Parkes main tool is historic aerial photography, coupled with hours of research in the National Australian Archives and the National Library of Australia. To that he adds geophysical surveys of the infrastructure. Parkes is undertaking the geophysical surveying and mapping using an Ashtech ProMark™ 100 GNSS receiver and a Willy Bayot PPM Mk 3 magnetometer.
Parkes runs the two units in parallel later processing both data sets. Parkes says, “it is absolutely critical that the GNSS receiver and magnetometer keep in synchronization during data collecting runs including under the frequently encountered tree canopies.” To improve accuracy, he avoids using RTK as “that would involve have another electronic device operating and emitting more noise in the signal spectrum.”
The dual constellation, GPS and GLONASS, reception of the ProMark 100 is essential to the success of Parkes’ work. After more than a hundred data collection passes with the magnetometer and ProMark 100 through groves of trees, at no time did the PDOP rise to more than three and at all times there were more than eight satellites available. The ProMark 100 data is post-processed to improve accuracy. Parkes notes that ironically many of the most interesting finds have been collected under heavy tree canopy. Without the quality of the geographic positions enabled by the ProMark100 under tree canopy, Parkes reports that much of his work would have been impossible to achieve.
In fact, when Parkes first began the project he used a single-constellation GPS system and post processed the results against the local International GNSS Service (IGS) reference station. The GPS only system worked very well until a grove of trees would interfere with the sky. Now with the ProMark 100 GNSS receiver, Parkes surveys using GPS L1 and GLONASS in continuous kinematic mode at a one second collection rate. He then post processes the data against another ProMark 100 used as a local reference station. To date, Parkes has mapped an underground railway, artillery observation posts, several shelters, fuel terminals and other yet to be identified pieces of the vast infrastructure.
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