Lab: GPS OPUS

Objectives:

1. To learn how to determine a geodetic location using the OPUS service of NGS.

2. To test the relationship between session length and (a) accuracy, and (b) precision.

3. To test the relationship between the CORS base location and (a) accuracy and (b) precision.

Procedure:

Three sessions:

Set up a dual frequency GPS receiver over a known control point. Occupy this point in static survey mode for three sessions of different lengths: Session 1: 10 minutes, Session 2: 30 minutes, and Session 3: 60 minutes. Download the observation file and export the data in RINEX format using the software provided by the GPS manufacturor. Upload the RINEX file to OPUS.

OPUS processing with three groups of CORS base stations:

1. Nearby CORS Stations:

First, select THREE NEARBY CORS STATIONS, within 0-25 miles if possible. Have OPUS give the positions in each of your three field sessions as an average from these nearby CORS points. You should have three positions from this processing.

2. Medium Distant CORS Stations: Select THREE MEDIUM DISTANT CORS STATIONS, from 25 to 300 miles. Calculate three positions for these CORS stations, one for each session length.

3. Far Distant CORS Stations:

Process the data again through OPUS, this time selecting THREE FAR DISTANT CORS STATIONS, up to 4000 miles away. You should have three positions from this processing.

Plot the results:

Make an x-y plot of your positions. First plot the "known position" in state plane coordinates N and E, using the NGS published values. Then plot your nine positions from the NEAR, MEDIUM, and FAR CORS stations.

Make conclusions about precision and accuracy.

You should have 10 points plotted. (1known, 3 near CORS, 3 medium CORS, 3 far distant CORS).

Session Length vs. Precision: Precision is measured by the "clustering" or "repeatability" of data points. Can you see a difference between the clustering of the 10m. vs the 30 m. vs the 60 m. data? (For this, ignore the "known" position, just look at how "tight" the pattern is.) Please explain your observations and conclusions.

Session Length vs. Accuracy: Plot a mean position of all the 10m points. Do likewise for the 30 m. points and the same for the 60 m. points. Now compare these mean positions with the known position. Is there any relation between session length and accuracy?

Base Location vs. Precision: Inspect the "clustering" of the points from the near base stations, and compare with the clustering from the medium distant base points, and then with those from the far base points. Is there a relationship between the "tightness" and the base station location?

Base Location vs. Accuracy: Plot a mean position of all the near base points, do likewise for all medium points, and similar for all the far points. Compare these locations with the published position. Is there a relation between the base location and accuracy??

Turn in a full write-up, addressing these questions in your analysis:

OPUS website:

Interactive map of CORS stations