SparkFun RTK Facet L-Band Hookup Guide
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How RTK Facet L-Band Works
The SparkFun RTK Facet L-Band is unique in that it receives its corrections from u-blox's PointPerfect service broadcast from a geosynchronous Inmarsat satellite. No base station is needed, no external radio link, no internet connection or RTCM corrections are needed to achieve RTK fix. The RTK Facet L-Band does not require creating an account anywhere, does not require copy and pasting of keys, certificates, or any other materials. Everything is built into one unit and made to be as easy as possible to use.
What magic is this?
A normal RTK setup requires a Base and Rover. The Base sits still and provides RTCM corrections over a radio or internet link to a rover. These corrections aid the Rover in its location calculations allowing it to remove small ionospheric and tropospheric distortions. When it's all working, you can achieve 14mm X, Y, and Z accuracy. For many applications this works fine, but for a variety of applications a unit is often located far from cellular connectivity or a base station setup is not possible. For this reason, u-blox has built a large network of base stations across the contiguous 48 states of the USA and the EU. The correction data from these stations are aggregated into a series of correction feeds. Those feeds are then uplinked to, and broadcast from, a geosynchronous satellite operated by another company call Inmarsat. In the USA the satellite is broadcasting at 1.55629GHz and in Europe a different Inmarsat is broadcasting at 1.54526GHz. These frequencies are handy in that they are in the same band as the GNSS satellites: L-Band. L-Band is any frequency from 1 to 2 GHz. These frequencies have the ability to penetrate clouds, fog, and other natural weather phenomena making them particularly useful for location applications.
The single antenna atop the RTK Facet L-Band is a unique combination of elements designed to receive the GNSS signals (L1/L2) alongside the 1.55GHz PointPerfect corrections. These signals are sent to the ZED-F9P for classic GNSS position calculation (e.g. producing latitude, longitude, and altitude of the device), and to the NEO-D9S. The NEO-D9S is a satellite receiver who's sole purpose is to be tuned to a frequency (in our case ~1.55GHz) and output data. This data is the encrypted correction data in the SPARTN format. The RTK Facet L-Band receives these packets and passes them to the ZED-F9P. At startup, the ESP32 provides the ZED-F9P with decryption keys. With the packets and the keys, the ZED-F9P is able to decrypt the data and use it to correct its GNSS fix allowing the accuracy to increase from ~300mm to the RTK accuracy of 10 to 20mm.
It is fun to point out the GNSS satellites have a medium earth orbit, circling the earth every 12 hours, and have a orbit above the earth at approximately 22,200 kilometers (12,551 miles). Whereas the Inmarsat satellites broadcasting the PointPerfect correction values at ~1.5GHz are geosynchronous and therefore must be located much further out, at about 35,786 kilometers (22,236 miles).
Where can the RTK Facet L-Band operate?
USA and EU Coverage Map of PointPerfect Corrections
Currently the u-blox PointPerfect service is only offered for the USA's contiguous 48 states and the EU. Please see u-blox website for additional information. If L-Band coverage is not available in your area, the RTK Facet L-Band is still capable of better than 300mm accuracy off the shelf and 14mm accuracy when provided RTCM corrections using a standard Base/Rover setup.
How much does this cost?
The cost of a RTK Facet L-Band includes a 12 months subscription to u-blox's PointPerfect L-Band service. u-blox does not currently offer PointPerfect L-Band service to individuals. SparkFun formed a partnership with u-blox to allow SparkFun users to obtain L-Band service through SparkFun's account.
How do the 'keys' work?
One of our mantras at SparkFun is Don't Make Me Think based on a book by Steve Krug. The RTK Facet L-Band does not require creating an account anywhere, does not require copy and pasting of keys, certificates, or any other materials. The provisioning process is complicated, so we've designed the firmware within the RTK Facet L-Band to deal with everything. The only thing a user must provide is a 'Home' WiFi network ID and password. We use the term 'Home' but it is the network the device is mostly likely to be turned on around, for example the surveyor's office, school, or residence. Once these details are entered via WiFi or serial, the device will take care of everything else.
How do the L-Band keys work?
The correction data that is broadcast via Inmarsats is encrypted. The decryption keys expire every 28 days and two keys are provided during each update process. This means the RTK Facet L-Band can operate without any internet connectivity for a maximum of 56 days best case, 29 days worst case. Every time the device is powered on the RTK Facet L-Band will attempt to connect to its 'Home' WiFi. If successful, the keys will be automatically be updated and stored. The whole process takes lass than 10 seconds (worst case). The RTK Facet L-Band will show a series screens indicating what its doing but the device is designed to be used without concern or supervision.
What happens after 12 months?
The retail price of the RTK Facet L-Band includes 12 months of PointPerfect corrections. After 12 months the device will be unable to obtain keys and the device will operate as a normal, very high accuracy GNSS receiver. If additional PointPerfect corrections are needed they can be purchased for $600 per year by filling out the form here.
Can I get a unit without the subscription?
u-blox does not currently offer PointPerfect L-Band service to individuals. If you are interested in purchasing a large number of RTK Facet L-Bands, we can work with you to provide subscription-less units but it is up to your organization to confirm that L-Band service will be provided. The RTK Facet L-Band allows for a custom Device Profile Token. Once you have an account setup with u-blox, this token will need to be entered into all RTK Facet L-Band devices and they can provision through the normal PointPerfect API. Please contact SparkFun customer service for more information.
What are the down sides to L-band corrections?
Cons:
- RTK fix time using L-Band is 45 to 60 seconds as opposed to RTCM based RTK which can take a few seconds.
- Increased cost via subscription fees to u-blox.
- Accuracy varies from 30 to 60mm guaranteed with 14 to 20mm regularly seen.
Pros:
- High precision RTK fixes without any setup.
- Very easy to setup and just use. No base, no radio, no internet needed.