1.1.3 Designing a Satellite Positioning System

IMPLICATIONS OF OPTING FOR A SATELLITE POSITIONING SYSTEM

Implications of the use of a Satellite-Based Positioning System: The satellites must be placed in suitable orbits so as to be "seen" by as many users as possible, from locations all over the earth. The reference coordinate datum to which they relate is largely defined by the system operators. Users do not have a direct interest in (or control of) the system, and policy decisions regarding the operation of the system tend to be made by the sponsoring agency. By replacing monumented ground control stations with satellites the well-established procedures for terrestrial surveying and navigation may no longer be valid. The orbiting control stations are invisible to the user. Position determination is most conveniently performed in relation to a three-dimensional datum. The coordinates of the orbiting control stations change with time (and must be continually updated). Errors in both the coordinates of the satellites (the ephemerides) and in the measurements themselves will affect the final quality of the position determination.

Further discussion on the consequences of a Satellite-Based Positioning System:

• Consequences for the satellite deployment strategy
• Consequences for geodetic datum definition
• Consequences of surrendering national control
• Consequences of measuring to satellites
• Consequences to conventional notions of positioning

Consequences for the Satellite Deployment Strategy

There are a important consequences in replacing hundreds of thousands of geodetic control stations on the ground (at separations ranging from tens to thousands of kilometres) by "orbiting control stations":

• To provide a global, 24-hour service, a constellation of satellites must be deployed. However, as satellites are expensive, their total number must be kept to a minimum.
• High altitude satellites give good coverage, but satellite-ground geometry may be weak unless observations are made over very long distances.
• Low altitude satellites most closely mimic the function of ground-based control stations, but too many would be required to ensure global, 24-hour coverage. They also travel at high speed relative to the ground and hence are only visible for short periods of time.
• A compromise needs to be reached as far as satellite altitude is concerned, as this most directly impacts the user availability of the satellite system.
• To ensure reliability, redundancy should be built into the system, for example by having more than the minimum number of satellites visible to a user at any one time. This implies that more than the minimum number of satellites should be deployed.

   

Consequences for Geodetic Datum Definition

As the datum is defined by the coordinates of the satellite tracking stations :

• A single global geodetic datum is ensured.
• National datums have no relevance in satellite positioning, except when the results must be compared with those obtained using conventional ground-based techniques.
• Most nations have no control over the satellite datum. If the system operators chose to change the datum in some way, users must accommodate these changes.
• The computation of the satellite ephemerides is an important part of the maintenance of the global satellite datum, and hence is usually the responsibility of the system operators. The quality of the ephemerides has an affect on the accuracy and reliability of the positioning results obtained by the user.

Consequences of Surrendering National Control

As a Satellite-Based Positioning System is often developed, funded and controlled by one nation (and usually the military agency within that country):

• Operation of the system is firmly vested in the hands of one agency within one country, and the influence of most users is small.
• Decisions that affect the coordinate datum of the system are not necessarily made with the global users in mind.
• If the system operators are opposed to a certain class of user (for example, on political grounds), efforts can be made to restrict system access or performance for such users.

   

Consequences of Measuring to Satellites

A Satellite-Based Positioning System is likely to have the following effects on the design of user equipment:

• The satellites are invisible, hence the user does not have direct "contact" with the system and its operations are largely of the "black-box" variety.
• In order to increase user acceptance of a satellite-based positioning system, the user equipment should be relatively inexpensive. This has the following consequences:
  • as much of the complexity should be placed in the satellites as at all possible,
  • omnidirectional, small antennas should be used,
  • receivers should be small, low power units, being essentially "passive", receiving the satellite signals, and not required to transmit a signal to the satellite(s), and
  • as many of the computations should be carried out within the user equipment, hence reinforcing the "black-box" nature of these systems.
• The most appropriate technology is that based on microwave, one-way (passive) ranging. This also satisfies the requirement for all-weather, night-and-day operation, as well as the military requirement for non-detectability of the user by an enemy.

Consequences to Conventional Notions of Positioning

A Satellite-Based Positioning System will have important implications for users:

• Position will be determined in three-dimensions.
• The artificial separation of horizontal and vertical positioning operations will no longer have relevance.
• Design of procedures for position determination will no longer be based on previous notions of ground network "geometry".
• The coordinates of the orbiting control stations must be easily accessible to users. This is most appropriately done by transmitting the satellite ephemerides with the signals themselves.

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© Chris Rizos, SNAP-UNSW, 1999