Satellite Vehicles-Based Positioning Technology

Modern localization satellite vehicles (SVs) techniques (or terrestrial equipments based) for positioning are based on electromagnetic impulses traveling time between the transmitters and receivers.

SVs positioning techniques are usually categorized in two main classes:

1. Mobile terminated (MT) 2. Mobile originated (MO)

Most of LBS known to mass market customers belong to the first family (GPS, Galileo, Glonass) in which the transmitters are installed on board of SVs and the receivers are held by users. With this technology solution, the localization mea-sure is available to customer equipments. This is often used to automatically route to a destination. Frequently, localization receivers are integrated with transmitters (GSM, satellite communicators, classic VHF radio link) to send the information to service centers.

Inmobile-originated techniques, the transmitters are installed on board of the user equipment. The services that use this kind of localization includesearch and rescue services, oriented to nautical, aeronautical, or other specialized applications.

They are calledSARSAT satellite services, where the user equipment transmits sig-nal to the satellites. From the SVs, sigsig-nals are immediately forwarded to safety centers. With this technology solution, customer equipments do not allow the users to know the position measure. The localization information is obtainable by a surveillance center of monitored vehicles (e.g., boats and airplanes). To increase the security and safety effects of people and vehicles, the current transmitters are equipped with a GPS receiver and with a radio frequency voice channel. In case of distress, therefore, the customer equipment transmits, besides the identification of the equipment that is strictly associated to the vehicle, also the GPS position. In addition, the first aid personnel are equipped with special receivers to better detect the exact distress point and to provide voice assistance. Therefore, GPS-equipped transmitter allows to automatically route to a destination.

3 Wireless Network Data Sources 79 In both cases, the position is computed defining a mathematical model of the media between the transmitters and receivers. However, the random positions of SVs and receivers make it difficult to compute the path between them. A highly precise localization can be obtained by using a sophisticated and computational expensive mathematical model of propagation paths. The challenging difficulty is due to the fact that, during the paths, the waves pass through a media changing their propagation characteristics as nonlinear multidimensional continuous function. The propagation parameters are function of the height above sea level, day time, seasons, pollution, and meteorological conditions.

3.2.2.1 GPS

The GPS project [16], funded by USA Department of Defense, is based on 24 SVs moving on six orbital plans, tilted 55^◦ respect of equator on an altitude of about 20,000 km. Actually 29 SVs are operative of which three are IIR-M class (the last generation). As already anticipated, GPS project implements a mobile-terminated solution, where transmitters are on board of SVs and therefore all the information (positioning, speed, and timing) are available only at receiver level. GPS works on two different frequencies: L₁=1,575.42 MHz andL₂=1,227.60 MHz. L₁ fre-quency is both for mass market and military (or special) applications whereasL₂ one is devoted only to military (or special) applications.

OverL₁frequency, two signals are coded: C/A for mass market and P(Y) for military (or special) applications. Over L₂, only the P(Y) signal for military (or special) applications is coded. In the near future, over L₂ frequency, a new civil signal, calledL2CS, will be broadcasted. This signal will allow higher precision and better availability.

The GPS receivers allow to measure, besides the location, two other important values: speed and time. It means that GPS can be used as tachometer and as a high-precise time reference instrument. This second feature is particularly useful in communication networks where data synchronization is essential.

The accuracy performances of the GPS system, for civil applications (low pro-file), are basically:

– Positioning: 32 m – Speed: 0.1 m s⁻¹ – Timing: 1µs

– Time to first fix (ttff): 1 min

Thettffparameter is the time elapsed from the receiver turning on to the posi-tion data availability. This value is particularly meaningful in all services where the position information must be available immediately turning on the receiver, such as services integrated with mobile phone.

The error sources can be SVs clock errors, SVs position errors, bad propagation model in the ionosphere, bad propagation model in the troposphere, and multipath effects. All these effects cause an error (e) of about 16 m per SV. Since four SVs

are necessary to get the position, in the worst case, the error is 64 m. Since the four errors are statistically independent, the resulting error is 32 m with 67% probability.

