To enable rapid collection of inspection data over several hundred kilometres of rail network at a time. This operation would be over diverse areas two or three times daily throughout the year. The method involved gathering imagery from a helicopter using a stabilised turret complete with Canon 1DC and 85mm/300mm/840mm lens options. A design aim was that inspection points could be positioned with an accuracy of better than 50cm at a range of 500m in real-time from a moving aircraft.
The data was required to be ready for uploading to a national database after landing. None of this had been done before and the overall specification was really only a well-thought out concept with new equipment. We were to be the system integrators. Finally we had to integrate the system such that complete installation could be achieved on any of three helicopters within a period of thirty minutes or so.
To achieve overall position accuracy on the ground it was necessary to achieve real-time accuracy in the air throughout the United Kingdom. To this end we chose to use Terrastar correction services in conjunction with NovAtel’s OEM638. We considered the use of cellular RTK correction services but discounted them as we could not guarantee cellular coverage throughout the United Kingdom. Terrastar, however, provides complete coverage with an accuracy better than our 15cm allowance. We also incorporated NovAtel’s SPAN INS.
The next challenge was ensuring the survey navigation and particularly the INS would continually give good data. Prior to this the customer expectation was to leave the survey line every six-seven minutes and inject some rapid dynamics into the INS to arrest and correct the IMU heading drift. This heading drift soon mounts up. Our application required a heading accuracy throughout surveys of 0.035 degs (RMS). Using any standard INS equipment this would not be achieved if heading drift were experienced. We used an enhanced GNSS baseline on the three Twin Squirrel helicopters to steady the INS heading. Using OPTOnav and NAVPak we ran constant quality control on the gathered data. A traffic light display in the GUI informs the user of marginal and unacceptable data.
NAVpak also gathers data from the camera and turret encoders such that orientation and focal length of the camera are always known. Images were collected as often as every two seconds triggering a chain of data collection from the camera, turret, INS and GPS. The data is stored in an OPTOnav directory for analysis. Positioning an object of interest involves finding that point in two or more images, naming them and marking them. The position is computed, annotated and stored with the metadata for the image along with quality statistics.
For those who have done this before they will be asking themselves how such accuracy is achieved in real-time when camera, turret, IMU and GNSS antenna alignment are so difficult to achieve in practice. An installation normally takes a day to complete with laser theodolites being used to establish offsets and alignments. Through the use of OPTOnav we require the air engineers to bolt the equipment to the aircraft and provide power. Beyond that OPTOnav takes over and conducts a patch survey (once in the air) using multiple images to align on common points. The result is a series of offsets, and axis alignments which are automatically applied to the survey data. The ensuing accuracy is quite often as good as 35cm at 500m and can always achieve the required 50cm accuracy.
For the system we used:
- Eurocopter AS355 (Twin Squirrel) helicopter
- Gyro Stabilized Systems C516 stabilised turret with combinations of Canon 1DC cameras using 85mm, 300mm and 840mm lens arrangements
- Forsberg Services MICROpod c/w NovAtel SPAN with Honeywell HG1900 IMU and second NovAtel OEM615 GNSS receiver card
- Terrastar C correction service
- Forsberg Services NAVpak for quality control display, data integration, collection, time synchronisation and storage
- Forsberg Services OPTOnav for system calibration, image analysis, real-time targeting and conversion to customer proprietary formats
- Achieved Target Position Accuracy at 500m range through the camera was 0.35m RMS (from a helicopter moving at ~60 knots)
As Featured in NovAtel’s Velocity Magazine
Due to the success of this project we were featured in NovAtel’s Velocity Magazine. You can read more about this project here: