FAQ – DGPS SURVEY OPTIMISATION WITH SXBLUE

How do I improve GPS accuracy?

A Differential Global Positioning System (DGPS) is a broad term to describe ways for enhancement to the Global Positioning System (GPS) which provides improved location accuracy from the 15-meter nominal GPS accuracy to about 1-3 cm in case of the best implementations. All SXblue’s receivers are DGPS survey systems using SBAS corrections.

Are SXblue receivers compatible across different operating systems and platforms?

Yes! All our receivers are compatible with Windows (XP, CE, Mobile, 7, 8, 10) SXblue series are compatible with Android since 2012 and/or Android 5.0. The iSXblue series and Platinum series are compatible with IOS 7.1 or up, Windows and Android.

How do SXblue receivers compare to competitor’s products?

SXblue receiver is a unique solution in that it is very compact, yet delivers very high-performance quality. There are a number of DGPS products on the marketplace. However, very few of them are suitable for commercial use in applications such as GIS & Mapping due to their positioning performance and their update rates. SXblue is up for this task and meets the needs for these applications. SXblue also offers the facility of outputting raw measurement data for post-processing in the office.

How to get the best satellite tracking with SXblue receivers?

For best performance, you have to be careful that the sky above the SXblue’s antenna is unobstructed for satellite tracking. SXblue tolerates a certain amount of signal blockage due to the availability of redundant satellites (only four are required for a position). However, the more satellites are blocked, the more impact this could have on your positioning accuracy.

How accurate are SXblue receivers?

The positions computed by your receiver are accurate (guaranteed) to within 1 meter 95% of the time (assuming a good view of the sky and minimal multipath). This accuracy is, conservatively, halve for z-values. With RTK corrections, it can go down to centimeter precision. Contact us for more details.

Can I use a vehicle’s power charger to operate the SXblue system?

Yes, it is designed for that purpose. Also, a cigarette lighter plug is available from your dealer or Geneq. This adapter recharges the integrated Li-Ion battery pack. It is important to use only the charger supplied by us.

How do I know if I can receive an SBAS signal in my area?

Refer to coverage maps for both WAAS (for Central and North America) and EGNOS (for Europe). It’s important to have both signal coverage and ionospheric map coverage. It’s desirable to have a few degrees of latitude and longitude of ionospheric map coverage around your location to ensure that the satellites available have these correctors.

Reading my receiver’s NMEA GPS data messages

SXblue supports standard NMEA data messages. The $GPGSV data message contains satellite tracking information. Since the GPS automatically tracks GPS satellites when powered, this will give you information on the tracking status. If your receiver has computed a position, this will be contained within the $GPGGA data message. Additionally, SXblue receiver has LEDs that give status indications.

Am I able to have the SXblue output different NMEA messages through the ports?

Yes, you may have different NMEA messages turned on for the ports. Further, these NMEA messages may also be at different update rates.

How to interpret NMEA sentences (GGA)?

GGA – essential fix data that provides 3D location and accuracy data. $GPGGA,123519,4807.038, N, 01131.000, E,1,08,0.9,545.4, M, 46.9, M,*47 Where:   - GP represents that it is a GPS position (GL would denote GLONASS). GGA Global Positioning System Fix Data 123519 Fix taken at 12:35:19 UTC 4807.038, N Latitude 48 deg 07.038′ N 01131.000, E Longitude 11 deg 31.000′ 1 Fix quality: 0 = invalid, 1 = GPS fix (SPS), 2 = DGPS fix, 3 = PPS fix, 4 = Real Time Kinematic, 5 = Float RTK, 6 = estimated (dead reckoning) (2.3 feature), 7 = Manual input mode, 8 = Simulation mode 08 Number of satellites being tracked 0.9 Horizontal dilutions of position 545.4, M Altitude, Meters, above mean sea level 46.9, M Height of geoid (mean sea level) above WGS84 ellipsoid (empty field) time in seconds since last DGPS update (empty field) DGPS station ID number *47 the checksum data, always begins with “   

Am I able to configure the various ports with different baud rates?

Yes, the Serial, USB and Bluetooth ports are independent.

How do I interpret the LED lights on the receiver?

Red: When the receiver is powered, this LED will light up. Green: Once the receiver retrieves his position from the satellites, this LED will light up. Orange: As the differential position is computed, this light will stay on. Yellow: as the receiver connects to SBAS or Atlas, this light will stay on. Blue: Stays full when receiver and data collector exchange data.

How can I be sure that the configuration will be saved for the subsequent power cycle?

The surest method is to query the receiver to make sure you’re satisfied with the current configuration, by issuing a $JSHOW<CR><LF> command (if not, make the necessary changes and repeat). If the current configuration is acceptable, issue a $JSAVE<CR><LF> command. Wait for the receiver to indicate that the save is complete.   

Do I need 10/20 Hz output?

Most of the time, NO. Only a few customers using receivers in a moving vehicle and seeking high precision will need these activations.

Why was Auto-Dif developed?

Once Selective Availability was removed in May of 2000, it opened the door to develop this technology as environmental errors change much more slowly than SA did. The purpose of Auto-Dif was to market an alternative to users in place of costly subscription-based differential services around the world where free services such as DGPS beacon and SBAS did not exist.

Will Auto-Dif perform as well as other correction services?

Auto-Dif will perform with a similar level of performance as other differential services at first. There will be a slow drift to the positioning performance with increasing time. We normally allow Auto-Dif to age for a maximum of 30 to 40 minutes; however, it’s up to you on how long you will allow it to age. A simple update to the reference position will again provide best performance.

Will Auto-Dif correct for multipath?

No, there’s no method of differentially correcting multipath signals. There are only ways of reducing their impact on the measurements gathered by the receiver.

Ensuring that the SXblue receiver tracks properly

For best performance, you have to be careful such that the sky above the SXblue’s antenna is unobstructed for satellite tracking. SXblue tolerates a certain amount of signal blockage due to the availability of redundant satellites (only four are required for a position). However, as more satellites are blocked, the more impact this could have on your positioning accuracy. SXblue has an excellent record in this regard, as this case study shows.

How do I know if the SXblue is offering a differentially corrected position?

The “DGPS” LED is the primary indicator that the receiver is outputting differentially corrected positions. SXblue outputs the GGA message as the main positioning data message. This message contains a quality fix value that describes the GPS status. If this value is a 2, then the position is differentially corrected. PocketMAX utility is a useful tool that provides this information without needing to use NMEA commands.

SXblue is frequently losing lock on SBAS due to obstructions and the low satellite elevation angles at my geographic location

No, provided that the receiver is receiving a full set of corrections relatively often. Using the COAST technology, the iSXblue/SXblue will be able to perform well for up to 40 minutes with old correction data (similar to Auto-Dif, accuracy towards the end will not be as accurate as the beginning.) In order to obtain a full set of corrections, the iSXblue/SXblue antenna receives the ionospheric map over a period of a few minutes (about 5 minutes). This is the minimum amount of time required to get a full set of corrections for SBAS operation. After this, the receiver can COAST until the next set of corrections has been received.