GPS Splitter

[PaulSS]

It looks like I'm going to end up with 4 GPS antennas in my machine. 1 for the MGL iEFIS Lite. 1 for the super duper MGL SP12 SIL = 3. 1 for my Funke BF157 'backup instrument' and, finally, 1 for my PAW.

I have read various things about some (SP12 in particular) requiring a ground plane and the distances required between GPS antennas. All of this means I could be struggling to find suitable locations. Now, this is not impossible and if it comes to it then I'm sure I'll cope but I was wondering about the viability of a GPS splitter. Something like this:

https://www.ebay.com/itm/Symmetricom-58536A-GPS-4-Way-Active-Antenna-Signal-1x4-Divider-Splitter-N-Female/283194294918?_trkparms=aid=555018&algo=PL.SIM&ao=1&asc=20131003132420&meid=fbb6b1e433e5445f9ed94b55dba9766d&pid=100005&rk=1&rkt=2&sd=361827897630&itm=283194294918&_trksid=p2047675.c100005.m1851

I was thinking that the SP12 GPS could be the 'master' input (since it's needed for the SIL = 3 etc) and then the other units could run off the splitter.

So, wise gurus of the Forum, what do you think? Am I worrying unnecessarily about the distance between GPS antennas? Is a ground plane really necessary for the SP12 GPS (it says so in their manual)? Do GPS splitters work or do they degrade the signal? In other words do I or don't I 

I have written to Mr MGL himself as he knows his stuff but realise there is a LOT of collective intelligence here on the matter, so thought I'd write twice.

[scsirob]

I would think that as soon as you mess with the SP12 on the RF side, it looses its certified status and its rights to transmit SIL=3.

From the installation manual:
"GPS antennas supplied with the SP-12 by MGL Avionics are the only approved GPS antennas to be used with the SP-12. Other antennas may work but may invalidate the SP-12's GPS approval for use as a GPS source for ADS-B purposes."

You might consider splitting the SP12 NMEA serial output. I had the same GPS sprawl until I found out that my AvMAP UltraEFIS supports NMEA Out. I've fed that into a USB-Serial line and into PAW. That eliminated the PAW dedicated GPS and works very well.

[PaulSS]

Thank you for your reply. I did post a message earlier but I think I must have had finger trouble as it didn't appear anywhere 

I've been looking further into your idea about using the NMEA GPS message output from the RS232 TX of the MGL SP12 GPS.

I have asked Rainier at MGL if this can be used at the same time as the CAN bus, which will connect the SP12 to my TT21 transponder but, assuming this is okay, it would seem you can connect more than one device to the RS232 output.

I am thinking RS232 wire to a USB adapter (makes a change from the other way round) to 'feed' my PAW with GPS position information and, possibly, an RS232 wire to SMA adapter to 'feed' the same position to my Funke BFI57 'back up' instrument (I haven't seen if this adapter is a real thing yet). Any snags with this please feel free to shout out 

Now, going a bit techie, does anyone know what baud rate would be acceptable to the PAW box, assuming it was being fed the GPS position as described above? The SP12 usually uses 115200 as the default and, I believe, this allows SDA 2, SIL 3. The baud rates of the SP12 can be changed and, obviously, this would not affect the PAW as it doesn't need those 'certified' numbers but I have no idea if going to a different baud rate reduces the SDA 2, SIL 3 of the SP12. I think I'm talking Greek at the moment 

I'll need to sort out the same thing with the BFI57 but one thing at a time.

So, does anyone have a view on all the above or am I on a hiding to nothing and will need to find room for 4 GPS antennas? 

[Admin]

Now, going a bit techie, does anyone know what baud rate would be acceptable to the PAW box, assuming it was being fed the GPS position as described above? The SP12 usually uses 115200 as the default and, I believe, this allows SDA 2, SIL 3.

115200 is fine, there are lots of other possibilities for selection as well

thx
Lee

[scsirob]

I am thinking RS232 wire to a USB adapter (makes a change from the other way round) to 'feed' my PAW with GPS position information

This is possible, and essentially what I did.

..and, possibly, an RS232 wire to SMA adapter to 'feed' the same position to my Funke BFI57 'back up' instrument (I haven't seen if this adapter is a real thing yet). Any snags with this please feel free to shout out 

SHOUT!! That's not going to work. The SMA connector isn't a serial input, it is an RF antenna input. The BFI has its own integral GPS receiver (pretty much like the SP12) and really needs its own dedicated GPS antenna. As it is your backup device, it would be best to reserve a spot for that antenna and not try to mix it with any other GPS RF side. You'd re-introduce a single point of failure, which means you risk losing the primary and backup at the same time.

Now, going a bit techie, does anyone know what baud rate would be acceptable to the PAW box, assuming it was being fed the GPS position as described above? The SP12 usually uses 115200 as the default and, I believe, this allows SDA 2, SIL 3. The baud rates of the SP12 can be changed and, obviously, this would not affect the PAW as it doesn't need those 'certified' numbers but I have no idea if going to a different baud rate reduces the SDA 2, SIL 3 of the SP12. I think I'm talking Greek at the moment 

As Lee already replied, PAW will work fine with 115200 Baud. Some other devices (notably RS-232 connected ELT's and transponders) require 4800 Baud or 9600 Baud. As the documentation of the SP12 shows, this will reduce the number of position messages per second. The simple reason is that at 4800 Baud you can only transmit approx 480 characters per second. If you add the length of all NMEA sentences ($GPGGA, $GPRMB etc) you simply can't fit two full sets of position data in 480 characters. At 19200 Baud you are already up to almost 2000 characters per second so you can have four full position sets in a second, enough to allow SIL=3. At 115200 there's plenty of room to transmit position from the SP12 to the attached devices several times per second.

So unless you must feed RS-232 at 4800 baud to some other device, leave the SP12 at default 115200. Better yet, if you have something else that wants 4800 or 9600 Baud, you might be able to feed that from your BFI57 which also supports NMEA out.

[PaulSS]

That's all fantastic info and thank you for taking the time to explain stuff so that even I can understand it :-)

I'm very pleased I'll be able to feed my PAW with NMEA messages from the SP12 and it'll all work without me having to adjust anything.

I certainly take your point regarding the single point failure for the BFI57 and may well end up just having it's own antenna (probably would be a better description). However, the SP12 will have it's own power supply and will be able to run off the battery, separate to the EFIS. If it gets to the stage where I've got no electricity then I'm buggered, 'cos it's a 912iS and they need wigglies to keep the prop turning, so I'll be looking for a field anyway and most likely not consulting my GPS  The other thing I discovered (once I'd manage to get hold of a BFI57 manual) is that they do have an RS232 Rx. I have written to Funke, explained the GPS messages transmitted by the SP12 NMEA Tx and asked them if the BFI would accept this as a GPS input. If they say no, then separate antenna it definitely is. The EFIS has its own GPS antenna, whereas the SP12 is basically for the transponder and acts as a backup GPS if the EFIS's's's's own fails.

I really appreciate the lesson in baud rates versus message length and understand now why the SIL 3 would need a high rate.

At least I'm down to 3 antennas now