SL871 Family Product User Guide
1VV0301170 Rev. 9
Page 20 of 89
2021-07-15
Not Subject to NDA
Differential GPS (DGPS) (SL871 and SL871L only)
DGPS is a Ground-Based Augmentation System (GBAS) for reducing position errors by
applying corrections from a set of accurately-surveyed ground stations located over a
wide area. These reference stations measure the range to each satellite and compare it
to the known-good range. The differences can then be used to compute a set of
corrections which are transmitted to a DGPS receiver either by radio or over the internet.
The DGPS receiver can then send them to serial port 2 (RX1) using the RTCM SC-104
protocol message types 1, 2, 3, and 9.
These corrections can significantly improve the accuracy of the position reported to the
user.
The receiver can accept either the RTCM SC-104 messages or SBAS differential data via
command $PMTK501.
Assisted GPS (AGPS)
Assisted GPS (or Aided GPS) is a method by which information from a source other than
broadcast GPS signals is used to improve (i.e. reduce) TTFF.
The necessary ephemeris data is calculated either by the receiver itself (locally-
generated ephemeris), or a server (server-generated ephemeris) and is then stored in
the module.
See section 2.3 Product Variants for applicability.
Locally generated AGPS - Embedded Assist System (EASY)
This feature is not supported on the SL871-S until ROM MT3337E (enhanced) version of
Oct 2015. It is supported on the SL871L-S.
Proprietary algorithms within the module perform GPS ephemeris prediction locally from
stored broadcast ephemeris data (received from tracked satellites). The algorithms
predict orbital parameters for up to three days. The module must operate in Full Power
mode for at least 5 minutes to collect ephemeris data from visible satellites, or 12 hours
for the full constellation.
EASY is disabled if the fix rate is > 1 Hz
EASY is on by default, but can be disabled by command PMTK869.
Server-generated AGPSS - Extended Prediction Orbit (EPO)
(SL871 and SL871L only)
