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Using OPUS

OPUS Guidelines | Height Measurements | Antenna Types | Output Description | Discussion

The following items describe in greater detail the guidelines for uploading data to OPUS.

1. EMAIL
You may enter either the email address for the machine that you are using to submit your data file or another email address where you would like the results to be sent.

2. DATA FILE
OPUS accepts data in either RINEX format or selected receiver formats. Your RINEX file may be UNIX compressed, gzipped or pkzipped. The file may also be Hatanaka compressed (yyd suffix). You may also submit multiple files in a zip archive, but your selected options will be applied to all of the files in the archive. NGS software will only process dual-frequency, carrier-phase data (L1 & L2). Single frequency data (L1 only) will not be processed. You may only submit data from a dual-frequency receiver. The data must have been collected for a minimum of 2 hours and from a stationary antenna. Your collection rate must be 1,2,3,5,10,15 or 30 seconds, and OPUS processes every 30 seconds of data.

The receiver formats that OPUS will accept are those that can be processed by Teqc.

3. ANTENNA TYPE
The vertical distance from the phase center of the GPS antenna to the Antenna Reference Point (ARP) is needed in order to connect the GPS measurements to the monument or point whose position you are trying to determine. NGS has measured these offsets as well as how these offsets change with direction to the GPS satellites (see GPS Height Measurements). Most GPS antennas suitable for geodetic work have been calibrated by NGS and new antennas are calibrated when they become available (see GPS Antenna Calibration). The NGS software applies these calibration corrections for the particular antenna type that you are using in order to ensure the greatest possible height accuracy.

The "Select the antenna type" box will display all the antennas that have been calibrated by NGS and whose phase center offsets and phase variations are known. The antenna type names are very similar and should be clearly recognizable from the GPS antenna model number. The model or part number included in the antenna type may be found in almost every case stamped on the antenna. In every case, the model number has been preceded by a 3-character abbreviation for the manufacturer. In many cases the model numbers in this list have an additional designation appended to their name. These designations are: "GP" or "gp" for groundplane, "rd" for radome; and "CR" or "cr" for choke ring. The + or - symbols indicate if these features have been added (+) or removed (-) from the normal configuration of that antenna model. To avoid confusion, a "-" symbol that may have literally appeared in the model number stamped on the antenna may have been changed to a "." symbol in this list.

For pictures, diagrams and more information on GPS antennas and calibrations see the NGS web site for GPS Antenna Calibration.

3 Char ID	Maufacturer
AER	Aeroantenna
AOA	Allan Osborne Associates
ASH	Ashtech
JPL	Jet Propulsion Lab
JPS	Javad
LEI	Leica
MAC	Macrometer
MAG	Magellan
MPL	Micro Pulse
NAV	NavCom
NOV	NovAtel
SEN	Sensor Systems
SOK	Sokkia
SPP	Spectra Precision
TOP or TPS	Topcon
TRM	Trimble
NGS	National Geodetic Survey

OPUS does not read the header of the RINEX for antenna or height information. With the exception of parentheses, periods, and hyphens, which may be omitted or replaced in order to have a more uniform naming convention, the model number stamped on your GPS antenna should very closely match a model number in this file. If you have trouble finding a match, look at GPS Antenna Calibration, which has photographs and engineering drawings of most of the calibrated antennas.

If you still do not find a match, you should select "NONE" (default). Selecting "NONE" causes OPUS to ignore the phase center variations (since the antenna is unknown). In this case your results will contain a warning that no antenna type was specified and that the computed position refers to the phase center of the antenna (if 0.0 is entered for the antenna height) rather than to the Antenna Reference Point. Missing or incorrect antenna calibrations primarily affect height measurments and have been seen to cause height errors as large as 10 cm.

4. ANTENNA HEIGHT
To complete the connection of the GPS height measurement to your monument or point, the Height of the Antenna Reference Point (ARP) in meters above your mark must be entered. OPUS does not read the information in the header of the RINEX file. The position that a user wants is most often that of a physical mark usually located directly beneath the antenna. The antenna phase center is not a physical point, but its location with respect to a physical feature on the antenna can be determined through a set of calibration measurements. The vector from this antenna reference point (ARP), which is usually the center of the bottom of the antenna, to the phase center is called the antenna offset. There are separate antenna offsets for the L1 and L2 carrier phase data.

