Trimble Outdoors SK8 - Manual (Page 55)

Lassen-SK8 Embedded GPS Module 4-3
Operation and Performance
The warm start time to first fix, when the receiver has been powered down for more than
60 minutes (i.e. the ephemeris data is old) is usually less than 45 seconds.
4.2.3 Garage Search Strategy
During a warm start search, the Lassen-SK8 knows which satellites to search for, based on
the system almanac, the initial position (last known position) and the current time. In some
cases, the receiver may not be able to acquire the expected satellite signals (e.g. a vehicle
parked in a garage or a vessel in a covered berth). Trimble's patented “garage search”
strategy, also known as a split search, is designed for such situations.
If the receiver does not acquire the expected set of satellites within 5 minutes of power-on,
some of the eight tracking channels will continue to search for the expected satellites
(warm search) while the remaining channels are directed in a cold start search. This
strategy minimizes the time to first fix in cases where the stored almanac, position and
time are invalid. The stored information is flushed from memory, if the cold start search
proves effective and the warm search fails.
4.2.4 Hot Start
A hot start strategy applies when the Lassen-SK8 has been powered down for less than 60
minutes, and the almanac, position, ephemeris, and time are valid. The hot start search
strategy is similar to a warm start, but since the ephemeris data in memory is considered
current and valid, the acquisition time is typically less than 20 seconds.
4.3 Satellite Mask Settings
Once the Lassen-SK8 has acquired and locked onto a set of satellites, which pass the mask
criteria listed in this section, and has obtained a valid ephemeris for each satellite, it will
output regular position, velocity and time reports according to the protocol selected.
The default satellite masks observed by the Lassen-SK8 are listed in Table 4-1. These
masks serve as the screening criteria for satellites used in fix computations and ensure that
position solutions meet a minimum level of accuracy. The Lassen-SK8 will only output
position, course, speed and time when a satellite set can be acquired which meets all of the
mask criteria. The satellite masks can be adjusted in GPS receivers accepting the TSIP
protocol. (See the section titled Key Setup Parameters, located in Appendix A.)
Table 4-1. Default Satellite Mask Settings
Mask Setting
Elevation 5°
SNR 2
PDOP 10
PDOP Switch 5