Aviation
Airborne GPS receivers are generally used for navigation and attitude determination. There are a wide range of receivers available to fit every budgetary requirement. There are handheld GPS receivers available which can accept data cartridges containing Jeppson charts. High-end GPS units are being built into jumbo jets and are being tested for automated landing.
Computer Boards
These receivers are designed to fit inside a computer of some sort. A general purpose IBM-PC based receiver is made by Novatel while another GPS designer kit is made by GEC-Plessey. These boards are fairly pricey, with none I'm aware of running below $1000.
Handheld
Quite a variety of these are now available. Many are intended to fit a specific purpose such as land navigation, boating, aviation and even industrial mapping. The range of features is therefore, fairly diverse. Some are available at prices as low as $300 with some high end mapping units running up to $4000.
Mapping
These receivers are intended for mapping items for later inclusion in databases, maps or drawings. All will have DGPS capability and most will have the ability to store lots of points and add text or menu information to the points. These units will start around $1000 and go up from there.
OEM Modules
OEM modules are bare GPS receiver boards that are intended to be incorporated into other equipment. Many will be outfitted with one or two RS-232 ports from which the user is responsible for programming the unit and interpreting the output. These can run from a few hundred dollars to several thousand dollars.
PC Card (PCMCIA)
There are only three of these that I am aware of. The NavCard is made by Rockwell while Trimble Navigation makes the GPScard and the Gold GPScard. The Gold card accepts differential correction while the regular card does not. All of these cards are priced between $300 and $1600.
Marine
These are used almost exclusively for navigation. Many will have NMEA-183 interfaces for connecting to other ship electronics.
Spaceborne
Spaceborn GPS receivers are used for satellite navigation and attitude determination. From what I've seen so far, most spaceborne receivers are radiation-hardened versions of ground-based receivers. I don't have the faintest idea what these things cost but it's got to be heartstopping (from my budgetary standpoint, anyway).
Surveying
This grade of receiver is used by surveyors to derive "measurements" rather than "position". For surveyors it is the relative relationship between two receivers which is important; from this relationship an absolute "position" may be derived if and as necessary, on whichever datum is appropriate for the survey. Surveying receivers are generally capable of the highest accuracies and cost the most (up to $30K per set).Time & Frequency
These units are intended for one purpose - to generate accurate time. There are some that have both GPS and Loran-C receivers built in for increased reliability. Others have attached rubidium or cesium atomic clocks for improved short term and long term stability. Most are furnished with with 1 pulse-per-second output while others have "Stratum" class outputs. (I don't remember what that means but I'll find out - jtb). High end timing units are used to syncronize digital telecommunications networks.
In this section I am going to discuss a subject that is fairly technical and that I do not understand particularly well. However, I am sure that anybody who reads this will be only too happy to mail me corrections (hint, hint). Put simply, when using a DGPS system, you can get better accuracy faster and over longer distances when using a dual frequency system then you can from a single frequency system. If you are willing to put up with a longer explanation, read on. If not, stop here. The NAVSTAR satellites actually transmit information on two radio frequencies. The L1 frequency carries both civilian and military grade information while the L2 frequency carries only coded military information. All civilian grade equipment listens to the L1 frequency and is able to use the un-encrypted information found there to calculate a position solution.There are two methods for calculating a position solution from the information transmitted from the NAVSTAR satellites. The most common is to listen to the coded time and satellite position information being transmitted on L1 coming from four (well, at least three) satellites. The received data is run through a rather complex matrix calculation. The output of the calculation is the position where the satellite has to be in order to have heard those particular inputs. A solution of this type can be as good as 15 meters.
To get a more accurate solution, you have to do carrier phase measurement. Carrier phase measurement is a technique wherein the receiver uses the characteristics of the radio signal to determine distance. The problem is that it assumes that the radio wave travels in a direct line from the satellite to the receiver. The problem with this assumption is that the radio waves do not in fact travel in straight lines. They are deflected by portions of the atmosphere, particularly the ionosphere. The ionosphere is that region of the atmosphere which is responsible for you sometimes hearing AM radio stations over several hundred miles.
Radio waves at the frequencies used by the GPS system are not subject to much deflection, but it doesn't take much to cause a error of a meter or more. However, there is a way to solve for much of the ionospheric error because the error due to the ionosphere is frequency dependent. So, if our GPS receiver could listen to both the civilian L1 frequency and the military L2 frequency, the receiver could determine much of the error due to the ionosphere and take it out of the position calculation.
The military code on the L2 frequency is encrypted so our civilian GPS receiver can't listen to it. But, it can still listen to the raw radio carrier. Civilian dual frequency radio equipment is designed to do this and to use whatever information it can to reduce as many error sources as possible. In the event that the military code is not encrypted (DoD does seem to shut it off from time to time) some receivers will use it to get an even better solution.
Historically, the user of a dual frequency DGPS system would have to perform some complicated procedure to initialize the rover against the base station. Recent advances have produced equipment with an ambiguity resolution on the fly (AROF) capability. AROF capability allows the equipment to syncronize within a minute of being turned on and does not require a complex initialization scheme.
The practical upshot of all this is that if you want to do precision DGPS work quickly, you'll want a dual frequency AROF system.
Time to First Fix is the amount of time it takes for the receiver to start producing solutions. This number is somewhat abused in the GPS industry. A GPS receiver will start producing solutions fairly quickly if it has a rough idea of the time, it's approximate position and the current satellite ephemeris. However, when you bring it home from the store and take it out of the box, it will probably know none of this.
The satellite radio signals are doppler shifted in frequency because of their motion. The actual frequency of the radio signals will depend on the velocity of the satellites with respect to you. If the receiver you are holding has some preprogrammed information time, satellite ephemeris and a rough location (within 200 miles or so) it can make a pretty good guess where the satellites are. Then it can calculate the doppler shift of the radio signals and the appropriate code. Getting a first fix this way, called a warm start, is relatively quick.
On the other hand, if the receiver knows none of this, it has to randomly scan the radio band looking for a possible signal. Any signal it finds it has to try to decode using each of the possible psuedo-random noise codes. Some receivers will keep doing this until they have enough satellites to calculate a solution, while others will stop to download an ephemeris. This method, called a cold start, can take a considerably longer period of time.
So if you see an advertisement that announces a fast time to first fix, look to see if that is under cold start or warm start conditions.