Heave compensation using time-differenced carrier observations from low cost GPS receivers

Blake, Stephen James (2008) Heave compensation using time-differenced carrier observations from low cost GPS receivers. PhD thesis, University of Nottingham.

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Vertical reference for hydrographic survey can be provided in two ways: through the use of an expensive and very accurate GPS-aided INS system, or through the classical method of compensating for heave motion measured on board the vessel and tide data taken from a nearby tide gauge. Whilst the GPS-aided INS approach offers significant advantages in terms of accuracy their high cost has prohibited their widespread use within the hydrographic survey industry and the classical method is still prevalent.

Heave motion of a survey vessel has traditionally been measured using inertial technologies, which can be expensive and have problems with usability and instability, resulting in higher survey costs and a significant hydrographer input burden. Heave can also be measured through the use GPS receivers by the differencing of measured carrier phase pseudo-range from adjacent epochs and the recent introduction by U-Blox of the Antaris AEK-4T, an off the shelf low cost GPS receiver capable of measuring and recording the carrier phase pseudo-range observable, has allowed the exploration of a novel method of measuring and compensating for vessel heave using off the shelf low cost GPS receivers.

The work presented in this thesis details a method of compensating for vessel heave motion in bathymetry data that has been developed specifically for use with the U-Blox Antaris receiver. The technique is based on the production of highly accurate velocity estimates using the carrier phase observable. Carrier phase measurements are differenced across adjacent epochs to give relative delta range estimates between receiver and satellite along the direct line of sight, which are then processed to calculate an accurate estimate of receiver delta position across the epoch, a measurement analogous to receiver velocity. This technique has been termed Temporal Double Differencing (TDD).

Integrated vertical velocity estimates produce the relative vertical displacement of the vessel over time. Because of bias errors in the velocity estimates from TDD, this vertical displacement is subject to drift. The drift is removed by passing the data through a high-pass filter designed to stop the drift frequencies yet pass the required frequencies of vertical vessel motion.

An obvious advantage of this technique over conventional technologies is cost. Instruments currently on the market are centred on inertial sensors and generally have prices ranging from £12,000 to £25,000. Low cost GPS receivers are priced at around £200 and so this technique can have sizeable cost implications for the hydrographic survey industry. In addition the nature of the TDD algorithm results in a heave sensing technology that is not subject to turn induced heave which can affect inertial based sensors, and also imposes no requirement on the user to account for parameters such as vessel heave characteristics and current heave state. A further advantage over interferometric GPS heave compensation techniques is that the TDD algorithm is stand-alone and requires no reference receiver.

Two trials have been undertaken to test the ability of the low cost U-Blox receiver to record accurate phase pseudo-range observables and subsequently produce a heave estimate: a Spirent GPS hardware simulator trial, and a sea trial. The simulator trial has been the first to quantify the errors associated with the measurement of carrier phase pseudo-range observables using low cost commercially available receivers. The trial used three separate receivers: a Novatel OEM4, a Leica 530 and a low cost U-Blox Antaris. Three scenarios were programmed into the simulator to rigorously test the effects of receiver quality and receiver dynamics on the resulting velocity estimates using the TDD algorithm. The sea trial involved fitting various sensors to the vessel including a Honeywell HG1700 IMU, an Applanix POS-RS GPS-aided INS system and the same three GPS receivers as used in the simulator trial. The POS-RS system and the inertial based heave sensor were used to provide a reference against which the novel low cost heave output could be compared. The comprehensive nature of the sea trial makes it the first work to compare the results from the TDD heave algorithm using varying grades of receiver, and against truth data from both an inertial based heave system and a GPS-aided INS.

The results of the simulator trial have shown that under static conditions the TDD velocity estimation using the U-Blox Antaris is of comparable quality to that produced using both the Novatel OEM4 and the Leica 530. Under dynamic conditions the performance of the U-Blox Antaris is greatly degraded when undergoing large accelerations, an artefact of the inferior componentry used in the signal tracking loops. The sea trial has demonstrated the ability of the TDD heave algorithm developed for use with commercially available low cost GPS receivers to measure vessel heave to a similar standard as inertial based technologies at a fraction of the cost and with greatly reduced instability and usability issues that are traditionally associated with inertial based heave sensors.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Hide, C.
Hill, C.J.
Moore, T.
Park, D.W.G.
Keywords: Hydrographic surveying, Bathymetry data, U-Blox Antaris receiver, Velocity estimates, Temporal Double Differencing
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Faculties/Schools: UK Campuses > Faculty of Engineering > Department of Civil Engineering
Item ID: 28703
Depositing User: Blore, Mrs Kathryn
Date Deposited: 21 Apr 2015 09:44
Last Modified: 17 Oct 2017 01:09
URI: https://eprints.nottingham.ac.uk/id/eprint/28703

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