 2011, 01:07 AM
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Daemon's Avatar
You can't expect to get a standalone GPS lock inside your house or workshop.
It needs a clear view of the sky. *Sometimes* if you have a really solid
lock you can maintain it inside a structure with a thin wood roof, or big open windows.

Your cell phone is cheating because it uses "assisted GPS" which means
1. It has a rough idea of where in the world it is immediately, due to cell tower triangulation. It initializes the GPS receiver with this information which gives it a huge head start in identifying which satellites in the constantly moving GPS constellation it should expect to hear.
2. It then receives the GPS signals and sends the raw timing data to the cellular network where computers on the network do the initial location calculations to narrow its position further and sends this back to the phone. This takes a lot of the initial workload off the GPS Rx's CPU.
3. Then once it pretty much knows exactly where it's at, it hands off the position calculations to the GPS CPU in the phone which can maintain GPS lock even if it temporarily moves off the cellular network.

You can generally verify this process by trying to get a GPS lock while you're out of cellular coverage. It will either not lock at all (most phones), or it will take a long time (as long as your standalone GPS). Even if they have the ability to get a GPS lock off network, most smart phones remember where they last had *any* position data and store this, so the next time GPS Rx is turned on it can start there.

A standalone GPS receiver without battery backup (most of those used for OSDs) has to start from scratch knowing nothing about where in the world it is, and thus no idea which GPS satellites to expect to see. It has to listen for a while to identify any of them and then get a solid lock on 3-4 satellites before it can even start to calculate its position, and it takes a while to narrow it down. This can take a few minutes. If the GPS receiver has battery backup then it may be able to speed the process up the next time it boots because it knows where it was last, and assumes its near that same location so can calculate which satellites it should expect to see. This can cut the initial lock time down to 30-45 seconds.

Last edited by Daemon; Jun 18, 2011 at 01:12 AM.
Jun 18, 2011, 01:22 AM
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BushmanLA's Avatar
Thanks. I didn't realize the phone had all those cheats. I know there was a position available based on the tower I was using etc.

So 15 mins is not out of the question for a cold start stand alone GPS?
Jun 18, 2011, 02:08 AM
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Daemon's Avatar
15 minutes sounds like a long time, but do you have clear coverage of the sky when
it takes that long?

Jun 18, 2011, 05:29 AM
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Gree's Avatar
Make sure that your plugs are secure. I had lock problems on my RVOSD when I first got it and it turned out the plug wasn't in as well as it should have been. Sometimes in can also take a while. You might find when you go to your flying field it actually picks up satellites a lot quicker.

When I live has a lot of high rises so there is a lot of shadowing making locks difficult. But if I go to the field off it goes happily.
Jun 18, 2011, 05:38 AM
Registered User
Well till now I always had the same problems. I have search and search and I found a solution what can help. First i have done this mod with my locosys LS20033 https://www.rcgroups.com/forums/show...5&postcount=16
Next I have replaced my patch antenne with a bigger one from an old bt gps receiver. Just simple desolder the old antenna in the middle and replace it with a bigger one. I have an original LS20033 to compare the results and here they are. The original with the small antenna and no patch and solder mod: no fix in my house. With the mod i have 5 sattelites in 3 minutes. Next thing is to solder a backup battery http://ikarusosd.wikispaces.com/FAQ#nueve

Last edited by marco899; Jun 18, 2011 at 06:17 AM. Reason: Photo's
Jun 18, 2011, 07:36 AM
FPV Desert Beta Test Center
I've had my share of gps problems over the years and believe there are two general causes.

1) The first known for a long time is wide physical separation from any other noise on the plane.
2) The second is gps damage which is less understood. From my experience they are easily damaged by rough handling and when damaged will give you fits with marginal performance. I used to velcro them in place and take them on and off all of the time. Now I only use a cushioned mount in which the module held softly and never moved.

If you have enough separation and are still having problems try a new unit. I'm using the RV 10Hz if your equipment will accept 10Hz. I don't know if this brand is any better just that I have used it for some time with zero issues.

I get 10 sats in my garage.
Jun 19, 2011, 03:31 AM
Registered User
Other causes may include the supply voltage. You typically get the voltage from an OSD or other board that outputs 3.3V. Some boards only output 3.1-3.2, causing problems with some GPS's.

There are GPS's with onboard little batteries (basically large caps), which allow the GPS to retain the position up to 1-2 weeks. These can go through a hot/warm start, yielding a lock in a second up to 30 seconds.
Jun 19, 2011, 10:18 AM
FPVing for fun...
rcall's Avatar
I've been working with GPS for years and had small modules take one to 40 minutes to fix from a cold start...
Jun 19, 2011, 11:03 AM
Kiwi in Germany
whakahere's Avatar
Do you have wire wrapped in a toroid core right next to it? Since I did this mine locks under a minute when outside and holds the signal much better.
Jun 19, 2011, 08:13 PM
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techspy's Avatar
I had horrible (10 plus minutes) lock times on my Mikrokopter and 5 mins or so (and horrible accuracy) with my Skywalker and my ET OSD and torrids made no difference. I now get almost instant locks on both. What did I do? Placed a sheet of copper (5"x5" on the Skywalker and approx 8"x8" on the MK) under the GPS. From what I understand this helps in 2 ways. It gives a good, solid ground plane and blocks RFI from the GPS antenna. I have a lot of RF on both craft and believe the man problem was RFI. I found out by holding the GPS in my hand while powered up. It made a huge difference and helped it lock much better. I got some copper sheeting (very thin, like paper thin) from the local craft store (Hobby Lobby or Michael's) and it was very cheap (less than 10$). I can even get locks inside my house, where I never got a lock before.

One of the best mods I have done to my FPV craft. Try it.
Jun 23, 2011, 06:41 AM
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Last edited by kyle7119; Jul 29, 2011 at 08:27 AM.
Jun 23, 2011, 06:45 AM
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techspy's Avatar
When is the last time you had yours powered up and locked? What about his? Do the DSOD have an onboard battery to keep the config info from being lost? If so, does yours show the correct voltage? Did you try putting some type of ground plane under it like the copper sheeting I mentioned?
Jun 23, 2011, 06:52 AM
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Last edited by kyle7119; Jul 29, 2011 at 08:27 AM.
Jun 23, 2011, 06:57 AM
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techspy's Avatar
I wasn't referring to the correct feed voltage. Most GPS have a small watch-type battery (like a cmos battery on a motherboard) to keep the location info in memory for future power-ups so they take less time. Basically on a "cold" power-up the GPS has to scan through a lot more info to figure out what part of the world it is in and what satellites it should be able to see. Once it has that info it stores it for future reference so it can get its location quicker on subsequent power-ons. If the memory battery is bad, that info will be lost every time and you have longer lock times. I had to replace one on my Mikrokopters GPS board as it would not hold a charge anymore (most are rechargeable as this one is, I had not powered it on for 6 months and the battery was completely dead).

If it never got the correct "part of the world location" the one satellite you see it losing in the air, may not be a satellite it should even be trying to connect to. I had issues with my smart phone getting confused about what part of the world it was in and even though it could "see" several satellites, it was trying to lock on to sats that where way too low on the horizon. I could see the issue when I looked at the time the sats were reporting (it was a about 5 time zones off!)

Anyway, just a couple things to check. Maybe leave the gps/dosd powered on over night in a location that it should be able to get a lock.
Last edited by techspy; Jun 23, 2011 at 07:02 AM.

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