Ionospheric measurement with GPS: Receiver techniques and methods

Accurate characterization of ionospheric parameters such as total electron content (TEC) and scintillation (signal fluctuation due to ionospheric irregularities) is critical to all users of GPS, whether the ultimate goal is measurement in navigation, geodesy, ionospheric or atmospheric studies. Improved absolute TEC measurement accuracy is demanded by many global ionospheric characterization schemes, where small errors can be magnified in 3D tomographic profile reconstructions. Our research indicates that there are three errors, or biases that typically result from characterizing TEC with GPS receiver data. The Center for Remote Sensing, Inc. has produced a GPS receiver, and researched techniques that mitigate these biases. These biases are: 1. Estimation, instead of measurement of receiver differential code bias (DCB) 2. Ionospheric divergence of pseudorange code derived TEC resulting from code smoothing 3. Delay of pseudorange TEC as a result of code smoothing We present results of ionospheric data collected with a receiver that mitigates these biases to demonstrate the utility of improved accuracy particularly for ingestion into tomographic reconstructions, but also for conversion from slant to vertical TEC. GPS carrier phase derived TEC provides a smooth but relative measurement of ionospheric TEC, while code derived TEC provides a noisy but absolute measurement. However, this absolute measurement is plagued by instrumental biases that must be accounted for before GPS data can be reliably used for ionospheric characterization, and these are the so-called receiver and satellite differential code biases or DCBs. The CRS ionospheric monitoring receiver contains a patented built in calibration of receiver code and phase bias. We will present a technique that uses this internal calibrator to measure receiver DCB to sub-centimeter level or less then 0.1 TECU. Demonstrations of data collected with multiple receivers and other techniques will be presented that verify the effectiveness of this calibrated receiver DCB. To mitigate inherent fluctuations in pseudorange due to bandwidth limited precision, receiver noise, and multipath, typical GPS receivers generally employ socalled phase smoothing or leveling of code. Phase smoothing is essentially some combination of the noisy code pseudorange with the comparatively smoothly varying carrier phase. These smoothed pseudoranges are typically the only pseudorange output available to a GPS receiver user. As we shall demonstrate, this smoothness is achieved at the expense of imposing bias on the code TEC estimate. A standard method for phase smoothing is the Hatch filter, and we will show that employing a Hatch filter delays the code TEC from the true code TEC by approximately 1/3 of the smoothing time (typically 200- 1000 seconds). We will present comparisons demonstrating that a standard GPS receiver employed for ionospheric monitoring possess a code TEC that is delayed by approximately 30 seconds. Therefore if the ionospheric delay is changing, the code estimate of TEC will be biased by an amount equal to the change in delay over that 30 second or longer period, which is often as large as 0.5-2 TEC (depending on filter time and ionospheric conditions). A Hatch filter also imposes a bias on the code derived TEC during ionospheric changes due to what is known as ionospheric divergence (phase advance versus group delay). We will derive and demonstrate using GPS measurements, that this bias is equal and magnitude and opposite in direction to the change in ionospheric delay over the course of a smoothing time, and presents between a 1 TECU and 20 TECU bias for normal to storm-time conditions.