The project requires a accurate GPS to identify the precise location of latitude and longitude with an Android Smart Phone. Most device claims to be inaccurate due to the stage of starts is undefined. Figure 1, shows GPS module have to connected to at least 4 GPS satellite in a cluster to retrieve Ephemeris data. Note that the time obtained from satellite will be saved to battery backup RAM and apply with real-time clock. The time @ precise will be based on orbital and clock correction data which retrieved from Ephemeris. The validity of Ephemeris is perishable up to 4 hours. However, if the device is retrieving new data, which is the orbital parameter, it had to retrieve the Almanac information. If the Almanac is still valid, it can decrease the time to first fix.
Figure 1: (Time To First Fix) TTFF of GPS Module
There are 3 modes of first fix. Cold, Warm, Hot. The modes will require an open sky and no interference or noise.
At cold start, it will be 30 second and above up to few minutes to prepare all the components as no information from last fix. This due to the device usually unable to verify or predict the overhead satellites or had been travelled >100km from previous fix.
At warm start, this stage contains little information from last fix where it contains little precise time and Almanac information and device had travelled <100km range, the signal levels still good from connected satellite clusters. It will take lesser time < 30 seconds up to a minute to retrieve new Ephemeris data (which had been expired > 4 hours ago).
At hot start, the device has all required information from last fix and the data is still valid. Device will be able to predict and continue in positioning challenges within 20 seconds.
To minimise TTFF, ensure the device is located under unobstructed view of sky. Next, by reduce the electrical noise on GNSS receiver by maximise the distance, introduce proper shielding and power with isolated power supply.
Lastly, introduce flash memory to GPS module with backup battery to have last fix information, reduce information retriever from GNSS provider. A new technique to reduce TTFF is by introduce A-GNSS. Assist info provider by networking provider or cloud it can accurately and actively retrieve latest information from GNSS then allow interval connection to GPS module.
Updated By:
Elia Godoong 