4.2.3 GPS Hardware for Surveying:

SOME ISSUES CONCERNING HARDWARE

The following are some hardware issues that need to be borne in mind when weighing up the advantages of one GPS surveying instrument over another, and whether the performance of an instrument is subject to factors beyond the operator's (or manufacturer's) control:

• CODE-CORRELATING receivers can measure both pseudo-range and carrier phase.
  
• Instrumentation has been developed to give decimetre precision pseudo-range data from C/A CODE RANGING, but there are still problems with multipath effects.
  
• Given a choice, CODE-CORRELATING receivers collect a lower noise phase measurement.
  
• SQUARING permits phase measurements on L1 or L2, however CODE-CORRELATING permits only L1 measurements (phase or pseudo-range) with C/A code.
  
• CODE-CORRELATING with P code permits dual-frequency measurements to be made.
  
• ANTI-SPOOFING* affects CODE-CORRELATING with P code, but not C/A code.
  
• Proprietary techniques have been developed to extract L2 phase and pseudo-range data under AS conditions WITHOUT THE NEED FOR SIGNAL SQUARING (see below).
  
• There is no hardware "fix" for SELECTIVE AVAILABILITY** --> only military receivers will have the "crypto-key" to overcome SA.

* AS is now fully implemented
** SA is fully implemented on Block II/IIA satellites though the decision is up for review

Single or Dual-Frequency?

One of the most common questions that arises with regard to the selection of a GPS receiver to purchase (or use) is whether dual-frequency instrumentation is in itself superior to single frequency instrumentation. The following comments can be made:

• Dual-frequency instruments eliminate ionospheric delays (a measurement bias).
  
• Ionospheric delay is largely a problem for baselines greater than about 20-30km in length.
  
• Single frequency observations are adequate for short baselines using conventional GPS surveying techniques.
  
• Dual-frequency instruments are more expensive than single frequency receivers.
  
• Different implementations of dual-frequency capability , for example, code-correlating, squaring or proprietary techniques (see below).
  
• Dual-frequency instruments usually are either:
  • hybrid (code-correlating on L1, squaring or proprietary technology on L2), or
  • special L2 tracking design which allows for switching (for example, between P code-correlating and squaring/proprietary technology when AS turned on).
    
• Dual-frequency instrumentation is essential for high productivity "rapid static", "stop & go" and "kinematic" GPS surveying procedures because of the advantages of dual-frequency phase and pseudo-range data for rapid ambiguity resolution.
  
• There is intensive R&D being invested to improve tracking performance on L2 carrier wave.

Selecting a GPS Receiver for Surveying Applications

The selection of a receiver that best suits the surveyor's needs should take the following factors into account:

• ACCURACY: What level of accuracy is sought? Over what baseline length? Perhaps a dual-frequency receiver is recommended.
  
• RUGGEDNESS: How portable must the instrumentation be? Will it be mainly used for fieldwork, or will it merely operate as a (semi-) permanent base station?
  
• RELIABILITY: What generation of instrument is it? Is it a revolutionary design newly released (with potential reliability problems)? Has the software been thoroughly debugged?
  
• POWER REQUIREMENTS: How do they compare with competitors's instruments? How many options for power input are there?
  
• SERVICING: What competitive advantage is there here? What is the local agent support like?
  
• FINANCING: How much GPS surveying work is available? Considered a consortium agreement whereby a pool of instruments is purchased by different partners? Lease or buy option?
  
• FLEXIBILITY OF USE: How many options for field operation are there? Are these options expensive? Do they involve the purchases of additional hardware/software? Will it be used for kinematic work? Used for real-time operation? etc.
  
• STATE-OF-THE-ART: Can it support all the modern GPS surveying techniques?
  
• PRODUCTIVITY: Can it do "on-the-fly" ambiguity resolution in a matters of a few tens of seconds?
  
• STORAGE MEDIUM & CAPACITY: Is it adequate? Is it appropriate?
  
• SUITABILITY OF SOFTWARE: Is it provided? At what cost? Is it up to the task or accuracy requirements? How reliable is it?
  
• EASE OF USE: How easy is the receiver to operate? How sophisticated (but easy to use) is the data post-processing software?

The actual hardware configuration selected for single frequency instrumentation is largely immaterial as all receivers are capable of measuring the raw L1 phase to a similar level of accuracy. The measurement of L2 phase is more problematic, and proprietary techniques are generally used. Each technique, however, has its advantages and disadvantages (though they may only be obvious after careful and thorough testing), but the most critical issue is signal-to-noise ratios for L2 measurements. Ultimately the quality of the results will probably be more influenced by the processing software and field procedures, than the phase measuring hardware.

