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I'm Using My Onstar Antenna For My Garmin GPS

J

jasgot

Charter Member
Full Member
Joined
Apr 6, 2002
Messages
102
Location
Detroit, MI
I have a Garmin 176C; when in my topless jeep, it works great. When in my Av, it works, not-so-great!

I made a cable to go from the Onstar Antenna to my Garmin. The results are nothing less than awesome!

With the stock antenna on my garmin, I get few sattelites, lose the signal often, and cannot get WAAS info. See this pic:

http://www.syo.com/mj/gps/GPS005.jpg
Notice is says, Searching for WAAS? It never finds it!

With the GPS Plugged in to the Onstar Antennea, which is a gain antenna, drawing power from the gps through the antenna connector, I get great reception, NEVER lose a signal AND get Differential Data from every satellite, providing highly accurate WAAS info.

See pic:
http://www.syo.com/mj/gps/GPS004.jpg
Notice it says Using WAAS, ?also, notice the "D" in every satellite bar!

Here is the back of the GPS, a BNC connector:
http://www.syo.com/mj/gps/GPS006.jpg

Here is the attachment to the Onstar antenna, a SMB connector:
http://www.syo.com/mj/gps/GPS010.jpg

The loose wire is the antenna wire that goes to the Onstar box in the dash. ?Onstar displays a red light to the left of the white dot button, no biggie, just plug onstar back in and it immediately goes to green.

I'm so happy!


images too large ... changed to urls by admin
 
Holly Crap!!! this is big.
Can we get more details?
 
We were wondering what that little black box was on the windshield. It seems to be different for different make dates as well.

I agree. THIS IS BIG.

 
mnr602 said:
Would it be possible to split the antenna signal with a Y Adapter?
Click to expand...

I would think, yes. My first thought is that a diode would be required to prevent one power source from getting into another.
 
This is a very important find. Unfortunately, I use my notebook for GPS navigation and merely connecting an antenna to my notebook will not work. (This is also the case with Chasman's setup, though he may also have a Garmin unit as well.)

The only other possibility for people in my situation is that you can toss the usual GPS beacon into the dash and pry it apart to connect the antenna to the beacon itself. I'm not sure if I want to tear apart a $400 GPS beacon though. :/
 
That little black box behind your mirror is indeed the GPS antenna for Onstar. The Onstar antenna is still on your roof. (It's a cell phone antenna.) That will only work on older model Av's cause in the newer models the GPS antenna is not on the windshield but built into the base of the cell phone on the roof. You may still be able to get at the newer ones. I believe there is a plug under the dash at the base of the RH "A" pillar for it. Great find ! :B:
 
Very interesting Jasgot. I had often wondered about the possiblity of doing that. However, before anybody else tries that, I would try to determine what is the OnStar antenna voltage, and make sure that the GPS unit you are using will not try to supply more voltage to the antenna than what OnStar supplies. For example, if the OnStar GPS antenna is expecting 3 volts, and your GPS unit puts out 5 or 12 volts to the antenna, you could damage the amplifier in the antenna and ruin it. Even if it doesn't burn out right away, extended usage at the wrong voltage can damage it in the long run.

mnr602 said:
Would it be possible to split the antenna signal with a Y Adapter?
Click to expand...
jasgot said:
I would think, yes. My first thought is that a diode would be required to prevent one power source from getting into another.
Click to expand...
I think it's going to be a little more complicated than using a splitter or a diode.

A bit of background for those not familiar with them: GPS antennas typically have a built in amplifier, which is powered by the GPS unit. They typically run off of 3, 3.3, or 5 volts. although I've seen 12 volts used in at least one application (TrueTime.) The cable between the antenna and the GPS unit is a siple coaxial cable with a center wire surrounded by insulator and a shield. The shield is grounded, and the center wire provides both the power from the GPS unit to the antenna, and the RF (radio frequency) signal from the antenna to the GPS unit.

Now, when you try to hook up two GPS units to a single antenna, you have the problem of two different power supplies fighting each other in their attempt to power the antenna. This would not be good.

While a pair of diodes may block the power from one unit being fed back to the other unit, it will also block half of the GPS signal from reaching either unit.

Placing a capacitor inline with the cable going to one of the units would block the DC power coming out of (or going into) that unit. However, that capacitor will also affect the loading and the transmission of the RF signal. Care would have to be taken to choose the proper value for that capacitor. I don't know the formulas that one would use to do that.

The actual units use a hybrid network to inject the DC onto the signal line, and to extract the RF from it. The best bet would be to do something like that in a coupler. The net result you want is to split the RF power from the antenna evenly to the two units, while also alowing DC from one unit to pass, and blocking DC in or out of the other unit.

