Now, what’s tomorrow’s challenge?

APN-011 Rev 1 August 15, 1997

1 of 2

TECHNICAL BULLETIN

GPS OBSERVATION AND POST-PROCESSING TECHNIQUES FOR DUAL

FREQUENCY RECEIVERS

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

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

primary advantage of dual frequency equipment is the ability to observe baselines using much

shorter occupation times. It is difficult to state exactly what this occupation time should be since

every observation session will be different. 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. Therefore, if the sky visibility at either station is poor, one may consider

increasing the occupation times. 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

time. Although NovAtel dual frequency receivers are capable of resolving baselines in less than

a minute under ideal conditions, we suggest the following conservative rule of thumb:

5 minutes for baselines up to 1 kilometer + 1 minute per additional kilometer.

2 of 2

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. The use of dual frequency receivers provides a number of processing options that

are not available with the use of single frequency equipment. Specifically, the data from the L2

or second carrier frequency can be applied in different ways to effectively obtain a baseline

solution. The following table provides some guidelines regarding the most commonly used

solutions:

Solution Type Solution Characteristics Solution Application

L1 Fixed

Uses both L1 and L2 data to fix the integer

ambiquities on the L1 carrier only. This

solution contains the lowest noise level.

Best solution for baselines less than 10 km.

L5 Fixed

(Narrowlane)

Fixes the integer ambiguities on the sum of the

L1 and L2 carrier. Resulting cycle is much

shorter than L1 or L2 alone (hence called

‘narrowlane’).

Sometimes useful for increasing the accuracy

of baselines less than 10 km. Increased noise

may mitigate the improved solution accuracy.

L3 Fixed

(Iono-free)

Combines the L1 and L2 data to eliminate

ionoshperic errors in the solution, but noise

levels are increased.

Useful for baselines over 10 km. At this point,

ionospheric errors become more significant

than the increased noise level.

L3 Float

This combination of L1 and L2 data is not as

accurate as the L3 Fixed, but will still reduce

ionospherice errors.

Next best solution if the above solutions are

not possible. May be only attainable solution

for very long baselines.

L4 Fixed

(Widelane)

Fixes the integer ambiguities on the difference

of the L1 and L2 carrier. Resulting cycle is

much longer than L1 or L2 alone (hence

called ‘widelane’).

Applied over very long baselines when an L3

solution is not attainable. These integer

ambiquities are easier to solve for, but are less

accurate.

L1 Float

Much less accurate than L1 fixed. Generally

indicates that the data collected was

inadequate to obtain a good baseline solution.

These solutions should be avoided. They may

be the only option if occupation times are too

short given the station conditions.

Once again, the above table is only intended to provide insight regarding the different solution

types. For medium baselines lengths it is often useful to try a couple of different solutions and

compare the results. This process will give the operator the experience necessary to determine

when one solution will be superior to the other. The following table is a summary of all the

above information that will give the novice user a running start.

Baseline Lenth Approximate Occupation Time Suggested Data Interval Best Processing Mode

0 – 10 km 5 to 10 minutes 5 seconds L1 or L5 Fixed

10 – 20 km 10 to 30 minutes 10 seconds L1, L3, or L5 Fixed

20 – 50 km 20 to 60 minutes 10 to 30 seconds L3 Fixed or L3 Float

50 – 100 km 45 to 120 minutes 30 seconds L3 Fixed or L3 Float

100+ km 60 to 300 minutes 30 seconds

L3 Fixed, L3 Float, or

L4 Fixed

For any 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.