INNOVATION INSIGHTS with Richard Langley

Richard Langley

GOOD NEWS, EVERYONE! Instant GPS positioning appears to be at hand. For better or worse, we live in fast-paced society with its fast food, fast communications, and fast cars, and have come to expect instant responses when we want something. Our TVs now turn on instantly. We push a single key on our mobile phones for speed dial or instant push-to-talk service. We press the shutter on our digital cameras to capture and view images instantly. But GPS? Not so fast.

After switching on our receiver, we typically have to wait for some time before we can start navigating. This time to first fix (TTFF) depends on the quality of the received signals and the age of the receiver's stored almanac and ephemerides used to determine the positions of the satellites. It's also affected by how well the receiver knows the exact time. So there are several kinds of TTFF.

If a receiver has no knowledge of its last position, doesn't know the approximate time, and has no almanac, it starts searching for signals blindly. This is called a cold start. Depending on signal quality and the design of the receiver, it can take anywhere from 60 seconds to 12 minutes or more before the receiver acquires signals, obtains ephemeris data, measures pseudoranges, and gets its first position fix.

If the receiver knows the approximate time as well as its approximate position and has a recent almanac but not a current ephemeris, it can produce a position fix within about 30 seconds or so after it is switched on — the time required to receive orbit and clock data from the tracked satellites. This is called a warm start.

A hot start occurs when a receiver is powered on with a current ephemeris (received within the past four hours). It can take up to 6 seconds or more before the first fix as the receiver must typically acquire time marks from the satellite navigation messages to resolve the pseudorange ambiguities.

Assisted GPS, or AGPS, can reduce TTFF by supplying current ephemeris data and accurate time over a mobile phone network. In some situations, TTFF can be reduced to just a second or two. However, the receiver does need to be connected to an AGPS network and so cannot operate autonomously.

Enter BGPS. In this month's column we learn about an innovative approach that can produce accurate first fixes within one second without a network connection. Within one second and without a network connection? Oh my, yes.

"Innovation" is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic ideas. To contact him, see the "Contributing Editors" section.

In an Expert Advice column in the October 2006 issue of this magazine, one of us wrote:

"With hundreds of navigation satellites in the sky, and a handful of GNSS-enabled gadgets, the taxpayer would expect to get the resulting service — his instant position at any time and at any place — without becoming a specialist in satellite navigation. This is our challenge today: providing seamless instant positioning at any location, at any time, at the touch of a button."

Since autumn 2007, we have been pooling our efforts to create the necessary technology, which would enable us to meet these goals. We have now developed a procedure that provides an immediate position fix at the touch of a button. This novel approach has been developed on the borderline between assisted GPS (AGPS) technology and that used for geodetic applications. We used an opportunity to implement this technology to create a software receiver, which meets the requirements of the Japan Aerospace Exploration Agency (JAXA) for an airborne software receiver. JAXA is investigating robust navigation systems for aircraft operation, including precision landing. The software receiver will be used for the development of an ultra-tightly-coupled GPS/inertial navigation system (INS) unit to cope with ionospheric anomalies and interference. This project also allowed us to validate the receiver and the associated technology.