To do this right will take time, and could end up costing more than the cost of an extra external GPS antenna, especially when you take the value of your time into account. For my personal Garmin GPS V, I bought one of the Garmin antennas (about $70) and mounted it to the center of the dash at the base of the windshield. It works great for me, and I don't have to worry about any interaction between it and OnStar. Click on the link in my sig for photos of my finished installation.

A very important point to keep in mind, as indicated by Boar-Ral: What we are talking about here is the radio frequency output of the antenna which must go to a GPS receiver/processor. This is not the NMEA serial data output that can be plugged into a laptop for use with a mapping program. You will still need a GPS receiver/processor unit that recieves the RF signal from tha antenna, decodes and processes it, and outputs location fixes though a serial output.

So far as I know, nobody has yet found somewhere to tap into a serial data line from OnStar so that it can be fed to a laptop/pamltop. There are those, including myself, who think that it is not possible because OnStar is designed to be hacker-unfriendly.

-- SS
 
ShapeShifter said:
So far as I know, nobody has yet found somewhere to tap into a serial data line from OnStar so that it can be fed to a laptop/pamltop. There are those, including myself, who think that it is not possible because OnStar is designed to be hacker-unfriendly.

-- SS
Click to expand...

I have this info...... ;D
 
ShapeShifter said:
Well, don't just leave us hanging, man. Spill it! :eek:

-- SS
Click to expand...

You'll have to decide if it is NMEA or not..... I'll post it in a few minutes.... The schematic simply says, Serial Data Send / Receive
 
jasgot said:
You'll have to decide if it is NMEA or not..... I'll post it in a few minutes.... The schematic simply says, Serial Data Send / Receive
Click to expand...

Just like one needs to be wary of voltages going into the GPS antenna amplifier, one needs to be wary of voltages going down the data lines. It may be 'data in' and 'data out' but if the voltages are not within tolerances, you can kiss the serial header goodbye on your notebook.
 
So would this work with a laptop. I'm getting ideas for my next mod. I want to run my GPS antenna the dash but I'd rather hook up to the OS GPS.

Manny
 
jasgot said:
I've tried twice and can't get the files attached, any ideas?
Click to expand...
Did you make sure it's less then 250 k bytes?

-- SS
 
Yes I am also very interested in this. I have been thinking about this for a bit and was going to probe the OS interface one of these days. I've been thinking about mounting a Cappuccino PC (http://www.cappuccinopc.com) in the AV and being able to interface to the OS GPS would be cool!
 
I am not positive about this, but I have seen schematics for the "Serial In and Out" connections and I believe that they are part of the Multiplexed Serial Bus on the AV.. Not RS232 serial signals exclusivly transmitting GPS data :( Anyone know otherwise ???
 
Leather said:
I would love to disconnect OS and hook my laptop to the OS antenna, please tell me we can do it simply and safe :)
Click to expand...
Leather, (and others who asked pretty much the same question) you cannot hook the antenna directly to a laptop as indicated in the first post. Apparently you can, with certain caveats mentioned earlier, hook the antenna to a GPS receiver, and then hook the serial output of the receiver to your laptop.

As for using the GPS receiver built into OnStar, that's a different question. Read on...

bmontini said:
I am not positive about this, but I have seen schematics for the "Serial In and Out" connections and I believe that they are part of the Multiplexed Serial Bus on the AV.. Not RS232 serial signals exclusivly transmitting GPS data :( Anyone know otherwise ???
Click to expand...
When I first investigated this, I saw those serial lines, and a description that called it "class 2 serial data" which is a multiplexed serial buss as you indicated. Now, when I go back to look at, I can't find that description.

I guess some more investigation is called for. If I have a chance, I'll try to borrow a scope from work and see if I can figure out what is on those wires.

No promises, however. I'll look into when I have a chance, but I can't promise that it'll be this weekend. Don't worry, if I can confirm this, I will let you all know.

If anyone else can confirm this independantly, please report.

-- SS
 
If one of you guys figure it out, I'll write the software. I'm one of the Palm & PC developers with way to much time on my hands and plenty of love for my AV & GPS!
 
SS,

Sounds like you're in the middle of a good one. We're gonna miss you at the GTG in WV. Takle care and have a happy holiday!
 
rvissers said:
I'm one of the Palm & PC developers with way to much time on my hands...
Click to expand...
That's good to hear. I'm also a S/W developer, but I don't have any time on my hands... :6:

(Any free time I have is taken up here! >:D )

-- SS
 
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