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Bharathidasan University
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SATELLITE-BASED POSITIONING SYSTEMS |
Some characteristics of satellite-based positioning systems, past, operational and proposed, are:
TSIKADA
NAVSTAR Global Positioning System
STARFIX
GEOSTAR/LOCSTAR
NAVSAT & other LEO systems
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The history of TRANSIT coincides with the start of the Space Age (4th October 1957). Sputnik I was launched and the Doppler shift of the signals were used to determine the satellite orbit. The method was inverted so that if the orbit were known, the position of the receiver could be determined (PARKINSON et al, 1995). Special features and milestones:
Figure 1. The TRANSIT Doppler satellite positioning system configuration.
ARGOS is another satellite system which uses the Doppler principle for positioning. ARGOS is a cooperative project between the French Centre National d'Etudes (CNES), NASA, and the U.S. National Oceanic and Atmospheric Administration (NOAA), and was first deployed in 1978. Transmitters are operated by users on variety of "platforms" (buoys, animal tracking, radiosondes, etc.), and satellites act as receivers (one of two U.S. TIROS weather stations). CNES computes position and velocity of platform, sends information (and the bill!) to user. Other such "subscriber" systems in place include the COSPAS-SARSAT search & rescue system and GEOSTAR. The important distinction is that ARGOS is essentially a satellite-based tracking system, while many of the other systems (including GPS) are self-navigating systems.
Russian equivalent to the GPS system, having the following characteristics (KLEUSBERG, 1990; IVANOV & SALISTCHEV, 1991; PARKINSON et al, 1995):
64.8 inclination, 19,100km altitude (11hr 15min period).
WHAT IS GPS? |
The NAVSTAR Global Positioning System (GPS) is a satellite-based positioning system, which by virtue of its special characteristics, is revolutionising the tasks of navigation and surveying:
Relatively high positioning accuracies, from the dekametre to the millimetre level.
Determination of velocity and time to an accuracy commensurate with position.
Available to users anywhere on the globe: in the air, on the ground, or at sea.
Relatively low cost system, with no user charges.
All-weather system, available 24 hours a day.
The position information is in three dimensions, that is, vertical as well as horizontal information is provided.
The Global Positioning System is, in the first instance, a military navigation system designed, financed, deployed and controlled by the U.S. Department of Defense. |
Development work on GPS commenced in 1973 as a result of the merger of several
R&D programs within the U.S. Department of Defense, namely the Navy's "TIMATION"
project, and the Air Force's "621B" project. The first satellite was launched in
1978. (For a background to the development of the GPS program the reader is
referred to
PARKINSON, 1994, and
PARKINSON et al, 1995.) The development and production program for the GPS
is managed by the U.S. Air Force (USAF) Systems Command, Space Systems Division,
Joint Program Office (JPO) at the Los Angeles Air Force Base, California. The
JPO is manned by personnel from the USAF, U.S. Navy, U.S. Army, U.S. Marine
Corps, U.S. Coast Guard, U.S. Defense Mapping Agency, NATO nations and
Australia.
The aim of the JPO was to develop an all-weather, 24-hour, truly global navigation system to support the positioning requirements for the armed forces of the U.S. and its allies. As such a system was designed to replace the large variety of navigational systems already in use, a great import was placed on the system's utility, reliability and survivability. A number of stringent conditions therefore had to be met. In addition to those listed above, they included:
This led to a design concept based on:
|
The
total investment by the U.S. military in the GPS system to date is well over
$10 BILLION (US)! However, although the primary goal of GPS is to provide land, air and marine positioning capabilities to the U.S. armed forces and its allies, GPS is freely available to all users. The number of civilian users is already far greater than the military users, and the applications of the positioning technology are growing rapidly. The civilian sector therefore represents an important user group that is increasingly lobbying in order to influence official GPS policy. The U.S. military however still operates several "levers" with which they control the performance of GPS. On the other hand, there is tremendous innovation occurring within the civilian sector, with the development of technology and procedures that are increasing making redundant many of the U.S. military procedures intended to restrict GPS performance. |
Satellite Positioning Systems are global. The signals can be "seen" over a large area, and are not interfered with by terrain or geography to the same extent as conventional ground-based positioning systems. |
GPS was designed to replace the TRANSIT Doppler satellite navigation system which has given good service to the navigation and geodetic community for over 20 years. The advantages of TRANSIT are essentially those of GPS as well. A microwave satellite-based system:
However, in addition, the NAVSTAR Global Positioning System has some further advantages over other satellite-based positioning systems:
It is a one-way (listen only) system, in which the satellites transmit signals, but are unaware who is using the signal (no receiving function). The user (or listener) does not transmit a signal, and therefore:
As GPS is a multi-satellite system, there is always a number of satellites visible simultaneously anywhere on the globe, and at any time.
