[Dick]

GPS is a useful tool to have to complete your navigation tools. It should not be used as an alternative to map and compass, but as a compliment.

The units are not particularly good for navigation (used like a compass) because if / when you lose your satellite signal you have lost your compass. Also, if you are stationary the GPS will not work as a compass. You need to be moving for it to work as a compass. The worst thing is that it will still give you an azimuth when it’s not working, just the wrong one!

Most useful features include the ability to record a “waypoint” or “landmark”. You can landmark a good fishing spot on a lake, your food cache, your vehicle, etc… Essentially you are getting a map grid for those locations.

Some units allow digital maps or orthophotos to be loaded into them, allowing real time positioning. Most will record “tracks”, so you can see exactly where you traveled, how far, average velocity, etc… These tracks (and “landmarks”) can be loaded back up onto a map if so desired.

You can use it to give you an azimuth between two landmarks, as well as a distance, and estimated travel time.

Positioning with GPS is accomplished by satellites triangulating on your position. The satellite signal has an assumed velocity. The satellites all know where they are relative to one another, so the time it takes for the satellite signal to reach your receiver represents a distance. You therefore need a minimum of 3 satellites for a position, although 4 will give you a more accurate position including elevation.

The assumed velocity of the sat signal is just that, assumed. Differences in atmospheric moisture and density can affect the time it takes and the strength of the signal reaching your receiver. Most, if not all, receiver will indicate the “accuracy” of the position based on the strength of the signal. On a good day it may say “Accurate to within 4 meters”, whereas on a bad day it may say “Accurate to within 36 meters”.

Accuracy is impacted by satellite orientation also. If the satellites you are using are all stacked together in the southern sky you will not get a good fix, whereas if they are all around you (N,S,E,W) you will get a much better position.

Most of the satellites are pseudo equatorial. The further north you travel, the more southern the satellites. This can be problematic if you have a ridge or mountain blocking the southern sky.

The receiver will automatically pick the best satellites based on signal strength and spatial orientation, and will switch satellites at any time to optimize the units performance.

The signal will not pass through water. Putting your hand over the receiver is enough to block the signal. Heavy forest cover (trees have water) will interfere and possibly block a signal. The worst I encountered was when the trees were heavily laden with wet snow.
The satellites are constantly moving in their orbits, so if you lose a signal chances are you will get it back if you wait. Alternately, just move from your present location. Sometimes a few meters is all that is necessary.

The beauty of the GPS is that you can navigate without having to pace and follow a precise azimuth. If you record a landmark and mark it on your map, you can wander around willy nilly en route to your destination. It is important to take additional positions along the way and mark them on your map, just in case you lose signal or your unit malfunctions. You can always go back to the last ‘known’ point and compass back to your home base. Your unit doesn’t need satellites to calculate distances and azimuths from pre-recorded landmarks.

You can get your map position either by location via grid reference, or you can landmark a known location (i.e. road intersection) and then ask the unit the azimuth and distance to that landmark. Then just use your compass (protractor and ruler) to mark your new location. Landmark again, and repeat.

Despite the potential “weak links” GPS is very reliable, just not entirely reliable.

Hope that all makes sense.