Good ridin' to ya,
Victor Johnson

Terminology

Signal - The GPS signal itself is relatively weak and does not pass through objects very well. Buildings, mountains, trees (especially if the leaves are wet) will block signals. The are, however, pretty good at getting through rain and/or snow.

Initialization - When your first turn on a GPS unit, it has to figure out where it is. Most have an initialization process they must go through when you first turn them on or have moved more than 500 miles from where they were last turned on. If you don't have a good view of the sky, you may not be able to initialize at all. If your unit is turned on and used frequently (and you don't let the batteries completely discharge) initialization won't be something you'll need to do very often.

Cold Start - If it has been over a half an hour since you last turned on your GPS, the stored almanac data stored in memory will have gone stale. It will need to download fresh almanac data over a period of being on for about a half an hour to get a fresh almanac restored. This is one of the reasons it is easier to get a fix in marginal conditions if your unit is left on rather than turning it on and off each time you want a fix.

Hot Start - If it has been less than about half an hour since you turned your unit off, the stored almanac data will still be fresh enough for the unit to be able to

Almanac - A stored timetable of satellite numbers and their orbits. Satellites being tracked by your GPS are moving across the sky both in and out of your viewing area. Embedded in the signal you unit is receiving are the orbital characteristics of all satellites and these are accumulated and store in your receiver. This way, your GPS knows about satellite's distance and position even if it isn't currently tracking the signal from that satellite.

Fix - A computed position given a solid lock on at least three or four satellites. Three satellites will give you a two-dimensional position and four satellites will give you a three-dimensional position. Once you have locked on a satellite, it is much easier for you receiver to keep that lock and track satellites for a continuous fix than it is to turn it on cold and have it try to acquire a fresh lock.

Bearing - A heading or direction based on 360 degrees in a circle in a clock-wise direction. A true north is a bearing of 0/360 degrees. East is a bearing of 90 degrees, south is a bearing of 180 degrees and west is a bearing of 270 degrees. So, if your bearing is 317 degrees you are headed in a N-NW direction. Bearings are real useful to pilots and seamen who can travel in any direction they want. The "as the crow flies" direction isn't normally as easy for land travel so your bearing is normally a "general direction" type of thing.

Waypoint - A stored position in your GPS memory. These can be obtained by storing your position into memory when you're out on the trail, gotten from someone else and keyed into your receiver manually or entered from map coordinates. Once you have waypoints in memory, your GPS unit will tell you how far you are from that waypoint and what bearing you need to get to that waypoint.

GoTo - A handy navigating feature that lets you specify a specific way to "go to" and your GPS will track your progress in getting towards that destination. Some units will display all kinds of handy statistics on this progress including estimated time of arrival, estimated time on route, bearing error, etc.

Route - A series of waypoints tied together in segments. Each segment between waypoints is a "leg" of that route. As you approach the end of a leg, some units will give you a visual and audio alert and will then automatically switch over to the next leg when you pass the endpoint. Routes are limited in how may legs they will allow. 30 is an average number.

Tracklog - This is a "bread crumb" trail. Many GPS receivers have a record mode where you can just turn them on and they will sample your position periodically and then store that position into memory. That sampling can either be a period of time, such as every few seconds, minutes, etc. or may be some kind of algorithm to decide when to store a point or save memory for future points. The unit I use has a sophisticated algorithm that computes your heading and velocity and only stores points into memory if either of these changes significantly. Using the automatic mode, I can fit about 35 miles of single-track riding into my 1024 point tracklog memory. Some newer and more expensive units have more memory like 2048 points or more. If you plan to do mapping of trails, tracklog memory capacity is a very important feature to consider.

Coordinates - Latitude/Longitude, UTM, etc. These are coordinates for determining just where your position is and on maps. Most people are familiar with latitude and longitude. I prefer UTM (Universal Transverse Mercator) coordinates since they are based on meters and decimal divisions making it easier for me to interpolate on maps than the typical degrees/minutes/seconds system used with lat/long coordinates. Most of the maps ruled for GPS are in lat/long coordinates so I'll use a ruler or an image editor to lay out my own UTM 1 kilometer (about 0.6 mile) grid on the map.

Datum - The mathematical model representing the curvature of the earth for your location and map. The earth isn't a perfect sphere and has warps and puckers that vary across the continents. The most popular modern datum is WGS-84 and is what most GPS units operate in by default. Most of the USGS maps of North America use an older datum named NAD27. If your GPS is in WGS84 and your map uses the NAD27 datum, you'll need to shift your point just a tad. Depending on what scale map you are using, that shift may or may not be significant. Be sure and check your reference map to know what datum you should be using.

Multipath - Since GPS receivers use triangulation and propagation delay to compute your current position, that calculation can be unduly influence by signals that have bounced off several objects before getting to your receiver. Since that zigzag path adds propagation delay that wouldn't otherwise be there if the signal traveled direct, multipath errors can give you bogus readings. This is particularly noticeable when you are down in deep rocky canyons. I've seen my GPS give me some pretty wild excursions of up to a mile or more when it got confused by multipath signal errors.

GDOP - Geometric Dilution of Precision. More triangulation stuff here. If the satellites you are tracking are bunched up in the sky, the accuracy of your fix is degraded since that accuracy is proportional to the spatial distribution of the satellites. The best scenario for a good fix is tracking satellites that are widely dispersed from one another. That will give you the best triangulation possible. If they are all clustered in the sky, which does happen from time to time, your circle of uncertainty gets larger.