GGPhi
A low-cost, low-power Galileo/GPS carrier phase positioning system

The motivation is to extend the capabilities of high-accuracy GNSS (Global Navigation Satellite System) positioning systems, used very successfully in the developed world, for applications such as landslide monitoring or building deformation monitoring, to the developing world. The difficulties in the developing world are to do with requirements for low-power, low-cost, robust equipment and constraints due to lack of infrastructure. Other potential pseudo-static monitoring applications for this type of technology are monitoring of avalanches, glacier movements, volcanic ground deformation, subsidence, tsunamis and built structures.

A low-cost, low-power, high-accuracy Galileo positioning system for use in remote areas in developing countries. Research covered:

• the design and characterisation of the remote carrier phase sensor;
• the design of the local area wireless network;
• algorithms and software for accurate positioning.

Description 

Technological innovations

1. The low-cost, low-power carrier phase sensor design relies exclusively upon tracking a double frequency component of the Galileo and GPS (modulation-suppressed) carrier frequencies by squaring the received satellite signal prior to tracking it in a phase-locked loop. The design and evaluation steps were reported in WP2 report RP3 ‘Receiver design document’. A study on the impact of the Multiplexed Binary Offset Carrier (MBOC) modulation is provided in TN1: ‘CBOC effect on GGPhi’. To satisfy the requirements for time referencing in the phase measurements and the impact of timing error in this reference for double- and triple-difference carrier phase observables, a solution was proposed in the form of having one single code tracking channel in the remote GGPhi sensor (as opposed to a total of 20 or more channels in the receiver).
2. The requirements for the GGPhi wireless network in terms of area of radio coverage, frequency bands, antenna types and cell deployment, network topologies and time and frequency synchronisation, under the overall system goals of low cost and power consumption, were studied in WP3 report TN 2: ‘Communications systems high level design’. Candidate radio technologies for the GGPhi data communication requirements given by the positioning software were evaluated for robustness and resilience, energy requirements, time synchronisation characteristics and system costs, and reported in WP3 report TN 3: ‘Communications systems low level design’. Three network configurations – ring topology, mesh topology and cluster topology – were proposed as appropriate to different network deployments. Examples of the three system deployments and suitable wireless network topologies were given providing complete estimated costs for the local elements, the link-to-home satellite connection and appropriate solar power devices.
3. Algorithms and software have been developed for accurate positioning using the GGPhi remote sensors carrier phase-only and reference station measurements and reported in WP4 report RP4: ‘Galileo/GPS carrier phase positioning algorithms’. The most important task was the development of the triple difference processing software using simulated data and based on criteria provided by the receiver development work. An end-to-end evaluation of the integrated system combining the outcomes of the three areas of work has been carried out in WP5 and reported in WP5 report RP5: ‘Evaluation of an integrated system’. To evaluate the expected quality of the GGPhi sensor measurements, satellite availability and the distribution of the GGPhi visible satellites, carrier to noise density ratio (CNR) was calculated for the Galileo and GPS constellations for an evaluation experiment period of two days.

Objectives

The low-cost, low-power Galileo/GPS carrier phase positioning system (GGPhi) project aimed at making high-precision positioning attainable for applications that at the moment are constrained by environmental and cost issues. In particular, in the target application of ground deformation monitoring the required accuracy is very high, while the sensor equipment will likely have no access to the power grid and be active for long periods of time, so will have to have a very low power consumption. Also the ranging sensors will be multiple, remotely located and might not be retrievable, so must be low cost. For this purpose, a measurement system composed of remote wireless-connected Galileo/GPS carrier phase-only measurement units supported by a GNSS aiding and processing unit was designed.