Now, what’s tomorrow’s challenge?

APN-012 Rev 1 November 15, 1997

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TECHNICAL BULLETIN

GPS OBSERVATION AND POST-PROCESSING TECHNIQUES FOR SINGLE

FREQUENCY RECEIVERS

This bulletin is intended to provide some guidelines and insight regarding appropriate

observation time and post-processing techniques when using single frequency GPS receivers.

There are two basic techniques that can be applied to single frequency processing:

1. Code only – the C/A code based pseudoranges are used for differential processing,

2. Full carrier phase – the carrier signal is used for processing.

Code only processing is a relatively simple and fast technique that is typically used for lower

accuracy applications such as mapping and GIS data collection. Full carrier phase data

processing is a much more complicated procedure that will improve differential GPS accuracy to

the level required for precise surveys. Both of these differential solution types will be

considered.

The accuracy of single frequency differential processing is much more dependent on occupation

times than dual frequency processing. It is important to keep the following factors in mind when

trying to determine how long a station should be occupied (occupation time refers to the

simultaneous observation time at both base and rover):

• The distance between rover and base station: As the distance between the base and rover

receivers increases, the occupation times should also increase.

• Sky visibility at each of the base and rover receiver: The accuracy and reliability of

differential GPS is proportional to the number of common satellites that are visible at the base

and rover. Effective differential processing usually requires at least 6 satellites visible above

10 degrees elevation. This condition is best measured by monitoring the number of visible

satellites during data collection along with the PDOP value (a value less than 3 is ideal).

• Time of day: The location and number of satellites in the sky is constantly changing. As a

result, some periods in the day are slightly better for GPS data collection then others. The

Planner utility that is included with the SoftSurv package is useful for monitoring the satellite

constellation at a particular place and time.

• Station environment: It is always good practice to observe the site conditions surrounding

the station to be occupied. Water bodies, buildings, trees and nearby vehicles can generate

noise in the GPS data. Any of these conditions may warrant an increased occupation time.

Although we usually wish to opt for the shortest occupation time possible, it is wise to rely on a

conservative time for all GPS operations. It will end up costing a great deal more in terms of

time and resources if a session or survey has to be repeated because of an insufficient occupation

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time. Although NovAtel single frequency receivers are capable of resolving baselines in only a

few minutes under ideal conditions, we suggest the following conservative rule of thumb:

20 minutes for baselines up to 1 kilometer + 2 minutes per additional kilometer.

Eventually the user will be able to determine, based on previous experience, when and where this

occupation time may be reduced and under what conditions it must be increased.

Once the data has been collected, post-processing must take place to obtain final station

coordinates. Depending on the type of processor used some or all of the solution options in the

following table will be available:

Solution Processor Solution Characteristics Solution Application

L1 Fixed SoftSurv

Fixes the integer ambiguities on the L1

signal. Accuracy will typically be at the

centimeter level.

Best solution for all applications.

L1 Float SoftSurv

Less accurate than the L1 fixed solution

(decimeter level), but may be the only

option over long baselines.

If an L1 fixed solution is not possible,

an L1 float will usually be the result.

Raw

Pseudorange

GIS Mobile

SoftSurv

This is a code-only solution. The result is

much less accurate than either of the two

options above – typically meter level.

Applies when using a code-only

processor or when carrier phase data is

extremely noisy (may happen in dense

trees).

Code-only processors use the pseudorange processing technique. A carrier smoothing function

will improve the result, but accuracy is typically limited to around 0.75 to 1 meter. An L1 fixed

solution involves full carrier phase processing and can provide geodetic level accuracy rivaling

that of dual frequency equipment. Single frequency receivers, however, are much more sensitive

to long baselines and short occupation times. The following table is a summary of single

frequency observation and processing techniques that can be used as a guideline until the user is

experienced with the equipment.

Baseline Lenth Suggested Occupation Suggested Data Rate Best Processing Mode Accuracy*

0 – 10 km 20 to 60 minutes 5 seconds L1 fixed 1 to 5 cm

10 – 20 km 40 to 90 minutes 10 seconds L1 fixed 2 to 10 cm

20 – 50 km 60 to 180 minutes 10 to 30 seconds L1 fixed (if possible) 5 to 20 cm

50 – 100 km 120 to 300 minutes 30 seconds L1 float (fixed not likely) 10 to 50 cm

100+ km 180+ minutes 30 seconds L1 float (fixed not likely) sub-meter

* Accuracies are a typical range given PDOP < 4.0 and mimimum 5 satellites tracked and may vary.

For further enquiries regarding this information or any other concerns, please contact NovAtel

Customer Service toll free at 1-800-NOVATEL (Canada and the US) or at (403) 295-4900.