Diallo,
Please do not change the GPS Protocol's projection. I will explain why, shortly, but, the GPS protocol should be in D_WGS_1984, and, only in special circumstances, should you change this.
Since you mentioned you are using ArcPad Data Manager to check out your data from ArcGIS Desktop to ArcPad, ArcPad should be loading your data in the correct projection. You need to check that your map projection of your map in ArcGIS Desktop is correct. Optionally, you need to ensure that the data sources in your map are also in the correct projection - it's optional, because checkout will reproject on check out and will reproject on check in.
ArcPad when dealing with projection:
1. When there's no data loaded, by default, it's in WGS84
2. If there is a projection defined in your ArcPad Map Project (.apm), it will use it, otherwise
3. When you load data, projection will adapt to the *FIRST* dataset loaded, so, you need to add a feature class, like a Shapefile or AXF of data in your projection
4. When you load more data, the projection will only adapt to secondary datasets, if the primary datasets were deemed reprojectable, e.g. redlining layers and ArcGIS Online basemaps are examples of reprojectable layers, otherwise, the projection will lock onto the primary dataset and attempt to reproject secondary datasets to it
ArcPad has a projection library but it only invokes it in the following scenarios:
- reprojecting ArcGIS Online basemaps to your map
- reprojecting GPS coordinates to your map
- reprojecting redlining layer to your map
The GPS Datum must be left as D_WGS_1984 so that the reprojection library can reproject GPS coordinates correctly onto your map. Some special GPS hardware allow you to change the GPS datum, and, only in that circumstances, should you change the GPS Datum in ArcPad to match your GPS hardware. i.e. this setting has nothing to do with your desired map's projection.
You mentioned that you're having an issue with ArcPad on your Trimble Juno 3D. Can you elaborate more details to the circumstances of your issue and what you've attempted to do to fix it? i.e. do you try loading your data on ArcPad on Windows? Did it work? Did you see the same problem? i.e. I want to know if your problem is specific to your Trimble Juno, or, if your problem is a general ArcPad issue.
Stephen