GPS can support a number of positioning and measurement modes in order to satisfy simultaneously a variety of users, from those only requiring navigation (dekametre) accuracies to those demanding very high (millimetre - centimetre) accuracies.
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SATELLITE-BASED POSITIONING SYSTEMS |
Some characteristics of satellite-based positioning systems, past, operational and proposed, are:
TSIKADA
NAVSTAR Global Positioning System
STARFIX
GEOSTAR/LOCSTAR
NAVSAT & other LEO systems
Back
to Chapter 2 Contents
/
Next Topic
/
Previous Topic
The history of TRANSIT coincides with the start of the Space Age (4th October 1957). Sputnik I was launched and the Doppler shift of the signals were used to determine the satellite orbit. The method was inverted so that if the orbit were known, the position of the receiver could be determined (PARKINSON et al, 1995). Special features and milestones:
Figure 1. The TRANSIT Doppler satellite positioning system configuration.
ARGOS is another satellite system which uses the Doppler principle for positioning. ARGOS is a cooperative project between the French Centre National d'Etudes (CNES), NASA, and the U.S. National Oceanic and Atmospheric Administration (NOAA), and was first deployed in 1978. Transmitters are operated by users on variety of "platforms" (buoys, animal tracking, radiosondes, etc.), and satellites act as receivers (one of two U.S. TIROS weather stations). CNES computes position and velocity of platform, sends information (and the bill!) to user. Other such "subscriber" systems in place include the COSPAS-SARSAT search & rescue system and GEOSTAR. The important distinction is that ARGOS is essentially a satellite-based tracking system, while many of the other systems (including GPS) are self-navigating systems.
Russian equivalent to the GPS system, having the following characteristics (KLEUSBERG, 1990; IVANOV & SALISTCHEV, 1991; PARKINSON et al, 1995):
Figure 2. The GLONASS satellite configuration.
Table Comparing GPS and GLONASS ( IVANON & SALISTCHEV, 1991 ) |
||
Parameter |
GLONASS |
GPS |
Ephemeris information presentation method |
9 parameters of s/c motion in the gecentric rectangular rotated coordinate system |
Interpolation coefficients of satelite orbits |
Geodesic coordinate system |
SGS 85 |
WGS 84 |
Referencing of the ranging signal phases |
To the timer of GLONASS system |
To the timer of GPS system |
System time corrections relative to the universal coordinates time ( UTC ) |
UTC ( SU ) |
UTC ( USNO ) |
Duration of the almanac transmission |
2.5 min |
12.5 min |
Number of satelites in the full operational system |
21 + 3 apares |
21 + 3 spares |
Number of orbital planes |
3 |
6 |
Inclination |
64.8 |
55 |
Orbit altitude |
19.100 km |
20.180 km |
Orbital period |
11 h 15 min |
12 h |
Satelite signal division method |
Frequency division |
Code division |
frequency band allocated |
1602.5625-1615.5 |
1575.42 1 MHz |
Type of ranging code |
PRN-sequence of maximal length |
Gold code |
Number of code elements |
511 |
1023 |
Timing frequency of code |
0.511 MHz |
1.023 MHz |
Crosstalk level between two neighboring channels |
- 48 dB |
- 21.6 dB |
Synchrocode repetition period |
2 sec |
6 sec |
Symbol number in the synchrocode |
30 |
8 |
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© Chris Rizos, SNAP-UNSW, 1999
© Chris Rizos, SNAP-UNSW, 1999
Figure 2. The GLONASS satellite configuration.
