Head-To-Head Comparison of Four SiRF-Based GPS Receivers Disclaimer
This info may be of interest to those using, or contemplating use of, SiRF-based GPS receivers, such as recent Garmin models, Trimble Nomad series, Juno series, and XC/XB receiver cards, and Magellan MobileMapper6. Forward as appropriate.
The SiRF Star 3 GPS chipset started appearing in commercially available GPS receivers around 2005. Its primary advantages are:
- Fast fix times;
- High sensitivity improves satellite lock in difficult GPS environments, like heavy tree canopy or steep terrain; and
- Low power drain prolongs battery life.
The trade-off necessary to gain that extra sensitivity, however, is that SiRF receivers will accept very weak, low-quality signals. For instance, Trimble’s SiRF implementation (includes the Nomad series, Juno series, and Pathfinder XB and XC receivers) is hard-wired to a maximum PDOP of 99, minimum SNR of 12, and an elevation mask of 5°. As such, SiRF receivers are likely to surrender some position quality in exchange for high productivity in difficult GPS environments.
Post-processing SiRF-derived data can occasionally produce poorer positions than the original autonomous positions. Here’s why.
SiRF receivers are hyper-sensitive, making them more susceptible to multi-path error, especially in heavy tree canopy or steep terrain. Multi-path errors cannot be corrected by post-processing.
Furthermore, SiRF receivers have a 5° elevation mask, while most base stations have a 10° elevation mask. This creates a situation where SiRF receivers may include satellites in their working constellation that base stations are ignoring. The post-processing engine can only use satellites common to both the base and rover, which has the effect of recalculating each rover position based on only the mutually-seen satellites. The result is a series of re-calculated rover positions having poorer DOP (lower resolution) than the original autonomous positions, which are then post-processed.
The bottom line is that post-processing SiRF-based data cannot always be relied upon to provide an improved result.
The results of this non-rigorous assessment is in relatively open sky and results may be different under heavier canopy.
This information and summary report is courtesy of Carl Beyerhelm