Next Generation of Satellites

There are a number of improvements being considered for the next generation of satellites. These include:

Increase in satellite EIRP: With 50 W satellite transmissions, the level received at the receiver is of the order of 160 dBW for C/A code and 166 dBW for the P Code, making the system susceptible to extraneous noise. Higher power and frequency diversity (see next paragraph) are being considered to improve robustness.

Provision of two additional frequencies: Next generation satellites, called Block 11F, are planned additionally to transmit C/A code in band L2 and, as yet, undecided frequency, perhaps in the L. band (960 1,215 MHz)currently used by the Aeronautical Radio Navigation Service. Use of frequency diversity improves transmission reliability and accuracy.

Auto navigation feature on GPS satellites: Satellites will cross link in space to generate their own ephemeris, enabling them to provide ephemeris data for up to 180 days without ground uploads. At present, the data uploaded from the ground can be saved from 14 to 180 days (depending on satellite generation), but in the absence of regular refresh from the ground the accuracy deteriorates with time.

Mitigation of multipath by use of directional antennas: Multipath noise degrades the accuracy of fixes, which may not be acceptable when very high, accuracy is needed (see Table 10.3). Directional antennas tend to reduce the multipath and therefore their applicability is under investigation.

As mentioned, the accuracy of fixes depends on a number of factors, such as the prevailing atmospheric and ionospheric conditions which are corrected to the extent possible by signal processing and downloaded correction data. It has been observed that ephemeris and measurement errors are correlated in time and spatially. Spatial correlation decreases with distance but is quite insensitive to variation in distance correlation is close even for a distance of a few tens of kilometres and temporal correlation decreases rather slowly with time, for propagation errors and S/A errors are correlated in a S 10 s time span. An accurate estimate of errors can be obtained when the position of a location is known; if these corrections are transferred to users in the vicinity of the measurement site, the accuracy of the fix is improved considerably. Application of this technique is known as differential GPS (DGPS), where errors in the navigation solution are derived at a reference site and transmitted over a radio link to the receivers. Reference sites can be located from less than a kilometre to over 1,000 km, and as mentioned above, the accuracy of the fix improves with a reduction in distance to the reference station. Measurements and simulation demonstrate errors ranging from tens of centimetres when the reference site is a few kilometres, to 5 m for a distance of 1,000 km. A number of commercial and other bodies responsible for safety operate DGPS systems in various parts of the world. Service is offered through a radio carrier relayed through satellite or terrestrial transmitters. Commercial users include offshore oil platform operators, fleet managers in the trucking industry, etc. Safety related services are offered, for example, by the US Coastguard, which provides differential corrections at 285 325 kHz in coastal areas free of charge. There are plans to extend the service throughout the USA. Potential beneficiaries include agriculture, forestry, the emergency services, etc.

The accuracy of fix estimates is reduced in the case of failure or unavailability of satellite(s) in the constellation, possibly without the user being aware. An external system which provides such an integrity monitor can be very useful. Geostationary satellites transmitting signals identical to GPS and GLONASS, containing the integrity of satellites and error estimates, can be used to improve the accuracy. inmarsat 3 geostationary sat I ellites include a navigation transponder and ate used for providing this type of overlay in many regions.

This type of overlay approach is being developed in the USA, Europe, Japan and the Federal Aviation Administration (FAA) for application to civil aviation. in fact, such a system is expected to revolutionize air traffic control in the next few years. At present, aircraft navigation relies on ground based transmitters or inertial navigation systems when aircraft are over oceans, which can cause them to drift several kilometres off course. Prior to landing, VOR and DME systems (see section 10.3 1) provide non precision navigation aid to pilots, which is particularly useful in adverse weather conditions; pilots then use either visual information or an instrument landing system (ILS) for landing. Clearly, GPS or an equivalent satellite system offers promising solutions for in flight and the approach part of landing, in particular when the cost of VOR systems is taken into consideration. Another limitation of the existing navigation aid is that it requires planes to fly wide apart separation of 5 8 km requires use of radars, which are not available for transoceanic flights, causing inefficient use of the air corridor; VOR based systems require separation of up to 8 miles (13 km). The solution being developed is called Automatic Dependent Surveillances (ADS B), where an augmented GPS receiver on aircraft broadcasts its own position to the ground receiver, thereby removing the need for radar, permitting separation to be reduced considerably. The technique is also being developed for the next generation traffic alert and collision av terns are based on radar monitoring. A GPS based navigation solution, planned to provide GPS based information for en route, departure and approaches under conditions where ceiling and visibility are 200 feet and 12 mile respectively.Systems compatible with WAAS are being developed in Europe and Japan. The FAA also intends to deploy the Local Area Augmentation System (LAAS), which will provide GPS navigation down to the surface, allowing near g, and airport surface navigation using ground receivers placed in blind landin areas within 30 45 km, giving an accuracy of 1 M.

GPS is used widely in mobile communications for applications where it is necessary for users to provide position updates, such as for tracking the positions of each mobile in a fleet, reporting the position for network mobility managernent, spot beam identification, etc. It is also used on yachts and ships and by explorers, mountaineers, scientists, surveyors, etc; specific examples include delivery or pick up points for an accurate delivery/pick up service; real time response to marine hazards such as oil spills; attitude and orbit control (AOC) of LEO/MEO satellites, etc. Novel applications are continually being developed around the system. An insurance company monitors the usage of cars for insurance charges; a journey at midnight is more expensive than during the day, as people are more alert during the day! Low power GPS receivers can now be installed in watches and personal digital assistants; GPS time is in use for synchronizing CDMA cellular base stations, essential for soft handover; medical services use it for responding to life-threatning emergencies etc.