In order to greatly simplify the correct use of antenna offsets, NGS has calibrated, and continues to calibrate, nearly all the GPS antennas likely to be used for geodetic measurements (see GPS Antenna Calibrations ). This eliminates the need for the user to ever know what these offsets are. All the antenna calibrations are available to the NGS software and are sorted by antenna type. The user only needs to identify the appropriate antenna type that was used to collect the data that is being submitted. Only the processing software ever needs to know the actual values of these offsets and calibrations.

Since the antenna offsets are obtained by selecting the correct antenna type, the height measurements are completed by entering the distance from the ARP to a mark or monument on the ground. The relationship between the antenna offsets and the antenna height is illustrated and discussed below.

5. SELECT OPTIONS
OPUS offers a number of options allowing the user some flexibility in the way solutions are performed and results reported back. These are described in links on the options page.

Antenna Offsets

A sample OPUS Output Page is shown below. An explanation of each of the items that appears in the output page follows the sample page.

Discussion

How are the solutions obtained?

OPUS-derived ITRF positional coordinates are the average of 3 distinct single-baseline solutions computed by double-differenced, carrier-phase measurements from 3 different National CORS sites using program PAGES. The reference ITRF coordinates for the CORS have been obtained from the NGS Integrated Data Base (IDB)and have been updated to the midpoint of the time interval when the submitted data were observed. Hence, OPUS-derived ITRF coordinates correspond to the position of the point at this instant in time. Points in the coterminous United States move between 9 and 22 mm/yr horizontally, relative to ITRF.

OPUS-derived NAD 83 positional coordinates are also the average of 3 distinct single-baseline solutions. The procedure followed to compute final NAD 83 coordinates at epoch 2002.0 is as follows:

First, the 3 derived ITRF intersite vector components, given at the midpoint of the data time interval, are individually transformed to the NAD 83 reference frame. Secondly, the NAD 83 coordinates of the three reference CORS stations, retrieved from the NGS IDB, are also updated to the midpoint of the interval, applying the NAD 83 velocities available from the data sheet. Vector components and CORS NAD 83 coordinates are added to determine 3 different values of the coordinates of the unknown point on the NAD 83 frame at the midpoint epoch. These 3 quantities are averaged to determine a unique value for the coordinates of the point at this epoch. Finally, these coordinates are then transformed in time to the epoch date of January 1, 2002 by using the NAD 83 velocity for the point as predicted by the HTDP (Horizontal Time-Dependent Positioning) software.

Because NAD 83 positional coordinates in the coterminous United States are referenced to the North American tectonic plate, NAD 83 velocities are typically very small. NAD 83 velocities in excess of 5 mm/yr, however, are prevalent in States along the Pacific Coast. Note that the OPUS-derived NAD 83 positional coordinates are not obtained by a direct transformation of their corresponding ITRF coordinates

While 3 single-baseline solutions are computed, the solutions can not be considered as completely independent. Local biases at the user 's submitted station will not be averaged away by the combination. For example, local multipath error, or error in the height of the Antenna Reference Point (ARP) will not be evident in looking at the solution variation. On the other hand, use of 3 single-baselines does provide a gauge of error contributions from the various National CORS stations.

What is the accuracy of the result?

Accuracy estimates for GPS reductions obtained by formal error propagation are notoriously optimistic. For this reason, OPUS does not rely only on the formal errors. Instead, the peak-to-peak error, or error range is provided for each coordinate component (XYZ and NEU). The peak-to-peak error is the difference between the maximum and the minimum value of a coordinate obtained from the 3 baseline solutions. A completely random population with a standard deviation of 1.0 cm, when sampled 3 times, will have a peak-to-peak error of 3.3 cm or less, 95% of the time. In other words, if you see a peak-to-peak variation in the ellipsoidal height of 3.3 cm or higher, there is a 5% chance that such a variation came from data that had a 1.0 cm (one sigma) precision. It is, of course, more likely that 3.3 cm or higher variation indicates a precision larger than 1.0 cm.

A key element, which bears repeating, is that accuracy estimates depend upon freedom from systematic error. For example, if there is an error in identification of the antenna type, the wrong antenna phase center variation model and wrong phase center-ARP offsets will be applied to the data. This could lead to errors of 10 cm or more that will not be displayed in the peak-to-peak error value.

The advantage of providing a peak-to-peak error measure obtained from 3 baselines solved from different National CORS is that the error range also reflects the errors in the reference coordinates of the CORS stations. The accuracies that can be obtained with modern GPS receivers and geodetic models are such that as your observational time spans get longer your results will improve so that the small errors in the reference coordinates can become a relatively more significant component of the total error. In fact, on the average, one should obtain larger peak-to-peak errors in the NAD 83 coordinates, when compared to the ITRF coordinates, from the same observational data. This is due to the procedures used to derive the CORS coordinates. To serve our users, the NAD 83 coordinates of the National CORS are updated less frequently than ITRF coordinates. However, this also results in the NAD 83 coordinates being somewhat less accurate.

