Pressing the help button: Drama on andromea

"Here is a real life example of multipath interference."

Stuart's map shows the result of multipath interference. Sometimes it is helpful to understand what actually causes it so that you can recogize the situation in the terrain and do things to avoid it. It is intended that GPS signals come down from space and hit the GPS antenna directly. Instead, if the GPS signals take a bounce off some reflective surface and then over to the GPS antenna, then one signal has taken a longer route to get to the antenna. The GPS receiver can read out of the signal when the signals left the satellite, and they should be arriving about 65 milliseconds later. If they take a bounce first, then they arrive (let's say) in 66 milliseconds, and that completely fouls up the accuracy of the position fix that the receiver computes. These are just an example.

1. It tends to show up as a problem more when you have a very limited number of satellites in view in difficult terrain.
2. Reflective surfaces include large smooth rock cliffs and large buildings with lots of glass and metal. It generally does not happen with surfaces on the ground, like smooth highway pavement.

Many GPS receivers have a display screen that can show you the relative positions of the satellites in the sky. So, you can look at that screen, then look up at the sky, and then look at any potential reflective surfaces, and then think about where the bounce could occur.

Using a powered high quality antenna is a good way to avoid multipath, since the antenna negates most of the signal problem. Too many receivers have only the internal patch antenna, which is the worst for this, and they have no external antenna port even if you wanted to use one.

–B.G.–

My wife has a topo map with all of the cow paths and my normal night time wondering routes on the about 4,000 acre deer lease. One very cold night I decided to sleep in one of my deer blinds located directly under a 150 year huge tree, on a cloudy night, deer blind roof consisted of galvanized steel and 1/2" sheeting on 2" X 4" frame.

I leave the SPOT in constant tracking mode that leaves a trail on the topo map, as required hy the wife when I am out alone. The next day my wife ask4d why I spent the night in the deer blind. Every 15 minutes,I believe, all night the spot binged my location onto the Topo map
I had no idea that SPOT would transmit through so much material.

There are two factors. One is the GPS signal has to come down from space to get you a GPS position reading. Second is the signal to the satellite has to make it out and up to the satellite.

Now, if it had your GPS signal going into the shelter, then it knows that you are there until it gets a different GPS signal to tell it otherwise. Transmitting out can get tricky, but if there are enough windows and doors, it might shoot a signal out that way, and if the satellite were low to the horizon, it might work fine.

I will bet a quarter that the signal to the satellite is not penetrating galvanized steel sheeting.

–B.G.–