P Code Availability

P code-correlation permits both the precise pseudo-range on the L1 and L2 frequencies, and the carrier phase on the L2 carrier wave to be measured. The C/A code-correlating receivers are used only to make L1 pseudo-range and phase observations.

P code derived L2 phase measurement:

• P code-correlating receivers, or ones capable of P1 and P2 measurement using proprietary techniques, are generally specified for geodetic work.
• L2 phase measurement should preserve full wavelength (24cm).
• A distinct advantage for "rapid static" and "kinematic" surveying techniques using dual-frequency data processing.

P code pseudo-range measurements, on their own or in combination with phase data:

• Have lower noise than C/A pseudo-ranges.
• Permit the use of new surveying techniques (for ambiguity resolution) that mix pseudo-range and phase data.
  

However, there is no P code-correlating capability while AS is turned on!

L2 Measurements Under AS Without Knowledge of the Y Code

Without knowledge of the Y code receivers have to apply codeless or quasi-codeless proprietary techniques to make measurements on the L2 carrier wave. The tracking techniques presently available are summarised below. (Much of the following information is taken from ASHJAEE, 1993; HOFMANN-WELLENHOF et al, 1998. The reader is also referred to VAN DIERENDONCK, 1995.)

The Squaring technique has already been discussed in section 4.1.3. In essence, the received signal is multiplied with itself resulting in an unmodulated carrier with twice the frequency (half the wavelength). Its disadvantages are so great that it is no longer used in any GPS instrument:

• All signal information is lost, hence no pseudo-ranging on L2 is possible and the Navigation Message is lost from the L2 signal (though not a problem if the code-correlating tracking technology is applied on L1).
• As the L2 wavelength is only of the order of 12cm, ambiguity resolution is much more difficult.
• The signal-to-noise-ratio is the worse of all the L2 tracking techniques, resulting in frequent signal loss, poor data quality and many cycle slips (especially in moving antenna applications, or when the ionosphere is particularly active).

The Cross-Correlation technique was proposed 10 years ago, and is now implemented within a number of GPS receivers, including the Rogue and Trimble instruments. The technique makes use of the fact that the unknown Y code modulation is the same on both carrier waves. Due to the frequency dependence of the ionospheric delay, the Y code on the L2 signal is slightly slower than the Y code on the L1 signal. Hence the time delay of the L2 signal in relation to the L1 signal can be measured. The following comments can be made:

• The L2 carrier measurement is obtained by adding the (L2 - L1) delay (in units of cycles) to the L1 (C/A code-correlated) measurement.
• The L2 pseudo-range measurement is obtained in a similar fashion, by adding the (L2 - L1) delay (in units of metres) to the C/A code pseudo-range measurement.
• The L2 carrier measurement is of full wavelength (24cm).
• However, the signal-to-noise ratio is only slightly improved over using the pure squaring technique.

The Code-Correlation Plus Squaring technique is an improved squaring technique developed about by the Magnavox Corporation (now owned by Leica Geosystems), and is used in the Leica GPS receivers. The technique requires the correlation of the received Y code on the L2 signal with a locally generated P code. This is possible because the Y code is formed by the modulo-2 addition of the P code and the encrypting W code. As a result of the W code frequency being about 20 times less than the P code, there will always exist Y Code portions which are identical to the original P code portions. Hence the P code correlation occurs between the internally generated P code and the underlying P code of the incoming signal, and the result can be low-pass filtered. Then the signal is squared to get rid of the code. A modified version of this technology has recently been implemented in the latest generation of Leica products. The following comments can be made:

• The L1 and L2 pseudo-range measurement can be obtained.
• The L2 wavelength (Leica System 200) was only of the order of 12cm, hence ambiguity resolution is much more difficult. This drawback is overcome in the new Leica systems.
• The correlation with the P code yields better jamming immunity and an improvement in multipath rejection performance.
• The signal-to-noise ratio is significantly improved over using the above two techniques.

The Z-Tracking technique is one of the more interesting quasi-codeless techniques, and is used in the Ashtech GPS receivers. The technique is based on the removal of the encrypting W code through a relative complex procedure described in ASHJAEE (1993). The following additional comments can be made:

• The L1 and L2 pseudo-range measurements are obtained in the same way as standard P code correlation.
• The L1 and L2 carrier phase measurements are obtained, both with full wavelength.
• The signal-to-noise ratio is significantly improved over the Code-Correlation Plus Squaring technique.
• In reality the Z-Tracking technique has not always delivered the best tracking performance.

All techniques developed to overcome AS are sub-optimal, compared with using the P code-correlation technique!!

Codeless techniques to track L2 under AS.
(HOFMANN-WELLENHOF et al, 1998)

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© Chris Rizos, SNAP-UNSW, 1999