3.4.6. Types of GNSS receivers

A wide variety of GPS receivers are commercially available today. Depending upon the type of application, accuracy requirements and cost, the users can select the type of GNSS receiver which best meets the requirements. These receivers cover a wide range from the high-precision receivers with built-in atomic clock, to the hand-held navigation receivers, which can give the precise position to few-metres. Even wrist-watches with built-in GNSS receivers are now commercially available. Three broad categories of GNSS are explained below (Garg, 2021).

GNSS receivers come in many types and functionalities to suit different users’ needs. Users can choose receivers based on their specific requirements concerning application, precision, and cost. Applications may range from high-precision scientific research to casual navigation. For instance, a research scientist might use a high-precision receiver with an atomic clock, while a hiker might opt for a simple handheld receiver. Interestingly, GNSS technology is so advanced that we can even find it in wristwatches.

Think of GNSS receivers like different types of cars. Just as a family may choose a minivan for camping trips or a sports car for speed, a user may select a receiver based on whether they need high precision for surveying or just a simple device for navigation while hiking.

(a) Navigation receivers:

Navigation in three dimensions is the primary function of GNSS. Navigational receivers are made for aircraft, ships, ground vehicles, and for hand carried by individuals. These are used for navigation, positioning, time dissemination, measuring atmospheric parameters, surveying, geodetic control, and plate tectonic studies. These receivers are normally single-frequency, C/A code, hand-held light weight receivers, which can give the position with a few metres to few tens of metres accuracy. These receivers are very much portable, weighing only few hundred grams, and are fairly cheap. Single channel receivers, which can track 4 or more satellites, are now being replaced by two or five channel receivers. The accuracies in positioning obtained by these type of receivers are in the range of few tens of metres in absolute positioning (in the absence of SA), and few tens of cm in relative positioning, over short baselines of few km.

Navigation receivers are designed primarily for 3D navigation, which includes tracking altitude, latitude, and longitude. They are ideal for various transport modes, from personal hand-held devices to navigational systems in commercial airplanes. These receivers generally use a single C/A frequency and provide positioning accuracy ranging from a few meters up to tens of meters. They are lightweight and quite affordable. As technology advances, we see a trend from simple single-channel receivers to multi-channel ones that enable more accurate tracking by connecting multiple satellites simultaneously.

Imagine you are sailing in a boat and need to navigate through the ocean. A navigation receiver behaves like a skilled sailor using a detailed map and compass. It helps ensure you reach your destination accurately, even as waves and winds try to push you off course.

(b) Surveying receivers:

The surveying type of receivers are single frequency, multi-channel receivers, which are useful for most surveying applications, including cadastral mapping applications, providing tertiary survey control, engineering surveys, etc. They are more expensive than the navigational receivers, but more versatile. The data from many of these receivers can be directly imported to most commonly used GIS software packages. Most of these receivers can also be used in DGNSS mode.

Surveying receivers enhance the capabilities of GNSS technology, offering multi-channel functionalities that allow for improved data collection and processing. They often find applications in fields such as land surveying and engineering, providing precise location data essential for mapping and planning. They can send collected data directly to GIS software for further analysis, highlighting their versatility. While these receivers are pricier than basic navigation ones, their accuracy and adaptability make them a vital tool for professionals.

Think of surveying receivers as advanced digital cameras compared to standard point-and-shoot models. Just like professional photographers use advanced equipment to capture the most detail and clarity, surveyors use specialized GNSS receivers to gather accurate data for large-scale projects like city planning or land development.

(c) Geodetic receivers:

The geodetic receivers are multi-channel, dual-frequency receivers, generally with the capability of receiving and decoding the P-code. They are heavier and more expensive than the navigation and surveying receivers. They are capable of giving accuracies of few cm in absolute positioning with precise post-processed satellite orbit information and of few mm in relative positioning. These receivers are usable for applications related to geodetic, geodynamic, detailed GIS and topographic engineering survey, etc. A modern geodetic receiver should be able to measure accurately and reliably anywhere under any condition.

Geodetic receivers represent the peak of GNSS technology, typically incorporating advanced dual-frequency capabilities for unparalleled accuracy. They can pinpoint a location within a few centimeters and are essential for applications that require high precision, such as scientific research in geodesy or studying Earth movement. Although they come with a higher price tag and are bulkier, their ability to deliver highly precise measurements across various environmental conditions makes them indispensable for professionals engaged in critical projects.

Geodetic receivers can be likened to a high-end GPS system used for space exploration. Just as scientists need exact data to navigate spacecraft accurately, geodetic receivers provide the required precision for mapping Earth's shifting landscape or understanding tectonic movements.

