Inside Frequency Control

While GPS technology plays an important role in modern navigation, it's not perfect. Although GPS might perform well enough consumer usage, signal jamming or other disruptions present serious threats to military operations. 

Enter assured positioning, navigation, and timing (A-PNT), new technology that enhances traditional GPS timing. Assured PNT uses additional sources to augment GPS and prevent vulnerabilities. Its goal is to ensure that accurate positioning, navigation, and timing (PNT) is always available.

In this blog, we'll review GPS, its flaws, and other PNT systems (like the GNSS) and give you an overview of how assured PNT technology uses other systems to improve effectiveness. 

GPS disciplined oscillators (GPSDO) are some of today's most trusted and accurate sources of timing. These powerful devices (sometimes called GPS clocks) consist of a high-quality stable oscillator and a GPS receiver. The GPSDO works by disciplining (or steering) the oscillator output to a GPS device or GNSS satellite signal via a tracking loop.

Before we dive into the differences between the 5 GNSS constellations... It's important that we're all on the same page with the difference between GNSS and GPS.

Many people get GNSS and GPS technology confused. A good way to think about the Global Navigation Satellite Systems (GNSS) is as the backbone (or underlying technology) behind GPS. The Global Positioning System (GPS) GPS is a GNSS constellation, but GNSS is not always GPS. GPS is one of the 5 GNSS constellations used around the world.

The 5 GNSS constellations include GPS (US), QZSS (Japan), BEIDOU (China), GALILEO (EU), and GLONASS (Russia). We'll cover each of these constellations in-depth in this post.

5G Fixed Wireless Access (FWA) will be one of the earliest uses of 5G. 5G FWA will provide lightning fast gigabit internet speeds to homes, apartments, and businesses. Better yet, 5G FWA will be a fraction of the cost compared to traditional cable & fiber systems.

The U.S Department of Homeland Security considers 13 of the nation’s 16 critical infrastructure sectors to be ‘critically dependent’ on PNT technology. Also, a loss of PNT would cost the U.S an estimated $10 BILLION a day. Not only do many everyday consumers use PNT & GPS on a daily basis, it's also crucial for military and intelligence operations. Other industries that strongly rely on GPS based PNT include

• Radio communications
• Network synchronization
• Intelligence systems
• Information gathering
• Cruise missiles
• Weapons systems
• Electronic warfare

Building resilience into Global Positioning System (GPS) timing and frequency receivers is crucial in the 21st century. Since the presidential directive on U.S Space-Based Position, Navigation, and Timing (PNT) and Critical Infrastructure Protection (PPD-21), federal agencies have been advancing their efforts when it comes to GPS and Global Navigation Satellite System (GNSS) capabilities. 

The critical component that is becoming more of a necessity for cubesatellite platforms is the GPS Disciplined Oscillator, GPSDO for short.  GPSDOs comprise of a GPS receiver, Oven Controlled Crystal Oscillator (OCXO), and a Phased Locked Loop (PLL). In this post, you'll get a deeper look into how GPSDOs are designed to help achieve mission success for your small satellites or cubesatellites. 

The Global Navigation Satellite System (GNSS), in geo-stationary orbit, provides a GPS signal that many use today on the ground for navigation.  With a GPS receiver (along with a GPS antenna) in place on board the satellite, a very precise 1 pulse per second signal can be generated from the signal transmitted from the GNSS (10^-12 depending on how well the GPS receiver was designed). 

With the 1PPS signal from the GPS receiver, the OCXO can be steered using a phased locked loop.  The PLL essentially compares a PPS signal derived from the OCXO to the 1PPS signal generated by the GPS receiver and a control circuit adjusts the OCXO frequency to synchronize both signals at the same timing offset (or phase).  The GPS receiver is a great solution for long-term stability because OCXOs degrade over time with respect to stability, known as aging (i.e. increase in deviations over time from the desired frequency), and over temperature as well. 

This degradation is being compensated over time with the control circuitry in the PLL which that adjusts the frequency of the OCXO as the PLL is locking the derived PPS signal from the OCXO to the GPS 1PPS signal.  However, short-term stability on the 1PPS signal from the GPS receiver is quite noisy due to atmospheric fluctuations, jitter within the GPS receiver, and other noise contributors.  The use of a very stable OCXO for the GPSDO remedies this issue (ultra-stable quartz OCXOs can have short-term precision on the order of 10^-12 or 10^-13).

The GPSDO design enables for a great short-term stability (from the OCXO) and long-term stability solution (from the 1PPS GPS signal).  A system block diagram is provided below:

One of the most significant changes to the way the military operates has been the integration of GPS for positioning, navigation, and timing purposes, which has given military forces unprecedented command and control over battle grounds. Troops and command centers are now in a state of continuous situational awareness, with an endless stream of data being added to military servers at all time.