One set of camera exposure coordinates are derived by post-processing Airborne GPS data. This data is with lag in the time of GPS recording.

The difference in coordinates of exposure stations derived from photogrammetric method compared with ABGPS derived exposure coordinates for the determination of Lag-time. Run wise mean difference in coordinates of all runs is shown in figure 1.

Delta X represents the difference in X coordinates in metres.

Delta Y represents the difference in Y coordinates in metres.

Delta Z represents the difference in Z coordinates in metres.

It is observed that the lag in time is more in X direction i.e., in the direction of flying compared to Y direction and very less lag is observed in the elevation.

The mean lag observed is 10.28333 metres. The aircraft’s speed during operational flying is 180 knots as per the flying report given by aircrew.

? the lag-time = [(10.28333 / 1000 ) ÷ 333.36] 60 x 60 = 0.11105 seconds
= 111.05 milliseconds

This lag-time correction can be applied during the processing of ABKGPS data, which improves the locational accuracy of the exposure station coordinates.

Flying has been carried in East – West direction. The average lag observed
in X direction i.e., in the direction of flying is 10.2616 metres, in Y direction it is 0.4071 metres and in Z direction it is 0.1645 metres. This is because, the kinematic ABGPS acquired data at one sec sampling rate and the instant of exposure time is recorded back to GPS system. The coordinates for that instant of exposure time are interpolated by GPS system. The time
difference in recording by the GPS system is one of the factor causing lag in time of recording GPS epoch. This is in the order of milliseconds.

It is also observed that the lag is not a constant amount all through out the
flying. This is because of the change in the flying speed, wind direction and wind speed. This clearly indicates that a mechanism to continuously record the aircraft speed, wind speed and direction is required.

The lag in lateral direction i.e., delta Y is changing abruptly at some instances of exposures. This phenomenon is observed in Run No.1; the lag is consistent, where as in Run No.2 fluctuations are more. It is noticed in Run No.2 the difference in Y is maximum. This is caused drift and it can be concluded that the cause is more lateral winds during flying.

Similar fluctuations are noticed in other runs also. Wherever drift occurred the difference in Y is more fluctuating. The difference in Z, which is along vertical axis, is very less compared to difference in X and difference Y. But in some instances the difference in the vertical direction also noticeable. This is because of fluctuation in flying height of aircraft.

It is advisable to record the speed of the aircraft continuously throughout the mission along with the wind out side the aircraft along with the direction. The lag in the GPS recording should be applied prior to the GPS post processing of the Air borne kinematic GPS for deriving photo exposure coordinates.

In aerial triangulation, the number of ground control points can be reduced
drastically, by using these accurate exposure station coordinates.

With the great strides being made in technology, the combination of airborne GPS with other technology such as videogrammetry, digital cameras, CCD, Inertial Navigation and LIDAR is bound to find very useful applications in measurement science. If these technologies become less expensive and more accurate then perhaps high accuracy mapping can even be performed without analytical aerotriangulation.

The authors are extremely thankful to Shri. K. Kalyanaraman, General Manager, Aerial Services and Digital Mapping, NRSA for his constant encouragement and support in carrying out this study. The authors are thankful to Dr. G. Madhavan Nair, Charman, ISRO/Secretary, DOS and Dr. K. Radhakrishnan, Director, NRSA. Thanks are especially due to Mr. V. Raghu Venkataraman, Head, AS&DPD, NRSA and Mr. Murali Mohan, Head, ATD & AD, NRSA for allowing us to carry out this study.

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In A Commercial Environment, A Ph D thesis submitted to The Graduate School, The University of Maine.