GPS Basics

GPS, the Global Positioning System, consists of three elements:
  1. The space segment
    This has 24 satellites orbiting the earth twice every sidereal day. These satellites are arranged into 6 high orbit planes at a height of 20,200 Km.. With no obstruction, there are typically 8-12 satellites visible at any one time from anywhere on earth. Each satellite contains a Rubidium atomic clock so between them they represent an extremely accurate time standard available for synchronisation at any point on the earth. It is this accurate timing that leads to an application of the GPS satellites separate from their function for navigation. The world's cellular and fibre communications use the time information derived from the GPS satellites for clock synchronisation. The business world also uses this reference to ensure that large global financial contracts are executed at precisely the same time at both ends of the transaction. Each satellite transmits a spread spectrum signal containing a BPSK (Bi-Phase Switched keyed) signal in which 1's & 0's are represented by reversal of the phase of the carrier. This message is transmitted at the L1 frequency 1575.42 MHz at a "chipping rate" of 50 bits per second. The message repeats every 30 minutes and is called the C/A signal (Coarse Acquisition signal). This message contains two important elements, the almanac and the ephemeris. The Almanac contains information about all the satellites in the constellation. This information is regularly updated from ground stations monitoring the system but almanac data remains useful for around one year. The Ephemeris contains short-lived information about the constellation and the particular satellite sending it. Its information is updated from the GPS ground stations every four hours. Its validity in calculating position deteriorates gradually over this period as satellites rise and fall above the horizon. There are also other encrypted signals; the P code and Y code that are used for military applications transmitted at frequencies L1 & L2.

  2. Control Segment
    The control segment monitors the signals from the satellites and transmits modifications to the Almanac and Ephemeris as small changes occur in the orbits and the nature of the Ionosphere etc.. It also monitors the satellite clocks and transmits corrections for these and other parameters necessary to maintain the accuracy of the system.

  3. The user segment
    From the information transmitted by the ephemeris and almanac a GPS receiver can determine just how long it took the transmitted signal to reach it. That time is proportional to the distance the signal travelled from the satellite (its range) so it can be used to determine an arc on which the receiver must lie. Calculating the intersection point of a number of such arcs derived from different satellites provides a solution to the receiver's position on the surface of the earth. The distances calculated for the ranges of the satellites from the raw variables data extracted from the GPS satellite transmissions are called pseudo-ranges , "pseudo" because the receiver clock's inaccuracy introduces a significant error. This error causes the arcs represented by each satellite not to intersect. Working with four satellites the common clock error can be calculated and corrected. This is called a 3d fix and allows, in addition, the calculation of the receiver's height above the imaginary surface of the earth. Without the additional height information and using just 3 satellites an approximate fix can be determined for objects on the surface of the earth by introducing into the mathematics a model for that surface. Such a fix is termed a 2d fix. The earth of course is not a perfect sphere and the imaginary surface must represent an appropriate model which is an approximation for the earth's surface. The common reference used is called WGS84 . (World Geodetic Reference 84.) This is a complete approximate model of the earth. Some GPS receivers also carry algorithms to convert these WGS84 based co-ordinates to other local references in order to relate the position to particular cartographic maps.

Accuracy and availability

The signals from the satellites that are available for commercial use are called C/A codes (Coarse Acquisition). The satellites also transmit signals for use in military applications. These signals are referred to as the P-Code signal (Private Code) and are transmitted using a different modulation scheme as well as being encrypted.

In April of 2000 the US government removed Selective Access from the C/A code. SA as it is known was a jittering of the satellite timing clocks as represented in the satellite ephemeris. This improved the short term accuracy of the system by a factor of 4 to around 10 metres. Access to CA code signals is available to anyone and is provided free of charge. Without SA, GPS accuracy is around 10m over a long period and has a repeatability of around 1m over short periods. The gross inaccuracies in the system come primarily from the changes of the speed of radio waves travelling through the different parts of the Ionosphere traversed by signals from each of the satellites.

