Why L1-Static GPS?
For most of the history of high-precision GPS surveying, there have been two choices to make when designing a monitoring solution:
- Whether to use single (L1) or dual frequency (L1/L2) hardware
- Whether to use static or kinematic data processing
Carrier Phase Capability
The first decision involves selecting the GPS observable(s) that best serves a project's size and purpose: the network baseline lengths and the expected deformation dynamics. The dominant concern is network baseline length: long-baseline processing requires the use of dual frequency GPS in order to model the ionospheric noise. Without this, the differential propagation delays make it very difficult to fix the integer ambiguities.
However, on short baselines, where the ionospheric effect is essentially the same at both ends of a baseline, the second frequency is not needed. In fact, the use of L2 actually introduces noise into the solution. Therefore, L1-only is the preferred, low-noise choice for short-baseline applications.
Processing method is another primary consideration in GPS monitoring network design. Although there are variants, processing method comes down to two basic types: static and kinematic:
- Static processing is the processing of a data set collected over time to generate a single baseline vector estimate for that time period. This approach is suited to stations experiencing negligible motion over the processed time span.
- Kinematic processing is the generation of a new baseline vector (or position) estimate for each new epoch of data. This approach is suited to stations in motion.
The decision on processing method typically comes down to the nature of expected deformation. If deformation is dynamic, engineers may need to employ kinematic processing to generate high rate data. Applications targeting slowly developing deformation can usually allow some delay in the delivery of their results. This favors the static method.
So, for short baseline projects applied to monitor slowly developing deformation, the L1-Static GPS method is an ideal solution:
- L1 GPS hardware is far less expensive than L1/L2. This can reduce budgets or allow more field stations to be deployed for the same expense.
- L2 carrier is noisier than L1; by excluding L2 on short baselines we exclude unnecessary noise.
- Static processing is more precise. The longer data spans aid ambiguity resolution and reduce the impact of short term noise. In fact, precision (repeatability) improves with the length of the data span. This is why our InteTrak software allows automated processing of multiple data spans.
- Static processing requires less data (less storage and network bandwidth). Server-based static processing requires carrier phase measurements at a 30 second interval. Kinematic processing will typically require measurements at at least a one-second interval, and possibly up to 100 samples per second.
Exceptions to the Rules
- Since kinematic processing would be required to monitor dynamic deformation, dual frequency hardware may provide a more reliable network. Although the kinematic method can be used with L1-only (and we have in our projects), ambiguity resolution is generally more reliable with L1/L2.
- Receiver-based kinematic processing should eliminate the requirement for raw carrier phase and ephemeris measurement collection, reducing the data packet size. However, receiver-based processing may require bi-directional network communications to transmit corrections and processed results, and raw data will not be available for later reprocessing.
- L1-Static GPS could benefit from the addition of GLONASS (and in time other constellations) satellite observations, especially to increase satellite counts in locations with poor sky visibility.
L1-Static GPS is very well suited to monitor slowly developing deformation on short-baseline applications such as dams, mines, landslides, etc. The lower cost-per-station of L1 hardware allows the customer to monitor more points. The resulting data allow precise indication of three-dimensional displacement of structures and natural hazards.