BSWA 308 - Page 27

BSWA 308/309 BSWA Technology Co., Ltd
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 Satellite Ephemeris: GPS satellites orbit information. According to ephemeris, satellite
positioning signal and time, the current location can be determined. Ephemeris need to
download from the GPS satellites, but the download speed is very low (approx. 50bps),
and vulnerable to the impact of satellite signal strength. The high bit error rate may lead to
a longer time of download ephemeris, and even download fail. The sound level meter can
keep the ephemeris data in memory for approx. 30 minute after turn off GPS module. The
ephemeris data is only is valid within 2 hours.
 Satellite Signal Noise Ratio: Satellite positioning signal intensity. In rainy days or indoor,
signal strength will be affected.
GPS have 3 boot modes: cold start, warm start and hot start:
 Cold Start: First location, need to download the latest ephemeris and spend longer time.
 Warm Start: GPS module has the last saved location information, but need to
re-download the ephemeris due to expired. Warm start needs almost same time as cold
start.
 Hot Start: GPS module has valid ephemeris and can reposition in a very short time.
The GPS of sound level meter is pure passive positioning, so it needs to take some times to
download the almanac from satellite before positioning. Please note it’s not same to the
A-GPS which is commonly most used in the mobile phone and need the right to access to the
internet. Mobile phone can use A-GPS to download the almanac from internet to significantly
speed up the positioning process.
Another important issue is the GPS need an open sky to receive the signal of satellite,
that’s means it should be place outside the building and no cover between the sound level
meter and the sky.
Additional, the bad weather such as rain, snow and cloud may lower the bit rate of
communication and GPS need longer time to positioning.