Table Comparing GPS and GLONASS ( IVANON & SALISTCHEV, 1991 ) |
||
Parameter |
GLONASS |
GPS |
Ephemeris information presentation method |
9 parameters of s/c motion in the gecentric rectangular rotated coordinate system |
Interpolation coefficients of satelite orbits |
Geodesic coordinate system |
SGS 85 |
WGS 84 |
Referencing of the ranging signal phases |
To the timer of GLONASS system |
To the timer of GPS system |
System time corrections relative to the universal coordinates time ( UTC ) |
UTC ( SU ) |
UTC ( USNO ) |
Duration of the almanac transmission |
2.5 min |
12.5 min |
Number of satelites in the full operational system |
21 + 3 apares |
21 + 3 spares |
Number of orbital planes |
3 |
6 |
Inclination |
64.8 |
55 |
Orbit altitude |
19.100 km |
20.180 km |
Orbital period |
11 h 15 min |
12 h |
Satelite signal division method |
Frequency division |
Code division |
frequency band allocated |
1602.5625-1615.5 |
1575.42 1 MHz |
Type of ranging code |
PRN-sequence of maximal length |
Gold code |
Number of code elements |
511 |
1023 |
Timing frequency of code |
0.511 MHz |
1.023 MHz |
Crosstalk level between two neighboring channels |
- 48 dB |
- 21.6 dB |
Synchrocode repetition period |
2 sec |
6 sec |
Symbol number in the synchrocode |
30 |
8 |
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© Chris Rizos, SNAP-UNSW, 1999
GPS SATELLITE SURVEYING |
Adopting the broadest definition of "GPS surveying", the following classes of
surveys can be identified:
What are the criteria for deciding if an application belongs to "surveying", "navigation", or "other"? This is not as easy as it may first appear. In general (and there are exceptions), a "surveying" application is a positioning task that:
In the case of land surveying applications, the characteristics of GPS satellite surveying are less contentious:
A convenient approach is to adopt an applications classification on the basis of
accuracy requirements. Three classes of applications can be identified on this
basis, for which a range of relative accuracies (it is assumed that single
receiver point positioning is not accurate enough to satisfy these applications)
ranging from low-to-moderate, 1 part in 104, through to the
ultra-high 1 part in 107 or better accuracies:
Category A (Scientific) |
: better than 1 ppm |
Category B (Geodetic) |
: 1 to 10 ppm |
Category C (General Surveying) |
: lower than 10 ppm. |
Category A surveys primarily encompass those surveys undertaken for precise engineering, deformation analysis, and geodynamic applications. Category B surveys include geodetic surveys undertaken for the establishment, densification and maintenance of control networks to support mapping. Category C surveys primarily encompass lower accuracy surveys, primarily undertaken for urban, cadastral, geophysical prospecting, GIS and other general purpose mapping applications. Users in the latter two categories form the majority of the GPS user community, while category A users often provide the primary "technology-pull" for the development of new instrumentation and processing strategies, which may ultimately be adopted by the category B and C users.
Note that this classification scheme is entirely arbitrary, and does not reflect any "order" of survey as defined by Survey Authorities. It does, however, provide a convenient breakdown of GPS survey "type", enabling the similarities and differences between the categories to be highlighted. Below are listed the advantages and disadvantages of the GPS technology (in the context of land survey applications) in broad-brush terms only.
There are several advantages of the GPS satellite surveying
technique:
It would be remiss not to also mention the disadvantages, some
of which will no doubt be overcome in time, others with some additional effort,
while others cannot be dismissed so easily:
The prospect for increased acceptance of GPS satellite surveying is very good,
particularly as the cost of GPS systems drops and new higher productivity
techniques are developed. Although GPS was initially used for high-order geodesy
and geodetic control surveys on the one hand, and geophysical exploration
surveys on the other, adoption of the GPS technology for applications such as
lower-order control densification, and even cadastral, engineering and detail
surveys, has already commenced.
However, for all its technical advantages, there remain a number of significant differences between the results of the GPS surveying technology and that of conventional terrestrial techniques. To reconcile these differences, and in order to ensure that GPS will complement these other technologies (and hence maintain compatibility with the geodetic framework established in many countries over a long period of time), a significant amount of post-processing of GPS results is necessary. This tends to make the GPS technology less attractive, and has the effect of raising the threshold of acceptance slightly higher than it would otherwise have been.
In addition there is an investment in human resource development that must be taken into account. GPS manufacturers are striving to make equipment that is ever more "user-friendly", which will mean that many other professionals apart from "qualified professional surveyors" will be able to carry out high accuracy GPS surveys. The challenge, however, to surveyors is to maintain the "edge", by seeking to use their best judgement and skills not only to achieve high GPS accuracy, but also to ensure that the quality and reliability of the results are at the level demanded by the client. A further advantage that surveyors enjoy over other professionals is that they often are the only ones skilled in integrating GPS results into previously coordinated networks.
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© Chris Rizos, SNAP-UNSW, 1999