Unit 15

1. Read and translate the following text

Differential gps (dgps)

Differential GPS (DGPS) services have been developed in response to inherent and imposed limitations of GPS. Despite the high order of accuracy available from the Precise Positioning Service, even this may not be sufficiently accurate to meet the stringent requirements of navigation in harbours and their approaches.

The fundamental principle of DGPS is the comparison of the position of a fixed point, referred to as the reference station, with positions obtained from a GPS receiver at that point. The observed difference can either be considered as a 2 or 3 dimensional geographical co-ordinate offset (position differential), or as a series of corrections to the satellite range data (pseudorange differential). Other methods of supplying corrections exists as satellite clock and almanac corrections or pseudorange corrections for a specific location with additional values to correct for user's position with respect to the specific location. The position differential technique is of limited application as it is necessary for the mobile observer to use the same satellites observed at the reference station. The pseudorange technique involves computing a unique correction to the range of each satellite observed at the reference station but does not necessarily require all of the same satellites to be the ones used by the mobile observer. Those pseudorange corrections calculated at the reference station which are not being observed at the mobile will be ignored.

Investigations into ways of transmitting the pseudorange corrections to the user, identified medium frequency marine radiobeacons operating in the 285 - 325 kHz band as the most suitable terrestrial method for use within the coastal zone. An American correction message format known as RTCM SC104 has become the industry standard for encoding DGPS corrections. The correction message is broadcast on the radiobeacon frequency by applying G1D modulation with Minimum Shift Keying (MSK). A Broadcast Standard document describing the broadcast signal structure and format has been produced by the US Coast Guard.

In order to make use of the DGPS corrections, users must have an MSK/radiobeacon receiver and a GPS receiver capable of incorporating DGPS correction data in the RTCM SC104 format.

Several commercial companies provide a DGPS service, but encrypt the RTCM format signal so that either a special receiver or a decoding device is required before the higher accuracy positions can be calculated.

DGPS systems using geostationary satellites have also been developed. A network of reference stations provides the pseudorange corrections, which are transmitted to the satellites for broadcasting to users. The coverage of the satellite broadcast is very much greater than MF radiobeacons and the "range" of the system is only constrained by the distance from the reference station over which the derived corrections remain valid.

Eurofix — Differential Global Positioning System (dgps) on Loran-c

The Northwest European Loran-C System (NELS) is established by Denmark, Germany, France, Ireland, the Netherlands and Norway to offer a regional, terrestrial radionavigation system for Northwest Europe.

The establishment of NELS has acted as a stimulus to Loran-C research and development at European universities and scientific laboratories. One result of this activity is the development by the Delft University of Technology in the Netherlands of an integrated navigational system called Eurofix. Eurofix is an integrated radionaviagtion and communication system, which combines Loran-C and DGPS by sending differential satellite corrections to users as time, modulated signal information. Loran-C or Chayka stations are upgraded to broadcast low-speed data over ranges of up to 1000km. Data is separated into eight channels which are assigned to DGPS, DGLONASS, DLoran-C/DChayka, navigation integrity messages and short message service. Three channels are reserved for future applications. The normal navigational operational mode of Loran-C is preserved which gives the Eurofix user, next to accurate DGPS positions, improved navigation reliability. If either GPS or Loran-C might fail, the other system still provides position information, however at a reduced accuracy of between 100 to 300 metres. As Loran-C and GPS are highly dissimilar in operational control and signal propagation characteristics, there is not much risk that both systems will fail simultaneously.

Eurofix test transmissions from the Loran-C station at Sylt in Germany have been successful, resulting in a decision to implement Eurofix on another three stations: Lessay (France), Vasrlandet and B0 (Norway) offering Eurofix coverage throughout the area of the current NELS Loran-C coverage.

A plan for full implementation of Eurofix on all NELS stations has now been accepted by the NELS Steering Committee, the system's supreme authority.

Subject to such implementation, there is a potential for further Eurofix coverage expansion in Europe by making use of the existing Loran-C infastructure in the Mediterranean Sea area and the Russian Chayka (Loran-C equivalent) infastructure.

Eurofix can act as a DGPS system when using a Loran-C standard GPS correction output message fed into a suitably equipped GPS receiver system. It does also allow position calculation using both DGPS and Loran-C and comparison of the two systems. If either system fails, the other can take over thus improving availability and continuity. Finally, Eurofix allows full DGPS/Loran-C integration which in a given situation will make for instance, Loran-C act as a satellite source. A user will, in this example, be able to calculate a three dimensional position with only three satellites available.

The coverage of Eurofix is estimated to be at least 1000 km from each Eurofix equipped Loran-C transmitter. Fully implemented, an absolute accuracy of better than 5 metres and an availability of better than 999996% per month is achievable.

The NELS Eurofix programme is fully in line with the EU's policy on radionavigation system development in Europe.

2. Answer the following questions

1. Why have differential GPS (DGPS) services been developed?

2. What is the fundamental principle of DGPS?

3. What methods of supplying corrections exist?

4. What does the pseudorange technique involve?

5. What countries is NELS established by and for what?

6. What is Eurofix?

7. What level can Eurpfix coverage be estimated?

3. Discuss recent radionavigation system developments.