Various means have been devised to improve these accuracies but they have been made redundant in many systems because of the improvement in accuracy coming from the removal of SA. Here are some of them that you may still encounter :

Differential correction

A fixed, accurately positioned, reference station transmits to local GPS receivers corrections to their calculated position based on the known errors received at the base station. Commercial differential correction services are transmitted from satellites services and as an RDS service from FM radio broadcasts. The naval coastguard transmitters send differential corrections for coastal shipping traffic and being low frequency these have ranges covering large areas of many countries. Differential corrections can also be provided locally within a user system. An important user of differential corrections is the farming industry for guidance of tractors working very large fields.

Reverse differential correction is often used in AVL (Automatic Vehicle Location Systems). The vehicles transmit to the base station only the pseudo ranges received from the satellite. The base station uses these for calculation of position. The differential errors calculated by the base station are applied in these calculations to correct the transmitted position.


WAAS - Wide Area Augmentation Signal. EGNOS and MSAS

This is a signal transmitted from additional geo-stationary satellites that mimics the GPS satellites. These satellites transmit on the same frequencies some additional corrections for the errors occurring at any point on earth for the signal coming from each satellite due to variations in the speed of travel of its radio signals through the ionosphere. These corrections are measured and updated on a regular basis by correction stations around the world. WAAS is a correction system deployed by the Civil Aviation Authority for the navigation of air traffic. Its most important function now that SA has been removed is to provide very rapid advice of any short-term malfunction in any part of the GPS system so that aircraft air traffic controllers are not unknowingly using inaccurate data. WAAS covers North America. A similar system is available in Europe and Russia called EGNOS and Japan and Australia provide a correction called MSAS . Together these form a complete global correction system.


Alternatives to GPS

The Russians have a satellite positioning system called Glonass and the Europeans are planning a new system called Galileo that is scheduled to roll out in 2008.

The United States will upgrade the current GPS system with additional frequencies. The 'Block IIF' satellites will add a third carrier signal designated 'L5' at 1,176.45 MHz. intended for civilian applications in air traffic control. Full operational capability of the Block IIF constellation is not expected before 2011.


Performance of GPS receivers

Typical modern GPS receivers are constrained in their performance only by the specification of the GPS system. Accuracy is determined by the amount of ionospheric variation. This is most noticeable at dawn and daybreak when the temperature in the ionosphere changes as the sun crosses the horizon. The addition of the second GPS signal L5 is intended to allow these changes to be removed in the calculation of position because the change in the velocity of radio waves through the ionosphere varies in a more or less predictable way with frequency.

An important measure of performance is defined as the Time To First Fix TTFF. This is defined for the following conditions:


Cold start

The GPS receiver has a valid almanac stored. The Almanac data is valid for at least a year and most receivers store this data in battery backed RAM or non-volatile memory. TTFF is determined largely by the time taken to download a full ephemeris packet. This is determined by the satellite data rate of 50 bps and takes around 45 seconds depending on where in the message the system is at switch-on.


Autonomous start

The GPS unit has no information of time, ephemeris or Almanac data.

This normally only occurs when the unit is first powered since the GPS can store this data in either battery backed memory or in non-volatile memory. The time is determined statistically based on the state of the satellite messages when the receiver is turned on and the time that it takes the satellites to transmit a complete set of data. The number and strength of the visible satellites will also affect it. In an open area with a good antenna that is well placed this time is about 90 seconds. This can be reduced by feeding the receiver with an approximate position (within 100Km) and the time of day


Warm start

The GPS receiver has valid ephemeris and almanac data but not accurate time. This can vary from 7 -15 seconds on the quality (age, up to four hours) of the ephemeris data stored.


Hot start

The GPS receiver has valid ephemeris, almanac and time

Obscuration

If a satellite being tracked and used in a navigation solution by a GPS unit is momentarily hidden from the GPS antenna then Obscuration recovery is the TTFF after the satellite reappears in line of sight. This is particularly relevant in a mobile receiver in an urban canyon situation where passing a tall building may temporarily obscure a satellite from the antenna.