1. Introduction
2. Principle of Bottom Tracking
3. Principle of (D)GPS
4. Our view on Bottom Tracking
5. Qliner River Discharge Measurement System
6. Conclusion

1.  Introduction (top)

Bottom Tracking (BT) together with (D)GPS is implemented by manufacturers of Doppler Profilers in their sensors to generally improve the accuracies for moving vessel operation.

In this document we summarize the arguments for not using Bottom Tracking in our Qliner discharge measurement systems.

2.  Principles of Bottom Tracking (top)

Bottom Tracking is a method for measuring speed-over-ground. A sonar emits acoustic pulses that are scattered by the bottom. Determining the Doppler shift in the returned echo provides a measure of the relative velocity between the Profiler and the bottom. This information is used to provide motion data required by the Profiler for its water-velocity measurement.

Whether on vehicles moving along the water surface or fully submerged, the Profiler measures velocity relative to itself, sometimes called apparent velocity. To derive actual velocities, one must correct the apparent velocity for it Profiler motion. Two applications apply depending on whether the source of the echoes received by the Profiler is fixed (sea-floor) or moving.

For the fixed position, the velocity sensed by the Profiler is caused by its own motion. Apparent and actual velocities are the same except for a change of sign. (Mobile river beds, "moving bottom", are an exception e.g. during floods).

For the moving position, which normally applies for water profiling, the Profiler velocity determined from bottom tracking is used to correct the profile of apparent water velocity to actual values.

Because the Profiler measures both the water motion and its own motion in the same reference frame, several potential error sources affecting the motion-corrected velocities are eliminated. This is not the case when other methods like (D)GPS are used to measure the Profiler motion. Correcting apparent velocities by using data from these other devices requires very careful orientation and calibration to avoid systematic errors in the resulting velocity estimates.

3.  Principles of (D)GPS (top)

(Differential) Global Positioning System is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these "man-made stars" as reference points to calculate positions accurate to a matter of meters, in fact with advanced forms of GPS you can make measurements to better than a centimeter. The basis of GPS is "triangulation" from satellites. To do this a GPS receiver measures distance using the travel time of radio signals. To measure travel time, GPS needs very accurate timing. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. Finally you must correct for any delays the signal experiences as it travels through the atmosphere. Basic GPS is the most accurate radio-based navigation system ever developed, but some engineers came up with "Differential" GPS, a way to correct the various inaccuracies in the GPS system, pushing its accuracy even farther. (D)GPS involves the cooperation of two receivers, one that's stationary and another that's roving around making position measurements, preferably with a communication link. Apart from private companies offering differential signals, nowadays other international agencies are establishing reference stations all over the world, especially around harbors and waterways. The latest improvements are using RTK systems pushing the accuracies even farther. As stated before the system is based on measuring distance using travel time, so a clear view from the receiver antenna to the satellite is essential. When working close to buildings, trees or bushes or even steep river banks the view to the satellites can be blocked causing accuracies dropping to unacceptable levels. It is also essential for high quality discharge measurements to use a very accurate system which needs mostly local differential stationary receivers to be set up, which is time consuming and expensive.

4.  Our view on Bottom Tracking (top)

Qmetrix, together with our mother company Nortek A/S, believe in using Bottom Tracking for most of the operating conditions for Profilers and has fitted this option in their Vessel-Mounted Doppler Profilers. However, for discharge measurements these are technical, operational and hydraulic arguments for not using Bottom Tracking.

Technical aspects and moving bottom

We do not need to give a long explanation of the phenomena "moving bottom". During relative high velocities there is a lot of suspended solids in the water due to erosion and this sediment moves mainly over the bottom. Systems using Bottom Tracking need a more or less solid bottom and especially higher frequencies produce louder returns from the water that can degrade bottom tracking in two ways:

*  The Doppler shift attributed to the bottom echo contains a much stronger contribution from moving water mass near the bottom. The bottom tracking velocity can become biased. In turn, this causes biased in velocity and discharge.

* The water-bottom interface can become difficult to discern in the bottom echo, resulting water depth becomes uncertain.

Operational aspects

We design sensors and systems that need to be used in many conditions. These systems must be robust and easy to operate. Unfortunately Bottom Tracking makes things more complex with an increased possibility of failure when you are on the site. Data processing is far more tedious.

* Keeping the instrument at fixed positions works fine under all conditions. The boat is more stable at a fixed position opposing the current than when it is dragged across the current and quite perpendicular to the main current direction.

* A Bottom Tracking system results in many shortly spaced flow profiles with quite low accuracies due to the short averaging time (seconds). Extensive data checking at the site is necessary.

Hydraulic aspects and accuracy

* Many rivers have a natural pulsation of some minutes. Making measurements in too short time does give systematic errors.

* Making a measurement at specified positions while averaging over periods of twenty seconds or one or two minutes results in accurate flow profiles which compare well with propeller or electromagnetic measurements. The methods to compute discharge from individual measurement cells are well established and are identical to the methods that are being used with propeller or electromagnetic measurements.

* The accuracy of a single Qliner measurement is many times as high as 1% (depending on conditions). This accuracy cannot be reached with Bottom Tracking systems and thus measurements have to be repeated at least four times. Total measurement time is then equivalent. The key element in measuring discharge is the accuracy of the measured total wet area, determined by depth and width. Even very accurate Bottom Tracking does not beat a simple distance measurement using a rope.

5.  Nortek Qliner River Discharge Measurement System (top)

The Nortek Qliner River Discharge Measurement System is specially developed for use in rivers and creeks where mostly a lot of suspended solids and (D)GPS errors can be expected, so taking into account the remarks made in chapter 4, it is apparent the Profiler used in the Qliner is not equipped with Bottom Tracking.

Other facts:

Facts out of USGS report 1):

The U.S. Geological Survey has made a comparison between the Qliner and a MBD (Moving-Boat Doppler) discharge measurement system. During test near Vernalis, the USGS collected 18 Qliner measurements and 141 MBD measurements. In the report is stated that the Qliner avoids the problems and errors associated with bottom tracking, is far lighter than Moving-Boat Doppler systems, and is designed for smaller rivers and streams with depth from 30 cm to 10 m.

The data of the two systems is compared against a USGS gage and shows the Qliner average is within 0,5% of the gage while te average MBD measurement is low by almost 4%.

Comparison of the BoogieDopp (Qliner) and MBD discharge with the USGS gage. Note that be MBD data above are individual measurements. The USGS typically uses averages of four measurements, which would reduce the scatter but not change the bias in the above data.

1) See report: BoogieDopp (Qliner) Discharge Measurements over a Moving Bottom in the San Joaquin River, Nortek USA, March 1, 2003

6.  Conclusion (top)

Many tests with the Qliner have produced accurate discharge measurements because its depth and velocity measurements are accurate and because it processes data carefully to compute discharge. Just as important, the Qliner keeps its method simple by leaving out bottom tracking velocity, (D)GPS and complex data collection modes. This means that the Qliner avoids some of the errors that Moving-Boat Dopplers are subject to.

In short:

The Qliner River Discharge Measurement System is simple and robust, easy to use, always succeeds and its accuracy is just 1 - 2%.