In last few years use of High Precision Positioning systems gained popularity in variety of application e.g. construction, agriculture and GIS. High precision GNSS systems with horizontal accuracy in centimeters helped in precision agriculture for planting in accurate positions as per plan and auto guidance for tractors. AutoGuidance in big farms helps farmers to save fertilsers and herbicides by preventing unnecessary reapplication over same patch of field. Vertical accuracy of GNSS service helps for land leveling applications.
But How they achieve centimeter level Accuracy. Do they use sane satellites ? Do they use same hardware architechture ?
Yes they use same satellites and same hardware but the methods to get the cordinates is different. They are generally called correction methods. In normal GNSS receivers there are various errors
- Satellite Clock Error
- Orbit Shift/ In accuracy in Laser Marking
- Variable Propagation Delay due to ionsperic variations
- Noise sourced from other sources
- Multipath Error
Few of these errors are static over time which can be easily corrected with a reference data of the Final cordinates or the PRN and azimuth angles. The oldest method use this technique its called Differential GPS or DGPS (DGNSS in Generic terms). Lets list out the correction techniques
The most accurate coorection method is RTK which need RAW Data from GNSS modules. Most of the GNSS?GPS module with RAW data costs very high and most of them are not available for individual buyers. Now these ublox Neo Series GPS Chips available abundantly in online shopping sites. These modules cost approx $15 or INR 1000/- . Although none of them seems to be a genuine product. The first one i ordered online was a ublox Neo-m8n which was a Neo-6M when received so i returned the item. Next time i purchased Neo-6M from local market(Chandni chowk) which was a ublox-7 version chip inside !! Again i ordered another one from amazon (REES52 Neo-7) this was a actual Neo-6 (but i was expecting Neo-7 inside )
Now how did i enabled RAW data from both Chips, The chips are not assumed to output RAW data as per the manufacturer and even they declare the discussion of related things on there official forum as illegal discussion. Fetching RAW data from these chips are actually a user level hack. They have some debug codes to check working of there GPS engines which spits out the RAW data and the exports decoded it as the data they actually need.
As the names are very confusing i tried the wrong commands from the ubloxraw page at last got the correct ones and some commands sniffed from the ucenter +serial sniffer. The ublox 7 series chip uses TRK-TRKD5+NAV-CLOCK+NAV-TIME to get RAW Data where as ublox 6 version chip gets the RAW data directly as RXM-RAW