Because of the automatic character of OPUS solutions, and the critical nature of elements such as antenna identification and ARP height measurement, NGS provides a disclaimer to all OPUS results, “This position was computed without any knowledge by the National Geodetic Survey regarding equipment characteristics or field operating procedures.”

How to get a more accurate result

The single best way to get a more accurate result is to submit a longer time span of data. While we currently accept a minimum of 2 hours of data, we recommend at least 4 hours of data. As an example, our height modernization surveys, which routinely achieve 1 cm, one sigma, ellipsoidal height accuracy, require three or more sessions, each at least 5.5 hours long, on two or more days, where two of the observation time spans are offset to sample different satellite geometries. While good results can be obtained with 2 hour solutions, we have found that longer time spans are consistently more reliable.

In addition, using a longer time span of data allows greater averaging of multipath error. Alternatively, if one is able to use a multipath-suppressing antenna and/or receiver, (e.g. choke-ring antennas, --- correlation receivers), then more accurate results should be obtained from a given amount of data. However, even if a such improved antennas/receivers are used locally, longer data sets are still useful, since such antennas and receivers are not always used at the National CORS sites, themselves.

No improvement will be obtained by submitting GPS data taken every 5 seconds or, even, every 1 second. Since the time correlation of multipath error is typically on the order of 10 to 20 minutes and given the variety of data rates within the CORS network, the data rates used for OPUS solutions are fixed at 30 seconds.

If you submitted your solution very soon after taking your data, then it is possible that predicted orbits, rather than precise, post-fit orbits were used to obtain your solution. The priority for orbit selection is:

• IGS precise orbits (typical delay 10-14 days) highest accuracy
• IGS rapid orbits (one day delay)
• IGS ultra-rapid (predicted) orbits (near real-time)

The orbit source will be indicated on your solution. There is essentially no difference in accuracy between OPUS solutions using the rapid or precise orbits due to the moderate lengths of the base lines. However, if the predicted orbit was used, you may wish to resubmit your solution at a later time when a more accurate orbit is available. In addition, not all CORS data are available at NGS within one hour, and sometimes, not available within one day. This means that more distant CORS stations may have been used for the OPUS solution. While baseline length is much less critical than measurement time span, this can sometimes also be a reason to resubmit your data .

It is possible that more accurate results can be obtained for a given set of data through manual processing through suitable software. Such processing can include manual cycle slip editing, deletion of outliers, incorporation of local meteorological measurements, and experimentation with allocation of tropospheric parameters, variable cut-off angle, and different constraints of the carrier phase ambiguities to integer values. However, manual processing alone is not a guarantee of accurate results. Accurate results, particularly for long lines, depend on the fidelity of the geodetic models incorporated in the GPS reduction software. OPUS has an extensive set of geodetic models in the PAGES software engine and the reliability of the automated processing (data editing, integer fixing, etc.) has been repeatedly demonstrated for NGS's own processing.

At the highest level of accuracies, one is limited by the accuracy of the reference coordinates of the National CORS. The NAD 83 datum is generally less accurate than that of the ITRF. One could take the ITRF coordinate, and apply the Helmert transformation to generate an “NAD 83” coordinate (this can be done with HTDP). If one does this, one will actually obtain a coordinate that has good consistency with the National CORS, but will have less consistency with the NAD 83 coordinates in the NGS Integrated Data Base. NGS is currently engaged in a new GPS survey across the country to obtain 2 cm (2 sigma) ellipsoid heights for 2002. When this effort is completed, the nation will have a uniformly high level of accuracy in both the NAD 83 and the ITRF reference systems. More information is available at our web page on the New Reference System.

As a final note, please, triple check the antenna type and the height of the ARP submitted to each OPUS run. A chain is only as strong as its weakest link. Antenna type and the height of the ARP are critical links in the GPS data reduction chain.

What to look for in a quality solution

There are no absolute rules, but we can certainly provide some guidance on OPUS solutions.

• Make sure the antenna type and the ARP height are correct.
• Review the solution statistics:
  • A good OPUS run should typically use 90% or more of your observations.
  • OPUS should have fixed at least 50% of the ambiguities
  • The overall RMS should seldom exceed 3 cm.
  • The peak to peak errors should seldom exceed 5 cm.
  • (This depends, of course, on the accuracy you are trying to achieve.)