Intro to GPS Apps
Types of GPS Recievers

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Contents

Introduction

A special radio receiver is used to detect and decode gps signals and to return the output to the user in a useable form. The characteristics of the receiver depend on the application for which it was designed. Consumer receivers meant for land navigation or boating will usually have an LCD display and may have an RS-232 digital output. Receivers intended for commercial airliners will usually only have a data port intended to integrate into the aircraft's navigation system and will be able to calculate the relative accuracy of the information it is receiving. Military receivers can decode a second encrypted radio frequency to obtain higher stand-alone accuracy than a civilian receiver.

Types of Receivers by Architecture

Sequential Receivers
The sequential receiver uses one or two hardware radio channels to sequentially provide individual satellite observations. These receivers are among the cheapest available due to the limited circuitry needed. However, they provide the poorest time-to first-fix and cannot track satellites while moving at high speeds.

Continuous Receivers
The continuous receiver has sufficient dedicated hardware radio channels to provide continuous satellite observations. This type of receiver has the best performance of all the receiver architectures. Four hardware radio channels at minimum are required for continuous operation. A five channel receiver can view four satellites and read the NAV message from a fifth, thus continously keeping the receiver's database of satellite orbital parameters up-to-date. A six channel receiver can read NAV messages, track four satellites and a keep a fifth in reserve in case one of the four is lost for any reason. An all-in-view receiver has sufficient hardware radio channels (sometimes 12) to lock onto all the satellites that happen to be in view at any time.

Multiplex Receivers
The multiplex receiver acts like a sequential receiver in that it switches between satellites being tracked; however, it does it at a fast sample rate (approximately 50 hz) and can track more satellites than a sequential receiver. It's performance is still lower than a continuous receiver because it can't integrate all of the satellites's transmitted spread spectrum power.

Types of Receivers by Method of Operation

Code Correlation
These receivers determine position by processing information found in the code that is transmitted by the satellites. The advantage to this method is low cost. The drawback is only moderate accuracy, on the order of 5 meters or worse.

Carrier Phase
These receivers determine position by processing measurements of the carrier phase of the satellite signals over time. They do not need to decode the information being transmitted except for locating the satellites. Some such receivers may have no code reception capability at all, in which case the receiver must be preloaded with that data from another source. The advantage to this method is very high accuracy. Such receivers can provide centimeter level accuracy in real time when used with differential correction. The drawback is high cost.

Types of Receivers by Application

General Purpose Handheld
These receivers are characterized by being small and portable, battery powered and having a built in display. The display is usually an LCD for low power consumption, and can be either alphanumeric or graphical. Some of these receivers may have the ability to display aeronautical or marine charts from data cards. The antenna may be detachable for mounting outside a vehicle.

Attitude Determination
These receivers are used to determine the three dimensional position an object with respect to the earth. These receivers use multiple antennas, the relative placement of which must be known.

Aviation
These receivers are optimized for aviation navigation and can display aeronautical charts. Accuracies vary depending on the class of aircraft in which the device is to be used. Receivers intended for use by general aviation may not use any correction, thus being limited to 100 meter 95% accuracies. Receivers integrated into the navigation suite of a commercial passenger aircraft may be capable of using local area differential broadcasts, increasing the precision of the aircraft's navigation system to the point that the aircraft can land automatically.

Automobile Navigation, IVHS and Fleet Location
These receivers are mounted in cars, trucks and trains. The purpose of the receiver may vary depending on the application but the characteristics will be similar. Receivers used in cars are generally intended for driver navigation or for sending the position of a car to an emergency response center in case of accident. GPS receivers used on busses, trucks and trains will generally be intended for fleet tracking.

Mapping & Data Collection
These receivers are optimized for collecting data to be exported to an external database. They often have moderate to good autonomous accuracy with differentially corrected accuracies as good as 1 meter. Often they will have an attached computer dedicated to data collection. Such data collection computers can be preloaded with feature libraries so that the operator has preset classes of items to choose from. These receivers may be carried a persons hand, with extra batteries and a the GPS antenna fixed to a backpack.

Marine
These receivers are intended for marine navigation, including the ability to display marine charts and connect to other navigation equipment.

OEM (Original Equipment Manufacturer)
These receivers are intended to be included in other equipment. They come from the manufacturer as a bare board or module, with no display or built in power supply. Technical characteristics of OEM receivers can vary widely depending on the market for the unit.

Space
These receivers are used on satellites both for navigation and for attitude determination. They may be radiation hardened and have special programming allowing them to operate at the high-relative velocities experienced by orbital spacecraft.

Surveying
These receivers are intended for high-accuracy measurements intended for some form of land surveying solution. Such receivers will have external tripod-mounted antennas and be able to swap power sources while operating.

Timing
This type of receiver is intended to act as a time and frequency reference. Position is secondary information to these receivers and is often ignored by the user. The primary benefits of GPS-derived time and frequency are long term stability and coordination with the world wide time network via the GPS time standard. These receivers are often used for applications such as:

  • Calibration of test instruments by calibration labs
  • Digital network syncronization by telecommunications providers
  • Syncronizing astronomical observations by observatories
  • Syncronizing fault recorders for electric utility grids
  • Syncronizing seismographs for accurate earthquake location

Because these receivers are often used in critical applications, the gps derived time is often paired with a different kind of time receiver, such as LORAN or WWV, or an additional high-accuracy clock such as a cesium atomic clock. Thus, if the GPS receiver were to fail, the output could still be assured over some period of time.

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