When the sea level is not considered, latitude and longitude can be computed by three SVs and therefore the positioning performance increases, reaching 28 m. The main source of error here is the bad ionosphere modeling that causes 10 m error.

Modern commercial GPS receivers can compute the positioning using more than four SVs, until 32, if viewable, achieving a better guess of measures [15].

To increase the GPS performances, several solutions have been designed. The most well known are European geostationary navigation overlay service (EGNOS), A-GPS (already described in Sect. 3.2.1.7), and the new satellites constellations.

3.2.2.2 European Geostationary Navigation Overlay Service

EGNOS is a project designed by Europe Space Agency (ESA) and by the main aerospace industries. It is based on the concept that the propagation characteristics of the atmosphere are quite stable in a wide area. This means that the error of the measures is prevalently the same in the entire area. The project architecture is based on a network of reference stations installed in Europe, in high precision georefer-enced sites. The reference stations are composed by a sophisticated GPS (called reference GPS) and a computer to measure and send, in real time, the GPS system error to the main center. The GPS system error can be measured as the difference between the known exact position of the reference station and the position measured by the reference GPS. All errors are sent to the main station, whereas they are packed and sent to the delivering infrastructure. The delivery infrastructure uses two types of media: three geostationary SVs (different from GPS constellation) and a Web server. The geostationary SVs transmit the information using L₁ frequency, thus allowing commercial GPS devices to receive the error information by just upgrad-ing the firmware. This solution can work very well in every part of the world, except city centers of old towns, since urban canyon makes it difficult to set up a link to geostationary SVs. The second solution uses a radio link or GSM (GPRS/UMTS) connection to the server hosting the error information. EGNOS project increases the precision performance up to 2 m. Other solution analogous to EGNOS are the American WAAS and the Japanese MSAS. In addition to that, other private and public solutions based on this principle (differential GPS – DGPS) are achievable.

3.2.2.3 New Satellites Constellations

By the end of February 2008, a new generation of SVs will be launched. It will provide a deep evolution of GPS services. They will transmit with higher power and will introduceL₂frequency for civil user (L2CS). TheL₂civil services will allow the advantages of increasing the GPS performances in terms of precision, availability, and reducing ofttff. Under military point of view, new encryption will allow to

3 Wireless Network Data Sources 81 benefit of a better protection antijamming. In the far future (2011), an additional frequency labeled L5 will be set up to further increase GPS performances.

3.2.2.4 Galileo

Galileo will be the European answer to American GPS. It will be operative by 2010;

it uses three frequency bands at around 1,500 MHz and will use 30 SVs to offer five services:

1. Open service(OS). It is targeted to general purpose customer similar to current GPS users.

2. Safety of life service(SoL). It is targeted to people involved in safety activities.

It is a double frequency service.

3. Commercial service (CS). It is targeted to service as parking, auto route pay-ments, or other commercial services. It is a double frequency service.

4. Public-regulated service(PRS). It is targeted to government applications: mili-tary, police, and similar customers. This is a high availability service, it is also protected by voluntary disturbs.

5. Search and rescue service(SAR). This service will enhance the quality of ser-vice actually available in search and rescue area. That is because it will integrate the Galileo receiver with current SARSAT transmitters that guarantee a higher response rapidity and a higher precision.

3.2.2.5 Glonass

Glonass is a satellite-based localization system delivered by ex-USSR. At the moment, the Glonass localization service is accessible only to less than 50% of the world, that is mainly over 50^◦North and under 50^◦South latitude. The full world wide coverage will be available by the end of 2010. Glonass localization perfor-mances are similar to GPS ones. Glonass receivers are not targeted to consumer market, since they are used for professional applications only. Most of Glonass receivers are therefore equipped also with GPS receivers, it means that the avail-ability and the precision of the Glonass localization improve with respect to GPS only receivers. The copresence of two receivers (Glonass and GPS) in the same localization set guarantees also a faster localization acquisition. This performance is really appreciated in all professional applications where high precision measures are required. Both GPS and Glonass use two frequency bands called L₁and L₂. The main difference between the two localization technologies is that GPS uses one channel of 20 MHz in each frequency, while Glonass uses 25 channels of 562.5 kHz in each frequency.