The GNSS receivers can be classified into two basic types: (i) Code phase receivers, and (ii) Carrier phase receivers (Seeber, 2003; Dhunta, 2001).

(i) Code phase receivers

These receivers are also called code correlating receivers as they access the satellite navigational P- or C/A-code signal for their operation. They have a unique capability to begin calculations without having an approximate location and time. Code phase receivers provide real-time navigation data using almanac data from satellite message for operation and signal processing. These receivers have anywhere-fix capability as they can synchronize themselves with GNSS time at a point with unknown coordinates. For this purpose, we need to lock the signals of four satellites to start the survey with a quicker start-up time. In code based receivers, phase position of the received code sequence is compared with the phase of an identical code replica, generated by the receiver (using the same algorithm as used for the code from the satellites) via a correlation technique. Hence, the observable is also called the code phase. These receivers can be used for the rapid calculation of baselines where high accuracy is not required, for example, in exploration or offshore work. The two code sequences are shifted stepwise in phase until a maximum correlation is obtained. These receivers have a complete code dependent correlation channel which produces: code phase, carrier phase, change of carrier phase (Doppler frequency), and satellite message.

(ii) Carrier phase receivers

These receivers utilize the actual GNSS signals to calculate a position. Two common types of such receivers are; (i) single frequency, and (ii) double frequency, which are compared together, as given in Table 3.4. The single frequency receivers track L1 frequency signal only, and are cheaper than dual frequency receivers. They can be effectively used in relative positioning mode for accurate baselines of less than 50 km or where ionosphere effects can normally be ignored. The double frequency receivers track both L1 and L2 frequency signals, and are more expensive than the single frequency receivers. They can effectively be used to measure longer baselines of more than 50 km where ionosphere effects have a larger impact, as ionosphere effects are eliminated by combining L1 and L2 observations.

GNSS receivers can be categorized into two types based on their operational principles. Code phase receivers determine positioning by correlating received signals with a stored satellite code, allowing them to operate without prior knowledge of location. This type is particularly beneficial for applications not requiring extreme accuracy, like general exploration. In contrast, carrier phase receivers use the phase of the actual GNSS signals, which allows for significantly improved accuracy, especially in applications where longitudinal measurements over greater distances are required. By separating L1 and L2 signals, these receivers can mitigate atmospheric influences, resulting in more reliable positioning.

Imagine code phase receivers as a compass that tells you which direction to go but doesn’t provide an exact location. Carrier phase receivers act like a detailed GPS map showing not just the route, but your precise location on it. For instance, in scientific research requiring precision mapping, relying on the clear, accurate paths provided by a GPS map (carrier phase) is essential, compared to general navigation (code phase) where just knowing the general direction would suffice.

Following factors should be kept in mind while selecting a receiver (Seeber, 2003, Spirent, 2011, https://www.sparkfun.com/GPS_Guide):

(i) Capability of tracking all the signals from each visible satellite at any time (as GPS+GLONASS system needs 20 dual frequency channels) (ii) Have low phase and code noise (iii) Contains high data rate (> 10 Hz) for kinematic applications with high memory capacity (iv) Low power consumption, low weight and small size (v) Full operational capability under Anti-Spoofing (AS). (vi) Capability to track weak signals (under foliage, and difficult environmental conditions) (vii) Able to do multipath mitigation, interference suppression, stable antenna phase centre (viii) DGNSS and RTK capability (ix) 1 pps timing output (x) Ability to accept external frequencies (xi) Few or no cable connection (xii) Availability of radio modem (xiii) Can operate over difficult meteorological conditions (xiv) Ease of interfacing with other GNSS systems. (xv) Flexible set up (tripod, pole, pillar, vehicle)

When selecting a GNSS receiver, several factors play a crucial role in ensuring efficient and effective performance. These factors include the ability to track multiple signals simultaneously to improve accuracy and speed, the low noise levels that affect signal clarity, and the receiver's capacity to process high data rates suitable for quick applications. Size and power consumption are other concerns for portability, while the ability to function in challenging environments and connect easily with other systems can significantly impact usability. Collectively, these attributes help users identify which receiver best meets their specific surveying or navigation needs.

Choosing a GNSS receiver is like selecting a smartphone. You seek one that supports all your required apps (tracking signals), has a long battery life (low power consumption), is lightweight (small size), capable of taking clear pictures even in low light (tracking weak signals), and can connect to the internet easily (ease of interfacing with other systems). Just as you wouldn’t settle for a phone without these features, users should be thorough when considering which GNSS receiver to purchase.

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