Homework No. 1

GS 609 Surveying with Satellites

Winter Quarter, 2001

January 12, 2000

Due: February 2, 2000

  1. Describe the concept of GPS point positioning with pseudoranges. How many range observable are needed to find 3D position of a point in space? State your assumptions and explain. What is a pseudorange, and how does the GPS receiver measure it?
  1. What is the accuracy of the GPS point positioning with P-code and C/A-code? Why is C/A code less accurate? Explain.
  1. The number and geometry of the GPS satellites observed from any location in the world vary during the course of the day. Thus, to find the best geometry and the higher number of satellites in order to achieve the highest positioning accuracy some “mission planning” is necessary. Visit the following web page: http://www.trimble.com/satview/, offering the online mission planning software, SatView. Following the online instructions create “sky plot”, “total in view” and “visibility period” plots for January 25, 2001 for a 6-hour time window starting at 11 GMT (Greenwich Mean Time) for a point at (39°59’58” north latitude and 276°57’32” east longitude), which is the approximated location of the NGS reference point across from the Center for Mapping.

What can you learn from these plots? Explain. Repeat the above tasks for the mask angle of 10 deg and 20 deg. Explain the difference. Enclose the plots with your report.

Note: Trimble Geomatics Office is an integrated GPS data processing package developed by Trimble. It’s installed in Room 217, Boltz Hall. This package also offers a Quick Plan function (Start, Programs, Trimble Geomatics Office, Trimble Utilities, QuickPlan), so you may use it instead of the on-line Trimble facility.

  1. Knowing GPS altitude and assuming that you are collecting data when a GPS satellite is at zenith of your location, calculate the time required for the GPS signal to travel from a satellite transmitter to your receiver. What is the required accuracy of the time measurement to obtain a range observable accurate to 1 cm? (Assume that satellite and the Earth are stationary when you collect the data; the clocks on the satellite and on the ground are identical; neglect the signal delay due to atmosphere).