Determination of lag-time in kinematic GPS recording
|Since the staring of GPS, many researchers have investigated its application in aerial photogrammetry. Today, with the full constellation of 24 GPS satellites operational, enabling excellent satellite geometry any time of the day, the need to apply the full potential of GPS for real time aircraft navigation and photogrammetric mapping can be realized. The use of GPS to determine relative positional data for ground control points in a photogrammetric block adjustment is widely accepted and practiced. The camera exposure station coordinates derived by Airborne Kinematic GPS drastically reduces, the number of horizontal and vertical control points needed in aerial triangulation.
In large-scale mapping, the accuracy level of control data required is very high. The lag in time between the camera exposure and the GPS epoch recording in the GPS receiver is critical in deriving accurate coordinates for the exposure station (principal point) coordinates. Due to delay in the electronic transfer of data from camera clicking to GPS receiver in recording the event makes the Lag in time to occur. To meet the high accuracy requirements for the largescale photography and mapping projects the lag in GPS recording time should be derived and applied. In this study, an attempt is made to compute the Lag-time in airborne kinematic GPS derived exposure stations from aerial triangulation.
Aerial triangulation is carried in Digital Photogrammetry work station with conventional method of using ground control points, and the exposure station coordinates are derived. Lag-time is computed by finding difference in coordinates of exposure stations derived from conventional aerial triangulation and from airborne kinematic GPS. The results of this project will help to improve the locational accuracy of GPS derived exposure stations in aerial triangulation.
Aerial photography is carried out in the study area on 1:6000 scale with forward overlap at 60% and lateral overlap at 20% using RMK TOP30/23 camera. During aerial photography the airborne GPS is operated to record the exposure coordinates.
The computer controlled navigation system (CCNS) is loaded with flight planning data from World Wide Mission Planning (WWMP). During aerial photography CCNS takes coordinates of aircraft position from navigation system and navigate pilot for alignment as per flight plan. Based on navigation coordinates, CCNS sends signals to camera for exposure. The camera exposure system is connected to Trimble 4000 SSI dual frequency GPS system on board, which records GPS data continuously at 1.0sec sampling rate. During the camera exposure, camera system sends signal to the onboard GPS system, which record each exposure as an event marker in the GPS