[MikeNH]

I'm looking forward to getting into fpv and have been struggling to get my 250 size quad ready for the warmer weather. Because I'm new I've been wrestling with getting the OSD (micro minimosd w/MWOSD) with Frsky XM receiver (S-BUS) working to display RSSI, battery voltage, altitude, distance, etc. Finally got all this working. I went outside and used Autotune to get some basic flight characteristics. So far, great.

I have a Devo 7e radio that has a trim switch configured as my 5 position flight mode switch. I tried to assign flight modes other than Stabilized and AutoTune. Because the quad has a NEO-6M U-BLOX GPS with no compass every other flight mode choice generated the "Configuration Error".

My question (finally): What does a GPS module without a compass offer? I've run out of ports to plug a DJI GPS into. I  believe I can get Altitude Vario at least.
https://librepilot.atlassian.net/wiki/spaces/LPDOC/pages/12058671/Altitude+Hold
The GPS does render some telemetry.

Thank you.

[TheOtherCliff]

A GPS without mag gives you a "bread crumb" trail on the GCS map, BUT ...

You can use the on board mag on some quads.  I personally set up a 500mm quad this way for a friend.  You must be very careful to twist and route all high current cables correctly.  Most people say that small quads (say 250 or smaller) should not be set up this way.  I have never tried, but I have a new toy I might try it with (especially with next, see below).

Also, "next" unreleased version, has a setting to let you use the mag as 2D ("compass only") which may allow you to use the on board mag.

Also, you can buy a mag only, but you are out of ports.  I would buy a DJI for $30 or so rather than try to add a mag to what I have.

Also, some "GPS only" units actually have a mag chip inside and only need a cable to be soldered on.  It is a tiny 16 pin square chip usually labeled "L83".  It is a 5883 magnetometer.  Careful, this needs to have 3.3v power, so take power from the GPS circuit board 3.3v regulator chip if the L83 needs power.  This doesn't help you if all your ports are already in use.

Remember that DJI and OPv9 GPSs give you GPS and mag but only need one port/cable, so if you have a GPS plugged in right now, you can have a GPS/mag plugged in in place of your current GPS.

a similar thread
https://forum.librepilot.org/index.php?PHPSESSID=56br3c9s476h9mlo6ek9ckfn01&topic=305.msg2103#msg2103

[MikeNH]

Thank you for the detailed explanation. I did see the thread that you referenced but my question, and a question I suspect many other new folks have is "what is the benefit of a GPS without a compass?" It's not cleat to me what a bread crumb trail is.

May I ask some specific questions?
1 - "GPS Assist" doesn't work without "Position Hold" according to doc (https://librepilot.atlassian.net/wiki/spaces/LPDOC/pages/12058683/Return+to+base). Position Hold won't work without GPS w/compass, therefore "GPS Assist" is not available without GPS AND compass. Is this true?

2 - PositionHold, CourseLock, VelocityRoam, HomeLeash, AbsolutePosition, ReturnToBase, Land, PathPlanner and AutoTakeOff.   All of these flight modes including Land and AutoTakeoff require GPS/compass. Is this true?

3 - GPS WITHOUT compass will provide telemetry such as altitude, air speed or/and ground speed and distance. Is that true?

4 - GPS WITHOUT compass will provide direction to home and distance from home.

5 - Without the compass/magnetometer, the GPS is oriented to the direction is was facing at the time it was armed. Is that true/complete?

6 - Is Longitude & Latitude available without a compass?

================================
Unrelated question...
with minimosd and micro minimosd connected to Revolution FC, the pass through capability is a great convenience but requires the DTR pin to also be connected. Is there any harm in keeping the DTR pin connected at all times so there is no need to get access to that pin when configuring MWOSD?
================================

I will update doc once I get a basic understanding of the value of GPS without compass.

[TheOtherCliff]

1 - "GPS Assist" doesn't work without "Position Hold" according to doc (https://librepilot.atlassian.net/wiki/spaces/LPDOC/pages/12058683/Return+to+base). Position Hold won't work without GPS w/compass, therefore "GPS Assist" is not available without GPS AND compass. Is this true?

True.  Think of GPS Assist as a GPS flight mode (position hold) that is automatically engaged at center stick.  All GPS flight modes require a working mag.

2 - PositionHold, CourseLock, VelocityRoam, HomeLeash, AbsolutePosition, ReturnToBase, Land, PathPlanner and AutoTakeOff.   All of these flight modes including Land and AutoTakeoff require GPS/compass. Is this true?

Yes.

3 - GPS WITHOUT compass will provide telemetry such as altitude, air speed or/and ground speed and distance. Is that true?

Yes.

4 - GPS WITHOUT compass will provide direction to home and distance from home.

No.  Not directly.  Or more accurately, it depends on the FPV OSD that you have.  And it can be seen on the GCS satellite map display that the model is there and home is here but you don't really know what direction the model is facing.  Both coordinates are known, but as far as I know, the distance is not displayed anywhere (it could be).  It should be fairly obvious from the satellite map how far it is from home.  GPS even gives a heading direction of travel when it is moving (but doesn't say what direction the aircraft is pointed; it could be flying sideways).

5 - Without the compass/magnetometer, the GPS is oriented to the direction is was facing at the time it was armed. Is that true/complete?

Yes.  But this heading drifts and can easily be completely wrong over the length of a flight.

6 - Is Longitude & Latitude available without a compass?

Yes.  GCS map has it displayed.

with minimosd and micro minimosd connected to Revolution FC, the pass through capability is a great convenience but requires the DTR pin to also be connected. Is there any harm in keeping the DTR pin connected at all times so there is no need to get access to that pin when configuring MWOSD?

I don't know.  I use FTDI for configuration and have never tried Revo DTR.  You can carefully slightly lift the tiny pin flap, slide the pin out, fold wire back and tape.

[Tongala]

Have had DTR pin connected to Revo now for about one year and have not noticed any problems.

[MikeNH]

Doc has been updated. Please review.

I have the DTR pin connected and will be trying this out once we get decent weather. Thanks for the reply.