BACKGROUND

International civil community has been continuously put in need of better and cheaper positioning system. Many sectors as land navigation, marine navigation, air navigation, surveying, mapping and geographical information system are the primary users of the system. Recently, the need is fulfilled by GPS (or GLONASS)). But the GPS (or GLONASS) is a single nation controlled system, which may eventually lead to denial of service for brief or extended periods of time.

Many proposals have been submitted to develop a satellite based global navigation system intended for civil purpose under control of an International Civil Authority. In general, the systems will still have 3 segments, space segment, Control segment and user segment. The difference is that the control Segment is under an International Civil Authority instead of a single nation.

The GPS Control Segment has functions as follows:

Ø Maintaining the satellites in their proper orbital position (station keeping)

Ø Monitoring satellite subsystem health and status

Ø Monitoring the satellite solar arrays, battery power levels, propellant level used for maneuvers

Ø Updating each satellite’s clock, ephemeris (satellite orbit parameters) and almanac

Ø Resolves satellite anomalies

Ø Control SA (selective availability) and AS (anti spoofing)

SA is implemented to deny full system accuracy to SPS (Standard Positioning Service) users. SA dithers the satellite’s clock, thereby corrupting TOA (time of arrival) measurement accuracy. Further, SA induces errors into the broadcast navigation data parameters. PPS (Precise Positioning Service) users remove SA effects through cryptography.

It is seen that feature like SA still can be applied on the proposed civil systems. In order to remove possibility of SA like feature applied in the system, the function to update satellite’s clock & ephemeris must be taken from the Operational Control Segment. This function is then to be carried out in the user’s receiver.

Assessment on all implications come up due to this change is the main objective of this research.

RESEARCH OBJECTIVES

This research aim is to assess feasibility and performance of a satellite based global positioning system if the ephemeris update function is put on user’s receiver (user segment) instead of in the control segment (an operational control station).

This change is to make the system independent of any institution controlling the operational control station. How this change carried out is described below.

Important to mention here, that in this satellite based position determination, satellite constellation act as references. Their positions are known at any time. So, position of x on earth can be calculated when its relative positions to the satellites are known. Therefore ephemeris update is an essential to maintain accuracy of the position determination.

In GPS, the update is carried out through a tracking process done by some tracking station. The data collected is then used to estimate the ephemeris which then uploaded to the satellite. From these updated ephemeris which is broadcasted by the satellites, the position of satellites at any time can be calculated.

In this system, signal from satellite brings sat-ID and clock information, but not the ephemeris. Satellite position at any time is obtained using orbit propagator software inside the receiver. Off course, there will be an accumulated integration error as the time elapsed. But the error can be limited by entering the updated satellite positions.

Updated satellites positions can be obtained by either:

Ø From an institution which give the service, by doing satellite tracking

Ø The user actively doing ephemeris update by using reverse method of the satellite based position determination

The reverse method compares a known location on earth coordinates with the ones obtained from the receiver calculation. This error is then used to make correction to the satellite ephemeris.

The main works to do in this research are:

Ø System configuration design (Space System Engineering)

Ø Algorithm of position determination to put in the receiver (Astrodynamics)

Ø Algorithm of ephemeris update to put in the receiver (Astrodynamics)

Ø Software and Hardware realization of the algorithms (Software & Instrumentation Eng)

Ø System Performance Test

PLANNED RESEARCH ACTIVITIES

1. System configuration design

2. Development of position determination Algorithm to put in the receiver

3. Development of ephemeris update Algorithm to put in the receiver

4. Software development for system simulation

5. Receiver and satellite payload prototype realization

6. System Performance Test