Bad GPS accuracy

Certainly, it is  not expected. Normal - well, that depends...

Can't tell enough about the topography. But you should at least consider the possibility that the device itself had a poor sky view resulting in a poor quality fix. You have to consider whether or not the mountain blocked a large portion of the sky, You ALSO have to consider the fact that your body does a fine job of blocking GPS signals. Body position relative to device/sky view is opposite going up and going down.

My experience with the messenger is that it works fine in a pocket (hip, shirt, jacket, or whatever). A thin mesh pocket is not going to be a problem.

Although anything is possible, I doubt that the device is defective. The simpler explanation is something blocking a large portion of the sky.

Thank you for the explanations, twolpert.

I am wondering whether the GPS in the Messenger is known to be particularly sensitive to the factors you mentioned (such as body positioning)? Would it be better to put it in the should pocket of my trail running vest, so that it points to the sky?

I am asking because my cheap Android phone was in the other (symmetrical) pocket of my trail running, and it does not show any issues. Similarly, I have also used other GPS devices before (including some older Garmin eTrex devices) and never had such issues with them, even if I put them in my pants pocket, so this is surprising to me.

On a different note, if I place the Messenger on the dashboard of my car, then sometimes I am also getting bad GPS data, such as wildly incorrect speed (e.g., 5 km/h when I'm driving 120 km/h on the motorway). I don't think that the windshield of my car has electric wires in it, or an shielding surface coating.

Where are you viewing speed? iR devices (including the Messenger) only acquire a fix when they need to. For example, to send a track point, to send a message, to log a track point, or when you interactively ask to view position. (Offhand, I don't recall if position is continuously updated once you ask to view it, or if it times out.) Point is that things which depend on two or more fixes (like speed) may be unreliable when using an iR device. iR devices try very hard to maximize battery life, which generally means minimizing GPS and iR radio use.

Most iR devices will warn you when you select a logging interval of 30 seconds or less. This results in keeping the GPS radio on at all times, which does consume power.

The Messenger is generally regarded as having a much superior antenna system (compared with other iR devices). I don't know much that impacts GPS performance.

ZorgroZHello, from everything you are explaining the biggest thing that would be causing this would be placement. The receiver on the messenger device is the square on top of the device. It is meant to be sat flat somewhere to connect to satellite. Poor Satellite Reception on an inReach. You may also find this article helpful to understand the satellite connection and delays How Long it Takes to Send or Receive inReach Messages. 

Although Leslie is concentrating on iR performance, iR and GPS share the antenna. This is a so-called patch antenna. It points outward from the face of the device, roughly under the Garmin logo.

Patch antenna reception can be sensitive to orientation. The ideal orientation is with the device horizontal, with the antenna pointing directly at the sky. In my experience, the it's not particularly sensitive to the exact orientation.

However, things NOT to do:

1) Point the antenna down (so that the orange back of the device is toward the sky). The antenna must point generally in the direction of the sky.

2) Put the device in a pocket with the antenna pointed at your body. The antenna must always face "out" in a pocket.

This is hard to visualize. For reference, here is a link to an image of the lobe pattern of a typical patch antenna. This is NOT the antenna in the Messenger. It's just an example.

https://en.wikipedia.org/wiki/Patch_antenna#/media/File:Patch_antenna_pattern.gif