GPSNav
NAVIGATION SYSTEM AND FLIGHT PATH DATA ACQUISITION
For
AIRBORNE SURVEYING

SGL has developed a flexible and simple navigation system, GPSNav, specifically designed for the airborne geophysical environment. The system uses the high accuracy of the GPS system and a navigation computer to allow precise navigation in three dimensions anywhere in the world.

SGL's GPSNav system steers the crew to the start of a specific line in the survey area, directs them along the survey line, and to the next line or any other specified line.  At the end of the flight, the GPSNav system can be used to fly back to the airport or any other entered waypoint. GPSNav can also be used with a preplanned drape surface to guide the aircraft's altitude in the survey area.  It provides steering information to the pilot in the form of a cross track and terrain clearance needle, the Positional Situation Indicator (PSI).

Sample of flight planning generated by our software and fed into aircraft navigation system

The GPSNav system is used to store the raw GPS measurements.  The raw measurements, together with the measurements taken at a ground station, are processed post-mission using the kinematic GPS positioning program GPSoft.  GPSoft uses a robust U-D factorized Kalman filter to process double-differenced observations which provides extremely accurate differentially corrected GPS flight path recovery.

The basic GPSNav system consists of the following items:

1) AIRBORNE GPS RECEIVER AND AIRCRAFT ANTENNA

• NovAtel 3951R 12-channel GPS receiver or Ashtech combined GPS/GLONASS GG24 twice 12-channel receiver
• Special flush mount aircraft antenna
• Provides position information at a maximum rate of 10 Hz
• Power requirement nominal, 10-35 VDC 10 WATTS

2) AIRBORNE NAVIGATION CONTROL AND FLIGHT PATH RECORDING COMPUTER

• Provides steering information to the PSI, a left/right and up/down indicator
• Records the position of the aircraft and the raw GPS satellite range data necessary for post-mission differential processing at up to 10 Hz
• The system can perform Real-Time Differential GPS position corrections for 2-5 m accuracy in flight
• Used to send UTC Time and/or timing pulses to a camera or data acquisition system
• Data are recorded on a Bernoulli or Jaz disk drive, which are dependable in high vibration environments
• The computer, designed for airborne use, uses an Intel CPU, and operates on 28 VDC, 60 WATTS
• RS232, optionally ARINC or RS422 data port
• Great Circle, UTM or Rhumb Line flying
• 97 possible waypoints

3) POST FLIGHT SOFTWARE

• Enables the crew to list or view the recorded GPS flight path positions
• This software can be run on any PC computer, including the airborne computer
• GPSoft - Differential GPS positioning software

4) GPS REFERENCE STATION FOR DIFFERENTIAL GPS POSITION SOLUTIONS
To take the advantage of the maximum accuracy of the GPS System, the following is needed at the ground reference station site.

• NovAtel 3951R
• Standard PC computer
• Ground Station Recording Software to record the data from the receiver

The recorded GPS measurements, together with measurements recorded on the aircraft, are processed after the flight using the GPSoft kinematic differential GPS positioning program.

We have reviewed various GPS receivers available and have concluded that the NovAtel 3951R is most suitable for this job. This 12-channel receiver will report actual positions per second; a feature that is not available with other smaller sequencing receivers.  Real time differential navigation is also possible with the GPSNav system.  The NovAtel 3951R has a sophisticated serial computer link which enables it to receive differential corrections and to record the differentially corrected position, raw range data, and time directly from the receiver.   The Ashtech combined GPS/Glonass GG24 receiver is used when Glonass capability is required.

NAVIGATION AND FLIGHT PATH RECOVERY SYSTEMS

The GPSNav navigation system can be configured to provide pilot guidance in any of the following manners:

1.  Real-time DGPS with corrections from a satellite service such as Landstar or Omnistar

2.  Real-time DGPS with correction from a radio link set up by SGL

3.  GLONASS satellite positional data

4.  Raw GPS data, using barometric or radar altimeter as an aid for height determination

All the normal navigation parameters are displayed on the miniterminal, such as:

In addition to this, the number of satellites being used and various other parameters of the GPS system are displayed.

GPSNav PERFORMANCE SPECIFICATIONS

Differential Position: The accuracy achieved by this method is 1 - 5 m.

Single Receiver Position: The accuracy achieved when SA is OFF is 25 m, when SA is ON it is 50 m - 60 m, most of the time. SA (Selective Availability) has been.turned off since 2000.

These specifications are given assuming that the GPS receivers are tracking a minimum of four satellites, with good satellite geometry.