Forsberg says the system can use oblique and vertical imagery in the form of still photos as well as video or other sensor imagery, a method that depends on the cameras’ and sensors’ focal points being accurately real-time oriented and positioned in 3D.
“OPTOnav operation relies on the principle of the software being told the precise location of the camera’s focal point and the target centroid pixel orientation with respect to the survey datum,” Forsberg says. “This requires a camera and GNSS receiver as a minimum and can go all the way up to a full inertial navigation system and stabilized camera turret.”
An aircraft or vehicle system can be custom-mounted in a stabilized turret such as the Gyro Stabilized Systems C516. The turret is instrumented with precision angular encoders and an externally mounted derivation of a NovAtel Synchronous Position, Attitude and Navigation (SPAN®) HG1900 Inertial Navigation System (INS).
Running through a combined Ethernet and RS422 link, imagery and navigation data is transmitted back to a Forsberg Services MICROpod-HDG. This houses two NovAtel GPS+GLONASS dual–frequency Real Time Kinematic (RTK) cards. The primary card is used to produce the INS data while the secondary card derives GNSS heading from a secondary antenna. Data is then taken from the NovAtel (SPAN®), GSS C516 turret, and Canon DSLR camera by MICROpod generated time-synchronisation pulses triggering data gathering from each component.
The HG1900 Inertial Measurement Unit (IMU) offers a hybrid package of Honeywell’s Micro Electromechanical Systems (MEMs) Gyros and RBA accelerometers. Economical, robust and small, the low power HG1900 provides high end tactical grade performance for commercial and military guidance and navigation applications. As a part of NovAtel’s SPAN GNSS+INS system, this IMU is ideal for airborne and ground applications that require accurate 3D position, velocity and attitude data.
A proprietary NovAtel MEMS Interface Card (MIC) couples the HG1900 IMU with SPAN receivers, offering a unique, powerful system for weight and size constrained applications. Designed as a board stack configuration for ease of integration, the MIC interfaces directly with NovAtel’s small form factor OEM615™ SPAN receiver.
The HG1900 is also available as a stand-alone product so integrators can easily pair it with an existing OEM6® SPAN receiver.
“Achieving best accuracy at the targets when moving at sixty knots needs millisecond level time-synchronization,” says Forsberg. “GNSS and INS synchronization are well known, and to a certain level so is camera synchronization. However, when you stray from the standard survey camera range, time synchronization has to be learned. For a surveyor on the ground the operation becomes simpler in that it’s enough to know the precise location of the camera and some ground reference points to orientate each image in space.”
For cameras, Forsberg has tended to go with digital SLRs. “They pack extensive capability into a compact size and can be enhanced with various prime lenses,” Forsberg says. “By example, we have used Canon 1DC, 1DX, 5D, 6D and 400D with 24mm, 50mm, 85mm, 300mm and 840mm prime lenses. Other cameras include the Nikon DSLR range and GoPro. All have their place and generally the more expensive the camera, the better the results.”
Overall accuracy achieved in real-time aerial survey is about 35 centimetres (two sigma) at ranges of 600 metres to the object. Using ground reference points, accuracy can be improved to a handful of centimetres. In particular, object mensuration is accurate to the centimetre level. For example, the gauge, gradient and camber of a railway can be measured from oblique images to within a couple of centimetres in length, a few hundredths of a degree in gradient and a few tenths of a degree in camber. The distance between measurement points dictates the angular accuracy. Also, precision is more dependent upon lighting conditions than pixel resolution.