Table of Contents
Table of Contents

Maybe you’re confused about what STL is, or perhaps you’re considering an STL-supported product such as the SecureSync. This is a great place to start to learn more about this Iridium-based technology.

Frequently Asked Questions About STL

What is STL?

STL, or Satellite Time and Location, is a low earth orbit satellite based time reference. It uses the Iridium constellation of satellites to deliver a burst signal specifically designed for position, navigation, and timing applications. The technology was developed by Satelles and is being delivered commercially in partnership with Orolia.

Q: How is possible for the STL signal to work indoors?

The STL signal is delivered using the high-power paging channels of Iridium, a one-way transmission from a very high system gain. Combined with the low earth orbit of the satellites – about 500 miles above the earth compared to more than 10,000 for others – the STL signal arrives at the ground about 1,000 times stronger than GNSS signals. It’s this strong, one-way signal that allows it to be successful in working indoors.

Q: How deep indoors will the STL signal operate?

It is difficult to define for all cases, but a general rule of thumb is that if you can receive a mobile phone signal at an indoor site without the use of a cellular signal repeater, then you will likely be able to receive the STL signal. Of course, this may not always be true, because many factors go into signal propagation indoors. Conversely, if you are deep indoors – for example, in a basement datacenter with no exterior walls – and cannot receive a mobile phone connection from nearby cell sites, then you will probably not receive STL. STL is received through a cabled antenna connected to the SecureSync unit, so in these difficult situations, the STL antenna can be extended to a more suitable location for receiving the signal.

Q: How can you claim indoor operation when Iridium Satellite phones do not operate indoors?

The STL service uses the high-power paging channels of Iridium, which is a one-way transmission from the satellite that has a very high system gain. An Iridium phone uses a lower-power, two-way channel that has lower system gain and requires a line of sight connection to the satellite. The STL signal is completely different and should not be compared to the Iridium phone. The only commonality is that both are carried on the Iridium satellites.

Q: How accurate is STL?

The accuracy of STL is specified at better than +/- 500 nsec offset to UTC. However, in our testing and in real-world deployments, we’ve found the accuracy to be closer +/- 200 nsec offset to UTC.

Q: Is STL traceable to UTC?

Yes. STL is traceable to UTC (USNO) through GPS. Satelles maintains precise time uplink facilities to the Iridium constellation with precision UTC Time derived from GPS through a highly redundant and secure set of ground monitoring stations, or “teleports.”

Q: Because STL ultimately gets its UTC time synchronization via GPS, what happens in the event of a GPS failure?

Today, Satelles measures GPS Time with Rubidium-disciplined timing receivers at up to 25 ground stations around the world, then compares and crosschecks them. In the event of a GPS failure (such as what occurred in January 2016), the system will detect the error in GPS and utilize the rubidium oscillators until the GPS system recovers from the error.

In addition, Satelles has a cooperative agreement with NIST for using STL to distribute UTC(NIST) at sub-microsecond-level accuracy. You can learn more about this agreement here.

Q: What capability will Satelles STL have with the new Iridium Next satellites?

STL will maintain the same accuracy, exceptional availability and indoor capability as it does today.

Q: Are there interference issues with STL?

Although there can be interference issues with any radio signal, the Iridium signal is well established, having been in operation for almost 20 years. The high-power level of the STL signal allows it to operate in environments with interference that disrupts GNSS signal reception.

Q: In what environments has STL reception been demonstrated or deployed?

To know if the signal will work in a specific location, a site survey is necessary. However, the STL signal has been received and is in use in several challenging environments.

Q: How does STL deal with Multipath inherent to Indoor reception?

In an indoor environment, much of the received energy is from multipath, so the timing precision dilution that it causes is factored into the +/-500 nsec error budget. Because the Iridium satellites move through the sky very quickly — orbiting the earth every 100 minutes compared to 12-hour orbits for GPS satellites — multipath effects of STL average out very quickly.

Q: Are there similar alternatives to STL?

As of today, no. STL is the only non-GNSS satellite based reference. It is also the only commercially available, global, and encrypted signal reference available today.

Q: Can anyone obtain an STL receiver or are these controlled?

STL is a licensed (and patented) signal, exclusive to Satelles. The receivers are not government controlled but they must be purchased through a Satelles-authorized channel such as Orolia. The receiver must also be properly licensed and registered to make use of the signal.

STL and SecureSync

Q: What features of SecureSync are available with STL?

All standard features are available in SecureSync with STL. The STL receiver is an option card that can be added to SecureSync.

Q: I already have a SecureSync. Do I need to send it in to have STL installed?

No, the STL option module is field upgradable. You simply install it into your SecureSync like you would other option modules.

Q: Will STL be offered in other Orolia products?

SecureSync is the first of several products available with STL service. Stay tuned for additional product News or Contact Us to learn more.

Q: Can the STL signal augment other GNSS systems besides GPS?

Yes! All the same advantages apply when adding resilience and trust to the precise time and frequency outputs of the SecureSync when it is operating with any of the GNSS constellations like Galileo or GLONASS.

Q: Can the SecureSync operate with only the STL signal, without any GNSS signal?

Yes. The STL signal provides SecureSync all the necessary date, time and traceability to UTC for it to perform its time and frequency server function standalone without any connection to GNSS signals or any other timing reference.

Q: Can I operate in Mobility Mode with STL in SecureSync?

Not yet. Expect further mobility developments coming soon, or for immediate project needs, Contact Us for a quote.

Q: I am putting my SecureSync on a secured network that cannot have any connection to the outside world. If STL is using the Iridium signal, does this mean there is an outside connection that could breach my network?

No. STL is a one-way signal only, from satellite to ground, and the STL receiver being a receive-only device. There is no transmit hardware in this device.